Potash Fertilizer: Is There a Problem?

In a publication brought to the public’s attention by news release from the University of Illinois several weeks ago, S.A. Khan, Richard Mulvaney and a colleague in the Department of Natural Resource and Environmental Sciences at the University of Illinois challenged a number of basic tenets of soil fertility, especially practices related to use of potassium (K) fertilizer. Citing hundreds of references and thousands of reported studies, they asserted that: K fertilizer is generally unnecessary in soils like most of those in Illinois; the soil test is not a reliable way to know how much K the soil will supply to a crop; K used as fertilizer can cause crops to have lower nutritional value; using K fertilizer can damage soil structure; and potassium chloride (KCl), which is the most commonly-used (and lowest-cost) K fertilizer material, is harmful to crops.

I won’t try to assess these assertions in detail, but will make some comments here to keep us focused on the larger issues of supplying nutrients to crops to prevent deficiencies and crop yield loss.

Can soils provide enough K without fertilizer?

Clay minerals in soils contain huge amounts of K, measured in tons per acre. Plant-available K – that found in soil solution or bound to exchange sites on soil particles – is typically measured in (low) hundreds of pounds per acre. The K in minerals is part of their chemical structure, and becomes plant-available very slowly – a few pounds per acre per year in many soils – as these minerals break down with weathering. Very low crop yields and K removal, or no crop removal (as was the case before Illinois soils were farmed) will result in plant-available K levels either not dropping or slowly increasing. Crop roots can pick up K from deeper soil layers, and over time will bring K up to the soil surface, where it will be released when plants die.

One of the items reported in the publication is that soil test K level in continuous corn in the Morrow Plots increased by about 67 percent over 50 years without any fertilizer application. They based this on a soil sample taken in 1955 and another in 2005. Soil test K levels in samples taken frequently from 1967 through 2008 in the Morrow Plots did not show such trends, however (Figure 1). Levels of K increased in both fertilized and unfertilized plots after deep tillage to relieve compaction in the late 1990s, but yields (and K removal) rose after that, and K levels came back down as removal rates rose.

Figure 1. Soil test K levels in fertilized and unfertilized continuous corn in the Morrow Plots, 1967-2008.


The key to whether soils can supply enough K to meet crop needs is whether the crop removes K faster than the soil can free up K through the weathering process. At the low yield levels common 75 to 100 years ago, when return of nutrients to the soil as manure was common, K levels probably dropped slowly if at all even without fertilizer K. Today, a corn-soybean rotation with good yields will remove as much as 100 lb of K over a 2-year period. Most soils in Illinois can supply nowhere near these amounts, and so K levels will drop if no K fertilizer is added. How long it will take for a deficiency to appear will depend on how much plant-available K is present. But let’s not fool ourselves – K deficiency will appear at some point if removal continuously exceeds replacement. The only reasonable way to replace removed K is with K fertilizer.

There are soils in the world, including in parts of the western Corn Belt, where plant-available K levels are naturally high, due to the mineralogy of the parent materials from which the soils developed and the age of the soils. There are also places where soils have been weathered and leached for so long that K levels are very low. The soils in Illinois, and in most of the eastern Corn Belt US Corn Belt, fall into neither of these categories. Here, soils free up K more slowly than crops remove it, so if we are going to produce crops that remove K, at some point we are going to have to add some K back.

Is the K soil test useful?

There is no question that measuring plant-available K in soils is difficult, and that soil-test K levels vary over time, often not very predictably. Part of this is a sampling issue – soil test K levels often change quickly over short distances, so samples show variability. Soil moisture affects K tieup in clay minerals, and even the way soils are dried before testing can affect the soil test level. Despite all this, low soil test K values are often predictive of crop deficiencies, and crop K deficiencies are rare (though possible, especially when soils around the roots are dry) when soil test levels are high. Yield increases from adding K fertilizer are much more common when soil test K levels are low than when they are high. So while it’s easy to find fault with the soil test for K, the test (if samples truly represent the soils in a field) does tell us whether soil K levels are high enough to support full yields or whether deficiency and yield reductions are likely without adding fertilizer K. That’s usually all we need to know, especially when fertilizer K is routinely added back to replace that removed by crops.

Is K fertilizer harmful?

This is the area in which the authors take a rather odd turn, claiming that adding fertilizer K reduces crop nutritional value, damages soil structure, and (as KCl) can be toxic to plants. Evidence given on all of these counts is rather flimsy, and there’s little to say other than that any such effects are, based simply on quantities, minor or so small as to be non-detectable, or at worst, detectable but unimportant.

Claims that adding fertilizer K will lower “quality” of corn or soybean grain are without merit, and can be ignored. Corn and soybean grain K content can increase with increasing K rate in studies, but there is some recent evidence that grain K levels in both crops might be even lower than the “book values” used for years. Levels, and any problems claimed to be associated with such levels, are certainly not increasing.

The next claim has to do with the idea that K “makes the soil hard” and damages structure. As a monovalent cation, K can, if added in large quantities and given time, displace some of the other cations on soil exchange sites. If most of these exchange sites carried monovalent cations such as K or sodium (Na), soils would tend to “puddle” and be difficult to manage. The number of exchange sites is measured as the CEC, which range from single digits to the 40s or higher, depending on soil texture and organic matter. Each CEC unit occupied by K translates to 780 lb of K in the top 7 inches of soil. The great majority of exchange sites carry divalent cations such as calcium and magnesium, and this will continue to be the case even if we add hundreds (and in many soils thousands) of pounds of fertilizer K per acre.

There have been a number of efforts over years to show that the chloride in KCl can lower crop yield. How this would happen in Corn Belt soils is not clear. Like its cousin NaCl (table salt), KCl is a salt, and roots that encounter high salt concentrations in the soil can be damaged. But KCl can usually be banded with no negative effect, though there have been some reports of lower yields with banded K. Reasons for this are often not clear, but are more likely to be due to salt effects than to the chloride itself. As a negatively-charged anion, chloride moves through the soil quickly as water moves. Chlorine gas (Cl2) is harmful to all life, but its formation in the soil is chemically unlikely, and only tiny quantities would ever form. Chlorine is actually an essential plant nutrient, though some crops (like wheat) respond more to it than do corn and soybeans. It’s also known to reduce damage from some plant diseases. Plants, like people can deal with unneeded Cl by simply excluding it or getting rid of it once it’s been taken up.

The authors mentioned, without offering much evidence, that potassium sulfate would be a better K source than KCl. There’s nothing wrong with K2SO4 as a K source, but it is considerably more expensive than KCl, and there is no evidence that it’s a better source of K for corn and soybeans than is KCl.

Can we get better at fertilizing with K?

Even though the authors asserted that K fertilization is unnecessary in most Illinois soils, they (rather inconsistently) also suggested that producers do their own on-farm strip trials to see if adding K will increase crop yields. This is not a bad idea, as long as we keep in mind that soil test K levels in the medium or higher ranges – say above 250 lb per acre (125 ppm) usually means low chances of seeing a response. We can add to this the usual warning that looking at a few strips might lead us to wrong conclusions just by random chance of where the strips are located. But we have proposed a project to organize just such an effort in Illinois under the Nutrient Research & Education Council (NREC), and will be giving further information about this if the proposal is funded.



Issues with Nitrogen Fertilizer: Fall 2013

With 85 percent of soybean and 74 percent of corn acres harvested by October 27, the annual process of deciding when and how to supply nitrogen fertilizer for the 2014 corn crop is underway.

Lessons from this past year

Following the very dry first half of 2012 and low corn yields, soil sampling last fall revealed an average of about 80 lb N present as nitrate in the top foot of soil in Illinois. With a lot of rain in late winter and early spring, soil N levels by spring had dropped considerably. And early spring application in 2013 was often followed by a lot of rain before the crop started to take up much N.

The 2013 season was wet early then dry late, but with good root systems that took up water and nutrients well to produce good yields. So far overall N loss this past season does not seem to have been greater than normal. Evidence to support this comes from N trial data just coming in. In an on-farm study coordinated in Champaign County by Dan Schaefer of C-BMP, corn responded almost exactly the same, and with the same yield levels, to fall-applied NH3 as to UAN sidedressed in June. In a study with continuous corn at Perry, sampling in June showed that most of the N applied as UAN in April was still present after more than 12 inches of rain, and yields barely increased going from 180 to 240 lb N, at a yield level of about 200 bushels per acre. At Monmouth, supplementing 165 lb of N as UAN applied in early April with 60 more lb of N in early June, after some 18 inches of rain, did not significantly increase yield of about 240 bushels per acre.

High corn yields mean that large amounts of N were taken up. In the 2013 UI corn hybrid trials, the average yield among the three sites where we measure grain protein was 228 bushels per acre, and the average protein content (at 15% moisture) was 8.4 percent. That calculates to removal of 0.74 lb N per bushel, or 169 lb N per acre. We typically estimate that 2/3rds of the N in the plant at maturity is in the grain; that would calculate N uptake at more than 250 lb N/acre. When we can get 250-bushel yields with 180 lb of N as we did in some cases in 2013, it’s clear that the soil supplied a great deal of N to the crop.

It does not appear that much of the “extra” N needed for the high corn yields in 2013 came from N that was left over after the drought of 2012. As is typical, soybean fields had very low amounts of soil N after harvest in 2012, yet corn following soybeans is – again, as usual – showing less response to N rates than corn following corn in 2013. What data I have so far indicates that corn following corn may not be taking a big yield hit like we’ve seen in some areas in recent years. It’s possible that some of this is because of leftover N, but corn following corn is also showing a typical response to N rate, so it’s not clear that leftover N was a major contributor.

With the weather dry over much of Illinois since August (until rainfall this week), can we expect soil N levels to be high again this fall? I’ve seen numbers from only a few soil samples so far this fall, and they seem mixed, but generally lower than we saw in fall 2012. The crop clearly had the root system to tap into water that was deep in the soil in order to produce the yields that it did, and it’s likely that it brought a considerable amount of N up along with this water. We might expect that this meant removal of much of the N mineralized from soil organic matter, but with some rain now and soil temperatures still in the 50s, there is still some mineralization going on, at least for a few more weeks. But in general, we expect soil N levels to be more or less this fall; we have typically measured only 20 or 30 lb per acre of nitrate-N in the top foot, or even less.

Fall Nitrogen?

The big question that many people have is whether or not to apply N fertilizer this fall or to wait until next spring. We know from nitrate levels in rivers that an appreciable amount of the N present last fall left fields through tiles lines last spring. Fall-applied N generally went out late enough last fall, and soils turned cold and stayed cold after application, so most of the N lost in tile lines last spring came from leftover N rather than from N fertilizer applied last fall. The fact that we’re seeing similar responses to fall- and spring-applied N would support that most fall-applied N stayed in the soil and available to this year’s crop. We are not seeing the unusually large responses to applied N this year that we would expect to see following high N losses.

The basics of fall N application have not changed: the form should be anhydrous ammonia (NH3); soil temperatures should be at or below 50 degree F at the time of application; using a stabilizer/nitrification will slow the activity of microorganisms that convert ammonium to nitrate (the form that can leach); soils should not be wet or very dry, but should have enough water to allow the ammonia to spread to a diameter of 4 to 6 inches as it is released in the soil; and application depth should be 6 inches or more so the NH3 gas doesn’t escape after application. Fall NH3 application should not be done in southern Illinois due to higher chances of soil warming in the fall and earlier warm-up in the spring, and N losses also go up on poorly-drained soils or very light soils, making fall application more risky.

Except when fall weather is warmer than usual, soil temperatures in Illinois reach the 50-degree mark by about November 1; if they don’t, they are usually on the way down and will reach that level soon. You can see soil temperatures in Illinois at a number of websites; the Illinois State Water Survey site at http://www.isws.illinois.edu/warm/soiltemp.asp gives daily minimum and maximum temperatures at the 4-inch depth under bare soil. Minimum values were below 50 in the northern part of the state on October 30, and maximum values were in the mid- to upper 50s through northern and central Illinois. Keep in mind that there will be some biological activity even at 50 degrees, and any warm spell after application will mean more conversion to nitrate. The goal should be to have soils approach freezing temperatures with as much of the N still in the ammonium form as possible.

It is certainly the case that nitrapyrin or other nitrification inhibitors may not be necessary when soil temperatures are low at the time of application and stay low into the spring. Inhibitors also begin to break down as soils warm in the spring (or get or stay unusually warm in the fall), so that by the time the plants are ready to take up N rapidly, often in early June, much of the N will be in the nitrate form. So what we really hope to get from an investment in an inhibitor is a delay in the conversion to nitrate, so that more of the N is still in the ammonium form as soils warm and water starts to move through the soil, taking nitrate with it. On the other hand, when NH3 is applied late enough, the winter is cold, and the spring is dry, there’s little N loss regardless of N form, and an inhibitor will provide little benefit. This means that using an inhibitor is an approach to managing risk.

Another approach that some take to managing risk of N loss is to apply high N rates in the fall, with or without inhibitor, with the idea that some N loss can happen but that there will still be enough N available the next spring. This can certainly work in terms of having enough N, but it comes at a high environmental cost. Not only do we know that high yields of corn grown in productive soils often do not need the high N rates that some producers apply, we also know that too-high N rates will, sooner or later, mean more loss of N to the environment. While loos to the environment may not seem to be a “real” cost (though the additional N is a real cost), it is a real cost, in terms of things like water treatment to remove nitrate, and in terms of image.

One practice that some have adopted is to apply only part of the N in the fall, with the rest applied the next spring. This approach should reduce the amount of N loss if soil conditions become conducive to loss of fall-applied N. And it provides one of the underappreciated benefits of fall-applied NH3, which is having N dispersed through the soil and easily accessible to the plant in the spring. The drawback is that NH3 application is rather slow and costly compared to most other methods of N application, and so applying lower rates increases the cost per lb of N applied.

I mentioned a few days ago the use of subsurface banding of P and K, accompanied in some cases by placement of NH3 beneath this band in the same operation. Making one trip to apply fertilizer increases efficiency, and as long as soil temperatures are low enough, this can work well. As is the case with banded P and K, there is little evidence that applying NH3 under the P-K band provides a yield advantage over applying dry and N fertilizers separately. There has been some tendency in the past for such “dual placement” operations to start before soils are cool enough for fall N application, which increases N loss potential.

One issue with fall NH3 application in recent years has been whether or if to combine application with tillage. Some have applied N and then tilled, while others have tilled first and then applied (and sometimes tilled again after that). Ammonia is extremely soluble in water, and once dissolved in soil water it converts to ammonium. So if there is a moderate amount of soil moisture present, losses of NH3 should be relatively small, even if soil is tilled. Tillage does form air pockets in the soil, however, and if a marginally dry soil is tilled before NH3 is applied, some NH3 could be released before it can dissolv. This would be noticeable as puffs of “smoke” (actually, water droplets attracted by ammonia) after the applicator, and as the smell of ammonia.

Tilling after application does turn soil up where the sun can warm it, and the warmer temperatures might increase conversion to nitrate. If the soil is dry, tilling after application could also release some NH3 that was not dissolved, especially if the soil dries even more as a result of tillage. It also is probably worth asking as well if soil following NH3 application really needs tillage.

N Rates

One of the advantages of applying less than full rates of N in the fall is that it delays the final rate decision until the spring, allowing us to note loss conditions, planting dates, and other factors that might affect the rate we apply. At the same time, as we have seen this past year, the amount of N loss can be difficult to determine. This means that the tendency to apply some “insurance” N to make sure there’s enough operates in the spring as well as in the (previous) fall. In fact, despite a lot of evidence to the contrary, there’s an abiding thought that bringing out full yield potential for a crop that gets off to a good start in the spring will require more than the usual amount of N. We may have had a little less of this than usual this past spring – with the crop planted as late as it was it didn’t seem to be off to a great start. The fact that we are hearing of some yields above 250 bushels per acre, often coming with “only” normal amounts of N, should help a few more of us to start to question whether high yields only come when we pour on the N.

The N rate calculator located at http://extension.agron.iastate.edu/soilfertility/nrate.aspx remains our best “range-finder” for guiding the process of determining N rates. For central Illinois corn following soybeans, and with NH3 priced at $680/ton ($0.41/lb N) the current guideline rate is 167 lb N/acre (204 lb of NH3), and the range is 154 to 183. In northern Illinois, the most profitable rate under these same prices calculates to 149 lb N/acre, with a range of 137 to 163. As we have noted before, the ratio of N to corn prices tends to stay relatively constant over time, with a bushel of corn equal in value to about 10 lb of N (in the form of NH3). Changes in fertilizer prices by next spring are likely not to greatly change the guideline rates, but of course adjustments in total N applied can be made in the spring.

Cover crops and N?

Cover crop seed has been dropped or drilled into a lot of Illinois fields this fall. Ongoing dry weather has meant delays in germination in many areas, and temperatures down into the low 20s last week might have damaged some small cover crop plants, especially in harvested fields with cover crop plants more exposed. Growth of the cover crops will hopefully pick up now, but delays at the start may mean less growth before cold weather sets in.

We expect that a cover crop with vigorous growth and a good root system will take up some N left in the soil in the fall, and more N next spring if the cover crop overwinters. Once taken up into cover crop biomass, N is less likely to be lost to tile lines. From a crop standpoint, the N in the cover crop will be of value only if the next crop is one like corn that requires N from outside sources. The breakdown and release of cover crop N and its supply to the next crop is a biological process that depends on the weather. Cover crop residue in the spring can also affect soil temperature and water content, and so can affect the planted crop in ways other than through N supply.

One idea that has been floated is that fall cover crops can help take up fall-applied N, thus keeping it safe from loss and preserving it until the corn crop is up and growing next spring. This may happen to some extent, but fall uptake will usually be limited if the cover crop starts to grow well only after harvest of the crop it’s planted into, and if fall N is applied only after soil temperatures reach 50 degrees; this will typically give only a few weeks of uptake, or even less. Applying NH3 into growing cover crops will cause some damage to roots, and when soil temperatures are cool and dropping, it’s not likely that such roots will be able to take up much of the fertilizer N up before soil temperatures get low enough to halt root activity.

Uptake of fall-applied N increases as soils warm in the spring, but vigorous cover crops like cereal rye need to be killed in March, and soil temperatures by that time are usually still cool. As an example, average soil temperature at the 4-inch depth reached 61 degrees near Champaign in mid-March of 2012, one of the warmest Marches on record, but reached only 46 degrees by the end of March in 2013 and did not reach 60 this year until the end of April. So while we think that cover crops can have beneficial effects on N nutrition, we don’t expect this to be consistent over years. We certainly cannot afford to get sloppy with fall N thinking that cover crops will bail us out to prevent loss.


Fall Fertilization for the 2014 Crop

Corn and soybean harvest continues to move along in Illinois, and as the 2013 crops come off, thoughts turn to fall fertilization. In this article we’ll discuss nutrients other than nitrogen. This will be followed soon by an article on nitrogen.

P and K

In areas with dry soils, we have in recent years had reports of lower than expected soil test K values. There might be some of this in 2013, but we’re also hearing some reports of soil test P and K levels higher than expected. Test levels lower or higher than previous test levels, additions, and removals would suggest are not uncommon, and reasons for this are not always clear. Unusually low soil test levels need not be taken as a sure indicator of deficiency; neither should high tests be taken as an indication that nutrients are not needed. As long as previous soil tests were not low, and nutrients removed in harvested crops are being replaced with fertilizer additions, crops should be getting the nutrients they need.

Some questions have been raised about whether the “book” values for P and K grain concentrations, used to calculate removal, are still accurate. Dr. Fabian Fernandez, now at the University of Minnesota, produced data from Illinois grain samples (taken mostly in 2009) showing average P and K concentrations in corn grain of only 0.27 and 0.19 lb per bushel, compared to the book values (from the Illinois Agronomy Handbook) of 0.43 lb P and 0.27 lb K per bushel. Soybean values were closer to book values, at 0.69 (new) compared to 0.83 (old) lb P and 1.17 (new) compared to 1.30 (old) lb K per bushel. Other states also have some data showing lower values than the ones commonly used.

It’s certainly possible that higher yields and different genetics over the decades since these book values were produced might have lowered these values. But until we have more data to confirm this, it seems prudent not to lower removal/replacement amounts by a lot. Let’s consider an example in which soybeans in 2012 yielded 45 bushels per acre and corn in 2013 yielded 180 bushels per acre. This would produce 2-year P removal totals of 115 lb P per acre under the old (book) values, and only 80 lb P per acre using the new values. Using the old values would calculate removal of 107 lb K per acre, and the new values would calculate to 87 lb K removed per acre.

A reasonable approach might be to split the difference, calculating replacement as the average of the old and new removal amounts. In this example, that would mean replacing (115 + 80)/2 = 98 lb P and (107 + 87)/2 = 97 lb K per acre. An exception to this might be where P and K test levels are likely to be on the low side, in which case we might want to base replacement on the old book values to minimize the chances of deficiency.

Materials and placement

Most P goes on as ammoniated phosphates MAP or DAP, but forms such as DAP manufactured to include other nutrients such as sulfur and zinc are also marketed. We have not seen much response to additions of S, Zn, or other micronutrients in Illinois field crops, but can’t rule out possible responses. Micronutrients such as manganese, iron, and zinc are required in small amounts and can be partially immobilized if they are applied long before crop uptake begins, so are often applied in the spring; applying these in the fall can probably work if rates are adequate. Sulfate can leach much like nitrate, so elemental S, which is gradually converted to sulfate in the soil, is a safer form of S for fall application than is sulfate.

Another practice that is growing some in Illinois is placement of P and K in subsurface bands, usually 4 or 5 inches deep. Some may call this strip-tillage, though much strip-tilling is done without fertilizer placement. Strip-tillage usually includes planting on top of tilled strips in the spring, a practice that is not always followed with nutrient banding. In some cases fields are tilled after banding, probably shallow enough not to disturb the band, and rows might or might not be on top of the bands. Banding P and K can also be coupled with NH3 application in the fall, with the NH3 knives usually running a few inches deeper than the dry fertilizer band. Equipment to do this was much in evidence at the Farm Progress Show this year.

The equipment expense, power requirement, and relatively slow speed of application compared to broadcast fertilizer make banding fertilizer more expensive than broadcast. This means that banding needs to increase yields or lower fertilizer costs in order to make it pay. There has been a fair amount of research done on this, and most has shown little effect of this practice on yield, compared to broadcast-applying the same fertilizer rate and form. When crops are planted over the band, roots of the crop reach the band of fertilizer quickly, and the crop can take up most of the P and K it needs from the band. But roots have to go out into the bulk soil to take up water, and there is no advantage to having the plant take up most of its P and K in only the small part of the root system – the roots in the band – rather than throughout the root system.

In soil with very low soil test levels, roots in the bulk soil might not encounter high enough P and K levels, in which case either band placement or broadcast fertilizer should help. Without tillage, broadcast fertilizer nutrients will take more time to get into the root zone than if nutrients are banded. Finally, in soils with a tendency to tie up nutrients, banding can increase availability compared to dispersing the nutrients throughout the soil. These factors are of limited importance in Illinois soils with medium or higher soil test levels of P and K – in other words, in soils where P and K are seldom limiting to yield.

As a final point, remember that banding is a tillage operation, and that there could be a tillage effect separate from the effect of fertilizer placement in such systems. Of course, one trip to till and apply fertilizer is efficient, but strip-till by itself is less costly and usually faster than is band-placing fertilizer.

Lime and gypsum

Lime corrects low pH in soil, and with dry soils and with time for the lime to react in the soil to raise pH before spring, fall is the best time to apply it. We don’t see much indication that more expensive forms of lime, such as that shipped from a distance to better balance calcium and magnesium in a soil, provide much return to what is typically their higher cost.

In some parts of Illinois, usually those within reasonable shipping distance of a coal-fired power plant, gypsum is actively marketed as a soil amendment. Calcium carbonate (lime) or material derived from lime is used to remove sulfur from flue gas, resulting in formation of calcium sulfate, or gypsum. Such scrubbing is required, and coal-fired plants produce large amounts of gypsum. This gypsum represents a disposal challenge for power plant operators, and finding uses for it has been an ongoing process. Some gypsum is used to produce wallboard and other products, but gypsum continues to accumulate, and in some cases is being stored in landfills.

The two major selling points for gypsum use as a soil amendment are that it can serve as a source of sulfur, and that calcium ions can help bind soil particles together, which can help improve soil structure. Some also promote gypsum as a calcium fertilizer, but we have no good evidence that, given the large amount of exchangeable Ca in Illinois soils (some natural, some from lime application) and low requirements of our field crops for Ca, there is any need for Ca fertilizer. We also don’t have much evidence that S deficiency is widespread in Illinois, but applying gypsum will certainly provide S to a crop that follows. The S in gypsum is in sulfate form so can be leached through the soil, but quantities of gypsum typically applied are large enough that any crop S requirement will still be met.

Use of gypsum as a way to improve soil structure and drainage has been promoted fairly heavily in recent years. The basis for this is that Ca in soil solution exists as a divalent cation, with two (positive) charges. Clay and organic matter surfaces are negatively charged, so Ca ions act as a sort of “glue” to hold these particles together, making soil structure more granular and improving tilth, aeration, and water movement. In soils with a lot of sodium, including the “slick spots” common in some parts of Illinois, Ca ions will, if added in large quantities and given time, replace some of the sodium ions and improve permeability and soil productivity.

The real question regarding addition of large amounts of gypsum to improve soil productivity is whether the improvement is enough to pay the cost. Soils of medium or heavy texture already contain a great deal of Ca, and adding a few hundred or even a thousand or more pounds of Ca in the form of gypsum (or lime) may not produce much noticeable effect. High-clay soils, often targeted for marketing of gypsum due to their perceived poor structure, would require a great deal of gypsum (Ca) to produce an effect. So while adding gypsum to soil may have some effect, it is not at all clear that productive silt loam or silty clay loam soils with adequate (tile) drainage and with pH maintained by adding lime will be noticeably improved by this addition.

Perhaps because both contain calcium, there continues to be some thought that, like lime, gypsum will raise pH when applied to soil. But it is the carbonate in lime that provides the neutralizing effect, and straight gypsum has no carbonate, so does not affect soil pH. Scrubbers in some older power plants might produce gypsum that contains some lime, but neutralizing ability of such material would need to be confirmed by testing, and in many cases is probably negligible.

Brownstown Agronomy Research Center Field Day – July 25

The 2013 Brownstown Agronomy Research Center Field Day, presented by the University Of Illinois Department Of Crop Sciences, will be held on Thursday, July 25. Extension researchers and specialists will address issues pertinent to the current growing season. Tours will start at 8 a.m., with the second and third groups leaving the headquarters around 8:20 a.m. and 8:40 a.m. The tours will last about two and a half hours and will be followed by lunch provided by U of I Extension.

Shaded tour wagons will take participants to each stop. These topics will be addressed:

  • Nitrogen Sensors & Variable-rate N Applications – Dennis Bowman
  • Wheat Disease I.D. & Management – Dr. Carl Bradley
  • Emerging Developments in Weed Management – Doug Maxwell
  • Crop Rotation:  Another Risk Management Tool – Dr. Emerson Nafziger
  • Agronomic and Environmental Assessment of Cover Crops – Dr. Angie Peltier

 The 208-acre Brownstown Agronomy Research Center has been conducting crop research on the claypan soils of southern Illinois since 1937. More than 30 research and demonstration projects are conducted at the Center every year. Visitors are always welcome.

The research center is located south of Brownstown on IL Route 185, approximately 4 miles east of the IL Route 40 / 185 junction.

For more information, contact Robert Bellm (618-427-3349); rcbellm@illinois.edu
Visit us on the web at http://web.extension.illinois.edu/barc/

Mark Your Calendars for the 2013 AGMasters Conference

The 2013 AGMasters Conference will be held at the i Hotel and Conference Center in Champaign, IL on December 2 and 3. The conference will begin with a morning general program followed with 1 1/2 days of specialized sessions. Participants will be able to pick and choose the sessions of most interest to them. These sessions are designed to encourage interaction between instructors and students and cover a broad range of topics including crop production challenges, soil fertility, water resource management, entomology, plant pathology, weed science, and introductory statistics. Each session is taught twice and is limited to 40 students (per session). Registration for the most popular topics is very competitive. The overall conference is limited to the first 160 registrants. The conference will be pieced together over the summer and registration information will become available by early fall. For now, please add these dates to your calendar and look for more conference information to follow in this Bulletin. Conference co-chairs include Dennis Bowman, Carl Bradley, Aaron Hager, Sandy Osterbur and me, all members of the Crop Sciences Department. As the growing season unfolds, please contact any of us with your suggestions for the 2013 program. We welcome your input.

Mike Gray

Supplemental Information about Soil-Residual Herbicides Applied to Emerged Corn

The following table provides supplemental information to an article (“Soil-Residual Herbicides Applied to Emerged Corn”) published last week.  Table 1 summarizes information about postemergence applications of more traditional soil-applied corn herbicides.  Please consult the respective product label for additional information.


Table 1.  Maximum Corn Size for Postemergence Applications of Soil-Residual Herbicides

Herbicide Maximum Corn Size for Broadcast Application
Prequel, Princep, Verdict Before corn emergence
Balance Flexx, Corvus 2 leaf collars
Bullet, Micro-Tech 5 inches
Bicep Lite II Magnum, Cinch ATZ, Cinch ATZ Lite, Parallel Plus, Stalwart Xtraa 5 inches
Breakfree, Breakfree ATZ, Breakfree ATZ Lite, Degree, Degree Xtra, FulTime, Harness, Harness Xtra, Keystone, Keystone LA, SureStart/TripleFLEX, Surpass, TopNotch 11 inches
Atrazine, Bicep II Magnum, Expert, Guardsman Max, G-Max Lite, Lexar EZ, Lumax EZ, Outlookb 12 inches
Resolve DF 12 inches (V5)
Hornet WDG 20 inches (V6)c
Python WDG 20 inches or V6
Resolve Q 20 inches (through V6)
Callisto, Prowl H2O, Zemax 30 inches (V8)
Cinch, Dual II Magnum, Me-Too-Lachlor II, Stalwart C, Parallel 40 inches
TriCor Prior to tassel emergence

aAll of these products are labeled for directed applications to corn up to 12 inches tall.

bOutlook is labeled for layby applications to corn up to 36 inches tall.

cHornet is labeled for directed application to corn up to 36 inches tall.

Determining How Much Nitrogen Is Present

Fall Nitrogen

With a still fresh memory of the drought conditions during last year, recent rains have reduced concerns over water availability for the start of the 2013 growing season, but at the same time, concerns over nitrogen (N) loss have increased. Nitrogen loss is difficult to predict because it depends in many factors such as time of N application, type of N source, soil type and temperature, and the amount of precipitation received. While it is difficult to know how much N is lost without a direct analysis of soil N, I would like to provide some information that can help you determine what to do about N applications this growing season.

Most of the fall-applied N is either ammonium (NH4+) or a form that transforms rapidly into ammonium. Nitrification, or the conversion of ammonium to nitrate (NO3), is a bacteria-mediated transformation. The bacterium Nitrosomonas converts NH4+ into nitrite (NO2) while the bacterium Nitrobacter converts NO2 to NO3. The activity of these bacteria is minimal at temperatures below 50ºF.  These bacteria also need aerobic conditions (unsaturated soil-water conditions) to nitrify ammonium. Thus, the amount of nitrification that occurs in the soil is largely dependent on soil temperature and the time elapsed from application until the soil becomes saturated with water. Further, the nitrification process can be reduced with the use of nitrification inhibitors that reduces the activity of Nitrosomonas and allow N to stay in the ammonium form for a longer period of time.

When soils become saturated, the potential for N losses is directly related to the amount of N present in the nitrate (NO3) form. While wet soil conditions this spring may be a reason for concern that some of the N applied last fall may be lost, the cold temperatures we had until recently likely substantially reduced nitrification.  From now on, as temperatures increase, nitrification will also increase. When nitrate is present and soils warm up, N loss will start under saturated water conditions mostly through denitrification in fine-textured soils and through leaching in coarse-textured soils or intensively drained soils.

An important point to keep in mind is that the portion of the applied N that is in nitrate form is only subject to denitrification or leaching. However, the fact that N is in the nitrate form does not mean that N is lost; it simply means that it is susceptible to loss. Data from a study conducted a number of years ago (Table 1) provides a measure of the percent of ammonium that was transformed to nitrate by the end of May from three locations in Illinois depending on whether a nitrification inhibitor was used and when the application was done.

Table 1. Percent of ammonium converted to nitrate from date of application until the end of May at three locations in Illinois.
Ammonia Application Date Ammonia without N-Serve Ammonia w/ N-Serve
DeKalb Bondville Brownstown DeKalb Bondville Brownstown
% of Ammonia Nitrified (present as nitrate on May 25)


90 100 55 65 88
Dec.1 60 65 100 45 55 70
Mar.15 50 60 100 20 30 53
Apr.1 35 40 48 15 20 48

After determining how much of the N is in the nitrate form, it is possible to estimate how much N is potentially lost through denitrification based on soil temperature and the number of days the soil has been saturated. Current 4-inch depth soil temperatures can be accessed at http://www.isws.illinois.edu/warm/soiltemp.asp. In Illinois we have seen that for each day the soil is saturated with water, 1 to 2 % of the N in the nitrate form is lost via denitrification when temperatures are below 55°F.  When temperatures are between 55 and 65°F the loss is 2 to 3%, and when temperatures are above 65 to 70°F losses are about 4 to 5%.  Again, these losses are not for the total nitrogen applied, but rather for the portion that is in the nitrate form.  Loss will vary depending on different factors, but these values are intended to provide an estimate.

The following calculation is a hypothetical situation given as an example using the data in Table 1 and current soil temperatures below 65°F:

Let’s assume that 180 lb N/acre were applied in early November with a nitrification inhibitor in a silty clay loam soil in DeKalb and soils were saturated for 5 days in late April.

First calculate N present as nitrate
N applied x % in nitrate form
180 lb N/acre x 0.55 = 99 lb N/acre

Second calculate N denitrified
N in nitrate form x % denitrified
99 x 0.15 (5 days x 3%/day)
15 lb N/acre lost

If leaching is a greater concern than denitrification, for each inch of precipitation, nitrate moves approximately 5 to 6 inches in silt loam and clay loam soils and approximately 12 inches in coarser textured-soils.

Carryover Nitrogen

The above discussion was for fall N applications. However, this year an important concern is the potential for loss of the carryover N for fields going back to corn this year. As we indicated in earlier bulletin articles some fields had substantial amounts of N after last growing season. Most of this N was in the nitrate form, thus the potential for loss by leaching since last fall was not related to temperature, but by the amount of excess precipitation. The fall and winter were relatively dry and most of the precipitation was used to replenish water in the soil profile. Unfortunately, excess precipitation in April has made it so that some of that carryover N is no longer available.

What To Do Now

Regardless of how much nitrogen may or may not be present, if you have not planted your field yet, it will be more important to plant now and apply additional N later so planting is not delayed further. One way to determine the need for additional N in fields where substantial carryover N is suspected or where the soil has high potential for mineralization (for example fields where manure was applied within the last 2-3 years) is to use the pre-sidedress nitrate test (PSNT) during late May to early June. This timing should work well to allow results to comeback from the testing lab with ample time for a sidedress application. An additional advantage to collecting soil samples starting in late May is that the test provides a good reliable measure of N because 1) the potential for N loss of all the N present in the soil is low by the end of May and 2) since soil temperatures are warm by then the test will measure mineralized N from the soil organic pool in addition to other sources such as carryover N. The sample needs to be collected from the top 12-inches of the soil. lf the field had a history of broadcast applications, randomly collect 20 to 25 samples from an area no greater than 10 acres. If band applications of fertilizer or manure were used to fertilize the previous crops, make a sample by collecting at least 10 sets of three cores each between two corn rows. The first core is collected 3 inches to the right of the corn row, the second core in the middle of the two rows, and the third core 3 inches to the left of the next corn row. Collecting a sample less than the full 12 inches or not collecting all the cores will produce unreli­able results. If the samples cannot be delivered to the labo­ratory the same day, either freeze or quickly air-dry the sample. Make sure to tell the labora­tory that you want to measure NO3 nitrogen. If the test results comeback above 25 parts per million (ppm) no additional nitrogen will be necessary. If results are less than 10 ppm a full rate will be needed (as long as no yield potential has been lost). If test levels are between 25 and 10 ppm, the N rate should be adjusted proportionally.

Another way to determine if additional N will be needed is to establish a couple strips with 10 to 25% more N than the full application amount planned, and then compare the color of the crop as the season progresses. If the strips are substantially greener than the rest of the field any time before tasseling, that would be an indication that additional N is needed. Some of the potential drawbacks to be aware of when using this approach are that color differences could develop too late for a timely application, or that there might not be sufficient rain to move the late-applied N into the root zone.

Spring Soil Nitrogen Following the Drought of 2012

Last fall, with funding provided through the Illinois Council for Best Management Practices (C-BMP), GROWMARK, C-BMP, and the University of Illinois initiated the N-Watch soil sampling program to see how much inorganic N remained in the soil following the drought of 2012.

Fall sampling revealed fairly high amounts of soil N, with 151 samples statewide averaging 19.5 ppm of nitrate-N in the top foot of soil. We multiply this time 4 to get lb of N per acre, so soils represented by these samples had an average of 78 lb of nitrate-N per acre. Samples from northern Illinois had higher levels (26 ppm) than those from central and southern Illinois (both 18 ppm), even though 2012 corn yields in northern Illinois were higher than in central or southern Illinois. The second foot of soil depth had more than 15 ppm of nitrate-N, which meant another 62 lb of nitrate-N per acre, for a total of 140 lb N per acre.

One reason for fall sampling is to inventory soil N in order to know the potential for loss of leftover soil N. If the weather stays dry through the fall and winter, we expect minimal loss of soil N. But precipitation returned to normal over most of Illinois during the past 6 months, most areas showing only small departures from normal over the October 2012 to March 2013 period (Figure 1). Normal precipitation from October 1 through March ranges from about 12 inches in the northern edge of Illinois to about 20 inches at the southern tip of the State.


Figure 1. Precipitation and departure from normal (in inches) in Illinois, October 1, 2012 through March 31, 2013. Source: Midwest Regional Climate Center.








While nitrate-N moves readily down into the soil profile as water moves down through the soil, the lack of rainfall during the 2012 growing season meant that nitrate from fertilizer N and from soil organic matter simply accumulated. Water only moves as far down as it takes to wet the soil, and in a very dry soil, it can take 6 to 10 inches or more of water to wet the soil. Even more water is needed to move through to deeper layers and into tile lines. Indications are that most tile lines in Illinois have been running for the past few weeks in the areas that were dry longer, and for the past couple of months in areas that received more rain and rain starting earlier.

Not surprisingly, a few reports in recent weeks from sampling tile line outflow show elevated levels of nitrate-N. This is normal for spring outflow, but with little or no tile line outflow as N accumulated last summer and fall, and with the large amounts of nitrate we found in fall sampling, the flush of nitrate-N may be larger than normal this spring.

How much soil N has been lost?

Dan Schaefer of C-BMP took both fall 2012 and spring (March) 2013 samples in a few fields in east central Illinois that can give us some estimates of N loss. In one field where N was applied in the early spring in 2012, October samples had about 16 and 13 ppm of nitrate-N in the top and second foot of depth. In mid-March 2013, those numbers had fallen to 6 and 10 ppm, respectively. Assuming no net conversion of N from ammonium to nitrate, this reflects a loss of about 50 lb of nitrate from the top two feet. Much of this probably remains in the soil below the top 2 feet, but some may have entered the drainage tile by now.

In a second field in which N was applied as spring sidedressed ammonia, nitrate-N went from 24 and 8 ppm in the first and second foot of soil last October, to 9 and 18 ppm, respectively, by mid-March of this year. That represents a net loss of only about 20 lb of N per acre, but a considerable amount of movement of nitrate from the top to the second foot. Nitrate losses have possibly been limited because it took so long (and so much precipitation) for dry soils to rewet to the point that water started to move through the profile.

Ammonium-N is immobile in soil, but in warm soils, microbes generally convert it rather quickly to nitrate-N. Because of this we normally see only 2 or 3 ppm of ammonium-N in either the fall or the spring. We found an average of about 5 ppm of ammonium-N in both the top and second foot of soil in the 2012 fall samples. In the second field described above, the fall sample had about 11 ppm of ammonium-N in the top foot. This could have come from spring-applied ammonia or from mineralization of soil organic matter after rain in September. The surprise is that the March sample still had 9 ppm of ammonium-N in the top foot. It is clear that there was limited net conversion of ammonium to nitrate in the top foot of soil between mid-October and mid-March.

Does this mean that ammonia applied in the fall of 2012 is still present? Most of the ammonia was applied after soil temperatures had dropped to below 50. Soils also stayed relatively cool, with average soil temperatures at the 4-inch depth of 49, 45, 34, and 34 degrees in November, December, January, and February, respectively. In contrast, 4-inch average soil temperatures in January and February were 39 and 41.5 degrees, respectively, in 2012. There were only 3 days in January and February 2013 with average 4-inch soil temperature of 40 degrees or more. So we think that soil temperatures have stayed low enough this winter to minimize the conversion of ammonium to nitrate.

We don’t have many direct measures of how much ammonia applied in the fall of 2012 is still present, but based on a few estimates, we believe that loss of NH3 applied in fall 2012 has been relatively small. One field that Dan Schaefer sampled in October 2012 had 11.5 and 7.6 ppm of nitrate-N, and 3.7 and 2 ppm of ammonium-N in the top and second foot, respectively. In November, 100 lb of N as NH3 was applied. March samples showed 6.5 and 11 ppm nitrate-N and 12.7 and 2.4 ppm of ammonium-N in the top and second foot, respectively. Probe samples from the application band are not a very reliable way to measure N, but if we assume a 35-lb loss of nitrate-N (average of the other two fields described above), the probe sample “found” about 2/3rds of the N that was applied last fall, and about half of that remains in the ammonium form. Compared to some fall-to-spring changes in ammonium-N reported in the literature, we think that nitrification (conversion of ammonium to nitrate) and loss of fall-applied N have been less than normal this winter.

Other sampling done over the past month, in some cases to see if winter wheat should have N topdressing rate adjusted, has shown nitrate-N to be fairly low, often less than 5 or 6 ppm. The winter wheat crop would not have taken up much N when samples were taken, but soil nitrate-N does drop by 1 ppm for each 4 lb. of N present in the wheat (or other) crop at the time of sampling.

Spring Sampling, 2013

The priority for spring sampling should be those fields where corn in 2013 will follow corn (from which fall samples were taken) in 2012. We suggested last fall that any spring sampling done in order to adjust rates for the 2013 corn crop should best be done close to corn planting time, or at sidedress time. However, it would be useful if at least some of those sites where fall soil N levels were high could be sampled within the next few weeks so that we could better guess how much additional sampling would be useful.

I suggest that sites with fall nitrate-N above 25 ppm nitrate-N in the top foot (that is about 25% of Illinois sites) be sampled first, but any and all sites can be re-sampled if volunteers are willing to do this. All those who coordinated sampling last fall and who requested forms and shipping materials through me will get an email in the next few days with brief instructions. The process will be the same as it was last fall, with samples sent to A&L for analysis, and funding provided through C-BMP. Those who sampled under GROWMARK’s direction last fall will continue in that program this spring.
Those who did not sample fields last fall but would like to do so this spring are also invited to participate. Instead of sending requests through me, however, we ask that those sampling this spring for the first time send requests to Jean Payne jeanp@ifca.com at the Illinois Fertilizer & Chemical Association, who will pass along the request for sampling materials and instructions.

It might be difficult to get the deep (1 to 2-ft) samples in fields where soils remain wet. We think the 1-ft sample will show us how much N rate adjustment might be appropriate. Those willing and able to take samples from the second foot are encouraged to do so, however.

It is best to take samples before any N has been applied in the spring, though we can avoid the band if, for example, some starter was used at planting and sampling is done after that. Combine in a bucket enough samples to represent the area you want to represent, and take a subsample of this to send to the lab for analysis. Soils should be sent for analysis as soon as possible, and kept refrigerated if needed, to minimize N transformations before analysis.

Adjusting N rates based on spring sampling

The pre-sidedress N test (PSNT) and the pre-plant N test (PPNT) were developed to make N rate adjustments based on N already present in the spring. Adjustments are not generally suggested if the soil has less than 10 ppm of nitrate-N in the top 6-7 inches (10 ppm is about 20 lb. of nitrate-N per acre), and no additional fertilizer N is suggested if the surface soil has more than 25 ppm nitrate-N (some states use 20 and some use 30 ppm as this limit). Fertilizer N rates are decreased as surface soil nitrate-N increases from 10 to 25 ppm.

Because of uncertainty in sampling, many universities suggest making adjustments as ranges; for example, the N application rate might be reduced by 30 to 50 lb. N per acre if the soil has 10 to 15 ppm of nitrate-N; by 60 to 120 lb. N if the soil has 15 to 20 ppm, etc. In practice, adjusting N rate based on nitrate-N present has often been most useful in fields where a lot of organic N – from manure or forage legumes were grown previously – was added, in which case it is a test for how much N mineralized. Spring soil N testing also requires sampling and then waiting for results during a busy time. But it can be used if we suspect that there are more than normal amounts of soil N. In cases where there has been wet weather and some N loss, it can also be used to help decide whether or not to make a supplemental N application.

While the University of Illinois has not actively promoted the use of the PPNT or PSNT, it is logical to apply less than full N rates if spring samples show an appreciable amount of soil N already present. We think that a reasonable way to do this is to calculate lb. nitrate-N per acre (ppm of nitrate-N in the top foot times 4) and to subtract this from the normal N rate. It may be safer not to make any adjustments if nitrate-N is less than 10 ppm, since we would consider low levels to be normal. But as an example, finding 20 ppm in a sample would suggest a reduction of 80 lb. N per acre in the fertilizer N rate. That’s conservative – 30 ppm would rule out any more N under the PSNT guidelines in use, but would mean lowering rate by 120 lb. based on what we’re proposing here.

Sampling uncertainty does mean some uncertainty in N rate adjustments. That’s a concern, especially if we sample only a small area in the field. So spring resampling in only the small area where fall samples were taken under the N-Watch program, while it meets the important objective of measuring changes from fall to spring, may not be adequate for making adjustments for a whole field. At minimum, samples taken for N rate adjustments should be taken from different soil types in a field to see if soil N levels have enough consistency to warrant adjustments.

Emerson Nafziger