Issue No. 11, Article 10/June 3, 2005
Is It Drought Yet?
The meteorological definition of "mild" drought is rainfall 45% to 60% of normal over a 3-month period, while "severe" drought is defined as less than 45% of normal precipitation over a 3-month period. By this definition, we seldom experience drought in Illinois, simply because most dry periods don't last long enough to qualify. The low rainfall over the past 3 months, however, has some of Illinois now in a period of mild drought. Many stations recorded less than half of average rainfall amounts in May. Here in Urbana, rainfall in May totaled about an inch.
Using a period of 3 months to determine that we have drought conditions may be meteorologically accurate, but it isn't very useful from a cropping standpoint, where less than normal rainfall for even a few weeks at certain times of the season can mean a loss in yield potential. If we could choose one month to have below-normal rainfall during the growing season, though, May is not a bad one to choose. We went into the spring with adequate soil moisture, which in Illinois means that the soil profile was at or near field capacity, meaning that water content was the maximum that could be held against gravity. While surface soils have dried out to a depth of several inches, there is no lack of soil moisture within most of the (eventual) rooting depth in the soil. While water does move slowly through the soil to evaporate at the surface, soil without plant roots does not dry out very fast, especially when air temperatures are normal to below normal in May, as they have been this past month.
The fact that there is water several inches down in the soil does not mean that young corn and soybean plants are growing as if there were adequate soil moisture, however. The fact that roots cannot grow into dry soil means that, as soil moisture recedes because of surface drying, the root in the surface few inches can't continue to elongate to tap more water and nutrients. Larger plants have roots that continue to grow into soil moisture deeper in the soil and thus have a real advantage, in that they don't need to depend on roots in the dry surface soil. As roots tap into soil water, however, and as water loss rates increase because of increasing leaf surface area, the soil around the roots starts to dry out; and unless roots can continue to expand into soils with water, plants will start to show stress symptoms when roots can't take up water as fast as the plant is losing water through the leaves.
As soils dry and plant roots remove the water around them, plants in some fields are starting to show visible symptoms of lack of water. Even before leaves curl in corn or wilt in soybean, photosynthesis decreases, reducing the ability of the plant to grow. Water loss from the leaves stops at night, giving the roots a chance to catch up so that plants often look fine in the morning but start to "fade" earlier each day as soils continue to dry. If this continues for very long, plant parts that develop during a period of water shortage are often smaller than normal, so we might see shorter, narrower leaves, and stem internodes might end up shorter than normal, often causing plants to be shorter than normal at their mature height. The extent to which yield potential is affected by early dryness depends partly on the effect on eventual canopy development. Leaves that are expanding now do not represent a large percentage of the mature crop canopy, so their ending up smaller than normal might not be very important. We also like to worry about the effect of dry weather on ear formation, which starts when corn plants have about six leaf collars. There isn't much direct evidence that mild to moderate dryness during the start of ear formation lowers eventual ear size and yield potential, but reduced photosynthetic activity during the long days of June is not a positive thing.
Is there anything we can do to help crops that are showing drought stress symptoms and growing very slowly? While many people do not cultivate corn, a shallow cultivation can mulch the surface and fill in dry cracks, thus decreasing the rate of water loss at the soil surface. In dry soils, water loss rates are already low, so this effect might be minimal. Be very careful not to prune any roots or to turn up any moist soil. Try to keep weeds from competing for moisture, and monitor for and manage insects that cut roots, mesocotyls, or stems. Such efforts may seem rather futile, but they can help keep us occupied until it rains, and that distraction can be helpful. Remember that good yields are still possible, but check on insurance coverage at the same time. Even if dryness hasn't greatly hurt crop yield potential much yet, droughts do start this way; if dryness continues in the coming weeks, losses in yield potential will kick in.
One of the effects of soil dryness in recent months has been an unusual incidence of root damage from anhydrous ammonia. This is happening even when there was a "safe" delay of several weeks between N applications and planting. We can blame the cool weather and drying soils. Anhydrous ammonia immediately dissolves in soil water when it is applied and is converted to the ammonium form, which is tightly held on soil particles. Ammonium starts to convert to the nitrate form as soon as soil temperatures are high enough. Nitrogen in the nitrate form can move with water in the soil, and it is not nearly as directly toxic to plant tissue as is ammonia. Soils have been cool and have dried out over recent weeks, slowing both the conversion of ammonium to nitrate and plant (root) growth. As soils dry, some ammonium can convert back to ammonia form and can even "undissolve" from the soil water, to move again through the soil as ammonia gas. Ammonia is highly toxic to young roots, and if it moves to (or stays in) the rooting zone, roots stop growing and look "burned off" at the ends.
Root damage of this type has added to water shortage to produce unusually large visible effects on plant growth this year. The damage shows up as stress because of dryness, with shorter plants and earlier onset of leaf curling and plant purpling. In many fields, the damage pattern appears as shortened plants visible on the diagonal across the rows, following the pattern of anhydrous preplant application. If anhydrous was applied in the same direction as the rows, stretches of row may have shortened plants. There is nothing to do about this damage now, though rainfall will encourage root growth into zones with less ammonia and will redissolve ammonia to prevent further damage.
Several people have noted that fields with fall-applied ammonia tend to show better growth than those where N was applied in the spring. Reasons for this might include good retention of N through the winter (there was considerable rainfall, but soils were cool and most N was likely in the immobile ammonium form), prevention of anhydrous burn, not having to apply anhydrous this spring so one less trip over the field, possibly better dispersal of N throughout the rooting zone, possible loss of some surface-applied nitrogen this spring, and in a few cases earlier planting in these fields. Like ammonia burn, though, this is an unusual phenomenon, and we don't think there's any reason to move nitrogen application back to the fall for those who have moved it to the spring.
Dry weather after planting brings on a whole new set of plant problems, and we are seeing many of those now. We can take some comfort in the fact that our crops are planted and most stands are good; the crops are still in decent shape for the start of June. While crop plants will eventually tap the soil moisture that is present in abundance, the low rainfall of the past month has created temporary conditions under which much of the crop acreage is not growing especially well. It will rain again, but we might have to look at some rather sickly crops in many fields until it does.--Emerson Nafziger