No. 1 Article 5/March 26, 2010

Anything New as the Wait for Dry Soil Continues?

The article I wrote the first week of March that was released on March 5 as an off-cycle Update appears as a regular article in this issue. I won't repeat what's there, but will add to it based on observations, comments, and questions since then.

The hope for some dry weather has been realized, with most of the state showing below-normal rainfall for March, and the northern part of the state showing average temperatures a few degrees above normal. This is not too different from March 2009, but the soils that were very wet last fall are still very wet, and the drying process is likely to remain slow as long as the weather is not much drier or warmer than normal. The weather pattern so far doesn't give us great hope for an early warm-up.

I traveled to northern Illinois this week and did not see any field work done. The good news is that water is not standing in many fields, even in the low spots; it's had time to drain. But soils remain at or very near field capacity, and soil temperatures are not rising very quickly. The soil temperature under bare soil at the 4-inch depth at 10 a.m. on March 23 ranged from the upper 30s in northern Illinois to the upper 40s in southern Illinois. This hasn't changed much over the past week.

Water has a higher heat-holding capacity than mineral or organic matter in the soil, so wetter soils warm more slowly than drier ones. As I mentioned in the article written in early March, it is easier for water to keep moving up to the surface in untilled soils, so untilled soils tend to stay wetter (and so cooler) on the surface than tilled soils. Another large factor in untilled cornstalks is that crop residue both insulates (from warming) and reflects sunlight much better than does the darker surface of tilled soil. Untilled soybean residue also reflects a considerable amount of sunlight, but it insulates much less than cornstalks.

How will we know when soils are dry enough to begin field operations? It's to be expected that some producers who have many acres to cover with tillage or N application will drive out onto fields well before soils are fit. So one way is to wait for several good drying days after the first tractor is spotted in a field in the area, especially if that field has soils that are better-drained than in most other fields.

For an individual field, one way to judge readiness is to dig a spade to a depth of 6 inches or so, in both low and higher spots in the field. If the soil falls apart when it's turned over, it's probably fit. If it stays attached to the spade and has to be scraped off, it's not fit. If it comes off the spade but stays intact as a clump, it's probably not fit. If the clump breaks up rather than just taking a different shape when you step on it or hit it with the spade, it's getting close.

I have already discussed compaction. The lack of much freezing and thawing this spring, along with the insulation from crop residue, means that not even surface compaction will be relieved much by planting time in many fields. I walked in a field of stalks in northern Illinois that had a wet, solid surface underneath the residue, looking much like it would have looked last November. That field also had been harvested with a stalk-chopping head, and the mat of residue provided nearly complete ground cover, which is likely to slow drying.--Emerson Nafziger

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