Soil Temperatures and Spring Prospects

We hope that we’ve seen the last of the snow by now, but both air and soil temperatures remain below average in Illinois heading into the second half of March. According to the Illinois State Water Survey (http://www.isws.illinois.edu/warm/soiltemp.asp) minimum temperatures 4 inches deep under bare soil ranged from the low 30s in northern Illinois to the mid-30s in southern Illinois the morning of March 17, and with some sunshine on that day, reached the upper 40s to low 50w in southern Illinois but did not get above the low 30s in the northern part of the state.

Soils froze deeper than normal this past winter, and stayed cold into March; frost is only now disappearing in the northern parts of Illinois, which accounts for their staying cold during a sunny day. Hopes that such deep freezing will relieve soil compaction from last year may not be realized; while repeated freezing and thawing result in repeated formation of ice crystals that force soil particles apart, soils that stay frozen don’t repeat this cycle often enough to do much good. The freezing and thawing of the surface soils that we’re seeing now will help loosen them some, but we can’t expect that effect to extend more than a few inches deep.

Though having soil temperatures only in the 30s this late in March is somewhat unusual, March soil temperatures are variable over years. At Champaign, March soil temperatures at 4 inches deep have ranged over the past five years from an average of 36/39 (minimum/maximum) in 2013 to 60/66 in 2011. Rainfall totals ranged from 1.47 inches in 2013 to 5.38 inches in 2011. The start and progress of planting were delayed in both 2011 and 2013, but that was based more on April rainfall than on conditions in March.

When it comes to getting soils to dry out, is warm and wet better or worse than cold and dry? Because water has a higher heat capacity than soil mineral matter, cold soils do not dry out very fast, and wet soils do not warm up very fast. We have seen some of the standing water in fields drain out this past week as soils thaw, but the drying process will be very slow until soil temperatures start to increase. Water loss rates are affected by soil texture and water content, but we would expect wet soil to lose 0.1 inch or so of water in a day if average soil temperature is 40, and at least twice that amount if the average soil temperature is 60 degrees. So having soils warm up is the key to enabling them dry out, though of course it has to stop raining for soils to dry.

Though having low soil temperatures at this point in March does not produce a lot of optimism that planting will start early, it is also not a very good predictor about how the spring will go, or of what kind of season we’ll have. If we’ve learned anything in recent years, it’s that what happens during the summer matters much more than what happens in March and April. We simply need to be ready to do fieldwork and plant as soon as conditions permit.

The likely delay in the start of field work this year may mean re-prioritizing operations once soils dry out. It has been common in wetter springs for the application of anhydrous ammonia to get underway before soils are considered fit to till or plant. That worked OK last year, when soil compaction, due to weather patterns, did not cause much problem for the crop. But we can’t count on that, and compaction from applying fertilizer or doing tillage in wet soils can leaves soils in worse condition than before, even if the surface looks a little drier afterwards.

As a reminder, planting in early April almost never produces yields higher than planting in late April, and can lower yields, even when stands are good. That being said, planting in early April into good soil conditions, with soil temperatures expected to be on the rise after planting, is a sound practice, especially when there are a lot of acres to plant and starting early is the only way to finish on time. But “mudding” corn into wet or marginally wet but cool soil conditions in early April is almost always a bad idea, with considerably more potential to do harm than to do good.

On the lighter side, I’m interested when I hear people say that they usually don’t start to plant until after Easter. Easter falls on the first Sunday after the first full moon after the spring equinox, so ranges between about March 23 and April 24. Easter was on March 31 in 2013 and it was wet until after Easter, so that wasn’t a factor. Easter is on April 20 this year, and we hope that soils get in shape to plant before that. If that happens, I imagine that some might want to set aside their hesitation to plant before Easter, as many did on 2012, when Easter fell on April 8 and we had some 20% of the state’s crop planted by then.