In the week ending on April 18, planted acreage moved up in Illinois, from 5 to 12% for corn and from 2 to 5% for soybean. The forecast for cool temperatures this week has turned even cooler, with highs in the 40s for several days, and with frost possible, including temperatures that may drop into the upper 20s in some places in mid-week. There may be some snow as well, which as it melts will move as cold water into the soil. Warmup is still expected, but it won’t begin until April 25.
Surface soils look dry in some fields, and surface soil moisture remains 20 to 30% below normal in much of Illinois (https://cloud.csiss.gmu.edu/Crop-CASMA/). Because “normal” is fairly wet this time of year, and we have not had the high temperatures and wind needed to rapidly dry soils, it is not clear that soils are too dry for emergence. Despite how the soil surface looks, it is critical to dig to find seeds after planting to see how well they are “settled” into the soil before deciding to change planting depth. Suggestions to plant as deep as 3 inches in order to reach moisture may make sense in warm soil when little rainfall is expected, but doing this in April when soils are in the 40s and likely to get colder is, I believe, too risky.
It takes a surprisingly small volume of soil to provide the water needed to initiate germination. A silt loam soil with a permanent wilting point (PWP) of 15% and field capacity of 35% soil moisture can hold the difference (20% of soil volume, or more than 2 inches of water per foot of depth) as plant-available water. In order to germinate, corn and soybean seeds need to take up about 30% and 50% of their weight, respectively. If seeds are in full contact with soil that is at 50% of its water-holding capacity, and if half of that water can move into the seed as the soil dries, the shell of soil around the seed needs to be only about 1/8th of an inch thick in order for the seed to take up the water it needs to germinate. Moisture locked into small clods with dry surfaces moves into seeds slowly, and there may not be enough pathways through and around clods to allow adequate water to move in. On the other hand, soil pressed against the seed by the closing mechanism may hold enough water for germination even if it doesn’t feel very moist.
Further lowering of temperatures this week, along with possible snow, increases the chance that seeds will take in cold water and so experience imbibitional chilling. Water uptake starts relatively quickly after planting, and while cool soil will slow this process some, it will not stop it. With little to gain and more to lose from planting during the early part of this week, I believe the needle has towards waiting until later in the week before planting. We can also watch the forecast to see how much rain is on the way next week, keeping in mind that dry soils can take n an inch of water without delaying planting by very much, especially after soil warming begins.
Having air temperatures drop below 30 in mid-week is more likely in parts of the state that do not have many fields emerged by now, which helps. Still, especially in fields with some surface cloddiness, it is possible that cold air might infiltrate the soil to affect seedlings of crops planted in early April but not yet emerged. Having temperatures drop to 32 to 34 can damage emerged seedlings, so might put fields planted in late March or early April at risk. There’s not much we can to do about this, but cold air will drain into lower parts of fields, and no-till fields are usually more prone to damage from frost because undisturbed soil radiates less than tilled soil at night to protect plants. Claims that small corn plants, or even plants not yet emerged, with their growing points protected beneath the soil won’t be damaged by frost do not always hold true, especially when air temperatures drop below freezing.
Favorable crop responses following such weather events often come more from luck than from skill, so let’s hope for the best luck of all: that our fears of might happen don’t come true.