In an article posted last week I described the effects of rainfall and standing water on corn plants, especially corn roots, and what this might mean for corn crop prospects. Here, we’ll consider the extent to which damage from wet soils can be addressed by management during the second half of the 2021 season.
A short trip on Saturday made clear that a lot of corn and soybeans in fields and parts of fields where water hasn’t stood are in outstanding shape, and shows no sign of needing anything more than sunshine and a little more water during the rest of the season in order to produce high yields. Between July 11 and July 18 in Illinois, good + excellent ratings moved up, from 60 to 65% for corn and from 56 to 60% for soybeans. We hope that this improvement can continue as soils dry this week and as quantities of sunshine return to normal.
It’s also worth noting that management decisions for fields covered by crop insurance are somewhat different than those without insurance. Decisions to take insurance, which are being made now in fields with large areas of standing water, generally means that no more inputs will be applied in those fields. It may be difficult, especially in fields with both very good and very poor areas as a result of drainage patterns, to make coverage (and therefore further management) decisions. Drones may help.
Above-normal rainfall during the season always brings questions about how much N has been lost, and whether we should apply more N to make up for such loss. From our 4-year study tracking soil N through the first half of the season, we found that soil N levels by late vegetative stages could be fairly low without much effect on yield, as long as the plants had taken up most of their N by then. To reach full yield, there needed to be about 15 ppm of N in the top 2 feet (that’s about 120 lb of N per acre) by stage V11-V14, but only about 9 ppm by tasseling/pollination stage (VT/R1). This drop in soil N requirement happens as plants take up N: about 70% of the total N needed is taken up by stage R1. Uptake rates slow after pollination, and in many soils, mineralization can provide most or all of N needed to be taken up during grainfill. The amount of N in the plant by pollination is typically about the same as the amount in the grain at harvest, but more than that is needed to keep leaves healthy during grainfill.
Dan Schaefer of IFCA took some soil samples over recent weeks to see if soil N levels were being maintained. With good mineralization and limited rainfall between planting and sampling, levels on June 23 (before the heavy rain) were about the same as the amount of fertilizer N applied, after subtracting the amount is unfertilized checks. That says that uptake during those three weeks was matched by mineralization, leaving plenty of N available. In another trial sampled on July 6 following about 8 inches of rain between planting (in early June) and sampling, plots that received 90 lb as UAN at planting and 90 lb sidedressed, or with all 180 lb applied as UAN at planting, had about 200 lb of N in the top 2 ft. Those with 60 lb N at planting and 120 lb sidedressed, or 120 lb applied as NH3 last fall and 60 lb N sidedressed, had only 140 and 112 lb N, respectively, in the soil on July 6. Nitrogen applied early, at or close to planting, provides N to the plants when they need it, and can sometimes provide an advantage over applying much of the N earlier or later.
Greg Steckel and Marty Johnson sampled soils at the Monmouth research center, and found that in a trial planted on May 6 and sampled on June 22, there was 118 lb N in the top 2 ft of the zero-N check, and 310 lb N with 200 lb N applied as UAN at planting. Rainfall between planting and sampling totaled about 11 inches, so finding that much N is good news. With nearly 9 inches of rain since the plots were sampled, levels are likely to be lower now, but probably still exceed the amount needed for full yield.
The question or whether applying nitrogen (as urea applied by airplane) in wet fields will increase yields and profits is not an easy one to answer. Corn plants that are yellow after standing in water may never have good enough root systems to take up enough water to support yields even if they pollinate and set some kernels; adding more N will not change that. Fields or parts of fields that are through or nearly through pollination and still have good canopy color have good root systems, and there is little concern that they will run out of N. These two situations together cover much of the flood-affected area. Plants that aren’t standing in water but have lost canopy color might, depending on how well their root system still works, be able to benefit from N applied on the surface, as long as the N becomes available to the (shallow) roots quickly. If dry weather settles in, N applied now may not get to the roots in time to do much good. Aerial application is costly, and it may make sense to apply no more than 40 lb or so of N if there appears to be a need to do so.
Can plants even respond to N applied this late if they’ve been deficient? We recently published a paper in which half of the N (100 lb) was applied at planting, with the rest applied at stages ranging from V3 to R3 (dough stage). With half applied at planting, the second half could be delayed to pollination without loss of yield, and waiting until after pollination meant only small yield losses. Applying the second 100 lb of N as late as R3 always added yield compared to applying only 100 lb at planting: applying at R2 or R3 added about 20 bushels (10%) of yield to corn following soybean, and about 34 bushels (20%) to corn following corn. So plants can utilize N applied late even if they’re mildly deficient by then. But plants damaged by wet soils may not have as much capacity to respond to late N, especially if soils dry out and water becomes limiting. This year, most fields had full amounts of N applied early, so using the 100-lb rate applied early as an approximation of the N supply left after heavy rains probably underestimates how much N the crop had access to. Any crop that took on a dark green color had a good supply of N, and any deficiency that appeared in such crops likely resulted from damage to roots rather than to loss of soil N.
Urea is safe to apply to standing corn, but its N gets into the crop slowly and is vulnerable to loss if the urea stays on the soil surface without rainfall for days or weeks after application. UAN dribbled near the row is likely to get into the plant sooner, but ground application is the only option for UAN, and many damaged fields remain too wet to drive through. Foliar N could be applied by air, perhaps with fungicide, but small spray volumes mean small amounts of N, and the effect on yield is uncertain. Using urease inhibitors with urea or UAN may be OK, but if movement into the soil is delayed due to dry weather, the N may not be very effective, and adding inhibitor may only add cost. Slow-release products such as ESN should not be used when uptake has to be as fast as possible, which is the case with “emergency” N applied after tasseling.
Soybeans with pale green color due to root problems may seem likely to respond to in-season N (usually urea by air), but we have no good evidence to show that this will help restore yield. Soybeans in wet fields or in wet places in fields need to take up a lot of N over the next few weeks in order to restore lost yield potential, and roots need to be healthy enough to make that possible. In heavier-textured soil, soybean canopies that remain pale green or yellow are not very likely to be restored quickly by anything we can do; even if good soil conditions return quickly, the amount of N needed will probably exceed what the roots will likely be able to take up. The extent to which N applied now might limit the restart of N fixation, or if this restart can happen at all once nodules are damaged, is a concern. The soybean crop takes up only about a third of its N by stage R3 (beginning podset); and, unlike corn, N uptake remains steady or even accelerates slightly as the crop moves through podsetting and into podfilling. Restoring the dark green color needed by early August will be difficult, and in many fields may be impossible, if soybeans are showing N deficiency now and root are still compromised. Having leaf color begin to improve as soils dry this week will improve prospects for good yields.
Some producers consider application of fungicides to both corn and soybean to be priorities, even (or especially) when the crop is under some stress. Foliar diseases remain low in most areas, and as the corn crop moves toward and past brown silk (9% of the Illinois crop is at “dough,” well past brown silk), the value of protecting leaf area diminishes, especially when disease incidence is low. So it still makes sense to scout to see if there is disease before applying a fungicide. It’s also the case that relieving stress through “plant health” applications of fungicide is more likely to have this effect when the canopy is green and photosynthetic rates are high. Plants with damage to roots and pale green color are not likely to benefit as much from fungicide, mainly because fungicides can’t help bring back healthy roots, and can’t do much to restore leaf chlorophyll content and color until the roots get healthy.
Pale leaves may prompt some to take leaf samples in corn or soybeans to see which nutrients are lacking, and to decide which to include in an in-season application. There are a lot of yellow or pale green corn and soybeans this year, and it’s important to identify why they show such colors before loading up to apply nutrients. If such spots in the field are only in low spots, it’s probably roots that are the problem, in which case applying nutrients is unlikely to help. Leaf tissue tests will nearly always show that several elements are below critical (threshold) levels, and reports from such tests typically suggest which ones to apply. It is rare, though, that yellow leaves form plants in low-lying, wet areas of field would show only N as being deficient: such leave basically stop working, and the movement of almost every nutrient into the leaf slows or stops. The need for such nutrients also slows or stops, and until leaf color is restored, low levels of these nutrients aren’t likely to be limiting to photosynthetic rates.
This year will be another one in which crops with great potential early were challenged by weather and soil conditions that were largely beyond human control. While we should manage crops much as we normally would in mid-season, our ability to restore lost yield potential is somewhat limited by the timing and severity of the damage. In a few weeks we’ll have a better idea of how well the crop has been able to recover: this will be largely determined by how well canopy color returns to the dark green that we need in early August to get crops to reach their full yield potential.