Early February warmth once again gave way to cool, wet conditions that persisted well into May. Luckily, it looks like the warmer weather may be here to stay, which means corn growth should start to take off. As some corn nears V3/V4, it is a good time to evaluate your crop in anticipation of sidedressing.
Cool, wet conditions this spring have likely resulted in slower mineralization of soil organic matter or manures. It would not be surprising to see nitrogen (N), sulfur (S), or even manganese (Mn) deficiencies on early growth, especially on sandy, manured ground. Scouting crops now can help growers make decisions about what fertilizer sources to use at sidedress. Nitrogen deficiencies will manifest in the older tissue because N is mobile in plant tissue; look for yellowing (chlorosis) or browning of the oldest leaves on the plant. In contrast, S and Mn deficiency will be evident in the new growth, showing up as yellow interveinal striping on the youngest plant leaves (see photo). In fact, we have already seen S deficiency on Delaware corn at V4.
If you see evidence yellowing on new growth, you can do some sleuthing to determine if the cause is due to S or Mn deficiency. Manganese deficiency is most common when soil pH is neutral to alkaline (>6.5). Check the results of your most recent soil test to determine is Mn might be a problem. In addition to pH, you can look for the Mn availability index value (included on your soil test report if you use the UD Soil testing laboratory). Alternatively, the Mn availability index can be calculate based on results from your commercial testing lab (see the UD nutrient recommendations for grain corn for more information).
Tissue testing, while useful to confirm S deficiency later in the season, will not be of much help this early in the season. However, if you can rule out Mn deficiency based on soil test results, then it is likely that S is a problem. Corn growing on manured soils will likely grow out of a S deficiency as warmer temperatures fuel microbial activity and break down of manure. Similarly, corn grown on silt loam soils will be able to tap into subsoil S stores as roots expand. If a field has a history of S deficiency, side dressing with liquid ammonium sulfate or ammonium thiosulfate can also correct early season S deficiency, while also adding N right before the period of rapid corn N uptake. Apply 30-40 lb/ac of elemental S to correct early season S deficiencies. Make sure to account for the N in these fertilizers when making sidedress N decisions.
For fields managed with commercial fertilizers (no manure history), plan to apply N at the rate based on your realistic yield goal minus any N you applied pre-plant or at-plant. For example, if your realistic yield goal is 275 bu/ac and you applied pre-plant N at a rate of 30 lb/ac, you can plan to apply up to 245 lb/ac of N at sidedress (or less if you plan to fertigate).
Yellowing of new growth on corn due to S deficiency.
We strongly urge growers to get a pre-sidedress nitrate test (PSNT) for corn fields that received manure and less than 50 lb/ac of pre-plant N from commercial sources this spring. Growers who use a PSNT can save time and money and reduce the potential for N losses to the environment. Collect a composite PSNT soil sample to a depth of 12 inches from 15 to 20 locations within a field (max 20 acres per composite) when corn is at V5-V6 (10-12 inches tall). Mix soil thoroughly and submit a dry subsample (1-2 cups) to your soil testing laboratory for analysis. Results of the PSNT can be used to adjust the sidedress N application as outlined in the UD factsheet, Nitrogen Management for Corn in Delaware: The Pre-sidedress Nitrate Test.