A modern 110-day corn hybrid in central Iowa produces about 19 leaves but can only maintain 12-14 leaves on the plant. The first leaves begin senescing around the V5 stage, and by silking (R1) approximately 6-8 bottom leaves have already senesced (Figure 1). From R1 to R5, leaf senescence progresses slowly, with about 2-3 additional leaves senescing. Most leaves are rapidly senescing near physiological maturity (R6) or black layer.

Rainfall this spring and into midsummer wasn't unusual in total amount, but it was unusual in how it arrived in central Iowa. Cumulative rainfall data from Boone, Iowa (April 1–July 20) was in the upper-middle range of the 30-year record at 19.5 inches (range: 6.5” in 2011 to 26.5” in 2008; Figure 1).

One of the largest and least visible sources of nitrogen (N) for Iowa corn is the soil itself. Through N mineralization, soil microbes convert organic N into plant-available forms every day. In many fields, this natural process supplies well over 100 lb N/acre during the growing season. 

Photo by: John Sawyer, ISU Extension Retired

It's a question that arrives in our inboxes every spring without fail: "It's been raining — have I lost nitrogen, and do I need to apply more?" This year is no exception. With a highly variable spring precipitation pattern across Iowa (see map below) and corn now approaching or past the critical V6 growth stage, many producers are seeing yellowing in their fields and wondering what to do next.

Residual soil nitrogen levels last year were relatively high and extremely variable from field to field. That means the Late Spring Nitrate Test (LSNT) can generate a large return on investment for in-season nitrogen decision-making this year. For those making in-season nitrogen applications, there may be significant opportunity for savings on fertilizer inputs.