Gordon Johnson, Extension Ag Agent, Kent Co.; gcjohn@udel.edu
Residual effects of wet weather in May and early June continue to be evident in vegetable crops across the state. Nitrogen deficiency is the most common nutrient related disorder being found. This was a difficult year to determine how much additional nitrogen was going to be needed at sidedressing. Leaching rains and waterlogged fields with high levels of denitrification complicated the issue as up to 60% of N applied preplant or at planting was lost, N mineralization from organic matter and manure additions was reduced, and any N that was mineralized was subject to further losses with heavy rains. In addition, in wet areas, corn roots were not functioning properly and N uptake was limited.
I recently looked at several fields of sweet corn with nitrogen deficiency that was severe enough to reduce ear number and size. Nitrogen deficiencies in sweet corn result in an overall pale color with lower leaves becoming yellow from the tips in a V pattern. In severe N deficiencies, these V shaped areas will become necrotic and entire leaves may dry up. These fields showed these classic N deficiency symptoms. A useful tool to determine the extent of N deficiencies is the chlorophyll meter which measures how “green” the plant is. In these N deficient field areas, chlorophyll meter readings were 25-50% lower than surrounding corn that was not showing any symptoms. The following are some pictures:
Nitrogen deficiency showing up as a yellowing of lower leaves in a V pattern from the tip backward
More severe N deficiency with necrotic V area on leaf and dead lower leaves
Chlorophyll meter reading on the ear leaf of a N deficient plant. The reading on this plant was more that 50% lower than plants without N deficiency symptoms in an adjacent field.
In the fields examined, the most severe problems were in very sandy areas and low spots. These are areas where the most N loss would be expected. Other field areas were not heavily affected and appeared normal with chlorophyll meter readings above 50. One grower reported that they used a Pre-Sidedress Nitrogen Test (PSNT) in areas that had received manure and values indicated that no additional N was needed. While the PSNT is a valuable tool to manage nitrogen in sweet corn, any recommendations should take into account weather at and after the time of sampling. Low PSNT values may result from heavy rains just prior to sampling (it is recommended to wait several days after heavy rains to take samples for PSNT’s). High PSNT values (>21 ppm) would indicate no additional N is needed. However, heavy leaching rains, waterlogging, and cold weather still could still render the crop N deficient even with these high values.
Chlorophyll meter reading on the ear leaf of a fully green plant that was not showing N deficiency. We would expect readings in the 50s or low 60s. Values lower than 50 would indicate a N deficiency.