As the growing season moves forward more questions have occurred about what products to use in side-dress situations. Although nitrogen is on the minds of many, sulfur deficiencies are starting to be seen in fields as well. Applying the right product in the right situation at the correct time can be crucial in order to maintain yields and minimize damage to growing plants.
Liquid UAN (28 or 32%) solutions banded between the rows either on the surface or coulter injected provide the least potential for foliar injury to the crop. Can UAN solutions be sprayed directly on the corn safely? Post applications of these products will likely produce some foliar injury to the crop. Research conducted by Dr. Gyles Randall looked at 30, 60, 90, and 120 lbs of N foliar applied to corn at V3-V4 and found that grain yields were decreased with application rates of more than the 60 lb N/acre. If at all possible broadcast foliar applications should be avoided if drop nozzles are available to stream UAN on the soil surface. If sulfur application is needed do not broadcast apply ammonium thiosulfate on plant tissues. Ammonium thiosulfate will injure the plant. Ammonium thiosulfate can be applied as a surface application between the rows or coulter injected.
At early plant growth stages, granular urea or ammonium sulfate may be applied as a broadcast with some risk. Any material that is broadcast applied and falls into the whorl can cause burning or leaf streaking as new leaves begin to unroll. In severe cases large application rates at later growth stages can cause significant damage and grain yield loss. Application during cooler temperatures also may lessen the foliar damage on the plant. Early or small application rates can help lessen the risk of damage to plants. Post application with ammonium sulfate in 2009 and 2010 showed little effect on grain yields at 10 or 20 lb. sulfur/acre or42 or 84 lbs of AMS per acre. In addition, Gypsum could be broadcast applied if sulfur is needed.
Surface applications of urea containing fertilizers are at risk for N volatilization. These risks are worse at higher soil pH levels. Rainfall amounts of 0.25" or more are generally adequate to sufficiently incorporate urea into the soil. Agrotain can be used on urea to prevent some loss and give a larger time window before rainfall occurs, and is the only product that has been consistently found to have some effect on reducing volatile N loss from urea. Since UAN solutions contain about 50% of their N as urea some losses can be expected from surface application without incorporation and there has not been a product that has been demonstrated to reduce N volatilization from liquid solutions.
A supplemental N worksheet was developed to determine if nitrogen should be applied. This tool is helpful when corn is at the V5 growth stage or beyond. Decisions for sulfur application can be more challenging. If sulfur was applied pre-plant, it may not be necessary to apply it side-dress. The only exception may be on fields receiving elemental sulfur, for the first time this year. Some striping may occur even though sulfur was applied as elemental S. This was observed in 2008 when sulfur was applied. The sulfur symptoms could be caused by rapid plant growth and possible limited uptake of sulfur. The corn will grow out of this and grain yields should not be affected. If sulfur was not applied this spring, current research results indicate that early season applications (V3-V4) of sulfur responded the same as those made at planting.
There is still time to correct any potential deficiencies. Paying attention to sources and rates can pay big dividends at the end of the season.