If symptoms of fallow syndrome appear on corn, you must identify them early to attempt any rescue treatment. Affected plants tend to show typical phosphorus deficiency symptoms. They have purplish leaves and are smaller and more spindly than unaffected plants.

Experiments evaluated post-plant applications of fertilizer phosphorus on research farms in Buchanan and Howard Counties in 1960 and in Buchanan County in 1961. Application of 30 lb P2O5 per acre was after the date of planting, not after the date of emergence. Fertilizer phosphorus was placed by hand 2 to 3 inches to the side of the row and 4 inches below the soil surface.

Corn yields did not exceed 80 bu per acre in the two experiments conducted in 1960, probably due to planting in late May. In one experiment, corn yields increased 17 percent with an application 2 weeks after planting. In both experiments, corn yields increased 5 percent with an application 5 weeks after planting. Phosphorus applied within 5 weeks after planting increased grain yield, total dry matter produced, and total plant uptake of phosphorus. In the 1961 experiment, corn grain yields ranged from 79 bu per acre (with no phosphorus applied) to 92 bu per acre (with 30 lb P2O5 per acre applied 3 weeks after planting), a 16 percent yield increase. Phosphorus applied 6 weeks after planting increased grain yield 10 percent.

This meager set of data suggests that phosphorus-deficient corn can respond to a post-plant application of fertilizer phosphorus, if it is placed into the soil in a band next to the corn row. If fallow syndrome is identified, and no band application of fertilizer phosphorus has been made, a cultivator-applied band of fertilizer phosphorus close to the corn row is a possible rescue treatment. I suggest 30 lb P2O5 per acre (for example, 100 lb of 10-34-0 per acre). Do this within 5 weeks after planting, before the period of rapid plant growth.