Soybean acres continue to march north and west into predominately small grain areas. The seeding equipment most growers have in these areas results in many soybean acres being planted in narrow rows (7-8″). With this equipment, “P” fertilizer is usually placed tight to the seed with little spread. In small grain production, most or all of the “P” fertilizer is placed safely with the seed with this same seeding equipment. Soybeans are much more sensitive to fertilizer salt and ammonium injury from fertilizer placed directly with the seed. Recent research has shown that too much “P” fertilizer placed with the seed, even with narrow rows, can reduce soybean stand (see figure).
Soil moisture is an important factor when determining the rate of “P” fertilizer that is safe for soybeans with narrow row spacing. If the soil moisture is good at seeding, the rate should be limited to 40 lb/a of MAP (20 lb/a P2O5/acre) with the seed. If soil moisture conditions become dryer during planting season, this rate should be reduced by 50% to avoid stand loss.
If soybeans are planted in rows 14″ or wider, no fertilizer should be placed with the seed to prevent stand loss (see figure). If the row spacing is wider than 14″, P fertilizer should be placed 2″ to the side and 2″ below the seed if possible. This placement away from the seed is very safe and eliminates any stand loss, even at high rates of P fertilizer.