It’s important that producers not overreact to lower crop prices for wheat by cutting back this fall on phosphate fertilizer if it is needed. Wheat is a highly responsive crop to phosphate fertilizers. At low soil test levels, good profits can be made by using the right rate of phosphorus applied at the right time and in the right manner.
Soil testing is the key tool to determine if wheat will respond to added phosphate fertilizers. A “critical” phosphorus soil test is the level below which a response to phosphate fertilizer is likely. This critical level may be slightly different for different regions based on sampling protocol and calibration research for different soils, and the appropriate guidelines should be followed. The lower the soil test level, the greater the probability of a yield response.
Considering the 4-R’s of fertilizer management — Right Time, Right Place, Right Rate and Right Source — phosphate fertilizer timing and placement are especially critical in wheat production. Since wheat is a fall-planted crop and makes critical growth during colder weather, the best and most profitable timing for phosphate fertilizers is at or before seeding. The first few weeks, after emergence in the fall, are critical since phosphorus has major impacts on tillering and rooting of wheat. An early-season phosphorus deficiency can slow root and shoot growth and reduce tillering and plant development.
Phosphorus fertilizer placement is also very important in wheat production. Cool soil temperatures during fall growth reduce the rate of phosphorus movement to developing wheat roots. By concentrating the phosphorus in a band near the seed, this problem can be overcome.
It doesn’t take much added phosphate fertilizer, with the proper timing and placement, to have a big effect on early-season development and yields. Research conducted at low soil test levels showed nearly twice the response from applying 20 pounds of P2O5 in the row with the seed compared to broadcasting the same amount. Banding fertilizer near the seed can be more efficient for crops like wheat, which make critical growth during cool weather.
The right phophate fertilizer rate should be determined by a well-calibrated soil test, such as the Mehlich 3, Bray P1, or Olsen. Soil test methods without local calibration data have little or no value to provide rate recommendations at a given test level. Wheat removes about 0.50 lbs P/bu. But at low soil test levels, the recommended phosphorus rate for economic optimum yield is often 2-3 times the removal rate. At higher soil test levels, approaching the critical level, the soil is capable of supplying most of the phosphorus needed, and recommended fertilizer rates are often less than crop removal.
The normally available phosphate fertilizer sources can be successfully used for wheat production. One word of caution concerning fertilizer sources relates to ammonia and salt injury. While wheat is relatively tolerant to salt injury, and the narrow row spacing commonly used dilutes the amount of material in each row, there is a possibility of injury from fertilizer applied with the seed. In many cases this is the result of urea being added to the fertilizer mix. As a general rule, never apply urea fertilizers directly with the seed as this can result in ammonia injury. Also keep the total amount of N plus K to less than 30 lbs/acre in 7.5-inch rows.
In summary, phosphorus is a critical part of a wheat fertilizer program on soils with low soil test phosphorus levels. Applying the fertilizer at or before planting, and using some banded phosphorus, normally placed with the seed in today’s planting equipment is efficient and profitable.