Phosphorus fertility for wheat
It’s important that producers not overreact to lower crop prices for wheat by cutting back this fall on phosphate fertilizer, if it’s needed, for the wheat crop. Wheat is the most highly responsive crop we grow to P fertilizers. At low soil test levels, good profits can be made by using the right rate of P applied at the right time and in the right manner.
Soil testing is the key tool to determine if any crop, but especially wheat, will respond to added P fertilizers. In Kansas the Critical P Soil Test level (the soil test level below which a response to P fertilizer is likely) is 20 ppm using the Mehlich 3 P test. When soil test P levels (Bray P1 or Mehlich 3) are below 20 ppm, the likelihood of a wheat yield response to P is high. The lower the test level, the greater the probability of a response.
Considering the 4-R’s of fertilizer management -- Right Time, Right Place, Right Rate and Right Source -- P 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 P fertilizers is at or before seeding. The first few weeks after emergence in the fall, are critical since P has major impacts on tillering and rooting of wheat. An early-season P deficiency can slow root and shoot growth and reduce tillering and plant development. A poorly developed plant is more susceptible to stresses in winter and spring. So applying P fertilizers at or before seeding is best.
Phosphorous fertilizer placement is also very important in wheat production. Cool soil temperatures during fall growth reduce the rate of P movement to developing wheat roots. By concentrating the P in a band near the seed, this problem can be overcome.
It doesn’t take much added P fertilizer, with the proper timing and placement, to have a big effect on early-season development and yields. Early work at K-State conducted in the 1930’s 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. Those results are still valid today. Banding fertilizer near the seed is more efficient than broadcasting P, especially for crops like wheat which make critical growth during cool weather. Broadcasting P can improve early-season wheat growth, but broadcasting is less efficient and requires a higher rate to obtain a similar response, making it more expensive.
The right P fertilizer rate should be determined by a well-calibrated soil test, such as the Mehlich 3, Bray P1, or Olson. Other tests may be good, but they are not calibrated for Kansas or Great Plains conditions, and don’t have the rate recommendations at a given test level developed. The rate needed for wheat is a function of soil test level. Wheat removes about 0.50 pounds P per bushel. But at low soil test levels, the recommended/required P rate for economic optimum yield is often 2-3 times the removal rate, especially when broadcast. At higher soil test levels, approaching the critical level, the soil is capable of supplying most of the P needed, and recommended fertilizer rates are often less than crop removal.
Essentially all of the normally available dry or liquid P sources, such as 11-52-0, 18-46-0, or 10-34-0 can be successfully used for wheat production, with little difference in performance seen between them.
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, as compared to corn or grain sorghum, each year a report will come in of fertilizer injury from fertilizer applied with the seed. In many cases this is the result of urea being added to the starter fertilizer mix. As a general rule, never apply urea fertilizers directly with the seed as this can result in ammonia injury. Also keep the salt load low. Ideally, the total amount of N plus K should be less than 30 pounds per acre in 7.5 inch rows.
In summary, P is a critical part of a wheat fertilizer program on soils with P soil test levels below 20 ppm. Applying the fertilizer at or before planting, and using some banded P, normally placed with the seed in today’s planting equipment, is the most efficient and profitable. The right P rate should be based on current soil tests.
Dave Mengel, Soil Fertility Specialist
Dorivar Ruiz Diaz, Nutrient Management Specialist