How important are early season phosphorus applications?

Excessive phosphorus (P) use can lead to significant environmental issues and unnecessary expense, but are there some situations where it would be helpful?

Since the 1960s, indiscriminant application of phosphorus (P) has decreased. Most agricultural sites have adequate if not excessive amounts of P. This is often due to the previous general use of balanced N-P-K fertilizers. Unlike N and K, P builds up in the soil and can be carried through erosion to waterways where it causes significant algae growth. Therefore, its use should be closely monitored.

Michigan State University Extension suggests growers get a soil test to determine P levels and then follow the recommendations. Any site having more than 300 pounds per acre is restricted to applications of only what the crop will take off that year. In most vegetables, this is in the area of 15 to 25 pounds per acre.

Phosphorus is often used in starter fertilizer applications for many early planted crops. This is because root growth in cool soils is slow, thus inhibiting P uptake. To overcome this, extra P is applied to the area around the seed or transplant generally as a liquid or banded application. Starter fertilizers are characterized by having a high P and low N and K analysis such as 5-30-5.

In plastic mulch situations used for many vegetable crops, growers may want to forego even early season P applications. The plastic mulch will generally heat the soil enough to encourage root growth even though air temperatures inhibit top growth.

For more information, see “Are phosphorus applications under plastic mulch really necessary?

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