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Interpreting Phosphorus and Potassium Levels

December 15, 2018
This agronomic image shows nutrient deficiency in corn leaves.

Phosphorus (P) and potassium (K) are some of the most essential nutrients for a healthy crop. When testing your soil, it is important to analyze these nutrients to determine whether fertilizer is necessary to maximize yields.

Crop production impact

Phosphorus is commonly a limiting nutrient in crop production. While most soils have adequate P, the amount of available P is low as mineralization of this nutrient is slow. Phosphorus moves very little in the soil and does not leach even with large amounts of precipitation.

pH impact

Soil solution pH impacts P availability because it changes its form (usable ionic form or unusable form). Soil pH between 6.5 and 7.0 allows the most P availability in soil solution. Soil test results report P as an estimate of what is available to plants, not the total P in soil.

Analyzing nutrient levels in soil

Healthy levels of P in soil ranges from 25 to 50 ppm. The University of Nebraska-Lincoln gives the following guidelines to consider when analyzing your P levels in soil:

  • Healthy levels between 25 and 50 ppm should receive light maintenance application of phosphorus annually (1/8-1/4 lb. of P2O5 per lb. N)
  • For soils below 25 ppm, apply a starter fertilizer
  • No fertilizer is necessary when levels reach about 50 ppm

Crop production impact

Potassium is an exchangeable ion that easily binds with charged soil particles, locking it in mineral structures. Nitrogen is the only other nutrient absorbed in larger quantities than K (depending on the plant). Potassium is vital to many plant functions and cycles back into the soil from crop residue with precipitation.

pH impact

Potassium is a major plant nutrient that is less directly impacted by soil pH, but still is to some extent (especially when soil pH drops below 5.5.)

Analyzing nutrient levels in soil

Healthy levels of potassium in soil range from 40 to 80 ppm.

  • Soils with K levels lower than 40 ppm should receive an application of slightly more K than N annually
  • Apply .75 to 1.0 lb of K2O per lb of N applied if K levels are in the desired range (40-80 ppm)
  • No fertilizer is necessary when levels reach greater than 80 ppm

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