Interpreting Phosphorus, Potassium Levels

Source: Syngenta

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.

Phosphorus 

Crop production impact

Phosphorus is commonly a limiting nutrient in crop production. While most soils have adequate phosphorus, the amount of available phosphorus 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 phosphorus availability because it changes its form (usable ionic form or unusable form). Soil pH between 6.5 and 7.0 allows the most phosphorus availability in soil solution. Soil test results report phosphorus as an estimate of what is available to plants, not the total phosphorus in soil.

Analyzing nutrient levels in soil

Healthy levels of phosphorus in soil ranges from 25 to 50 parts per million (ppm). The University of Nebraska-Lincoln gives the following guidelines to consider when analyzing phosphorus levels in soil:

• Healthy levels between 25 and 50 ppm should receive light maintenance application of phosphorus annually (one-eigth to one-quarter pound of P₂O₅ per pound of nitrogen)
• For soils below 25 ppm, apply a starter fertilizer
• No fertilizer is necessary when levels reach about 50 ppm

Potassium

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 potassium (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 potassium levels lower than 40 ppm should receive an application of slightly more potassium than nitrogen annually
• Apply .75 to 1 pound pound of K₂O per pound of nitrogen applied if potassium levels are in the desired range (40 to 80 ppm)
• No fertilizer is necessary when levels reach greater than 80 ppm.