Building Soil Potassium and Phosphorus

Long-term fertility validation experiments for soil
potassium and phosphorus buildup are being
conducted on low-testing fields of cotton at
Wardell, Mo, rice and soybean at Qulin, Mo, and
fescue hay and pasture at West Plains, Mo.

University of Missouri (MU) soil test laboratory recommendations for P and K fertilizer are based on three components: target level, crop removal, and buildup. Target level is the amount of extractable nutrient found in a soil at which point applying more fertilizer containing the nutrient will probably not increase crop yields. Crop removal is how much the nutrient is reduced in the soil annually from harvested forage, grain, or fiber. Buildup is the additional fertilizer needed above crop removal to increase low and medium P and K testing soil into the optimum fertility levels for crop production.

Soil P and K buildup can be slow or fast depending on the economic situation of the farmer. Total fertilizer applied in slow and fast buildup programs is about the same amount. But, depending on the buildup period, the cost may be spread out over more years in slow buildups. Many soil test laboratories in the United States allow farmers to select the number of years they want to build low or medium P or K soil levels in fields to optimum levels. This decision has a large effect on the amount of fertilizer that a farmer will purchase and apply in a given year. The current soil test recommendations system used by MU allows growers to select the number of years over which to buildup soils. If a grower does not check an option box on a soil test submission form, the soil test lab uses an 8-year default buildup time to calculate fertilizer recommendations.

Shown below are the equations used at MU
to calculate the P and K buildup component of soil test recommendations.

Xd = desired soil test level in lb P or K per acre
Xo = observed soil test level in lb P or K per acre
Years = desired time period for build up

Soil potassium and phosphorus buildup from fertilizer is being studied in a rice/soybean rotation, and fescue hay and pasture. Standard treatments include an untreated check, 1-year, 4-year, and 8-year buildup fertilizer programs. Composite soil samples are collected from each plot and analyzed at the MU Delta Center Soil Lab. After three years, fertilizer treatments increased soil test levels in rice and soybean plots compared to the untreated checks, but one year P and K buildup applications failed to increase P or K above critical target levels. The general order from high to low was 1 year buildup, 4 year buildup, 8 year buildup, and checks (untreated and N only). Rice and soybeans yields were higher with P and K treatments than untreated checks. Averaged across years, no difference was found between 4 and 8 year buildup programs. In 2006, nitrogen fertilizer alone increased fescue dry matter yields 33 percent compared to the no fertilizer check. Applying P and K fertilizer with nitrogen increased hay yields an additional 35 percent. Nitrogen content in the hay, an indicator of crude protein, was increase by N but not P or K fertilizer. Tissue P content (possibly related to energy) in the 1 year build program was higher than other treatments. READ MORE