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Timing of Phosphorus Application for Corn and Soybean Production

Start Date: 2019
Principal Investigator: Dr. Daniel Kaiser
Organization: University of Minnesota
Status: Ongoing

Background Info

Phosphorus is a plant nutrient which, if deficient, can significantly limit crop growth and development. Phosphorus is considered a primary macronutrient. Primary macronutrients are elements that are essential for plant growth which more commonly require fertilizers to be applied to satisfy crop requirements. Orthophosphate is the form of phosphorus taken up by plants. The concentration of orthophosphate in the soil solution is low as orthophosphate is highly reactive with metal elements. Iron, aluminum, and calcium all can react with orthophosphate creating compounds varying in solubility. The ion which reacts with orthophosphate depends greatly on the pH of the soil.

Soils in major cropping regions in Minnesota predominantly formed under calcareous parent materials. Carbonates deposited in the material left following glaciation are still present near the soil surface in areas of the state. These soils with greater carbonate and calcium contents present challenges when managing phosphorus as it is difficult to increase available soil test phosphorus of calcareous soils. Soils with high calcium contents can fix phosphorus. Fixation is a process where orthophosphate reacts with calcium forming compounds like di- and tri-calcium phosphate. While fixed phosphorus is not technically lost from the soil it is rendered unavailable for plant uptake.

The relative rate of phosphorus fixation is not known in soils. Short term P sorption tests can be run to determine the amount of P which a soil will sorb, which can be substantial for some calcareous soils. Management of fertilizer P is common in the fall which gives more time for P to react and potentially bind phosphorus. Studies have been conducted focused on timing of P application but many were conducted in soils with a neutral to slightly alkaline pH which did not contain appreciable amounts of calcium carbonate.

In a previous study funded by AFREC, on-farm strip trials established to determine corn and soybean response to a single rate of phosphorous fertilizer showed that a high rate of P applied one year can have multiple years’ benefits for crops in a two-year rotation. The exception was one location with a calcium carbonate equivalency of 20% where there was a yield benefit to P applied both years for a two-year corn soybean rotation and P applied the previous year had not impact on the crop grown. Fall application provides more flexibility for farmers but there are questions as to whether spring is better under some circumstances.

Objectives

The objective of this study is to establish whether there is a difference between fall and spring application of P fertilizer for corn or soybean production.

The objective of this study is to establish whether potential differences may be tied to calcium carbonate content of the soil.

Key Findings

Both Bray-P1 and Olsen soil P in June was impacted consistently by P application rate and timing. Soil test P was greater in June following spring P application as indicated by significant P timing main effects and significant rate by timing interaction.

Corn leaf P concentration were consistently impacted by P application rate and seldom impacted by timing. Soybean trifoliate P concentration was not impacted by P application rate or timing.

Corn and soybean yield were impacted by P application rate at 2 of 3 corn and 2 of 3 soybean locations. Grain yield was increased when up to 60 lb P2O5 were applied.

Phosphorus application timing did not affect soybean grain yield. Corn grain yield was greater with spring P application at both locations where P rate affected grain yield.

There was a significant rate by timing interaction affecting grain yield at Morris. Corn grain yield was similar at Morris when 90 lbs P2O5 were applied in Fall versus 60 lb P2O5 in spring.

Grain P concentration and P removed in the harvested grain were inconsistently impacted by P application rate and timing. Soybean grain quality (protein and oil concentration) were no consistently impacted by P application rate and timing.

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