Closing the Phosphorus Cycle on Vegetable Farms: Releasing Soil-Bound Phosphorus to Support Springtime Seedling Growth

Closing the Phosphorus Cycle on Vegetable Farms: Releasing Soil-Bound Phosphorus to Support Springtime Seedling Growth

Summary

Summary
Vegetables are often grown on high-phosphorus soils, but additional phosphorus is often required for early sowings. We have demonstrated that a new granular potassium bicarbonate can release phosphate quickly in such soils and potentially replace the phosphorus addition. We are developing a protocol for using this material, determining on which vegetable soil types it is usable, as well as investigating high-phosphorus cover crop residue and seed-applied phosphorus as alternatives for supplying needed phosphorus until mineralization from the soil is sufficient.

Objectives
In high-phosphorus soils, identify which pools of P are available for release to spring-sown vegetable crops.

Test specific treatments that may make P available without further increasing soil P.

Methods and Approach
For objective one: Identify which pools of P are available for early release. The main questions are: In what form is the unavailable P in those vegetable soils in which P has been accumulating? Which of these forms of bound P is most easily made available at planting through cultural practices?

For objective two: Test specific treatments that may make P available without further increasing soil P. We will test several potential methods that have promise, based on grower experience, knowledge of soil chemistry, and knowledge of rhizosphere biology. The experiments will first be done with snap beans, the crop where the need for a solution is most acute. The methods include releasing bound phosphorus with citric acid or bicarbonate, adding small amounts of P directly to seeds, selecting P-releasing microbes to work at lower temperatures, and by moving phosphorus to cover crop residues that rare rapidly release after tillage.

Results
Focus group. A focus group with leading snap bean growers and processor field representatives revealed significant interest in the problem of excessive phosphorus. Their eagerness to implement alternatives to starter P is high, if those alternatives are inexpensive, easy, and effective. As a group, we prioritized the objectives based on their promise of the greatest adoption: bicarbonate in the furrow, cover crops, and seed treatments.

Citric acid. Citric acid at 50 µmol per kg soil solubilized phosphorus from three representative vegetable soils. However, at this concentration there was substantial phytotoxicity.

Bicarbonate counterion. Sodium bicarbonate (10 µmol/kg soil) solubilized phosphorus from three representative vegetable soils. We found slight phytotoxicity at 50 µmol/kg soil, which is too small a safety margin. Two other bicarbonates, potassium and ammonium, were not phytotoxic. Potassium bicarbonate has several advantages and is being investigated further.

Bicarbonate physical structure. Metering the material accurately will be important. Our focus group identified granular applicators as the obvious tool because most growers’ bean planters are equipped with them. The dispensers meter powders imprecisely and powders can clog the delivery tube. To improve accuracy and delivery, we sought to formulate the potassium bicarbonate into granules of 0.5 to 1.5 mm, the same size as the clay carrier granules that are normally dispensed with these applicators. Church and Dwight Company made a test batch of such granules for this experiment, and they worked well.

Bicarbonate field trial. Continual rain during the latter half of May limited us to a single planting. The effect of bicarbonate was small, but similar to starter phosphorus.

Need for Starter P. Beans in cold soils with high soil test phosphorus showed a small response to starter P fertilizer.

Impacts and Potential Contributions
We have identified the best material to use. Granules of potassium bicarbonate have the ability to release phosphate from our test soils at concentrations that do not cause phytotoxicity. This materials can be made into granules that are easy to meter and that don’t clump in tubing. The actual impact on production will not be known until tests on the phosphorus uptake in plants are done.

Reported November 2000

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