It’s a dichotomy – the impact of calcium on phosphorus retention and availability.
Fields that receive large amounts of manure or litter contain large amounts of phosphorus. Phosphorus that isn’t taken up by plants remains in the subsoil and is displaced by runoff. If it isn’t tied up by soil particles or minerals, it can leach down into the groundwater. Phosphates can leave the field in tile lines and drainage ditches and can end up in surface waters.
Gypsum can be added to a treatment ditch to trap phosphate before it is discharged into a drainage ditch. When the runoff or drainage water passes through the gypsum trap, soluble calcium in the gypsum immobilizes, or traps, the soluble phosphorus in the water, forming calcium phosphate. After several years of trapping, the material can be excavated and reapplied as phosphorus fertilizer.
Plants need calcium and phosphorus. However, their interaction in the soil can limit their availability. At the same time, this interaction can also scavenge up phosphorus so that it doesn’t end up in surface waters and contribute to hypoxia. Hypoxia occurs when algae blooms occur in surface waters or the ocean (Gulf of Mexico) due to high amounts of nitrogen and phosphorus. Algal blooms deplete oxygen levels in the water, killing off large amounts of fish and turning the water reddish in color.
Phosphorus, at commercial fertilizer application rate, is generally considered non-mobile in the soil, meaning it doesn’t leach or move with soil water down through the soil profile into groundwater or out tile lines. Phosphorus, though, can move off the soil surface with runoff along with soil particles.
Calcium and phosphorus (phosphate) have an affinity for each other. Soluble phosphate is an anion, meaning it has a negative charge. Any free calcium reacts with phosphate to form insoluble (or very slowly soluble) calcium phosphate compounds that are not readily available to plants. Soils have ample amounts of calcium either on the exchange complex or as calcite (limestone), and they can interact to form minerals. The two principal minerals are dicalcium phosphate and octacalcium phosphate. Since there is more calcium available in the soil than phosphorus, these interactions result in less phosphorus availability.
All soils immobilize phosphorus to some degree. Calcareous, high pH soils will tie up phosphate quickly and particularly at high phosphorus concentrations. At low phosphorus concentrations, non-carbonate, low pH clays with iron oxides will tie up phosphate. With either type of soil, phosphate is immobilized as a mineral and becomes unavailable. Animal manures, compost and cover crops provide humic materials and organic acids, which increase recovery of phosphorus that is tied up and immobilized.
Dr. Davidson posts articles on soil management and subjects to gypsum. If you have suggestions for topics or questions, feel free to contact him at [email protected] or call 402-649-5919.