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.
The relationship between calcium and phosphorus in the soil can create significant challenges for farmers, especially in soils with high calcium levels. When phosphorus is added to such soils, the excess calcium binds with it, forming insoluble calcium phosphate compounds that plants cannot readily absorb. This process significantly reduces the availability of phosphorus, a crucial nutrient for plant growth. As a result, even when phosphorus is present in the soil, plants may still struggle to access it, leading to deficiencies and suboptimal crop yields. The mineral forms of calcium phosphate, such as dicalcium phosphate and octacalcium phosphate, remain largely unavailable to plants, exacerbating nutrient management issues.
Soil conditions further influence phosphorus availability. In calcareous, high pH soils, the immobilization process occurs rapidly, especially when phosphorus concentrations are high. This means that the soil effectively “locks up” much of the phosphorus, making it difficult for plants to benefit from the nutrient. On the other hand, in soils with low pH or high concentrations of iron oxides, phosphate becomes bound in a different way, yet still remains unavailable to plants. To address this, organic amendments like animal manures, compost, and cover crops are often used to introduce humic substances and organic acids. These materials help to release phosphorus that has been immobilized by binding with minerals, thereby improving nutrient availability and enhancing soil fertility for future crops.
Dr. Davidson posts articles on soil management and gypsum-related subjects. If you have suggestions for topics or questions, feel free to contact him at [email protected] or call 402-649-5919.
