Phosphate performs a number of essential roles in plants. Primarily, it acts as a catalyst for photosynthesis, the process that converts the sun’s energy into simple sugars. In turn, these sugars provide a source of energy for most functions in plants and animals. It also transports and regulates nutrient uptake into the plant by acting as the major carrier of essential cations, such as calcium. Phosphate sugars are involved in DNA strands.
Building and maintaining a good phosphate bank it is essential to improve pasture performance and provide flow-on benefits in the cropping phase. The ratio of available phosphate to potash in the soil is also important for soil fertility. As this ratio approaches 2:1, there will be less leaching of nutrients, less erosion and reduced broadleaf weed pressure.
Ironically, phosphate is plentiful in many soils, although it is functionally deficient as it is “locked up” in forms that are unavailable to plants. This is because most conventional phosphate fertilisers are derived from rock phosphate, which consists of tri-calcium phosphate, a stable and insoluble compound. To make the phosphate more available to the plant, fertiliser manufacturers use strong acids (e.g. sulphuric acid or phosphoric acid) to remove the calcium, resulting in a soluble but unstable phosphate ion. Applied to the soil, this highly acidic, negatively-charged phosphate ion is drawn to unattached positively-charged ions (e.g. iron, aluminium, calcium and magnesium) in order to return to a stable state. In effect, it becomes “locked up” and unavailable to plants.
Calcium is another integral part of soil, plant and animal nutrition. Basic to every living cell, calcium contributes to the strength and thickness of the cell wall and helps to regulate cell functions and water content. In plants, it stimulates root, stem and foliar growth. In animals, it is essential for bone and muscle development in animals. Calcium interacts with other nutrients and micro-organisms in the soil to improve the availability of nutrients and metabolites. Optimum soil conditions exist when the calcium to magnesium ratio is 7:1. A low Ca:Mg ratio (e.g. 2:1) results in a tight soil that is sticky and slick when wet, and hard and compact when dry. Nutritionally, the soil will have excess magnesium and be deficient in calcium and nitrogen. The lack of oxygen will also reduce microbial activity. Conversely, a high Ca:Mg ratio (e.g. 10:1) will result in a flocculating soil. The soil opens up, but there is no structure and leaching occurs. Nitrogen moves into the plant in a soluble form allowing nitrates to accumulate, possibly leading to bloat and grass tetany in livestock. In conventional farming systems, lime (CaCO3) is often applied to correct soil pH. Ideally, the functional availability of calcium should be maintained throughout the year via small, frequent applications of lime rather than massive amounts once a year.
Healthy soils also contain a diverse and active array of macro- and micro-organisms, including bacteria, fungi, protozoa, nematodes, earthworms and arthopods. The “soil foodweb” performs a number of vital roles:
- Fixing nitrogen and carbon from the air for plant uptake;
- Regulating the soil environment, especially the functional availability of nutrients and buffering the soil pH;
- Digesting organic matter into humus to release plant-available nutrients;
- Providing metabolites essential for efficient plant growth; and,
- Holding soil particles together to slow erosion and for moisture holding ability.