Phosphorus, like coal, or oil, is a finite resource on earth, and it has been suggested that all known mineral reserves will be depleted within this century. What then, will be the fate of commercial farming, which has come to rely heavily on external sources of this macronutrient to fulfill the requirements of the crop plants that feed and clothe the world? Before possible solutions can be contemplated, an understanding of the cycling of phosphorus through the environment, particularly the soil, is needed.
Most of the world’s soils have the capacity to hold on to huge quantities of phosphorus in a state that is not available to plants. Less than one percent of the phosphate to be found in the soil is in a plant available state, which is the free orthophosphate (H2PO4) form. The rest of the phosphate is found in either an organic or inorganic state, both highly stable, but not usable by plants.
Phosphorus is used by plants to form new roots, seeds, fruits and flowers. It plays a large role in the transfer of energy within plant structures. Phosphorus stimulates early growth and hastens maturity.
Organic Soil Phosphate
The organic fraction of the soil phosphate usually makes up between 30% and 80% of the total phosphate in the soil, divided between stable soil organic matter and contained in the “bodies” of living microbes. The release of this organic phosphate to a form which plants can take up is known as mineralisation, which usually occurs when microbes die. The phosphate contained in the organic matter itself is extremely stable and is almost never released. Plant available phosphate gets turned into this unavailable organic form when organic matter with a low phosphate content is added to the soil and needs to be decomposed, to eventually end up as stable organic matter.
Stable soil organic matter tends to have a carbon to phosphate ratio of between 200 and 300 to 1, and if the material to be decomposed has a higher ratio (i.e. lower Phosphates), free phosphate will be sucked up from the soil and immobilised as part of the organic matter. This is not considering the phosphorus taken up by the decomposer microbes that proliferate when dead organic matter is present. Some of this phosphate will eventually be released. The timing is however dependent on the nature of the material to be decomposed, the type of microbes, and other factors like temperature and pH. Organic matter added to the soil will only act as a phosphate fertiliser if it is very high in phosphate, such as animal manure or sewerage effluent, so that there is enough left over for plants after decomposition. Even then, however, depending on the soil type and general phosphate fertility of the soil, much of the newly added phosphate may be taken up by the soil itself, in an inorganic form.
Inorganic Soil Phosphorus
Highly weathered red and yellow soils, such as this Magwa soil (Humic A on Yellow-Brown Apedal B), typically have a high phosphorus buffer capacity
When phosphate is added to the soil, it interacts with other elements and minerals in the soil that make it unavailable to plants. The longer the phosphate is associated with these minerals, the more irreversible the process. The exact nature of the problem depends very much on the pH of the soil. At a high pH, calcium holds the phosphate, while in acidic soil, the issue becomes far more serious, with iron and aluminium minerals being the main culprits, through a process known as adsorption. Lowering the pH will always release phosphate held by calcium, however, increasing the pH only works for a short while to release phosphate held by metallic minerals. The longer phosphate is associated with them, the stronger the bonds. Eventually, the phosphate becomes a part of the mineral itself, and is basically impossible to release.
The amount of phosphate that is associated with other soil elements is soil specific and is known as the phosphate buffer capacity of the soil. Phosphate adsorption and bonding proceed rapidly until close to the soils buffer capacity, after which the process slows down and added phosphorus will stay available for longer. In South Africa, highly weathered red and yellow soils tend to have the highest buffer capacity, due to the presence of numerous small aluminium and iron metallic crystals with lots of surface area to bond to phosphate. In these soils, any free phosphorus is very quickly taken up by the soil when the phosphate fertility of the soil is low, and there are many free adsorption sites. Adding enough phosphate to these soils to fill the buffer capacity and slow the rate of bonding can be an extremely expensive undertaking.
Managing Soil Phosphate
This has implications for the management of phosphate fertiliser applications. The longer applied phosphate remains in the soil solution before being taken up by the crop, the more likely it is to never be available to the crop, especially if the phosphate level of the soil is low and there are many free adsorption sites. Phosphate is often lost in this way during fallow periods, when there are no roots in the soil to take up the free phosphate, or when fertiliser is applied too far from the roots of young plants. Unlike nitrogen, even when phosphate is free in solution it is relatively immobile, and roots are not able to take it up unless they grow into the space where the phosphate is present. Until then, available phosphate is continuously converted to unavailable inorganic forms. Therefore, the least wastage of phosphate occurs when there are always roots present in the soil to take up any free phosphate and cause the desorption of loosely held phosphate into the solution for further uptake. This necessitates there always being a living crop on the land to ensure that there are always living roots in the soil to take up phosphate. It is far better that phosphate be lost temporarily to a cover crop, than permanently lost to the elements. It should also be ensured that the soil is never allowed to acidify, as phosphate availability is highest in the neutral range.
Special Plants
Since roots need to be present in close proximity to free phosphate before uptake can occur, it follows that plants with more root mass or complexity will be more efficient at taking up phosphate. Difference in root mass between genotypes / cultivars have been found for most common crop species, including maize and soybean. As a result, some varieties of these crops do better at lower soil phosphate levels. Many plant species also form associations with mycorrhizae to increase the soil volume which is available to the plant, and efforts should be made to ensure that soil conditions remain conductive to these fungi.
Some plant species which are adapted to low phosphate environments are known for some rather interesting strategies of acquiring phosphate, which may be usefully employed in agriculture. One of the species that is most well-known for several of these strategies is lupins. The new root growth of lupins is organised in the form of cluster roots, which looks like a bottle brush. This allows the plant access to more soil in which to “scavenge” for phosphates. Lupins, together with some summer legumes like pigeon pea and sensitive partridge pea release acids with a negative charge from their roots, most notably citric acid. These acids lower the pH of the soil around the roots. Chemical changes of the compounds holding the phosphate or complete breakdown thereof cause phosphate to be released ready for plant uptake. Since these acids have a negative charge, just like phosphate, they can also replace some of the phosphate held by the soil. Certain microbes are also known to release phytase enzymes, which can break up the most stable of the organic phosphate compounds. It is therefore advantageous to encourage a diverse microbe community to develop in the soil.
Bottlebrush-like Cluster roots of Lupins, that allow the plant access to a greater volume of soil to take up phosphorus
The effectiveness of lupins and pigeon pea, amongst other species, at releasing soil-held phosphorus has been shown to free enough phosphorus, which will be available for the intercrop or for the following crop. This has been tested for maize and wheat. It would be a shame to have access to an almost free source of phosphorus, and not use it.
Wheat benefits from the phosphate released from the soil by lupin under low phosphate conditions
Soil health for phosphorus savings
In conclusion, good soil health, including high biological activity, good soil organic matter levels and a stable pH will all help reduce phosphorus input costs. Phosphorus may become 4 to 5 times more available to plants when surrounded by soil organic matter while biology can solubilise phosphorus that is otherwise not available to plants.
Phosphorus reserves are going to run out at some point, whether in 100 years or 1,000 years. Either way, it’s up to us to use this nutrient responsibly. Despite debate on the date on which reserves will likely run out, there is consensus that the costs of mining, refining storing and transporting this resource are rising and more importantly, that quality reserves are running out. The lower quality reserves that are already being used contain minerals such as zinc and cadmium which are harmful to soil health when in over-supply.
In light of this, planting cover crops can be a key strategy in effective phosphorus cycling, which supports sustainable crop production.