Because phosphorus levels usually do not meet agricultural requirements, phosphorus fertilizers are essential and used extensively to increase bioavailable P. Manures can contain significant amounts of P and can be used to add P to the soil, but long-term manure application can actually lead to an excess of phosphorus and a deficiency of nitrogen due to the relative amounts of these nutrients consumed by plants. Due to the risk of nutrient imbalances from relying solely on manures, almost all growers use mineral fertilizers as well.

Phosphorus fertilizers are derived from ancient phosphorus-rich deposits known as rock phosphates. When added to the soil, rock phosphates slowly dissolve and release bioavailable P. Rock phosphate deposits can be found globally, but they are non-renewable and the massive need for P fertilizers is only speeding up the rate at which they are being depleted. Additionally, they are often improperly or excessively applied: only 15-30% of applied phosphorus fertilizer is actually taken up by harvested crops. The remaining 70-85% of the fertilizers is lost to the environment via wind erosion or runoff. This lost phosphorus enriches nearby bodies of water, causing massive algal blooms which eventually die off and decompose, leading to oxygen depletion and “dead zones.” This process is called eutrophication. Using fertilizers is unavoidable, but growers must take care to not over-apply them. Excessive fertilizer applications are costly not only for the health of the surrounding environment, but for the farmer as well.

Phosphate deficiency is common in agricultural settings: it is estimated that P deficiency reduces global crop yields by 30-40%. Phosphorus deficiency first presents in new leaves, as new growth has greater P demand than old growth. New leaves first appear healthy but are stunted and have reduced leaf size. Leaves often develop a blue-green, violet, or reddish color. Red leaves can occur under N deficiency as well, but no general chlorosis is seen in P-deficient plants. Under P deficiency, plants allocate more energy to root growth in the topsoil in an attempt to locate phosphate. This results in reduced above-ground growth, a shallower root system, and a higher root-to-shoot ratio.

Phosphorus toxicity is rarer than its respective deficiency, but it does occur. P toxicity is more common in hydroponic growing environments, where there are high concentrations of free phosphate and few adsorption sites. Excess phosphates accumulate in old leaves first, causing necrotic symptoms and stunting overall growth. When plants are experiencing P toxicity, other nutrient deficiencies, such as Zn deficiency, exacerbate the issue and can drastically worsen plant health.

As explained above, lower pH values encourage greater phosphate dissolution. However, a soil pH between 6 and 7 tends to be optimal for overall nutrient uptake, so reducing the pH of the soil is usually not a viable option to increase phosphorus availability. To increase phosphorus use-efficiency and availability in the soil, the three main goals should be to 1) improve soil structure, 2) increase anion exchange capacity of the soil, and 3) boost microbial activity. Adding organic matter to the soil can help accomplish all of these goals.

High organic matter decreases bulk density and makes the soil more porous. In porous soil, it is easier for plants to send out new roots and root hairs to scout for phosphate. High organic matter also increases the cation- and anion-exchange capacity of the soil. These anion-exchange sites adsorb phosphate ions and hold them against soil particles, where they can be later accessed by roots and are prevented from being lost to erosion. By increasing the anion-exchange capacity in your soil, you can improve the soil’s ability to retain phosphate. Organic matter, iron oxides and hydroxides, and silica clays (among others) all increase anion-exchange capacity and promote phosphate adsorption in the soil.

Organic matter in the soil also boosts microbial activity, increasing the amount of P in the organic P pool. These organic forms of P are constantly cycled and remineralized by microbes, providing plants with a steady source of P that is unlikely to be lost to runoff. There is evidence to suggest that applying organic material such as green manure at the same time as mineral P applications enhances P uptake and boosts plant growth by promoting microbial activity in the soil. Similarly, if you are applying mineral P fertilizers during the fall or winter, growing a cover crop at this time can significantly increase P retention. The cover crop will take up P released from rock phosphates during the off-season, and in the spring the cover crop can be used as a green manure to re-incorporate that P back into the soil.

Certain species of microbes are better suited to phosphate-deficient soils than others. For a precise approach, you can inoculate your soils with known phosphate solubilizing bacteria (PSBs). These beneficial bacteria produce low molecular weight organic acids, which chelate the cations bound to mineral forms of phosphate, releasing phosphate ions into the soil solution. This is the same mechanism by which root exudates increase P solubility. PSBs can dramatically increase P availability in the soil, boosting plant growth and reducing the need for more expensive fertilizers. Products such as Tainio® Spectrum PSB contain a diverse mix of bacteria which will colonize your soil and break down the bound or insoluble forms of phosphorus, making them available for use by the plant.