Manure has been used as a nutrient source for crop production for many centuries. However, only in the last few decades have we started to fully understand the chemistry involved in manure nutrient availability.
Phosphorus in manure is a valuable resource for crop production when applied correctly and has proven to improve crop yield to higher levels than commercial fertilizer. In contrast, inappropriate use of animal manure has been reported to negatively impact the environment. Therefore, it's important to understand the forms of phosphorus in manure and how manure interacts with soil before improved manure management strategies can be developed.
Phosphorus in manure is present in the organic and inorganic forms. Those phosphorus forms can be further divided into dissolved and precipitated as inorganic minerals and organic compounds. The amount of phosphorus present in a manure sample is highly variable and depends on the animal species, animal age, duration of manure storage, type of manure storage and other factors. Manure testing is essential to determine the real concentration of phosphorus in a given sample at any given time.
The availability of the manure phosphorus is dependent on the quantity of the dissolved form, and also on the solubility of the precipitated minerals and organic compounds. The dissolved phosphorus is already in solution, and is highly mobile until it is in contact with soil particles, which provide sites where the phosphorus can attach and become less mobile. However, manure placed on the soil surface and without being incorporated is highly susceptible to runoff with rainwater. To assure that phosphorus stays in the field where it belongs, incorporation soon after manure application is very important.
Once manure is applied to soils, the soil pH and soil texture (clay content) will also have an important effect on how available manure phosphorus is. Soils with low pH (less than 7.0) will promote minerals to dissolve fast and the manure phosphorus will become available rather quickly, whereas soils with high pH (greater than 7.0, like soils in the western part of Minnesota) will dissolve more slowly. Studies are starting to report that the clay present in soils may interact with microbes and as a result affect mineralization of the organic phosphorus. Phosphorus in organic compounds must first be mineralized before it can become available for plant uptake. Research has shown that soils high in clay content (greater than 12 percent) might have lower mineralization of organic phosphorus.
The organic phosphorus may stay stored in the soil for more than one cropping season, which would provide a source of phosphorus for following growing seasons. This is also one of the forms of manure phosphorus that has been called residual manure phosphorus. The residual phosphorus from manure can be any form of the inorganic or organic phosphorus that did not dissolve and reacted with the soil in the first year after the manure application.