Variety and solubility of phosphorus forms in animal manure and their effects on soil test phosphorus

Animal manure has been recognized as one of the best sources of nutrients for crop production for centuries. However, only over the last few decades have we started to fully understand and recognize the negative environmental impacts of agricultural use of manure. Although all essential nutrients required for plant growth can be found in a manure sample, there are primarily two that are well known to negatively impact the environment the most, nitrogen (N) and phosphorus (P). Both of these nutrients have considerably high mobility while within the manure matrix; however, their mobility significantly decreases once manure is incorporated into the soil. Phosphorus is found in two forms in animal manure, inorganic (P_i) and organic (P_o). The P_o can be further categorized as enzymatically hydrolysable or non-hydrolysable P_o. The P_i and P_o can be found dissolved in solution and also precipitated as minerals or complexed with organic compounds and metals in the manure matrix. The relative amount of each P form is highly variable and depends among other factors on the animal species, animal age, growth stage, animal diet, and manure storage conditions. When manure is applied to soils, most often there is an increase in soil test phosphorus (STP) levels. However, the magnitude of the increase in STP will depend on soil and manure properties. Understanding how manure and soil properties interact is key to minimize the negative impact of manure on the environment. This chapter will discuss the variety and solubility of manure P and also how the different forms of P_i and P_o can affect the increase in STP.