The Soil Phosphorus Problem
Phosphorus is adsorbed by plants in the ionic forms H2PO4– and HPO4=. General knowledge of ion exchange in soils would predict that these anions are not retained by the negative charged soil colloids, but move in the soil similar to nitrogen. However, phosphorus does not leach. In fact, it moves very little, even with large amounts of precipitation or irrigation. The reason for this apparent anomaly is that the soil solution contains only a very small amount of available phosphorus in these ionic forms at any one time. In fact, most soils contain less than 0.00005 grams phosphorus per liter or 0.0000068 ounces phosphorus per gallon of soil. It has been estimated that the phosphorus in the soil solution must be replenished on an average of about twice every day for normal crop growth. This is the basic phosphorus problem — to adequately re-supply the soil solution as the crop roots remove available phosphorus from the soil solution. It is the soil’s ability to re-supply the soil solution that dictates whether the crop will need additions of fertilizer phosphorus and whether those additions will be effective in the forms applied.
The ability of the soil to re-supply the soil solution with phosphorus is dependent on the complex chemistry of the soil system. However, the system can be viewed very simply with the following diagram:
|slowly soluble or insoluble P form||soluble or plant available P forms|
|Relatively unavailable ----------------------------------------------------->|
|P minerals and compounds of Ca, Fe, and Al Organic P <----------------------------------------------Soil Solution P|
This is an equilibrium reaction. As soil solution phosphorus is removed by crop roots, more phosphorus becomes available from the slowly soluble sources. However, if soluble fertilizer phosphorus is placed in the soil, it reverts into slowly soluble or insoluble forms, removing soluble phosphorus from the soil solution. This phenomenon is often called “fixation.” Fixation is the primary reason why placement of phosphorus fertilizer is important. Placement of phosphorus is an attempt to limit fixation. This is done by banding the phosphorus fertilizer near the seed or by dual placement with anhydrous ammonia bands. The goal is to limit soil-fertilizer contact, while placing available sources of phosphorus from the fertilizer in a position of a high probability root contact.
The above relationship is sometimes shown in terms of labile and non-labile phosphorus forms according to the following relationship:
Non-labile P <—> Labile P <—> Soil solution P
In this relationship, non-labile phosphorus refers to slowly available forms, while labile phosphorus is an intermediate form that is rather weakly adsorbed or bound to various compounds and clay in the soil (solid phase). This is the primary phosphorus source supplying the soil solution.
The equilibrium relationship shown above between non-labile or insoluble phosphorus forms and labile phosphorus is affected by many factors, such as size of the slowly available pool, soil temperatures, kind of compounds in the pool, kind and amount of clay in soil, and the pH of the soil solution. Figure 6.1 shows the general relationship between soil pH and phosphorus availability, which is based on the kinds of phosphorus compounds associated with the various pHs. At high soil pH, most phosphorus is in the form of calcium compounds. At low or acid pH, phosphorus is combined with iron and aluminum compounds. Maximum phosphorus availability occurs at a soil pH between 6.5 to 7.0. This is why one of the most important benefits of liming acid soils is improving phosphorus availability. Reducing the pH of calcareous soils would also increase the availability of phosphorus in the soil solution by changing some of the solid phase compounds into compounds of higher solubility. Sulfur will reduce the soil pH; however, the cost is prohibitive for field crops because of the high sulfur rates required.
Figure 6.2 characterizes phosphorus additions and removals from the soil system in addition to the inorganic minerals. Organic phosphorus in the form of residues, manures, or from the soil organic matter can contribute greatly to the phosphorus in the soil solution for crop growth. In some soils organic phosphorus can contribute 50 percent of the available phosphorus. Since availability of organic phosphorus is dependent on decomposition of the organic matter, soil temperature and moisture are important factors regulating how fast organic phosphorus is made available.
As previously indicated, available or soil solution phosphorus can revert to slowly soluble mineral forms. This fixation may also occur when available phosphorus is used by microorganisms in the decomposition of residues. This type of fixation is called immobilization and can be either long- or short-term.