The three major forms of phosphorus in mineral soils are organic phosphorus associated with humus, insoluble forms of mineral phosphorus, and plant-available phosphorus in soil solution. Phosphorus may be added to the soil by fertilizer, manure or residue application. Phosphorus is removed from the soil by plant uptake and erosion. Figure 3:2-1 shows the major components of the phosphorus cycle.

Unlike nitrogen which is highly mobile, phosphorus solubility is limited in most environments. Phosphorus combines with other ions to form a number of insoluble compounds that precipitate out of solution. These characteristics contribute to a build-up of phosphorus near the soil surface that is readily available for transport in surface runoff. Sharpley and Syers (1979) observed that surface runoff is the primary mechanism by which phosphorus is exported from most catchments.

SWAT+ monitors six different pools of phosphorus in the soil (Figure 3:2-2). Three pools are inorganic forms of phosphorus while the other three pools are organic forms of phosphorus. Fresh organic P is associated with crop residue and microbial biomass while the active and stable organic P pools are associated with the soil humus. The organic phosphorus associated with humus is partitioned into two pools to account for the variation in availability of humic substances to mineralization. Soil inorganic P is divided into solution, active, and stable pools. The solution pool is in rapid equilibrium (several days or weeks) with the active pool. The active pool is in slow equilibrium with the stable pool.

Users may define the amount of soluble P and organic phosphorus contained in humic substances for all soil layers at the beginning of the simulation. If the user does not specify initial phosphorus concentrations, SWAT+ will initialize levels of phosphorus in the different pools.

The concentration of solution phosphorus in all layers is initially set to 5 mg/kg soil. This concentration is representative of unmanaged land under native vegetation. A concentration of 25 mg/kg soil in the plow layer is considered representative of cropland (Cope et al., 1981).

The concentration of phosphorus in the active mineral pool is initialized to (Jones et al., 1984):

$minP_{act,ly}=P_{solution,ly}*\frac{1-pai}{pai}$ 3:2.1.1

where $minP_{act,ly}$ is the amount of phosphorus in the active mineral pool (mg/kg), $P_{solution,ly}$ is the amount of phosphorus in solution (mg/kg), and $pai$ is the phosphorus availability index.

The concentration of phosphorus in the stable mineral pool is initialized to (Jones et al., 1984):

$minP_{sta,ly}=4*minP_{act,ly}$ 3:2.1.2

where $minP_{sta,ly}$ is the amount of phosphorus in the stable mineral pool (mg/kg), and $minP_{act,ly}$ is the amount of phosphorus in the active mineral pool (mg/kg).

Organic phosphorus levels are assigned assuming that the N:P ratio for humic materials is 8:1. The concentration of humic organic phosphorus in a soil layer is calculated:

$orgP_{hum,ly}=0.125*orgN_{hum,ly}$ 3:2.1.3

where $orgP_{hum,ly}$ is the concentration of humic organic phosphorus in the layer (mg/kg) and $orgN_{hum,ly}$ is the concentration of humic organic nitrogen in the layer (mg/kg).

Phosphorus in the fresh organic pool is set to zero in all layers except the top 10mm of soil. In the top 10 mm, the fresh organic phosphorus pool is set to 0.03% of the initial amount of residue on the soil surface.

$orgP_{frsh,surf}=0.0003*rsd_{surf}$ 3:2.1.4

where $orgP_{frsh,surf}$ is the phosphorus in the fresh organic pool in the top 10mm (kg P/ha), and $rsd_{surf}$ is material in the residue pool for the top 10mm of soil (kg/ha).

While SWAT+ allows nutrient levels to be input as concentrations, it performs all calculations on a mass basis. To convert a concentration to a mass, the concentration is multiplied by the bulk density and depth of the layer and divided by 100:

$\frac{conc_P*\rho_b*depth_{ly}}{100}=\frac{kgP}{ha}$ 3:2.1.5

where $conc_P$ is the concentration of phosphorus in a layer (mg/kg or ppm), $\rho_b$ is the bulk density of the layer (Mg/m$^3$), and $depth_{ly}$ is the depth of the layer (mm).

Table 3:2-1: SWAT+ input variables that pertain to nitrogen pools.