> For the complete documentation index, see [llms.txt](https://swatplus.gitbook.io/io-docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://swatplus.gitbook.io/io-docs/theoretical-documentation/section-3-nutrients-pesticides/phosphorus/leaching.md).

# Leaching

The primary mechanism of phosphorus movement in the soil is by diffusion. Diffusion is the migration of ions over small distances (1-2 mm) in the soil solution in response to a concentration gradient. The concentration gradient is created when plant roots remove soluble phosphorus from soil solution, depleting solution P in the root zone.

           Due to the low mobility of phosphorus, SWAT+ allows soluble P to leach only from the top 10 mm of soil into the first soil layer. The amount of solution P moving from the top 10 mm into the first soil layer is:

          $$P\_{perc}=\frac{P\_{solution,surf}*w\_{perc,surf}}{10*\rho\_b\*depth\_{surf}\*k\_{d,perc}}$$                                 3:2.4.1

where $$P\_{perc}$$ is the amount of phosphorus moving from the top 10 mm into the first soil layer (kg P/ha), $$P\_{solution,surf}$$ is the amount of phosphorus in solution in the top 10 mm (kg P/ha), $$w\_{perc,surf}$$ is the amount of water percolating to the first soil layer from the top 10 mm on a given day        (mm H$$*2$$O), $$\rho*{b}$$ is the bulk density of the top 10 mm (Mg/m$$^3$$) (assumed to be equivalent to bulk density of first soil layer), $$depth\_{surf}$$ is the depth of the “surface” layer (10 mm), and $$k\_{d,perc}$$ is the phosphorus percolation coefficient (m$$^3$$/Mg). The phosphorus percolation coefficient is the ratio of the phosphorus concentration in the surface 10 mm of soil to the concentration of phosphorus in percolate.

Table 3:2-4: SWAT+ input variables that pertain to phosphorus leaching.

| Variable Name | Definition                                                       | Input File |
| ------------- | ---------------------------------------------------------------- | ---------- |
| SOL\_BD       | $$\rho\_b$$: Bulk density of the layer (Mg/m$$^3$$)              | .sol       |
| PPERCO        | $$k\_{d,perc}$$: Phosphorus percolation coefficient (m$$^3$$/Mg) | .bsn       |


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