> 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-5-land-cover-plant/actual-growth/growth-constraints/phosphorus-stress.md).

# Phosphorus Stress

As with nitrogen, phosphorus stress is quantified by comparing actual and optimal plant phosphorus levels. Phosphorus stress varies non-linearly between 0.0 at optimal phosphorus content and 1.0 when the phosphorus content of the plant is 50% or less of the optimal value. Phosphorus stress is computed with the equation:

             $$pstrs=1-\frac{\phi\_p}{\phi\_p +exp\[3.535-0.02597\*\phi\_p]}$$                                                         5:3.1.8

where $$pstrs$$ is the phosphorus stress for a given day, and $$\phi\_p$$ is a scaling factor for phosphorus stress. The scaling factor is calculated:

           $$\phi\_p=200\*(\frac{bio\_P}{bio\_{P,opt}}-0.5)$$                                                                         5:3.1.9

where $$bio\_{P,opt}$$ is the optimal mass of phosphorus stored in plant material for the current growth stage (kg N/ha) and $$bio\_P$$ is the actual mass of phosphorus stored in plant material (kg N/ha).

Table 5:3-1: SWAT+ input variables that pertain to stress on plant growth.

| Variable Name | Definition                                              | Input File |
| ------------- | ------------------------------------------------------- | ---------- |
| T\_BASE       | $$T\_{base}$$: Base temperature for plant growth (°C)   | crop.dat   |
| T\_OPT        | $$T\_{opt}$$: Optimal temperature for plant growth (°C) | crop.dat   |