# 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 | --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://swatplus.gitbook.io/io-docs/theoretical-documentation/section-5-land-cover-plant/actual-growth/growth-constraints/phosphorus-stress.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.