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.