Garden with Insight v1.0 Help: Soil patch next day functions: allocate nutrients to plants

Allocation of nitrogen and phosphorus to plants in a soil patch works in much the same way as allocation of water: each plant declares a N and P demand, then the soil patch divides up the available nutrients according to demand among all plants in the patch.

Nitrogen demand by each plant works by the potential-actual method. The optimal nitrogen uptake for the day is calculated from an S curve of optimal nitrogen concentration over the life of the plant (as determined by heat unit index). The shape of the S curve is determined by plant parameters. Next the N demand is reduced to account for the amount of nitrate in the layer and the amount of water taken up from the layer. Because the plant takes up most of its nitrogen passively in the water solution, N uptake is limited by water uptake. (Actually, plant roots also actively take up nitrogen by pumping ions out of the root that replace the nitrate ions in solution, but we do not simulate that smaller portion of N uptake here.)

For leguminous plants (like peas and beans), an adjustment is made to N demand for nitrogen fixation. Nitrogen fixation is simulated simply as a fraction of the daily nitrogen demand. The fraction of N demand met by fixation increases with the presence of soil conditions that promote the growth of the symbiotic bacteria: soil water (more is better, up to a point), heat unit index (midrange is best -- seedlings haven't established the symbiosis yet, and old plants are senescing), and available nitrate (as more is available in the soil, less is fixed). Any nitrogen fixation reduces the plant's N demand, though it cannot exceed it. This means that nitrogen-fixing plants cannot add nitrogen to the soil.

To start the calculation of phosphorus demand, the optimal P uptake is calculated in the same way as the optimal N uptake. Because phosphorus is more closely associated with the mineral phase than is nitrogen, the supply of P available to the plant in each soil layer depends not on water uptake but on the root biomass in each layer and on the amount of labile P available. The plant's P uptake in response to the amount of labile P available is determined by an S curve. Finally the plant attempts to compensate for inadequate P availability in some layers by increasing the demand in other layers.

Once the N and P demands have been calculated for each plant, the soil patch partitions its available nitrate and labile P to the plants on the basis of their demands. Plants add the N and P taken up to the total N and P in their live biomass. The soil patch subtracts the N and P taken up from its nitrate and labile P pools. After this is finished the plants check to see if their "optimal N uptake" and "optimal P uptake" options are turned on. If so, each plant sets its N and/or P uptake to the optimal amount calculated as the first estimate of demand. Plants that reset their uptake for an optimal setting do not tell the soil patch about this, because that would deplete the soil patch and cause other changes in the soil.

calculation of water allocation to plants
EPIC Nitrogen
EPIC Phosphorus
Model contents