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Garden with Insight v1.0 Help: demand/supply/uptake

In this simulation we divide demand for nutrients (N and P) into two stages of demand: first a "demand for optimal growth," and then a more restricted general "demand" which takes supply into account.

Demand for optimal growth is the amount each plant would prefer to absorb based on the cumulative total uptake over its lifetime as compared to the optimal cumulative uptake at this stage in its life. Optimal cumulative uptake at any time is based on an S curve parameter that determines the optimal concentration of N or P over the life of the plant (the heat unit index).

Demand for optimal growth is modified to produce general demand by considering supply. By supply here we mean the amount of nutrient (as nitrate or labile P) available to this plant if it were the only plant in the soil patch. Supply is based on two things: how much nutrient is available in the soil patch, and how quickly the plant's roots can absorb the nutrient. The ability of the plant to absorb nutrients is determined by the plant's water uptake (because nutrients are taken up with water), which is in turn determined by the distribution and biomass of the plant's roots. Thus general demand is the lesser of the plant's demand for optimal growth and the supply available to this plant if it were alone.

Uptake is the actual amount of nutrient taken from the soil patch by this plant. Each plant is given a proportion of its demand equal to the ratio of available nutrient to the total demand of all plants in the patch. For example, if one plant demands 10 units and another demands 6, and the soil patch has 8 units of nutrient available, each plant will get half of its demand (8/16), so the first plant will receive 5 units and the other 3.

For parts of the plant (leaves, flowers, etc., as modeled for drawing only), demand has a similar meaning. Each day the plant asks each of its parts how much biomass they demand to grow or make other plant parts. The new photosynthate for the day is partitioned to the plant parts according to their demands. As with whole plants no weighting is done.

How it works:
plant nutrient uptake
plant water uptake

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Updated: May 4, 1998. Questions/comments on site to
Copyright © 1998 Paul D. Fernhout & Cynthia F. Kurtz.