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Garden with Insight v1.0 Help: Soil patch end year functions: attempt to auto-control pH

Two approaches are used to raising pH depending on the type of soil.

First, highly weathered soils (usually in areas of high rainfall) normally contain a lot of clay. These soils usually have a high cation exchange capacity for holding ions and therefore are well buffered. Raising the pH to around neutral (7.0) requires a lot of lime in these soils, so instead the simulation raises the pH only enough to counteract the aluminum toxicity that results when the pH is below 5.4. (As the number of free hydrogen ions decreases when the pH rises, the Al3+ ions take up the places abandoned by the H+ ions on the clay micelles and therefore move out of solution in the soil water where they are more toxic to plants.) The amount of lime needed to counteract aluminum toxicity and raise the pH to 5.4 depends on the soil weight and bulk density, the current aluminum saturation, and the cation exchange capacity.

Second, soils that are not highly weathered are limed so that their pH is maintained at around 6.5. The amount of lime needed to raise the soil pH to 6.5 depends on the soil weight, the cation exchange capacity, and the current soil pH.

The pH, aluminum saturation, and base-forming cations used to calculate the amount of lime needed to raise the pH are not those of each soil layer, but an average of all the soil layer values weighted by the thickness of each soil layer. Since only one number is used for the calculations, only one number results, so all the soil layer values are set at this number. In effect, the simulation sets all these soil layer values as if the total amount of lime added is divided among the soil layers according to what is needed to bring each layer to the correct value. The application of lime, then, is not one that could be realistically expected in a field application.

EPIC does not include any pH control for the case of alkaline soil, since that condition is less frequent than the reverse especially in conventional farming. A model for automatically applying sulfur when the pH is too alkaline is being developed but is not in use yet.

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Updated: March 10, 1999. Questions/comments on site to
Copyright © 1998, 1999 Paul D. Fernhout & Cynthia F. Kurtz.