Garden with Insight
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.
EPIC Auto Operations