Not all the water in a soil is available. Above field capacity it drains away; below the wilting point the plant cannot pull it out. What is left between them, over the depth the roots reach, is the whole of a soil's drought resistance.
Soil loss is estimated as A = R·K·LS·C·P. The rainfall and the soil you cannot change; the slope you rarely can. Cover and practice you can — and because it is a product, halving either halves the loss.
Compaction is the damage that does not show. Bulk density rises, porosity falls, and roots stop at a layer that looks like ordinary soil. Porosity is one minus the bulk density over the particle density — arithmetic anyone can do with a ring and a balance.
Capability classes say what land can carry indefinitely, not what can be forced from it for a season. The class is set by the worst limitation — one severe factor is enough, and no amount of good in the others makes up for it.
| class | what it will carry |
|---|
Texture, Atterberg limits and the engineering classification of the same soil are in Soil; whether the irrigation water will sodify it is Water chemistry and its sodium adsorption ratio. This instrument is about what the land can carry, year after year.