Choosing a soil amendment

yet another great site I chanced upon
http://www.ext.colostate.edu/Pubs/Garden/07235.html

Table 1: Decomposition rate of various amendments.
Amendment	Decomposition rate
Grass clippings, manures	Rapid decomposition (days to weeks)
Composts	Moderate decomposition (about six months)
Wood chips (redwood, cedar), hardwood bark, peat	Slow decomposition (possibly years)

Soil Texture

Soil texture, or the way a soil feels, reflects the size of the soil particles. Sandy soils have large soil particles and feel gritty. Clay soils have small soil particles and feel sticky. Both sandy soils and clay soils are a challenge for Gardeners. Loam soils have the ideal mixture of different size soil particles.

When amending sandy soils, the goal is to increase the soil's ability to hold moisture and store nutrients. To achieve this, use organic amendments that are well decomposed, like composts or aged manures.

With clay soils, the goal is to improve soil aggregation, increase porosity and permeability, and improve aeration and drainage. Fibrous amendments like peat, wood chips, tree bark or straw are most effective in this situation.

Table 2: Permeability and water retention of various soil types.
Soil Texture	Permeability	Water Retention
Sand	high	low
Loam	medium	medium
Silt	low	high
Clay	low	high

Table 3: Permeability and water retention of various soil amendments.
Amendment	Permeability	Water Retention
Fibrous Peat Wood chips Hardwood bark	low-medium high high	very high low-medium low-medium
Humus Compost Aged manure	low-medium low-medium	medium-high medium
Inorganic Vermiculite Perlite	high high	high low