Soil amendments can include virtually any substance that improves the growth of plants in soil. Generally, we separate fertilizers from soil amendments by their higher nutrient content, but the distinction is not always clear since soil amendments can improve both chemical and physical properties of a soil. The chemical properties that may be altered by soil amendments include the soil fertility and pH.
Why use soil amendments?
A fertile loam soil that is easily tilled and retains moisture may require little if any soil amendments to support excellent plant growth. However, such soils are increasingly rare, and are almost non-existent in most urban environments.
Loamy soil takes years of adding organic matter to the soil from falling leaves, other decaying plant matter, and earthworm casting. When housing developments are constructed, most of this bio mass is stripped away during the construction phase. Even housing developments built over farm lands usually do not retain the original soil. The loam only goes down about 6 inches and sometimes less. You can see how easy it is for that to be pushed away when grading takes place.
Improving soil physical properties.
The physical properties of soil that are improved by amendments include soil structure, porosity, and water-holding capacity. Poor soil physical characteristics directly constrain root growth and are probably the biggest problem in landscaping newly constructed sites.
Organic soil amendments are currently the most common solution for improving physical characteristics of these sites. The beauty of organic soil amendments is that they will, on one hand, increase aeration and porosity in heavy clay soils, while increasing water retention in sandy soils.
A great variety of composted materials are available at a reasonable cost throughout the various regions of the United States. Composted sludge, composted animal manure, composted leaves, and composted yardwaste are some of the most common materials that make excellent amendments.
One word of caution: the composting process and the suitability and consistency of the resulting products can vary a great deal between producers. First experiment with using a compost product in the landscape before using it on a wide scale.