APPENDIX N -- THE COMPOSTING HANDBOOK

SOIL AND ORGANIC MATTER PRIMER

Richard Stehouwer, Professor, Pennsylvania State University

Latest revision: 12/01/2021

Since compost use is tied integrally to soil and growing media properties, it is important understand the basic properties of soils and soil organic matter. Compost is a product that, when used properly, can improve soil quality in many situations. Several such soil-based uses for compost are described in this chapter. Soils are complex, dynamic, living bodies that undergo constant change in response to internal physical, chemical and biological processes that depend on inherent soil properties and on external inputs such as water, heat, nutrients and the myriad of management practices imposed by humans. Adding compost to soil is one such external input that can profoundly change soil properties and the processes that occur within the soil. In this section we provide a greatly abbreviated description of a few key soil properties and processes that are influenced by compost addition. For a more complete presentation of these and many other soil properties and processes not mentioned here the reader is referred to any of several soil science textbooks (see suggested reading list at the end of this chapter).

Soil Quality

The term quality connotes two meanings; an essential or inherent feature of a substance, as well as the degree of excellence of that substance. Quality, then, is both that which makes a soil a soil and that which determines how good the soil is. There are many defining qualities of soil spanning physical, chemical, and biological realms. What constitutes a good soil can only be described in relation to the function or use of the soil. Here we are primarily considering soils from the perspective of their function as media to support plant growth. In this context the critical qualities of soil are those properties and processes that provide the essentials for plant growth: physical support, and supply of water, air, and nutrients. In addition, soils are part of a larger ecosystem and must perform important functions within that ecosystem. Thus soil quality is also defined by the ability and capacity of the soil to perform functions such as partitioning rainfall between surface runoff, infiltration, soil storage, and groundwater; releasing water and air that does not contain excessive levels of sediments, nutrients, other chemicals, or pathogens; absorbing or degrading environmentally hazardous chemicals; and, suppressing plant, animal, and human pathogens. Proper use of compost requires an awareness of what soil properties or qualities will be changed by the compost addition, some idea of how large and in what direction the change will be, and an assessment of what the short- and long-term effects will be overall soil quality.