Section 1: Introduction
Whether they are referred to as soils, root media, growing media, root substrates or substrates, the materials that are placed into containers and in which the plants' roots develop can have a profound effect on the development of the crop. This is because, regardless of its composition, the substrate may fulfill up to five basic functions:
- Provide physical support for the plant
- Retain water in a form available to the plant
- Provide for gas exchange between the roots and the atmosphere outside of the container
- Serve as a reservoir for plant nutrients
- Sustain a microorganism population important in nutrient cycles and disease suppression
Under certain production situations, the root substrate (substrate) might provide for all of these functions. However, in some production situations, the substrate may not be required to fulfill all of these functions, and as such may not be designed to fulfill all of these functions. For example, in hydroponic and some pseudo-hydroponic production systems, the substrate (if one exists at all) is generally not expected to retain nutrients for uptake by the plant. The substrate holds water and thus the nutrient solution that is applied during each irrigation (fertigation) cycle. However, this solution is replaced frequently and thus the root substrate is not specifically required to serve as a reservoir of nutrients for the plant. The substrate in this case is simply serving the functions of physical support and water retention. The importance of each of these functions may change depending upon the crop being grown and the cropping system being used.
There are many components that may be used in the formulation of a substrate. Because these individual components rarely have the optimal physical and chemical properties required when used alone, two or more components are often blended together to produce a composite material that will have the appropriate properties for the crop being grown and the specific cultural conditions under which the substrate will be used. In order for the substrate to fulfill the desired functions, it must have the appropriate physical and chemical properties. Therefore, understanding substrates begins with an understanding of their important physical and chemical properties.
