Section 3: Important Chemical Properties of Substrates
pH
The pH impacts many aspects of the substrate environment. The pH affects the availability of mineral nutrients required by the plant. Some mineral elements such as iron, zinc, manganese and copper are more available at low pH while others such as molybdenum, calcium and magnesium become more available for uptake as the pH increases. The pH of the root substrate can also impact the incidence of some soil-borne diseases. Although the optimal pH for a new unused substrate varies depending upon the specific root substrate components, the quality of the irrigation water to be used, the fertilization program to be used and the specific crop being grown, an initial substrate pH within a range of 5.3 to 6.3 is generally desirable. The exception to this recommendation would be for ericaceous crops such as azalea that require a lower pH. Additional information regarding substrate pH is discussed under the "Mineral Nutrition" and "Irrigation Systems" learning units.
Electrical conductivity
The electrical conductivity (E.C.) is a measure of the total ions (K+, Ca++, NO3-, NH4+, Cl-, etc.) in the substrate solution (the liquid phase of the substrate). The E.C. is determined as a measure of the substrate solution's ability to conduct an electric current across a distance of one cm and is therefore commonly expressed as mmho.cm-1, mS.cm-1 or dS.m-1. These units of measure are equivalent to one another and require no conversion (1.0 mmho.cm-1 = 1.0 mS.cm-1 = 1.0 dS.m-1). The higher the ion concentration in the substrate solution, the higher the E.C. Because E.C. cannot be used to measure the level of any specific ions in the substrate solution, it can only be used as a measure of the total ion concentration. However, because many of these ions are plant mineral nutrients, E.C. may be used as an overall measure of the mineral nutrient content of the substrate.
Recommendations for optimal E.C. of unused substrates can be difficult because the desired level will depend upon the crop, stage of the crop, production conditions, and the specific ions that make up the E.C. In cases where the substrate will be used for germination, a very low E.C. is generally desirable. When transplanting rooted cuttings or well developed plugs, a higher E.C. may be tolerated or desired. However, this is dependent upon the crop. For example, the desired E.C. of an unused substrate in which New Guinea impatiens will be transplanted will be lower than the desired substrate E.C. in which a large well-rooted geranium cutting will be transplanted. As a general rule of thumb, for rooting of cuttings and germinating seed, an E.C. of less than 2.0 is desirable. For well rooted crops that are not sensitive to E.C., an initial E.C. of 2.5 or less is desirable.
In some cases, a nutrient starter charge may be added to an unused substrate to increase the mineral element content of the substrate (and thus the E.C.). This starter charge is designed to provide a source of rapidly available nutrients for the developing new crop and to allow the crop to more quickly become established. More information is provided about nutrient starter charges under "Common Substrate Amendments" in the "Mineral Nutrition" learning unit.
Carbon:Nitrogen ratio
Organic matter serves as an energy source and provides the basic building blocks used by microorganisms in the substrate. As microorganisms break down organic matter, they require 1 nitrogen for every 25 carbons that they utilize. If the organic matter being broken down has a carbon:nitrogen ratio of greater than 25:1, the microorganisms must obtain nitrogen from another source in the surrounding substrate environment. Under such conditions, the microorganisms will utilize nitrogen in the substrate supplied by the fertilizer and intended for use by the plant. If the carbon:nitrogen ratio is very high, significant amounts of nitrogen must be taken from the fertilizer sources. In these situations, microorganisms may out-compete the plant for the available nitrogen and induce a nitrogen deficiency (often referred to as nitrogen tie up). However, a high carbon:nitrogen ratio alone is not sufficient to induce a nitrogen deficiency in a crop. The organic matter must have a high carbon:nitrogen ratio and must decompose relatively quickly for significant nitrogen depletion to occur. Therefore, organic materials that have a high C:N ratio and decompose quickly (i.e. sawdust, cotton gin trash, bark) are generally not used in substrates, or they are composted before use to lower the C:N ratio and allow for the readily available carbon to be utilized by microorganisms during the composting process.
Cation-exchange capacity
Many components used in the formulation of substrates have negatively charged sites on their surfaces that allow them to retain cations. Many of the mineral nutrients required by plants are cations (Ca++, K+, NH4+, Mg++, etc.). These negatively charged sites allow the substrate components to retain mineral nutrients for uptake by the plant and thus provide a reservoir of nutrients. The negatively charged sites also retain protons (H+). These protons can be exchanged with the substrate solution and thus help to buffer the substrate solution from rapid pH changes. Therefore, the C.E.C. provides the substrate with a certain buffering capacity and is sometimes referred to as the substrate buffering capacity. The higher the substrate buffering capacity, the more cations the substrate can retain and the more resistant the substrate is to pH changes. Cation-exhange capacity is expressed as meq.100ml-1.
Specifications
Mineral ions may occur in phytotoxic concentrations. This can be an issue with some potential substrate components and may make a component unsuitable for use in greenhouse substrates. In these cases, the overall E.C. may not be too high, but the level of a specific ion may occur at a concentration that can be phytotoxic to plants.
For example, ground rubber tires often contain phytotoxic levels of zinc. Some composts may contain phytotoxic levels of ammonium. When using any new or unfamiliar material in substrates, the mineral element content should be determined prior to use.
