ON THE STANDARDS OF GARDEN PEAT
Using peat as a growing medium in peat culture is a fairly new application, so not much is known about the quality requirements of peat for peat culture. Therefore it is necessary to classify peat according to the properties that are the most important for horticulture. It is also desirable to evolve easily measurable standards indicating its suitability for peat culture. Plants absorb water, oxygen and nutrients from their medium. So the principal physical and chemical factors affecting plant development are those that determine the water, air and nutrient economy of the medium.
Let us first consider water. Where the water supply is concerned, it would be desirable for the substrate to hold as much water as possible. Thus pure water would be an ideal culture medium. An argument against this is that, in preventing the supply of oxygen, the water supply itself would be affected and would eventually stop altogether. So a part of the substrate must be reserved for oxygen, or, in practice, air. In regard to the oxygen supply, it would be desirable for as large a part of the substrate as possible to be filled with air. But the air capacity reduces the water capacity. Therefore, both the water and air capacities should be the correct ones. Thus the water and air capacities, indicating the water and oxygen economy of the substrate, can be used as very important peat standards.
Besides water and oxygen, a plant takes up from the substrate the nutrients required. The more nutrients a substrate can store in an available form, the more easily a plant can take them up. In the first place it can take up water-soluble nutrients. The water-soluble nutrients increase the osmotic pressure of the soil solution. An increase in osmotic pressure impedes the plant's water supply. As a result of this, the amount of the water-soluble nutrients in a substrate may not be beyond a certain limit. A high water capacity therefore means not only increased water economy but also increased nutrient economy. In addition to water-soluble nutrients, a plant can take up exchangeable ions. Thus, besides water capacity, the exchange capacity of the substrate material can be used as a peat standard indicating its nutrient economy.
In Finland at present, the garden peat available for sale is controlled by the following standards:
- weight per volume,