ON THE STANDARDS OF GARDEN PEAT
During the last ten years the use of pure peat as a substrate has become more and more common in Finland. This culture method has come to be known as peat culture and the peat used as garden peat, literally translated growing peat.
It is only since the beginning of the 1960's that peat culture has become very common, due mainly to the excellent results obtained in peat culture experimentation. This is shown by the fact that, during the period in question, the use of peat in gardening has increased twentyfold. Besides this increase many different types of peat have come into use and many new problems have arisen.
One type of peat has proved suitable for one particular use, another type for another use. The grower accustomed to using one particular type of peat obtains the best results by using just this type. A change to another type may result in difficulties.
It is desirable, therefore, that peat would be classified according to those qualities which from the horticultural point of view, are the most important. It is further desirable that easily determined standards showing the most important qualities of the peat would be developed. And what are these qualities? This question should, of course, be considered from the plant point of view.
The most important feature of a substrate is that it can store water, oxygen and all the necessary nutrients and supply the plant with them.
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.
It is impossible to divide mere empty space adequately between water and air. This can be best accomplished by using a porous solid material as substrate. When the porous material is saturated with water the micro or capillary pores are filled with water while the macro pores are filled with air.
From the point of view of the plant's water and oxygen supply, it is desirable that the volume of micro and macro pores, i.e. the total pore space, would be as large as possible. The pore space, therefore, may be taken as a standard expressing the substrate's quality.
The water economy of the substrate is determined by its water capacity and its oxygen economy by its air capacity. 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