SUBSTRATUM FOR CONTAINER GROWN PLANTS
To be able to evaluate the physical properties, we had to find suitable methods for analysis, as the standard methods for soil were not satisfactory for materials like peat, organic debris and plastics products.
As the substrates normally are used under conditions of root restriction (container size) the characteristics must be related to the root-volume - in danish expressed as "dyrkningsrumfang" - : the volume of a certain amount of the wetted compost when plants are established in the pot, container or bench, here after called "growing volume".
Under standard procedure, not to be described here, a certain packing of the soil is carried out similating potting firmness, and the sample is then after watersaturation placed on a wet sand layer exposed to a suction of 0 and 10 cm capillary tension. The first giving a drainage similar to pots placed on a firm surface, the second as placed on a water saturated capillary bench drained to a depth of 10 cm. This to establish the worse drainage and also the standard conditions.
By measuring and weighing the following physical properties could be analysed and calculated:
- Weight of growing volume in kg/m3 (or g/l)
- Dry matter expressed in kg/m3 (or g/l)
- Dry matter expressed in % of volume
- Water content expressed in % of volume
- Air content expressed in % of volume
- Total pore space in % of volume
An analysis of 4 characteristic samples of types of sphagnum peat is shown in table 1 (see next page)
A good container compost should have a high water capacity combined with sufficient air space for ample root respiration allowing water and nutrient uptake. These figures indicate clearly that sphagnum peat in "growing volume" condition mainly consist of water and air.
The more decomposed and the finer the peat, the more water and less air will be held. Whether problems will arise, depends on the placement of the container, as seen in the difference of air content of 0 and 10 cm tension.