PEAT AS A STANDARDIZED MEDIUM FOR GREENHOUSE CROPS

Peat is probably one of the first substances used as soil additives. Since 1960 peat has no longer - in the Finnish cultivator's vocabulary - meant solely a soil additive and an ingredient of soil mixtures, but in hothouses it is a growth medium in itself. Today, pure peat has almost entirely replaced humus and other materials in Finland as a growth medium.

Yields from peat culture have been promising. In Puustjärvi's tests 34 kg of tomatoes and 63 kg of cucumbers were obtained per square meter in one cropping period. In two-year carnation growing the crop was 977/m2. Considering that these crops were grown north of latitude 60°, they must be considered as unusually good.

Research at the Helsinki University Horticultural Institute showed that in a peat medium plants grow strongly and rapidly. It has been suspected that over-rapid vegetative growth might have a deleterious effect on the blooming of flower plants and on the quality of the crop. In the following we therefore report on some results of experiments carried out with chrysanthemums and poinsettias.

Cut chrysanthemums. The quality of cut chrysanthemums grown in peat medium and in soil mixture has been compared in a number of experiments since 1961. The peat used in the experiments had the following properties: volumetric weight 70–84 kg/m3; pore volume 94–95%; air capacity 24–33%; exchange capacity 133–140 me/100 g. The chrysanthemum soil was prepared by mixing 2 volume parts of clay soil, 1 volume part of peat and 1 volume part of decomposed farmyard manure.

Peat and soil were added by the following fertilizers:

Fertilizer	Peat	Soil
Dolomite	5.5 kg/m3	3.5 kg/m3
Rock phosphate	1.0 kg/m3	-
Superphosphate	0.5 kg/m3	1.2 kg/m3
Potassium carbonate	1.0 kg/m3	-
Potassium sulphate	-	1.0 kg/m3
Cal-nitro	0.25 kg/m3	0.25 kg/m3
Trace nutrients	150 g/m3	150 g/m3

Once a week the soil was irrigated by a 0.02% fertilizer solution; during the first 6 weeks this contained only nitrogen, and afterwards nitrogen and potash (N:P = 1:5). During the growth season the peat was given only nitrogen if soil analysis showed that the nitrogen reading was clearly below 15 mg/l substratum. Table 1 gives the results of experiments conducted with many chrysanthemum varieties in unheated greenhouses between May and October. Each variety was included in at least three experiments.