Analysis of growing media by means of a 1: 1½ volume extract

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Abstract
Analysis of the salt and nutrient status of potting composts and growing substrates with the aid of the 1:25 weight extract ‐ previously used in the Netherlands ‐ has several disadvantages. The interpretation of analytical data based on the organic matter content is not very accurate because this content does not provide a true estimation of the moisture capacity. In addition the conductivity of growing media rich in gypsum gives an unfavourable impression of the salt status. Very good results were obtained in an investigation at the research station at Naaldwijk with a 1:1½ volume extract. For the preparation of this extract potting composts and growing substrates were standardised as regards moisture content, after which one part by volume was measured and mixed with 1½ parts of water. The conductivity of the extract and the contents of chloride, nitrogen, nitrate, ammonium, phosphate, potassium and magnesium were determined. The analytical results were closely correlated with those of the soil solution. The correlation coefficients were between 0.957 and 0.986. The data obtained for the 1:25 weight extract were not as closely correlated with those of the soil solution; the correlation coefficients varied between 0.775 and 0.980. The weight ratio water : soil of the 1 s 1½ volume suspension was closely correlated with the moisture content at pF 1.5. In the case of growing media with low moisture capacity the ratio was about 3:1 and with high moisture capacity about 30:1.