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FERTILISING STANDARDS FOR POTTING COMPOSTS BASED ON THE 1 : 1 ½ VOLUME EXTRACTION METHOD OF SOIL TESTING
The extent to which the standards conform to available trial data is discussed.
In order to assess the chemical properties of the factory made compost like those presented in the authors’ paper “Potting substrates in the Netherlands” (this symposium), soil analysis standards are set up based on the 1 : 1 ½ volume extraction method.
Sonneveld et al. (1974) gave a full account of this method, stressing its advantages over the previously used 1 : 25 weight extraction method like the independence of the interpretation of the analytical data of the organic matter content, the superior quality of the salt index and the greater speed of the analysis, since drying of the soil samples is no longer needed.
The standards for the pH – water of potting compost are 5.0 6.5 for vegetable crops and most pot plants and 4.8 5.3 for iron chlorosis sensitive potplants; that of seed compost (e.g. cyclamen) is 5.3 5.9 and of compost for cuttings (e.g. for begonia) 4.7 5.1. The standards for the chloride and total salt contents of potting composts are given in Table 1, those for nitrogen, phosphorus, potassium and magnesium content in Table 2. For convenience, in the following the 1 : 1 ½ volume extract values just mentioned will be referred to as Cl-water, EC (short for electrical conductivity), N-water, P-water, K-water and Mg-water, respectively.
The higher limit of the range termed moderate in Table 1 is the maximum allowable, so if higher, the potting compost is considered unsuitable.
The lower limit of the range termed normal in Table 2 is the minimum allowable, so if lower the potting compost is regarded as unfit for use as such.
Since there is only one single set of standards for each analytical value, regardless of crop differences, it is obvious that the designations of the respective ranges for the various analytical values cannot be but more or less arbitrary.
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