Reliable and repeatable analyses that measure nutrient reserves present in soil and allow reasonable predictions of the effects of fertilizer application are available for most elements with the general exception of nitrogen. Soil mineral nitrogen (N) levels vary continuously due to the effects of mineralization, fertilizer dressings, leaching, denitrification and plant uptake. In consequence, the timing of soil sampling is critical, and measurements are only of use for crop management decisions immediately after sampling.
Laboratory analyses of soil samples are used to determine available macronutrients such as phosphorus (P), potassium (K) and magnesium (Mg) expressed in mg/l. For practical simplicity, these values are converted to indices (Table 5.1) which indicate the relative quantities of nutrient available to the crop ranging from 0 (deficiency) to 9 (excess). Soils intended for Brassica production should be maintained at index = 3 for phosphorus and index = 2 for potassium and magnesium. Soils with these indices require only maintenance quantities of additional fertilizer; below these values, larger amounts are necessary to ensure economic returns from the crop and to restore the nutrient reserves of the soil.
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