Different testing laboratories use different solutions in taking P from a soil sample. These different techniques can result in very different numeric values being reported by different laboratories. This increases the importance of the index concept. Direct comparison of P levels between laboratories is of limited value unless they use the same procedures, solutions, techniques, and indices. However, it is hoped each laboratory can determine roughly equivalent indices.
P are very high, high, medium, low, or very low.
Each category should reflect the probability of response to phosphate fertilizer application. Crops grown on soils with a very high P index normally should not respond to phosphate fertilizer application, but crops on very low P soils should usually respond. Management of Phosphate Fertilizers Phosphorus does not move readily within soils, so placement of phosphate fertilizers is a major management decision in crop production systems. There is no ideal special placement for all crops. Decisions about phosphate fertilizer placement depend on the intended crop, soil test P level, and environmental considerations.