Threshold phosphorus level of acidic soils of eastern India

It is imperative to have a practical indicator for assessing the potential for phosphorus movement from soil to surface waters causing environmental pollution. The present study was undertaken with two groups of acidic soils from the terai and red and laterite agro-climatic zone of eastern India to estimate their phosphorus threshold values and establish a simple model with the clay content as the principal variable. The mean phosphorus adsorption maximum and phosphorus buffering capacity were higher in lateritic than terai soil. The change-point soil test values at which water soluble phosphorus enhanced abruptly ranged from 32 to 68 mg kg^-1 and 28 to 63 mg kg^-1 with Bray-1 and Mehlich-1 method, respectively, for the soils of the terai zone. Similarly, it varied from 47 to 90 mg kg^-1 and 44 to 89 mg kg^-1, respectively, for the lateritic soils. Application of phosphatic fertilizers should not be allowed beyond the threshold level, which was considered 75% of the change-point soil test value to avoid the risk of the soil becoming a source of phosphorus pollution for surface water bodies. The simplified models of phosphorus threshold level (mg kg^-1) developed with either of the extractants were "4.75 × clay content (%) - 30" and "6.00 × clay content (%) - 75" for terai and lateritic soil, respectively. These models can be extended to the soils with similar mineralogy but varying in clay content for sustainable phosphorus management without limiting crop production.