Effect of continuous cropping for twenty years on some properties of the intensively cultivated alluvial soils and nutrient indexing of rice

Soil samples were collected after wheat harvest from a rice-wheat cropping system of an intensively cultivated Nabha soil series in Punjab, while leaf-samples were taken from rice during crop season of year 2003 in order to monitor changes in pH, electrical conductivity, organic carbon, available P and K status of soil and create nutrient indexing of rice crop growing in the area. These samples were taken from the same 100 benchmark farmers’ fields from two villages, which had earlier been sampled during 1983. Continuous cultivation of high yielding varieties of rice and wheat for 20 years without K fertilizer application depleted NH₄OAc-K in soil to a level of 60+18 mg kg⁻¹ (2003) from its initial (1983) level of 108+66 mg kg⁻¹. Rice leaf samples showed 65% K deficiency against 49% fields deficient in NH₄OAc-K (available K). Water-soluble K decreased to 13.6+9.4 mg kg⁻¹ from the initial value of 35.7+35.2 mg kg⁻¹; boiling 1N HNO3-K remained more or less unchanged, indicating the substantial release of structural K due to weathering of K- minerals as a result of cropping for 20 years. Application of heavy doses of P to wheat for 20 years increased the Olsen-P to 34.1+10.5 from a level of 8.7+4.1 mg kg⁻¹. Thirty-seven per cent rice leaf samples were found to be high and 63% sufficient in P. Twenty-six per cent of soil samples were found to be deficient in CaCl₂ extractable S, while rice leaf-samples showed 32% S deficiency. Organic carbon upgraded in medium category from its low status recorded in 1983. Long-term applications of chemical fertilizers to rice and wheat crops along with good quality of irrigation water did not deteriorate pH and electrical conductivity (in terms of acidity/sodicity and salinity) of Nabha-soil.