There is significant phosphorus accumulation in Punjab’s soils due to the continuous overuse of DAP
To optimize fertiliser usage in the rice-wheat agricultural system, the Punjab Agricultural University (PAU), Ludhiana has emphasised the judicious use of diammonium phosphate (DAP). The institution’s soil test-based recommendations are aimed at promoting efficient use of this costly input.
The PAU Vice Chancellor Satbir Singh Gosal underlined DAP’s role as the costliest fertiliser input in the rice-wheat system, stressing the potential for cost savings through soil test-based recommendations. Recent studies by PAU have brought to light a significant phosphorus accumulation in Punjab’s soils due to the continuous overuse of DAP and other phosphatic fertilisers.
Gosal said excessive DAP usage had resulted in 31% of soils being classified as ‘very high’ and 30% as ‘high’ in crop-available phosphorus, with only 19% falling into the ‘medium category’, necessitating the recommended dose of this fertiliser. He said, “Higher phosphorus categories yield substantial discounts in P use for all crops. Additionally, the retention or incorporation of paddy straw contributes to increased soil organic carbon, enhancing phosphorus availability.”
The PAU Director of Research Ajmer Singh Dhatt pointed to the significance of adhering to soil test results for phosphorus application. For medium phosphorus soils, PAU recommends 55 kg of DAP per acre in wheat (or 65 kg when residue is retained or incorporated) and potato crops, with a 25% dose increase only when the soil tests low in P.
Dhatt outlined guidelines for DAP application in wheat, such as a 25% reduction for soils with high phosphorus levels (9-20 kg per acre) in low-organic carbon soils (less than 0.4%) or medium phosphorus (5-9 kg per acre) in soils with moderate organic carbon content (0.4 to 0.6%). He also recommended a 50% reduction in DAP use for soils with high phosphorus (9-20 kg per acre) and medium organic carbon (0.4 to 0.6%) or medium phosphorus in soils with high organic carbon (over 0.6%).
The PAU Department of Soil Science head Dhanwinder Singh revealed that under certain conditions, wheat requires no DAP application, such as in high phosphorus soils (9-20 kg per acre) with high organic carbon (over 0.6%) or soils with very high phosphorus levels (over 20 kg/acre), regardless of organic carbon content. “Integrated nutrient management practices allow a 50% reduction in DAP use for wheat, provided alternative sources like poultry manure or dried biogas slurry are applied at 2.5 tonnes per acre in the previous rice crop or by using 4 tonnes per acre of rice husk ash or bagasse ash in wheat. Also in fields, where organic carbon content of soil comes under high category after continuous retention/incorporation of paddy straw, the dose of DAP can be reduced by 50%. In addition, no DAP is necessary in wheat if farmyard manure is applied at a rate of 10 tonnes per acre to the previous potato crop,” Singh explained.
Singh also cautioned against the excessive use of phosphatic fertilisers, which can lead to zinc deficiency in field crops. In cases where DAP is unavailable, he suggested alternatives like single superphosphate (16% phosphorus) or nitro-phosphate (20% phosphorus) to address the phosphorus requirements of the crops.
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