Surface soil aggregation, trace, and heavy metal enrichment under long-term application of farm yard manure and mineral fertilizers

The DNPK trial was started at Samaru in 1950 to assess the nutrient imbalances resulting from the increasing use of mineral fertilizers on some poorly buffered soils, improve and optimize productivity of these soils under intensive agricultural land use. Aggregation, trace, and heavy metals enrichment were studied in some selected plots under this trial, with a view to identify treatment(s) that best improve soil quality over a long-term period. Soils under treatments involving sole cowdung (D), cowdung + phosphorus (DP) fertilizer, and cowdung + nitrogen, phosphorus, and potassium (DNPK) fertilizers recorded higher mean weight diameter, lower soil bulk density, and higher percentage of aggregates greater than 1-mm diameter. Hence, these three treatments are better aggregated to withstand wind erosion. However, the DNPK treatment was the only one, which consistently, recorded lower trace and heavy metals content.