The long-term dung (D), Nitrogen (N), Phosphorus (P) and Potassium (K) commonly referred to DNPK experiments in northern guinea savannah of Nigeria was subjected to different fertilization regimes from 1950 to 1996 under continuous cultivation. Experimental plots have been under fallow from 1997 till now. Surface soils (0 to 5 and 5 to15 cm) were sampled and analyzed for physical and chemical properties under different combinations and application rates with organic and or inorganic fertilizers; that is, DNP, DNK, DNPK, NPK, DPK, and control (CT) received no fertilization treatments. From 0 to 5 cm soil depth, bulk density was high except for NPK and DNPK which recorded the lowest values of 1.46 and 1.48 Mg m-3, and decreased by 14 and 16%, respectively compared to the control. Mean weight diameter (MWD) indicate water stable aggregates of 0.55 and 0.66 mm in DNPK and NPK, and dry macroaggregates between 1.08 to 1.71 mm across all treatments. Saturated hydraulic conductivity (Ks) was higher with 150.4 and 104.1 cm hr-1 in DNK and DNP, which increased by 6 and 9 folds, respectively, compared to CT while total porosities of 32 to 44% was observed for all treatments except DNP and CT, and increased by 47 to 88% in the other dung treatments compared to CT at 0 to 5 cm. Soils organic matter content was moderate to high (28.7 to 44.1 g kg-1) across all the treatments while soil (pH) with calcium chloride was lowest in NPK. Most of the moisture is retained in the soils at field capacity and permanent wilting point in dung treatments and CT, while it was lowest for NPK. Taking all the properties into consideration, soil quality decreases in the order DNPK > DNK > DNP > DPK = NPK > CT and implies better quality under dung fertilization with mineral fertilizer in the long term. The positive effect of long-term integrated application of organic and inorganic fertilizers is further emphasized as a recommended sustainable soil management practice for tropical soils.
Key words: Soil quality, long-term experiments, mean weight diameter, soil organic matter (SOM), inorganic fertilizers
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