Effects of soil fertility management practices on selected soil physical properties, water use efficiency and crop productivity in Murewa smallholder farming area

Abstract

Several studies have been conducted to restore the fertility of degraded soils in sub-Saharan Africa using combinations of inorganic and organic fertilizers. The studies mainly concentrated on nutrient uptake and balances in the soil. This study was carried out to determine the effects of cattle manure and inorganic fertilizer application (fertility treatment) on soil organic carbon (SOC), bulk density, aggregate stability, aggregate protected carbon, steady state infiltration rates, porosity, unsaturated hydraulic conductivity, moisture retention characteristics, crop water productivity (CWP) and grain yields of two contrasting soils in Murewa smallholder farming area, Zimbabwe. Four long-term (6 years) and short-term (2 years) fields, sandy homefield and outfield and clayey homefield and outfield were used to take into consideration the spatial variability in soil fertility induced by farmer management practices (field-type treatment). The fields were conventionally tilled (ox-drawn mould-board ploughed annually) and maize monocrop was grown in the long-term fields under the following soil fertility amendments, control (no fertility amelioration), 5, 15 and 25 t ha-1 manure + 100 kg ha-1 N applied annually. Soybean-maize rotation was practiced in the shortterm fields under the treatments 100 kg ha-1 N (maize)/ 40 kg ha-1 N (soybean), 30 kg ha-1 P, 29 kg ha-1 K, 20 kg ha-1 Ca, 10 kg ha-1 Mg, 5 kg ha-1 Zn and 5 kg ha-1 Mn in combination with cattle manure at 5, 10, 15 and 20 t ha-1. Cattle manure application resulted in significant increases in SOC, macro-aggregation index (Ima), aggregate protected carbon (APC), steady state infiltration rates (I.R), total effective porosity, unsaturated hydraulic conductivity (Ko) at 5 cm tension. Soil organic carbon ranged between 0.5-3.2 % on clay soils, in contrast to sandy soil’s SOC which was between 0.3-2.4 % under combined cattle manure and inorganic fertilizer application. Mean pore sizes that were significantly improved by cattle manure application were 0.58 mm in 25 t ha-1 manure while control was 0.43 mm. Moisture retention at 5 and 10 kPa was improved (p<0.05) by cattle manure application. Soil organic carbon was significantly (p<0.05) and positively correlated with macro-aggregation indices, aggregate protected carbon, steady state infiltration rates, total effective porosity, hydraulic conductivity and grain yield (r>0.6). Multiple regression analysis revealed that SOC, macro-aggregation indices, aggregate protected carbon, steady state infiltration rates accounted for some of the variability in grain yield in the long-term clay fields (p<0.001, r2=95.3). However, only SOC could account for the yield variability on sandy soils (p<0.05, r2=0.89). Generally, grain yield increased in the order control < 5< 15 ≈ 25 t ha-1 cattle manure application rates on both soil types. On the short term fields, crop yields were significantly lower in control and highest in 20 t ha-1 cattle manure treatment while the intermediate treatments did not significantly differ. AquaCrop model satisfactorily simulated actual crop transpiration with higher CWP observed in 25 t ha-1 cattle manure treatments relative to control. Combined cattle manure and inorganic fertilizer application significantly improved clay soils’ physical and bio-chemical environment which ultimately improved crop yields. Cattle manure rates at 5 t ha-1 yr-1 improved crop yields on sandy soils and 15 t ha-1 was required for physical properties’ improvement and yield on clay soil. In addition, soil fertility gradients were revealed to be a short-term phenomenon, whereby they disappeared after 6 years on clay soils after equal application of soil fertility amendments while sandy soil’s physical properties did not respond to cattle manure application.