Abstract

Deteriorating soil fertility and the concomitant decline in agricultural productivity are major concerns in many parts of the world. A pot experiment was conducted with a Ferralsol to test the hypothesis that application of biochar improves soil fertility, fertiliser-use efficiency, plant growth and productivity, particularly when combined with compost. Treatments comprised: untreated control; mineral fertiliser at rates of 280mg nitrogen, 70mg phosphorus and 180mg potassium pot(-1) (F); 75% F+40g compost pot(-1) (F+Com); 100% F+20g willow biochar pot(-1) (F+WB); 75% F+10g willow biochar+20g compost pot(-1) (F+WB+Com); 100% F+20g acacia biochar pot(-1) (F+AB); and 75% F+10g acacia biochar+20g compost pot(-1) (F+AB+Com). Application of compost with fertiliser significantly increased plant growth, soil nutrient status and plant nutrient content, with shoot biomass (as a ratio of control value) decreasing in the order F+Com (4.0)>F+WB+Com (3.6)>F+WB (3.3)>F+AB+Com (3.1)>F+AB (3.1)>F (2.9)>control (1.0). Maize shoot biomass was positively significantly correlated with chlorophyll content, root biomass, plant height, and specific leaf weight (r=0.99, 0.98, 0.96 and 0.92, respectively). Shoot and root biomass had significant correlations with soil water content, plant nutrient concentration, and soil nutrient content after harvesting. Principal component analysis (PCA) showed that the first component provided a reasonable summary of the data, accounting for 84% of the total variance. As the plants grew, compost and biochar additions significantly reduced leaching of nutrients. In summary, separate or combined application of compost and biochar together with fertiliser increased soil fertility and plant growth. Application of compost and biochar improved the retention of water and nutrients by the soil and thereby uptake of water and nutrients by the plants; however, little or no synergistic effect was observed.