THE POTENTIAL OF ALLEY CROPPING IN IMPROVEMENT OF CULTIVATION SYSTEMS IN THE HIGH RAINFALL AREAS OF ZAMBIA .3. EFFECTS ON SOIL CHEMICAL AND PHYSICAL-PROPERTIES

A detailed study of the soil chemical and physical properties in seven-year-old alley cropping trial containing Leucaena leucocephala and Flemingia congesta in Northern Zambia is described. There was a strong correlation between the maize yield and the total amount of nitrogen applied, both from prunings and fertiliser, suggesting that a major reason for the observed benefit from alley cropping, particularly with Leucaena, was due to an improvement in nitrogen supply. Leucaena produced significantly more biomass, and its leaves had higher concentrations of nitrogen, phosphorous and potassium and lower C/N and C/P ratios than did those of Flemingia. There was also evidence that the trees had a beneficial effect on other soil chemical properties; under the hedgerows, particularly those of Leucaena, there were higher levels of organic carbon, Mg, K and ECEC, and pH values were also highest. It is suggested that higher levels of organic carbon in the alley crop treatments were responsible for the improvements observed in soil physical properties. Lower bulk density, lower penetration resistance, and a higher infiltration rate and pore volume fraction were measured in the alley crops, although there was no significant change in the soil water release parameters. A deteriorating effect of constant applications of nitrogen fertiliser on soil fertility was observed; as the level of urea application increased, there were significant decreases in Mg, K and pH, increases in Al and soil acidity, and higher penetrometer resistance. These results highlight the urgent need for further research on biological methods of maintaining soil fertility.