Chemical and Biological Properties of Soils Under Maize-Cowpea Cropping Systems in Conservation Agriculture

The paper reports findings from an evaluation of the effects of selected chemical and biological properties of soils under maize-cowpea cropping systems in Conservation Agriculture (CA) and their relationship to biological nitrogen fixation capabilities of cowpea. Soils from Kayowozi Agriculture Camp of Chipata District of Zambia where CA had been practiced for six years were evaluated. Cropping systems studied included conventional tillage (control), maize monocropping (sole maize), maize-cowpea intercrop, maize-cowpea rotation- maize phase and maize-cowpea- cowpea phase. Standard laboratory procedures were used to determine the changes in the selected soil properties as a result of these cropping sequences under CA. The study showed that maize- cowpea intercrop and rotation-maize phase under conservation agriculture could result in a significant increase in soil organic carbon, total nitrogen and exchangeable calcium after six years of practice. This increase can be associated with the amount and type of residue retained and the contribution of biologically fixed nitrogen from the cowpea. Having cowpea as the immediate previous crop in sequence can result in a depression of soil pH. Soil pH, total nitrogen, available phosphorus and exchangeable calcium in maize-cowpea cropping sequences can influence the amount of biologically fixed nitrogen. Changes in soil activity and microbial biomass might need more than six years to be apparent. The sequencing pattern of crops in a rotation, the choices and characteristics of crops, and the length of time of practice, all play an important role in determining interactions and processes leading to changes in soil properties and crop performance over time.