Dynamics of Soil Properties and Crop Yields under Conservation Agriculture Practices in a Humic Nitisol, Eastern Kenya

Abstract

Soil nutrient depletion is one of the challenges limiting food production in Eastern Kenya where maize and legumes are grown under complex and risky farming systems. The main soil type is a humic Nitisol, characterized by moderate inherent fertility. Rainfall variability and unfavourable socio-economic environment adds up to the uncertainties farmers face. In the effort of providing sustainable approaches to alleviating food shortage problem, a four season study was conducted between 2011 and 2013 at KALRO - Embu farm situated on South-Eastern slopes of Mt. Kenya at 00 0 33.18’S; 037 0 53.27’E; 1425 m asl. The study’s main objective was to evaluate and make recommendations on the effect of CA practices on soil properties and crop yields in a humic Nitisol. The experimental site had for over 50 years been tilled conventionally for maize production. Results from the initial site characterization showed soil texture to be clay loam with 59.3% clay, 20.8% sand and 20.0% silt particle distribution. The average soil bulk density was high at 1.2 kg m . The soil pH was acidic at 4.8 and extractable soil P was low at 4.0 mg kg xviii -1 . The total soil organic carbon and soil nitrogen was 1.9 and 2.00%, respectively. The trial was based on a randomized complete split-split-split-plot block design with three blocks. Three tillage methods, three cropping systems, six nitrogen application rates and two crop residue management methods were the fourteen tested independent variables. The tillage methods made up the main plots. Maize hybrid, DK8031 and common bean, Embean14 were the two test crops planted either as sole or as intercrops. Data collected included soil quality before and after the four seasons of CA application. Seasonal crop growth and grain yields resulting from the application of CA practices were also measured, together with the crop leaf area index -1 from 4.0 mg kg -1 -3 under FR tillage practices combined with soil liming. Apart from improving the crop yields, FR tillage system significantly improved the populations of soil micro-organisms and also profitability of maize and bean crops. Indeed, higher net-benefits under FR and to some extent, ZT were observed resulting from labour saving costs on land preparation/weeding. The benefits of CA were further improved by combining the CA tillage practices with soil liming and herbicides weed control. In particular, soil pH was raised from 4.8 to above 5.0 due to soil liming. The soil exchangeable acidity was reduced from 3.9 to 3.0 cmol kg 1 , hydrogen ions from 0.5 to 0.4 cmol kg -1 and aluminium ions from 1.2 to 1.1 cmol kg -1 . In addition, liming significantly raised the soil available P concentration from 4.0 to 14.3 mg kg -1 , Ca from 2.0 to 2.1 cmol kg xix -1 , Mg from 3.8 to 3.9 cmol kg and Fe ions from 24.4 to 24.9 cmol kg -1 -1 . Glyphosate herbicides led to over 80% weeds suppression and subsequently increased crop yields and net benefits under CA farming methods. APSIM computer model simulated crop yields that were not significantly different from those observed in the field. The study concluded that the application of CA practices is a feasible option for improving soil productivity and crop yields in Eastern Kenya. The APSIM model provided appreciable crop yield predictions under conventional and CA tillage systems. In-crop rainfall variability, rather than the amount is one of the key factors to define the crop in the region. The study recommended that the soil biology should be considered together with physical and chemical properties when defining soil fertility. A need was felt to support APSIM computer model towards interpreting the farm system observed and predicted biophysical research scenarios. Simplifying, packaging and scaling-out the validated CA practices such as furrows/ridges tillage, soil liming and herbicide weed control approaches to farmers and other land users was suggested as one of the immediate activities. It was also suggested that further studies be conducted to monitor long-term