| There is considerable interest in soil organic matter technologies and fertility sources in intensified maize-based cropping systems in Southern Africa. This study investigated the effect of the Gliricidia sepium (Jacq.) Walp and maize intercrop on soil organic matter (SOM) fractions and soil fertility indices in southern Malawi. Part 1 investigated the response of SOM fractions to the intercrop, inorganic nitrogen (N), and phosphorus (P) on a long-term trial established in 1991 on a Ferric Lixisol in southern Malawi. Soil was sampled to a 20 cm depth in July of 2006. SOM, available P, exchangeable K+, and CEC were determined on whole soil samples. Particulate organic matter (POM) was separated from the soil by size and density fractionation and analyzed for C and N. The intercrop had a positive effect on all SOM fractions. With the intercrop, POM was increased by 37%, carbon in POM by 60% and nitrogen in POM by 78% compared to sole maize. C/N ratio of POM decreased from 17.9 with sole maize to 15.5 with the intercrop. After 14 years, predictors of soil fertility and SOM fractions were significantly greater under the intercrop than under sole maize. In part 2, an on-farm study in the same district described characteristics of intercrop users and their placement of the intercrop on different soils. Thirty-seven households using the intercrop and 28 households not using the intercrop were interviewed in June and July of 2006. Questionnaires were completed with each household concerning demographic statistics and the soil types found and cropping systems used in their fields. In female-headed households use of the intercrop was 66% compared to 49% in households headed by males. Intercrop use was 50% among households in the lower half of the socioeconomic scale and 63 % in the upper halt. Households using the intercrop had an average of 0.4 hectares of land, while those not using the intercrop had an average of 0.83 hectares. Placement of the gliricidia/maize intercrop on sandy M'chenga soils was less common than on Katondo and Makande soils. In part 3, the SOM and soil nutrient effects of the intercrop were compared to prevailing soil management practices in smallholder's fields. Soil samples were taken from the 2005/6 planting rows from the fields with and without the gliricidia intercrop. Soil samples were analyzed for SOM, sand content, available P, exchangeable K + and CEC. Soil organic matter values associated with the intercrop were not significantly different from those for other cropping systems. When the gliricidia/maize intercrop fields were analyzed separately, soil type, elevation, and their interaction affected trends in soil organic matter. A positive relationship between soil organic matter and elevation was evident in the two finer-textured soil types, Makande and Katondo. The addition of pigeonpea, Cajanus cajan (L.), to the maize cropping system at elevations below 877 meters was associated with an increasing trend in SOM compared to the sole maize cropping system. The emergence of increasing trends in SOM correlated with increasing elevation and fineness of soil texture in fields with the gliricidia/maize intercrop may indicate that a SOM benefit will develop over time in fields at relatively higher elevations with finer soil textures. |
