Exploring mechanisms to restore fertility of degraded lixisols for enhanced crop productivity under Smallholder Farmer Management Systems

Abstract

Croplands in smallholder farming areas of Southern Africa are in a degraded state, with most cereal and legume crops yielding less than 1 t ha-1 of grain. Restoring productivity of these degraded croplands is therefore key to intensifying crop production to meet household food and nutritional security needs. The objective of this study was to evaluate sequences of integrated soil fertility management (ISFM) technology components for rehabilitation of degraded sandy soils and intensification of maize (Zea mays L.)-based cropping systems in Hwedza and Makoni smallholder farming areas in Eastern Zimbabwe. Farmer participatory research approaches, laboratory characterization, remote sensing and Geographic Information Systems were first employed to formulate criteria for assessing degradation of croplands. Second, field experiments were conducted to evaluate alternate sequences of ISFM options based on nitrogen (N)-fixing green manure and grain legumes, low quality organic resources and mineral N and phosphorus (P) fertilizers for rehabilitating the degraded croplands. Third, biophysical data generated from field experimentation were combined with simulation modelling to best-fit the ISFM sequences to different resource categories of farmers. Farmers delineated croplands as productive, moderately productive, degraded and severely degraded based on common indicator weeds, crop performance and soil physical attributes. Overall, laboratory-based soil chemical and biological properties, and most of the weed species closely matched farmer categorization. Spatially, forty percent of the arable land was classified as degraded to severely degraded. Over a 3-year period, rehabilitation of the degraded sandy soils was positively influenced by high quality organic resource application and P fertilization. Indigenous legumes and sunnhemp (Crotalaria juncea L.) planted on otherwise degraded soils, with addition of mineral P fertilizer, led to higher biomass C and N production than under natural fallow. A combination of biomass generated under indigenous legume fallow (indifallow) and sunnhemp fallow, and cattle manure increased soil basal respiration and microbial biomass. Maize grain yields averaged 2.5 t ha-1 under the legume-based sequences compared with 1 t ha-1 under continuous fertilized maize and natural fallow-based options. Over a 4-year period, ISFM sequences based on cattle manure/woodland litter-, NP fertilizer- and legume-based rotations accumulated the highest maize and soyabean grain yields. This was largely explained by a significant accumulation of soil P. These sequences were more productive than farmers’ designated most-and least-productive fields. Cattle manure-based sequences gave the highest cumulative crop yields, while sunnhemp-based sequences attained the highest increase in soil P. Based on costs of seed, fertilizers and labour, the ISFM sequences gave better financial returns than the farmers’ most-and least-productive fields. When assessed over a 49-year period (1962-2011) using Agricultural Production Systems Simulator (APSIM) under agronomic management of different resource categories of farmers, simulated maize yields of the ISFM sequencing options averaged 3.8, 2.4 and 1.8 t ha-1 for resource-endowed, resource-intermediate and resource-constrained households, respectively. The major conclusions from this study were as follows: (i) ethnopedological approaches could aid assessment of soil degradation on smallholder farms to inform decision-making, (ii) degraded croplands are not beyond remedy, and ISFM sequences based on herbaceous legumes are potential entry points to rehabilitate them and (iii) ISFM sequences are a suitable option for intensifying smallholder cropping systems in Southern Africa through increasing crop yields to meet energy and protein requirements of households, and allowing for systematic allocation of the often limited and variable nutrient, seed and labour resources in space and time.