Soil is a fundamental resource and it is crucial to manage its quality in order to enhance agricultural productivity and environmental quality. Soil enzymes catalyze several biochemical reactions which result in the transformation of organic matter, and the release of inorganic nutrients for plant growth and nutrient cycling. Soil enzyme activities are useful biological soil quality indicators since they are operationally practical, very sensitive, integrative, easy to measure and more responsive to soil tillage and structure than other soil variables. There are several enzymes in soil, but those involved in hydrolases and the degradation of main litter components are used most often for evaluating soil quality. This paper reviews the roles of soil enzymes such as β-glucosidase, phosphatase and urease, as well as the implications of their activities for soil quality.

South Africa is classified as a water-scarce country, and depends on agriculture for food production. The irrigation sector is the largest consumer of water in the country, accounting for about 62% of water utilisation, but also losing 30–40%. Given the threat of drought and climate change, efficient irrigation systems have become a necessity, especially in the smallholder farming sector where most losses occur. Smallholder irrigation schemes (SIS) were developed to improve rural livelihoods through sustainable food production for food security and poverty alleviation, but these development objectives remain largely unfulfilled. The objectives of this review were to assess challenges facing SIS and explore opportunities for revitalising the schemes. The focus was on government policy and strategies to support smallholder farmers. A review of government policy showed that although the needs and interests of smallholder farmers are high on the national agenda, there is insufficient financial support to the sector, suggesting that smallholder agriculture is not really seen as a potential driver of the economy. The core focus of the government on repairing irrigation infrastructure while neglecting the soft components relating to capacity building has partly been blamed for the failure of SIS in South Africa. Capacity building is one of the missing links in smallholder irrigation development and many failures have been attributed to lack of adequately trained farmers and extension staff, particularly in irrigation water management. Land tenure insecurity has been singled out as a major institutional challenge leading to poor performance of irrigation schemes. The diversity of schemes means that different kinds of interventions are needed to respond to varying farmers’ needs, resources and agricultural contexts. These findings point to the need to balance the soft and hard components of the irrigation schemes for sustainability. It is therefore evident that the government needs to review its priorities in revitalisation of SIS. Land tenure policies allowing increased access to arable land need to be developed urgently, together with the promotion of alternative cropping systems that are suitable forthe smallholder farming sector.

The productivity and residual benefits of four grain legumes to sorghum (Sorghum bicolor) grown in rotation were measured under semi-arid conditions over three cropping seasons. Two varieties of each of the grain legumes; cowpea (Vigna unguiculata); groundnut (Arachis hypogaea); pigeon pea (Cajanus cajan); Bambara groundnut (Vigna subterranea), and sorghum were grown during the first season. The same experiment was implemented three times in different, but adjacent fields that had similar soil types. At the end of the season the original plots were split in two and residues were either removed or incorporated into the subplots. The following season sorghum was planted in all subplots. In 2002/03 (314 mm rainfall) cowpeas produced the largest dry grain yield (0.98 and 1.36 t ha⁻¹) among the legumes. During the wettest year (2003/04, 650 mm rainfall) groundnut had the highest yields (0.76 to 1.02 t ha⁻¹). In 2004/05 (301 mm rainfall) most legume yields were less than 0.5 t ha⁻¹, except for pigeon pea. Estimates of % N from N₂-fixation from the legumes were 15–50% (2002/03), 16–61% (2003/04) and 29–83% (2004/05). Soil water changes during the legume growth cycle were proportional to varietal differences in total legume biomass. Sorghum grain yield after legumes reached up to 1.62 t ha⁻¹ in 2003/04 compared with 0.42 t ha⁻¹ when following sorghum. In 2004/05, sorghum yields after legumes were also higher (up to 1.26 t ha⁻¹) than sorghum after sorghum. Incorporation of crop residues had no significant effect on sorghum yield. Beneficial effect of legumes on yields of the subsequent sorghum crop were more readily explained by improvements in soil nitrogen supply than by the small observed changes in soil water relations. Our results demonstrate clear potential benefits for increasing grain legume cultivation in semi-arid environments through the use of improved germplasm, which also gave substantial increases in subsequent sorghum productivity (up 200% in a wet season and 30–100% in a dry season), compared with an unfertilized sorghum crop following sorghum.

The best management options for cover cropping are largely unknown, including the growth patterns of cover crop (CC) species, optimum termination stages and termination methods. A greenhouse experiment was conducted to explore the following: (i) Effect of two termination stages (vegetative and flowering) on the chemical composition (N and C:N) of four CCs; (ii) Short-term impacts of living CCs and residues on soil pH, total N, urease and phosphatase activities at the two termination stages, and under two termination methods (slash and spray). Species tested as CCs were, vetch (Vicia dasycarpa L.), field pea (Pisum sativum L.), oats (Avena sativa L.), rye (Secale cereal L.) and a control (no CC). The experiment was set up in a randomized block design with three replications. Soil was sampled at kill and one year after CC kill. Delaying termination from vegetative till flowering stage decreased N in the tissue of P. sativum, A. sativa, V. dasycarpa and S. cereal by 59%, 65%, 44% and 56%, respectively, while their C:N ratios increased. Cover crop presence had no effect on soil pH. Living CCs had no significant effect on soil N concentration. The activities of urease and phosphatase were stimulated by all the living CC species. Unlike urease, all CC residues had a positive impact on phosphatase activity at one year. Only P. sativum and V. dasycarpa residues increased soil N concentration in the short-term. Compared to flowering, termination at vegetative stage improved soil N concentrations and phosphatase activity at both sampling times. Termination method had no effect on soil N, urease and phosphatase activity at one year. The significant interaction (P < 0.05) of sampling time, CC and termination stage effects on soil N concentration and phosphatase activity observed in this study indicates that these management approaches can optimize CC benefits and improve soil chemical and biological properties.