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.

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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 N 2 -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.

Next to drought, poor soil fertility is the single biggest cause of hunger in Africa. ICRISATZimbabwe has been working for the past 10 years to encourage small-scale farmers to increase inorganic fertiliser use as the first step towards Africa's own Green Revolution. The program of work is founded on promoting small quantities of inorganic nitrogen (N) fertiliser (micro-dosing) in drought-prone cropping regions. Results from initial on-farm trials showed that smallholder farmers could increase their yields by 30-100% through application of micro doses, as little as 10 kg Nitrogen ha -1 . The question remained whether these results could be replicated across much larger numbers of farmers. Wide scale testing of the micro-dosing (17 kg Nitrogen ha -1 ) concept was initiated in 2003/2004, across multiple locations in southern Zimbabwe through relief and recovery programs. Each year more than 160,000 low resourced households received at least 25 kg of nitrogen fertiliser and a simple flyer in the vernacular explaining how to apply the fertiliser to a cereal crop. This distribution was accompanied by a series of simple paired plot demonstration with or without fertiliser, hosted by farmers selected by the community, where trainings were carried out and detailed labour and crop records were kept. Over a 3 year period more than 2,000 paired-plot trials were established and quality data collected from more than 1,200. In addition, experimentation to derive N response curves of maize (Zea mays L.), sorghum (Sorghum bicolor (L.) Moench) and pearl millet (Pennisetum glaucum (L.) R.Br.) in these environments under farmer management was conducted. The results consistently showed that micro-dosing (17 kg Nitrogen ha -1 ) with nitrogen fertiliser can increase grain yields by 30-50% across a broad spectrum of soil, farmer management and seasonal climate conditions. In order for a household to make a profit, farmers needed to obtain between 4 and 7 kg of grain for every kg of N applied depending on season. In fact farmers commonly obtained 15-45 kg of grain per kg of N input. The result provides strong evidence that lack of N, rather than lack of rainfall, is the primary constraint to cereal crop yields and that

Participatory on-farm trials were conducted for three seasons to assess the benefits of small rates of manure and nitrogen fertilizer on maize grain yield in semi-arid Tsholotsho, Zimbabwe. Two farmer resource groups conducted trials based on available amounts of manure, 3 t ha -1 (low resource group) and 6 t ha -1 (high resource group). Maize yields varied between 0.15 t ha -1 and 4.28 t ha -1 and both absolute yields and response to manure were strongly related to rainfall received across seasons (P < 0.001). The first two seasons were dry while the third season received above average rainfall. Maize yields within the seasons were strongly related to N applied (R 2 = 0.77 in season 1, and R 2 = 0.88 and 0.83 in season 3) and other beneficial effects of manure, possibly availability of cations and P. In the 2001-2002 season (total rainfall 478 mm), application of 3 and 6 t ha -1 of manure in combination with N fertilizer increased grain yield by about 0.14 and 0.18 t ha -1 , respectively. The trend was similar for the high resource group in 2002-2003 although the season was very dry (334 mm). In 2003-2004, with good rainfall (672 mm), grain yields were high even for the control plots (average 1.2 and 2.7 t ha -1 ). Maize yields due to manure applications at 3 and 6 t ha -1 were 1.96 and 3.44 t ha -1 , respectively. Application of 8.5 kg N ha -1 increased yields to 2.5 t ha -1 with 3 t ha -1 of manure, and to 4.28 t ha -1 with 6 t ha -1 of manure. In this area farmers do not traditionally use either manure or fertilizer on their crops, but they actively participated in this research during three consecutive seasons and were positive about using the outcomes of the research in future. The results showed that there is potential to improve livelihoods of smallholder farmers through the use of small rates of manure and N under semi-arid conditions.

Poor soil fertility and erratic rains are major constraints to crop production in semi‐arid environments. In the smallholder farming systems of sub‐Saharan Africa, these constraints are manifested in frequent crop failures and endemic food insecurity. We characterized a semi‐arid smallholder farming system in south‐western Zimbabwe to assess crop production, nutrient use and factors that constrain productivity. The farming system was studied using resource flow mapping, farmer interviews and calculations of crop production over three cropping seasons (2002/2003, 2003/2004 and 2004/2005) to capture variability between years. Farmers were categorized into three groups: better resourced, medium resourced and poorly resourced. Better resourced farmers produced adequate grain for basic household consumption, except in the drought year (2002/2003). Poorly resourced farmers had large grain deficits, whereas the medium resourced class had smaller deficits. Better resourced and medium resourced farmers produced adequate amounts of staple cereal in two of the seasons, while poorly resourced farmers produced inadequate amounts of food in all three seasons. All farmers produced less than 300 kg/ha of legumes per season. Lack of seed was cited as the main reason for poor legume production. Better resourced farmers used animal manure (2000–5000 kg per season) and some fertilizer on their cereal crops, while the medium resourced group used less manure (1000 kg or less) and no fertilizer. The use of manure varied strongly across the years. Poorly resourced farmers used no nutrient inputs on any of their crops. All groups had negative nitrogen balances during the three cropping seasons, although the values varied strongly between seasons. Investigation of the potential strategies for developing sustainable production systems are required to address the problems of food security in the semi‐arid parts of the country and the region.

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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.