Soil Organic Matter Along a Degradation Gradient in a Semi-Arid Rangeland of South Africa

In a semi-arid South Africa rangeland on a sandy loam soil, artificial manipulations to create various rangeland conditions typical of different levels of livestock impact were shown to result in considerable losses of organic matter after 15 years (Du Preez & Snyman, 1993). In this follow-up study on a similar soil, we regularly measured the organic matter content following the conversion of rangeland in a good to a poor ecological condition, an undisturbed bare soil and a cultivated soil respectively. The changes in soil organic matter content after five years are reported

Establishing Baseline Values for Soil Quality Indicators in the Southern Cape of South Africa

Commercial dairy farming on pasture is commonly practiced in the southern Cape region of South Africa. In terms of its sustainability, quantification of the impact of dairy-pasture management on soil quality is essential. To comprehend the behaviour of soils in terms of quality, indicators should be assessed to screen the general direction of soil quality within a management system. Development of a soil quality assessment framework necessitates establishment of baseline values for soil quality indicators. The aim of this study was to establish baseline values for soil quality indicators for dairy-pasture soils in the southern Cape. Physical, chemical and biological indicators on a no-till kikuyu (Pennisetum clandestinum)-ryegrass (Lolium spp.) pasture were compared to those of a virgin soil. Stratification ratios, which demonstrated the rate at which the indicator decreased with depth, were assessed for soil organic matter (SOM) related indicators. Biological indicators showed a well-established and well-functioning microbial population in the improved pasture soil. Stratification ratios demonstrated that the soil quality of the improved pasture system was improved relative to the virgin soil. Chemical indicators showed that the impact of nutrient management was favourable to pasture productivity. The association between the chemical and biological indicator were better established in the managed pasture. Physical indicators demonstrated that management practices adversely impacted the ability of the soil to provide physical support and structural functionality. This data were used in developing a first approximation of baseline values. Further research is warranted to validate them on representative commercial dairy farms in the southern Cape

Seasonal variations of transpiration efficiency coefficient of irrigated wheat

Global diminishing water resources, especially due to climate change have serious impacts on evaporation (E) from the soil surface, transpiration (T) from plants (crops) and grain yield, which relates to water use efficiency of different crops. A study was conducted at Kenilworth over two wheat cropping seasons (2007 and 2008) with the objectives of: (i) evaluating the effect of soils and seasons on T, E and yield, and (ii) relating these parameters to transpiration efficiency coefficient. The treatments included two soil types and two soil surface treatments (bare and mulched), which were all replicated four times. Weekly irrigation was done using a surface drip system while maintaining the water table at a constant depth. Soil water content was monitored using a neutron probe. Neither soils nor seasons were found to significantly influence the partitioning of evapotranspiration (ET), and T varied from 74 to 76% of ET while E varied between 24 and 26%. Surface treatments caused significant differences in grain yield in both seasons. Reducing evaporative loss improves the water productivity of wheat, which has an important implication in dryland farming

Evaluation of the effects of phosphorus and nitrogen source on aerial and subsoil parameters of maize (Zea mays L.) during early growth and development

Understanding the nutrient requirements of maize is especially important during early vegetative growth and development, particularly when produced on highly weathered soils with low organic matter contents. Therefore, the growth response of maize to different nitrogen (N) sources viz. limestone ammonium nitrate and urea, phosphorus (P) sources viz. monoammonium phosphate, nitrophosphate and ammonium polyphosphate, as well as P application levels viz. 0, 10, 20, 30 and 40 kg ha−1 during early growth were evaluated. In pot trials with a sandy loam topsoil, aerial and subsoil plant parameters were measured to evaluate the effect of treatment and treatment combinations of P and N fertilisers on the vegetative growth and development of maize during the first five weeks after emergence. Plants treated with limestone ammonium nitrate had significantly greater plant parameter measurements compared with urea, primarily ascribed to immediate availability after application in addition to ease of uptake. Monoammonium phosphate and nitrophosphate (orthophosphate sources) yielded significantly greater aerial measurements compared with ammonium polyphosphate (polyphosphate source). Differences in subsoil plant parameter results for P source were not that clear. Aerial and subsoil plant parameter measurements were overall significantly greater with P applied at 40 kg ha−1 than the majority of lesser application rates.Keywords: nitrogen source, orthophosphate, plant growth parameters, polyphosphat

Land use and soil organic matter in South Africa 1: A review on spatial variability and the influence of rangeland stock production

Degradation of soil as a consequence of land use poses a threat to sustainable agriculture in South Africa, resulting in the need for a soil protection strategy and policy. Development of such a strategy and policy require cognisance of the extent and impact of soil degradation processes. One of the identified processes is the decline of soil organic matter, which also plays a central role in soil health or quality. The spatial variability of organic matter and the impact of grazing and burning under rangeland stock production are addressed in this first part of the review. Data from uncoordinated studies showed that South African soils have low organic matter levels. About 58% of soils contain less than 0.5% organic carbon and only 4% contain more than 2% organic carbon. Furthermore, there are large differences in organic matter content within and between soil forms, depending on climatic conditions, vegetative cover, topographical position and soil texture. A countrywide baseline study to quantify organic matter contents within and between soil forms is suggested for future reference. Degradation of rangeland because of overgrazing has resulted in significant losses of soil organic matter, mainly as a result of lower biomass production. The use of fire in rangeland management decreases soil organic matter because litter is destroyed by burning. Maintaining or increasing organic matter levels in degraded rangeland soils by preventing overgrazing and restricting burning could contribute to the restoration of degraded rangelands. This restoration is of the utmost importance because stock farming uses the majority of land in South Africa

A critical view on the soil fertility status of minimum-till kikuyu–ryegrass pastures in South Africa

Cultivated pastures provide increased productivity and contribute inter alia to food security. Rangelands in the southern Cape region of South Africa have had a low animal production potential and were therefore improved through time as cultivated pastures. Initially, annual pastures were established by conventional tillage methods, but from the 1990s permanent pastures were established on a minimum-tillage regime. Lime and fertiliser guidelines, which were developed for annual pastures established by conventional tillage methods, were followed on minimumtillage systems, despite changes in the soil physical properties and stratification of biological parameters. The study aim was to survey the soil fertility of irrigated minimum-till kikuyu–ryegrass pastures in the southern Cape region. This study highlights changes in soil fertility and identified potential detrimental effects of elevated levels of extractable phosphorus and zinc, which were drastically increased in the topsoil layer. The necessity for remedial and preventative strategies to mitigate nutrient loading in cultivated pasture soil are stressed. Fertiliser guidelines should be applied strictly and should fit the farming system. In doing so, the dual goals of economic and ecological sustainability should be achieved.Keywords: cultivated pastures, fertilisation, nutrient cycling, phosphorus, soil fertilit

Managing cultivated pastures for improving soil quality in South Africa: challenges and opportunities

Loss of productive agricultural land due to soil degradation poses a serious threat to agricultural output and sustainability. Soil degradation of cultivated pastures manifests as a long-term decline in production potential. There are concerns that soils under pastures in certain regions of South Africa are degrading as a result of mismanagement, which include practising continuous tillage, improper grazing management, injudicious application of fertilisers and poor irrigation management. Soil quality indicators, which include physical, chemical and biological parameters in soil, are science-based methods to monitor sustainability and protect soil. Soil quality data informs the land manager in the decision-making process to address practices that degrade agro-ecological systems and mitigate any loss of function. The aim of this paper is to provide a synthesis of the challenges and opportunities in soil management of pastures. The value of assessing soil chemical, physical and biological indicators to monitor soil quality is stressed.Keywords: fertilisation, grazing management, irrigation management, soil degradation, tillag

Are There Consistent Grazing Indicators in Drylands? Testing Plant Functional Types of Various Complexity in South Africa's Grassland and Savanna Biomes

Despite our growing knowledge on plants' functional responses to grazing, there is no consensus if an optimum level of functional aggregation exists for detecting grazing effects in drylands. With a comparative approach we searched for plant functional types (PFTs) with a consistent response to grazing across two areas differing in climatic aridity, situated in South Africa's grassland and savanna biomes. We aggregated herbaceous species into PFTs, using hierarchical combinations of traits (from single-to three-trait PFTs). Traits relate to life history, growth form and leaf width. We first confirmed that soil and grazing gradients were largely independent from each other, and then searched in each biome for PFTs with a sensitive response to grazing, avoiding confounding with soil conditions. We found no response consistency, but biome-specific optimum aggregation levels. Three-trait PFTs (e.g. broad-leaved perennial grasses) and two-trait PFTs (e. g. perennial grasses) performed best as indicators of grazing effects in the semi-arid grassland and in the arid savanna biome, respectively. Some PFTs increased with grazing pressure in the grassland, but decreased in the savanna. We applied biome-specific grazing indicators to evaluate if differences in grazing management related to land tenure (communal versus freehold) had effects on vegetation. Tenure effects were small, which we mainly attributed to large variability in grazing pressure across farms. We conclude that the striking lack of generalizable PFT responses to grazing is due to a convergence of aridity and grazing effects, and unlikely to be overcome by more refined classification approaches. Hence, PFTs with an opposite response to grazing in the two biomes rather have a unimodal response along a gradient of additive forces of aridity and grazing. The study advocates for hierarchical trait combinations to identify localized indicator sets for grazing effects. Its methodological approach may also be useful for identifying ecological indicators in other ecosystems