C. C. du Preez

Soil fertility in South Africa: the last twenty five years

Introduction In attempting an overview of developments in a subject such as soil fertility over the past twenty five years, it is inevitable that there will be some overlap with other topics, especially soil chemistry, degradation, biology and the environment. Soil fertility is not only at the interface of these soil-related topics, but it is also at the interface of disciplines such as soil microbiology, plant physiology and other plant sciences. It is an extremely important subject, dictating the growth and quality of plants in a direct link to human health and welfare. Its key role in the global food chain is being increasingly realized and appreciated.

Long-term effects of conservation practices on the nitrogen fertility of a soil cropped annually to wheat

Sustainable farming in the semi-arid interior of South Africa requires the adoption of conservation practices to maintain and, if possible, increase soil fertility. The effects of some such practices on the nitrogen fertility of a hydromorphic sandy clay loam soil were examined in a field trial at Bethlehem, in the eastern Orange Free State. Soil from selected plots was analysed, after this trial had been run for 11 and 12 years, for organic carbon and total nitrogen to use as measures of soil organic matter, and for residual inorganic and mineralizable nitrogen to use as measures of plant-available nitrogen. The trial includes 36 treatments, namely, all combinations of straw burning (burnt or unburnt), three primary cultivation methods (ploughing, stubble mulch or no cultivation), two weed control methods (mechanical or chemical) and three levels of nitrogen fertilization (20, 30 or 40 kg N ha−1). The 30 kg N ha−1 level was selected, reducing the number of treatments to 12. Similar measurements were made...

Changes in the organic matter and nutrient contents of some South African irrigated soils

Cultivation caused, without exception, a decline in the organic matter content of dryland soils in the central regions of South Africa. The question arose as to how cultivation affects the organic matter content of irrigated soils from these regions. The main aim, therefore, was to quantify the effect of cultivation on the organic matter content of soils from three irrigation schemes with virgin soils serving as reference. Virgin and cultivated topsoils (0–200 mm) were sampled from seven sites at Riet River, eight sites at Ramah and six sites at Vaalharts. The sites at each irrigation scheme represented varying periods of cultivation. Soil samples were analysed for particle size distribution, pH, exchangeable Na, K, Mg and Ca, extractable P and Zn, CEC, organic C and total N. The K, P and Zn contents differed significantly between virgin and cultivated soils: K decreased, while P and Zn increased with cultivation and irrigation. Organic C and total N significantly increased at seven sites and decreased at...

Loss of nitrogen fertility and its restoration in some Orange Free State soils.

The effects of present or past cultivation on nitrogen fertility in some central Orange Free State soils were investigated by the comparison of paired samples of cultivated or reverted soils with uncultivated soils situated as closely as possible to one another and belonging to the same soil form. The parameters measured were texture, pH, organic carbon, total nitrogen content and net nitrogen mineralization under uniform controlled conditions in the laboratory. Paired samples were taken at selected sites at depths of 0–0,15; 0,15–0,50 and 0,50–1,00 m. Effects of cultivation were small downward displacements of silt and clay, slight acidification and large losses of organic carbon and nitrogen from the surface layer. The easily mineralized fraction of nitrogen decreased more than the more resistant fractions. The rate of depletion decreased as the period of cropping increased but data were insufficient to show whether a new equilibrium had been reached. Reversion to pasture appeared to restore fertility w...

Effect of cultivation on the nitrogen fertility of selected dryland soils in South Africa

This study is part of an investigation into the effect of cultivation on the organic matter content of topsoils (0 - 200 mm) from commercial farms practising dryland agriculture in the summer rainfall area of South Africa (24°-30°S; 24°-30°E). The aim was to examine the effects of cultivation, period of cultivation and soil properties on potential mineralizable and residual inorganic N as parameters of the nitrogen fertility of the soils under discussion. Virgin and cultivated soils were sampled at 50 sites. Of these, 27 were located within five ecotopes. The sites in each ecotope varied as to cultivation period. The remaining 23 sites were chosen to obtain a wider range of climatic conditions and soil properties. Incubation tubes, packed in triplicate with a mixture of a soil and filter sand, were leached before incubation (residual inorganic N) and again after 2, 5, 9, 14 and 22 weeks of incubation (mineralizable N) at 30°C with 0.01 M CaCl2. The data was used to calculate cumulative net mineralization ...

Effect of cultivation on the nitrogen fertility of selected agro-ecosystems in South Africa

Surveys of soils on commercial farms practising rainfed agriculture in South Africa, found large losses of N fertility. Information regarding the pattern of this loss is not available for different agro-ecosystems. Thus, the aim was to quantify the effect of cultivation on the N fertility of soils in five agro-ecosystems with virgin soils serving as reference, find the relation between the period of cultivation and the decrease in N fertility for each agro-ecosystem and determine whether this decline differed between the agro-ecosystems. Five to six sites with different cultivation periods were sampled within each agro-ecosystem. At each site a cultivated soil and its virgin counterpart were sampled to 200 mm depth and total and mineralizable N were determined as indices of N fertility. Cultivation, irrespective of the period, caused a significant decrease in the N fertility of all five agro-ecosystems and the rate of loss was rapid during the first few years of cultivation. Thereafter, the rate decreased until an equilibrium was approached. In agro-ecosystems from the warmer, drier regions, an equilibrium was approached much sooner than in agro-ecosystems from the cooler, wetter regions, but the percentage of N fertility loss was larger in the latter regions. The percentage loss of mineralizable N was larger than that of total N. This depletion of N fertility in cultivated soils may result in unsustainable crop production.

Concentration, accumulation and uptake rate of macro-nutrients by winter wheat under irrigation

The objective of this study was to obtain information on the nutrient uptake patterns of winter wheat (Triticum aestivum L.) grown under normal irrigation farming conditions. Except for irrigation scheduling, 13 farmers were allowed to follow their normal farming practices. At each site biomass production, nutrient content of the top growth and root length were measured regularly throughout the growing season, starting 35–45 days after planting. The nutrient content of the soils was determined on samples collected 35–45 days after planting at 300-mm depth intervals to a depth of 1.8 m. As early as 40 days after planting the concentration of N, P and Ca in wheat plants started to decrease, while the concentration of K, Mg and S started to decrease 10–20 days later. Accumulation of the nutrients in plants increased until 120 days after planting and then stabilized at an average of about 214, 40, 326, 33, 21 and 18 kg ha−1 for N, P, K, Ca, Mg and S, respectively. Nutrient uptake rates reached a maximum betwe...

Assessment of tillage effects on soil quality of pastures in South Africa with indexing methods

Soil quality of pastures changes through time because of management practices. Excessive soil disturbance usually leads to the decline in soil quality, and this has resulted in concerns about kikuyu (Pennisetum clandestinum)–ryegrass (Lolium spp.) pasture systems in the southern Cape region of South Africa. This study aimed to understand the effects of tillage on soil quality. The soil management assessment framework (SMAF) and the locally developed soil quality index for pastures (SQIP) were used to assess five tillage systems and were evaluated at a scale inclusive of variation in topography, pedogenic characteristics and local anthropogenic influences. Along with assessment of overall soil quality, the quality of the physical, chemical and biological components of soil were considered individually. Soil physical quality was largely a function of inherent pedogenic characteristics but tillage affected physical quality adversely. Elevated levels of certain nutrients may be warning signs to soil chemical degradation; however, tillage practice did not affect soil chemical quality. Soil disturbance and the use of herbicides to establish annual pastures has lowered soil biological quality. The SQIP was a more suitable tool than SMAF for assessing soil quality of high-input, dairy-pasture systems. SQIP could facilitate adaptive management by land managers, environmentalists, extension officers and policy makers to assess soil quality and enhance understanding of processes affecting soil quality.

Influence of long-term wheat residue management on organic matter in an Avalon soil

This study was initiated to determine the influence of different wheat residue management practices, that were sustained for about 20 years, on the organic matter content of an Avalon soil in a long-term trial near Bethlehem in the Eastern Free State. The treatments were two methods of straw disposal (burned and unburned) × three methods of tillage (ploughing, stubble mulch and no tillage) × two methods of weed control (mechanical and chemical). Soil samples were collected at various depth intervals and organic C and total N, as indices of organic matter, were determined on these samples. The effect of either straw burning or weeding method on organic matter was small compared to that of tillage practice. A slightly higher organic C and lower total N content were measured in the unburned than burned plots to a depth of 450 mm. The tillage practices affected organic C and total N significantly in a similar manner to 100 mm depth with no-tilled plots having the highest organic matter content, followed in decreasing order by the mulched and ploughed plots. The organic matter content of the chemically-weeded plots was significantly higher than that of the mechanically-weeded plots to 100 mm depth, as indicated by organic C and total N. Significant interactions between the treatments on either organic C or total N were virtually absent. However, no tillage combined with chemical weeding resulted in the highest organic matter content. The latter combination is therefore recommended to maintain and even increase the organic matter content of this Avalon soil when cropped annually with wheat.

Ecological and agro-economic study of small farms in sub-Saharan Africa

Land degradation, rising population and poverty in sub-Saharan Africa threatens the agricultural sustainability and productivity, quality of the environment and socio-economic wellbeing of rural populations. We studied farm ecological, economic and social sustainability, productivity and production risks in the Mbeere District of Eastern Kenya. We used a soil nutrient monitoring methodology to collect data from 30 households. Ecological sustainability was threatened by soil nutrient decline at rates of 1.7 kg P and 5.4 kg K ha−1 half year−1 while N was nearly balanced in soils. Soil phosphorus and potassium stocks, in the cultivated soils, declined at rates of 0.3% and 0.1% half year−1, respectively. Farm economic returns were positive, albeit low, and could not sustain the livelihoods of the households. All the 30 households were living below the poverty line of 1 US dollar a day. Farm productivity was low, with livestock and yields of major staple food crops below on-farm target yields. To spread out the risks of production, farming households were cultivating an average of 4.7 crop fields, keeping more than two types of livestock and practising intercropping systems. Intercropping maize-beans reduced nutrient decline and raised household incomes compared with monocropping of either of the two crops. Despite the low rates of nutrient decline, high risks of production and the low crop yields, the livestock productivity and farm economic performance put the sustainability of these farming systems into question. The low levels of nutrient decline in small farms averaging at 1.7 kg P and 5.4 kg K ha−1 half year−1 contrasts with the high nutrient depletion rates on macro-scale levels, e.g. 20–40 N, 3.5–6.6 kg P and 20–40 kg K ha−1 year−1 for Eastern African countries and 22 kg N, 2.5 kg P and 15 kg K ha−1 year−1 for sub-Saharan Africa. These findings indicate that the extent of nutrient decline and conservation differs across sub-Saharan Africa. The positive contribution of intercropping to nutrient balances suggests the need to encourage farmers to adopt such systems rather than monocropping.