E. Kotzé

SOILS: THE FREE STATE'S AGRICULTURAL BASE

ABSTRACT At least one third of South Africas total grain crop, mainly maize and wheat, is produced in the Free State. Crop production potential and sensitivity to degradation depend largely on the interaction of climate and soil. Climatic conditions vary from arid in the south and southwestern region to nearly sub-humid in the extreme east on the edge of the Drakensberg escarpment. A map of soil distribution in the Province shows groups of land types, each one characterised by a particular soil distribution pattern. The soil components of the map units is also presented. The crop production potential of the Province is presented in the form of a map. Delineations are based on interpretation of land type data and range from arable to non-arable. The total potentially arable area of the Free State is 3.82 million ha. In spite of a detailed understanding of the Free States soils, degradation continues to occur, especially on cultivated land, where the most serious forms are erosion, acidification and organic matter decline.

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.

Influence of long-term wheat residue management on acidity and some macronutrients 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 some fertility indicators 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) and two methods of weed control (mechanical and chemical). Soil samples were collected at various depth intervals and pH, P, K, Ca, Mg and Na contents were determined. The effect of either straw burning or tillage practice on these indicators was much greater than that of weeding method. Burning of straw increased pH, P, K and Ca contents to substantial depths, although only pH and K were influenced significantly. The tillage practices affected pH significantly to 450 mm depth, with no-tilled plots having the highest pH values compared to the mulched and ploughed plots. No tillage, and to a lesser extent mulched tillage resulted in an accumulation of P, K, Ca and Mg in the upper 100–150 mm soil when ploughing served as reference. Chemical weeding also followed the same trend of accumulation of nutrients in the upper 100 mm soil when compared with mechanical weeding. This trend was also observed in pH, although it was not significant. No tillage combined with chemical weeding was the most beneficial to restrict acidification. In the case of the nutrients, significant interactions were virtually absent, except for K where burning combined with either no tillage or chemical weeding resulted in the highest K values in the upper 100–150 mm soil.

Impact of long-term wheat production management practices on soil acidity, phosphorus and some micronutrients in a semi-arid Plinthosol

Farmers continuously remove crop residues for use as building materials, fuel and animal feed or bedding as well as to avoid difficulties during tillage operations. Therefore, demonstrations of the benefits of recycling crop residues are necessary. The aim with this study was to evaluate the influence of different wheat production management practices on acidity and some essential nutrients from a long-term trial on a Plinthosol in semi-arid South Africa. The trial was set up in 1979, and since then two methods of straw management (unburned and burned), three methods of tillage (no-tillage, stubble mulch, and plough), two methods of weed control (chemical and mechanical), and three levels of nitrogen (N) fertiliser (20, 40 and 60 kg ha–1) have been applied. Soil samples were collected in June 2010 at depths of 0–50, 50–100, 100–150, 150–250, 250–350 and 350–450 mm from plots that received 40 kg N ha–1 and were analysed for pH, phosphorus (P), copper (Cu), iron (Fe), manganese (Mn) and zinc (Zn). Results obtained showed that straw burning resulted in higher P and Mn but lower Cu than no-burning. No-tillage, and to some extent stubble mulch, suppressed soil acidification and increased P and Zn compared with ploughing, especially in the surface layers where crop residues accumulate. In contrast, mouldboard ploughing and stubble mulch increased Cu more than no-tillage, possibly due to the strong affinity of organic matter for Cu. Tillage effects on Mn were inconsistent and difficult to explain. Chemical weeding also improved P, probably because of the pesticide application, but resulted in lower pH and Cu values compared with mechanical weeding. Treatment combinations also had an influence on P and, to a lesser extent, on soil pH and Cu, which might be due to the higher organic matter present in no-tilled soils. Irrespective of straw management or weed-control methods, no-tillage resulted in higher P than did ploughing and stubble mulch. Nutrient concentrations and pH values were sufficient for wheat growth under all treatments. However, although the nutrients were highest under straw burning, no-tillage and, to some extent, stubble mulch, wheat yield was higher with unburned straw and mouldboard ploughing. Therefore, an integrated approach from various disciplines is recommended to identify and rectify yield-limiting factors under conservation tillage systems.

Long-Term Effects of Wheat Production Management Practices on Exchangeable Base Cations and Cation Exchange Capacity of a Plinthosol in Semi-arid South Africa

Changes in soil fertility indicators are mainly the result of management practices and usually influence crop yields over the long term. This study shows the effects of long-term wheat production management practices on exchangeable base cations and cation exchange capacity (CEC). Applied field treatments included two methods of straw management (unburned, burned), three methods of tillage (no tillage, stubble mulch, plowing), and two methods of weeding (chemical, mechanical). Samples were collected at six soil depths and analyzed for potassium (K), calcium (Ca), magnesium (Mg), sodium (Na), and CEC. Burned wheat straw resulted in greater K and lower Ca concentrations compared to unburned wheat straw. No-tillage treatment increased K, Ca, Mg, Na, and CEC compared to both stubble mulch and plowing. Chemical weeding improved Na and CEC compared to mechanical weeding. The treatment combinations had positive influences mainly on CEC. Unburned straw and moldboard plowing with respect to burned straw and no tillage enhanced grain yield with 8%.

Soil microbial communities in different rangeland management systems of a sandy savanna and clayey grassland ecosystem, South Africa

Soil nutrient supply in rangelands depends on the maintenance and performance of soil microbiological communities. In this study we investigated how different rangeland management systems affects the structure and function of soil microbial communities in the clayey grassland and sandy savanna ecosystems, South Africa. These ecosystems differ in climate, soil and vegetation, with the sandy savanna ecosystem being drier, and encroached by bush. Soils were sampled under continuous and rotational grazing systems along a gradient with increasing grazing pressure. Analyses comprised of enzyme activities and phospholipid fatty acids (PLFA). The results revealed that the clayey grassland ecosystem displayed elevated enzyme activities and PLFA contents compared with the drier, sandy savanna ecosystem, irrespective of the rangeland management practices, likely because soil texture played a significant role in maintaining microbial communities. However, when microbial activity was normalized to carbon, nitrogen and microbial biomass, specific enzyme activities were significantly higher in the sandy savanna ecosystem, indicating a more efficient functioning of microbes here. Furthermore, these microbial parameters were more sensitive to grazing pressure in the clayey grassland ecosystem than other chemical or physical soil properties, whereas in the sandy savanna ecosystem this was not the case. Decreasing the grazing pressure on rangeland, as, e.g., done by commercial farmers practicing rotational grazing, appeared to stimulate microbial performance and thus microbial mediated nutrient mineralization with positive consequences on plant growth.

Soils, Agriculture and Food

Pre-1994 land policies caused South Africa to have a well-developed commercial agricultural sector and an underdeveloped communal agricultural sector. Both sectors are still operational today for either crop or livestock production despite significant political and social changes that have taken place since 1994. Climate, irrespective of agricultural sector, dictates together with soil the location of crop production regions and, together with vegetation, the location of livestock production regions. In recent years the production of crops and livestock has intensified. This intensification can be devastating to soil and vegetation because both natural resources are very susceptible to degradation. In addition to land degradation, climate change and land claims may impact negatively on future food production and security. A combined approach between agricultural and governmental stakeholders is therefore essential to counteract these potential negative impacts.

Long-term effects of wheat production management practices on some carbon fractions of a semiarid Plinthustalfs

This study evaluated wheat production effects on carbon (C) fractions and soil organic C (SOC) molecular composition of a semiarid Plinthustalf in a trial established near Bethlehem. Treatments applied for 20 consecutive years included two straw management (unburned and burned), three tillage (no-tillage, ploughing and stubble mulch) and two weed control (chemical and mechanical) methods. Samples collected from 0–50 mm depth of specific treatment combinations were analyzed for SOC, soil inorganic C (SIC), permanganate oxidizable C (POXC), cold (CWEC) and hot (HWEC) water extractable C, extractable humic substances (CEX), humic acids, fulvic acids (CFA) and SOC functional groups. Humification (HI) and polymerization (PI) indices and alkyl C/O-alkyl C ratios were calculated. No-tillage combinations demonstrated potential to reverse losses of soil C fractions in the 0–50 mm layer. Increased POXC, CWEC, CEX and CFA revealed the labile nature of accumulated SOC in no-tillage, suggesting that SOC therein could be rapidly lost if no-tilled soils were again cultivated. Although the HI and PI were not always significant, their decrease in the no-tillage combinations suggest minimal decomposition, which is a benefit in Plinthustalfs with low storage capacity. Positive correlations between SIC and SOC fractions implied that an increase in SOC fractions protected SIC, resulting in its accumulation in no-tillage combinations. Functional groups seemed to mimic fractionated SOC fractions because O-alkyl C decreased with concomitant increase in alkyl, aromatic and carbonyl C. These responses highlighted that no-tillage combinations could be ideal to restore SOC quality in drought-prone agro-ecosystems dominated by sandy soils.