Tillage systems and soil compaction in Africa

Introduction of mechanized agriculture induces profound changes in soil characteristics. Soil compaction originating from mechanical land clearing, mechanized cultivation, and continuous cropping is aggravated by crusting and hard-setting phenomena of soils, and widespread occurrence of naturally compacted upland soils and subsoil gravel horizons. Natural and anthropogenically induced soil compaction has detrimental effects on growth and yields of a wide range of crops. Furthermore, compaction can persist for a long time if no adequate measures are taken to minimize or alleviate it.

In humid and subhumid regions of Africa, the no-tillage system with crop residue mulch is an important method of controlling soil compaction, followed in significance, by biological and mechanical loosening where motorized land clearing is the causative agent. Biological methods involve cover crops and alley cropping or agroforestry. Where new land areas need to be opened up, land clearing should be done by the slash-and-burn method, so that most of the nutrients in the vegetation are returned to the soil. Where mechanical land clearing is inevitable, forest removal should be done by the use of shear blade, whereby most of the roots and stumps are left in the ground intact, and the forest litter is not removed.

In semi-arid and arid regions of Africa, alleviation of soil compaction can be done by two methods. One method is to use the controlled traffic tillage system. Controlled traffic results in both a loose-rooting zone and a firmed traffic lane, thereby providing good plant growth and trafficability for timely field operations. The second method is to use mechanical loosening techniques, i.e. ploughing by animal traction or tractor power, chiseling, deep ripping, subsoiling, and tied-ride system. The effect of mechanical loosening, however, tends to be of short duration if the ensuing field traffic is not controlled.     

Effects of cover crops on soil structure and on yield of subsequent arable crops grown under strip tillage on an eroded alfisol

Effects of weed fallow and of three grasses and five leguminous cover crops were investigated on soil structure of an eroded Alfisol. Crop growth and yields of subsequently grown arable crops were assessed under strip-tillage through the mechanically or chemically suppressed sods. Cover crops and fallowing improved soil organic matter content, total N, water retention and transmission properties, and decreased bulk density only in the top 0–10 cm depth. The improvements rendered were, however, slight. Grasses were difficult to suppress with paraquat or mechanical mowing, which resulted in low or negligible yield of maize, cowpea, and cassava. Leguminous covers were easily suppressed with paraquat application, and resulted in good yield of maize and cowpea. Mechanical mowing was as successful as herbicide application for suppressing Stylosanthes guianensis and resulted in satisfactory yield of maize and cowpea. Yield of cassava tubers was extremely low due to shallow surface soil, compacted sub-soil horizons, and competition from weeds and regrown cover crops. Results are discussed in terms of the amelioration of eroded and degraded soil.     

Surface clod size distribution as a factor influencing soil erosion under simulated rain

The influence of five ranges of surface clod size distribution (≤2 mm; 2–10 mm; 10–20 mm; 20–50 mm and 50–100 mm) upon the rate and total amount of runoff, soil loss, percolation water and splash sediments was studied using a rainfall simulator. The equipment is described and procedures for running simulation tests are given. The method of sample preparation is discussed. For an air-dried soil sample, soil loss and runoff decreased and total percolation increased with clod sizes up to and equal to 20 mm. Similar results were obtained with a wet run made 24 h after the dry run, although total soil loss and runoff were higher, and percolation lower. No changes were observed as clod size was increased above 20 mm.     

Diurnal fluctuations in hydro-thermal regime of a tropical alfisol as influenced by methods of land development and tillage systems

The nature of diurnal fluctuations in soil moisture profile, moisture potential profile, and soil temperature regime was investigated for soils under five different methods of land clearing and post clearing tillage systems on an Alfisol in south-west Nigeria. The treatments consisted of manual clearing with terraced conventional tillage, manual clearing with no tillage/mulch, shear blade – no tillage/mulch, tree pusher – terraced conventional tillage, and traditional clearing and management. Soils under no-tillage mulch and traditional managements had more favourable hydrothermal regime than mechanically cleared plowed treatments. Compared with traditional farming, the maximum soil temperature at 5 cm depth was more by 2°C for tree pusher conventional tillage, 1.5°C for manual clearing conventional tillage and 0.7°C for manual clearing no-tillage and shear blade no-tillage, respectively. Water loss in the 1 to 4 cm depth increment in 24 hours was 2.9 mm for the manual clearing conventional tillage, 3.1 mm for the tree pusher conventional tillage, 2.0 mm for the manual clearing no tillage and 2.2 mm for the traditional clearing and management.     

Tillage effect on partial budget analysis of cropping intensification under dryland farming in Punjab,Pakistan

A 2-year research experiment was laid out in a split-plot design with moldboard ploughing (MP, control) and minimum tillage (MT) as main plots and crop sequences as sub-plots. Summer-winter cropping sequences were fallow-wheat (Triticum aestivum L.) (FW, control), mungbean (Vigna radiata L.)-wheat (MW), sorghum (Sorghum bicolor L.)-wheat (SW), green manure-wheat (GW) and mungbean-chickpea (MC) (Cicer arietinum L.). During 2010–2011, in summer, mungbean crop produced the highest above ground biomass in mungbean-chickpea sequence under MP tillage (4.24 Mg ha^–1 mungbean) and in mungbean-wheat sequence under MT tillage (4.11 Mg ha^–1 mungbean). In winter, chickpea gave the highest biomass in mungbean-chickpea sequence under both tillage systems. During 2011–2012, mungbean produced the highest above ground biomass in mungbean-wheat sequence under MT tillage (4.43 Mg ha^–1 mungbean) and in both the mungbean-chickpea and mungbean-wheat sequences under MP tillage (4.24 and 4.17 Mg ha^–1 mungbean, respectively). In winter, fallow-wheat sequence gave the highest biomass in both tillage systems. The differences in grain yields were statistically non-significant in both the years. The gross marginal benefit ranged between ?190 and 548$ ha^–1 in the first year and ?165 and 1124$ ha^–1 in the second year. The net benefit values were the highest in mung-chickpea sequence under both tillage systems (1008 and 596$ ha^–1 under MP and MT, respectively), which gave cost–benefit ratios of 5.45 and 3.68, respectively. Use of legume-based cropping sequences is a sustainable and cost-effective practice in drylands of northern Punjab, Pakistan.     

Impairment of chlorophyll content in leaves ofDiospyros melanoxylon by fluoride pollution

The effects of fluoride pollution on the chlorophyll content of leaves ofDiospyros melanoxylon, an economically important tree, have been determined under field conditions. Chlorophylla, chlorophyllb, total chlorophyll, individual leaf biomass and size were reduced in polluted plants compared to unpolluted ones. The rate of increase of chlorophyll content up to September was comparatively less in polluted plants. Reduction in individual leaf biomass and size was about 32.3 and 27% respectively.     

Subsoil compaction effects on crops in Punjab, Pakistan:II. Root growth and nutrient uptake of wheat and sorghum

Crop yields can be reduced by soil compaction due to increased resistance to root growth, and decrease in water and nutrient use efficiencies. A field experiment was conducted during 1997–1998 and 1998–1999 on a sandy clay loam (fine-loamy, mixed, hyperthermic Typic Haplargids, USDA; Luvic Yermosol, FAO) to study subsoil compaction effects on root growth, nutrient uptake and chemical composition of wheat (Triticum aestivum L.) and sorghum (Sorghum bicolor L. Moench). Soil compaction was artificially created once at the start of the study. The 0.00–0.15 m soil was manually removed with a spade. The exposed layer was compacted with a mechanical compactor from 1.65 Mg m⁻³ (control plot) to a bulk density of 1.93 Mg m⁻³ (compacted plot). The topsoil was then again replaced above the compacted subsoil and levelled. Both compacted and control plots were hoed manually and levelled. Root length density, measured at flowering stage, decreased markedly with compaction during 1997–1998 but there was little effect during 1998–1999. The reduction in nutrient uptake by wheat due to compaction of the subsoil was 12–35% for N, 17–27% for P and up to 24% for K. The reduction in nutrient uptake in sorghum due to subsoil compaction was 23% for N, 16% for P, and 12% for K. Subsoil compaction increased N content in wheat grains in 1997–1998, but there was no effect on P and K contents of grains and N and P content of wheat straw or sorghum stover. During 1997–1998, K content of wheat straw was statistically higher in control treatment compared with compacted treatment. In 1998, P-content of sorghum leaves was higher in compacted treatment than uncompacted control. Root length density of wheat below 0.15 m depth was significantly reduced and was significantly and negatively correlated with soil bulk density. Therefore, appropriate measures such as periodic chiselling, controlled traffic, conservation tillage, and incorporating of crops with deep tap root system in rotation cycle is necessary to minimize the risks of subsoil compaction.     

Effect of cropland management and slope position on soil organic carbon pool at the North Appalachian Experimental Watersheds

Soil organic matter is strongly related to soil type, landscape morphology, and soil and crop management practices. Therefore, long-term (15–36-years) effects of six cropland management systems on soil organic carbon (SOC) pool in 0–30 cm depth were studied for the period of 1939–1999 at the North Appalachian Experimental Watersheds (<3 ha, Dystric Cambisol, Haplic Luvisol, and Haplic Alisol) near Coshocton, OH, USA. Six management treatments were: (1) no tillage continuous corn with NPK (NC); (2) no tillage continuous corn with NPK and manure (NTC-M); (3) no tillage corn–soybean rotation (NTR); (4) chisel tillage corn–soybean rotation (CTR); (5) moldboard tillage with corn–wheat–meadow–meadow rotation with improved practices (MTR-I); (6) moldboard tillage with corn–wheat–meadow–meadow rotation with prevalent practices (MTR-P). The SOC pool ranged from 24.5 Mg ha⁻¹ in the 32-years moldboard tillage corn (Zea mays L.)–wheat (Triticum aestivum L.)–meadow–meadow rotation with straight row farming and annual application of fertilizer (N:P:K=5:9:17) of 56–112 kg ha⁻¹ and cattle (Bos taurus) manure of 9 Mg ha⁻¹ as the prevalent system (MTR-P) to 65.5 Mg ha⁻¹ in the 36-years no tillage continuous corn with contour row farming and annual application of 170–225 kg N ha⁻¹ and appropriate amounts of P and K, and 6–11 Mg ha⁻¹ of cattle manure as the improved system (NTC-M). The difference in SOC pool among management systems ranged from 2.4 to 41 Mg ha⁻¹ and was greater than 25 Mg ha⁻¹ between NTC-M and the other five management systems. The difference in the SOC pool of NTC-M and that of no tillage continuous corn (NTC) were 16–21 Mg ha⁻¹ higher at the lower slope position than at the middle and upper slope positions. The effect of slope positions on SOC pools of the other management systems was significantly less (<5 Mg ha⁻¹). The effects of manure application, tillage, crop rotation, fertilizer rate, and soil and water conservation farming on SOC pool were accumulative. The NTC-M treatment with application of NPK fertilizer, lime, and cattle manure is an effective cropland management system for SOC sequestration.     

Microbial Population and Yield of Wheat in Relation to Soil Salinity

The studies on isolations of microorganisms from four different saline soils (ECe 1.85, 5.8, 9.4 and 13.4 dS/m) and roots of wheat ( Kharchia-65 , a salt tolerant variety) plants grown in above soils reveal an interesting correlation between microbial population and soil salinity Vis-a-vis growth and yield. Though growth and yield of wheat decreased with an increase in the salinity of the soils, the fungal and bacterial populations concomitantly increased and the maximum microbial colonies were recorded from the soil, having highest salinity status. A maximum number of fungal colonies (predominantly of Aspergillus sp., Paecilomyces sp., Penicillium sp. etc.) was isolated from the root surfaces of the plants grown on soil having ECe of 1.85 dS/m, whereas a single colony of Fusarium oxysporum occurred from roots of plants of soils having an ECe of 13. 4dS/m.     

The oceanic microcosm of particles

Analyses of suspended particulate matter larger than 1 micrometer, filtered from thousands of liters of surface and deep waters during GEOSECS expeditions to the Atlantic and Pacific oceans, have provided new information on the nature and time scales of chemical processes associated with the particles. Trace element and radionuclide data show that particles scavenge trace elements such as Th, Pu, Fe, Pb, and Cu from the ocean column, thereby controlling their concentrations. For other elements, however, particles are a source: carbon and silicon, for example, are introduced at depths as sinking particles dissolve. Studies of both particulate concentrations by filtration of seawater and particulate fluxes by using sediment traps seem necessary to delineate the intricate nature of chemical processes in the oceans.