Isaiah I.C. Wakindiki

Legume-wheat rotation effects on residual soil moisture, nitrogen and wheat yield in tropical regions.

Abstract Grain legumes grown in rotation with annual cereal crops contribute to the total pool of nitrogen in the soil and improve the yields of cereals. However, the anticipated N benefits of the legume may be positive or negative depending on legume species and its interaction with the environment. Such erratic response may result from excessive water use by the legume phase, its symbiotic performance, effects of soil pH on legume growth and biomass returned N, harvest index and immobilization of nitrate during decomposition of legume residues. A review of the effects of legume–wheat rotation on residual moisture exploitation for enhanced soil N productivity of the tropical soils, including factors affecting the decomposition of incorporated residues are presented. Whereas soil water storage in the soil profile during the fallow period has been an important consideration in dry land agriculture where water is often limiting crop yields, the growing of a short-term legume in rotation with cereal in a humid climate, and its depletion of fallow water does not adversely affect yield of following cereal crop mainly because of adequate rainfall during the main season for wheat. Soil N is enriched by various grain legumes through biological N fixation which subsequently enhances wheat yields. Non-N benefit includes reduction of wheat root rot incidence which enhances added N uptake, wheat leaf disease and pests. The strategy of using legumes in rotation with wheat in the humid tropics for enhanced soil-N supply, and pest, disease, and weeds-break effects should therefore be encouraged. Its concluded that introduction of legumes such as chickpea, dolichos, field bean, faba beans in wheat-based cropping is a viable strategy for the reduction of inorganic fertilizer use for the resource poor small and medium scale farmers in Africa.

Particulate organic matter, soil texture and mineralogy relations in some Eastern Cape ecotopes in South Africa

Contrasting reports have been given on the relationships between soil texture, mineralogy and particulate organic matter (POM). The objective of this study was to determine the effects of soil texture and mineralogy on aggregate POM in several ecotopes in the Eastern Cape province of South Africa. Soil from the surface 0–20 cm was collected from 14 ecotopes. Particulate organic matter was fractionated into litter POM, coarse POM and fine POM. Isolation of the POM fraction was done by dispersing the soil in 5 g l-1 sodium hexametaphosphate and passing the dispersed soil samples through a 0.053 mm sieve. Both total soil organic matter (SOM) and the POM in each fraction were determined using the weight loss on ignition procedure. Most ecotopes were either sandy loam or sandy clay loam and primary minerals especially quartz dominated the soil mineralogy. The clay content was significantly related to the total SOM (r = 0.78), hematite (r = 0.83) and quartz (r = -0.74). The results suggest that in soils that are dominated by primary minerals such as quartz, the clay content, rather than the clay plus silt content, is a better indicator of fine POM and total SOM.

Physical indicators of soil erosion, aggregate stability and erodibility

The sustained interest in soil erosion research is an indication of both its importance and the lack of definite solutions that can halt its negative impacts on the environment. This study reviewed the literature on trends, new perspectives, gaps and conflicts in soil erosion studies in the South African context. The suitability of using the relationship between aggregate stability and interrill erodibility as a predictor of the soil susceptibility to erosion was also investigated. This relationship is often used instead of the expensive and time-consuming in situ soil erosion studies and models. There are contradictory reports on its ability to offer quick results on the susceptibility of soil to erosion. However, the reviewed South African and international literature showed that aggregate stability is a widely used physical indicator of soil interrill erodibility. Nevertheless, there is no general agreement on the most suitable aggregate stability indices to use.

Soil sealing and crusting effects on infiltration rate: a critical review of shortfalls in prediction models and solutions

Soil sealing and crusting is a common feature in most arid and semi-arid soils with severe agricultural and environmental implications. One of the main effects of soil sealing and crusting is a marked reduction in hydraulic conductivity and infiltration rate (IR), which triggers run-off and erosion. However, measurement of crust IR in the field is cumbersome, time-consuming and expensive. The extremely dynamic nature of processes involved in crusting and crust characteristics makes it very difficult to measure and predict infiltration. Resultantly, many researches aimed at identifying alternative indices of soil sealing and crusting or at improving the methods used to measure IR on crusted soils directly or indirectly have been conducted. Of particular importance is the use of models developed for estimating IR and hydraulic conductivity, but most of them do not account for the presence of a crust layer and therefore fall short on crusted soils. These methods and in particular models and their suitability in estimating IRs on crusted soils are discussed in this article. Trends and perspectives, gaps and conflicts relating to soil sealing and crusting effects on infiltration are also described.

Tillage and crop rotation effects on carbon sequestration and aggregate stability in two contrasting soils at the Zanyokwe Irrigation Scheme, Eastern Cape province, South Africa

Intensive tillage and monocropping have adversely affected the quality of soils in South Africa through accelerated loss of soil organic matter. Two clay loam soils, a Bonheim at Burnshill and a Shortlands at Lenye, at the Zanyokwe Irrigation Scheme in the Eastern Cape province were used to evaluate the short-term effects of tillage and crop rotations on carbon sequestration and aggregate stability under sprinkler-irrigated crop production. A split-plot arrangement of treatments in a randomised complete block design was used with tillage as the main plots and crop rotations as subplots. Conventional tillage (CT) was compared to no-till (NT) under maize–fallow–maize (MFM), maize– wheat–maize (MWM) and maize–oat–maize (MOM) rotations. Carbon sequestration was monitored by measuring changes in soil organic carbon (SOC) and the stability index (SI) was used for monitoring aggregate stability. No-till had inconsistent effects on SOC relative to CT but resulted in improved soil SI on both soils, especially on the Shortlands soil. The MOM rotation enhanced SOC relative to the MWM and MFM rotations on both the Bonheim and Shortlands soils. Across tillage practices, the MOM rotation significantly increased the soil aggregate SI compared with the MWM and MFM rotations on the Shortlands and to a lesser extent on the Bonheim soils. Scanning electron microscopy revealed that soil aggregates under MOM had dense organic coatings and bridges compared with the MFM rotation, indicating the positive effect of carbon sequestration on aggregate stability. Generally, the results indicated that, in the short term, cover crops, especially oats, have greater influence on SOC accumulation and aggregate stability than tillage, irrespective of soil type.

Chickpea residue properties controlling decomposition dynamics and nitrogen availability in some tropical acid soils

Chickpea (Cicer arietinum) grown in a rotation can contribute significant nitrogen (N) if its decomposition and nutrient-release dynamics are known and synchronised with the maximum demand by the succeeding crop. The aims of the study were to investigate the decomposition rates of two chickpea residues, mature (CHR) and green manure (GM), and use their properties to predict N dynamics in acid soils. The N dynamics were predicted by the NCSOIL model using neutral and acidic detergent fibres (NDF and ADF, respectively) measured by near-infrared reflectance spectroscopy (NIRS) for defining residue pools. The GM released 50–60% of total N in 84 d, whereas CHR immobilised N. Simulations based on the two residue pools indicated that NIRS predicted the carbon (C) and N dynamics reasonably well for both residues. The decay rate constants of the NDF – soluble pool varied between 0.5 and 0.2 d-1. Adding an intermediate pool NDF + ADF improved the prediction of C and N dynamics for CHR but not for GM. Therefore, successful prediction of N dynamics required the search of N partitioning among pools by NCSOIL, as long as no chemical analysis of N was suitable for this purpose.

Assessment of indigenous soil and water conservation technology for smallholder farms in semi-arid areas in Africa and close spaced trash lines effect on erosion and crop yield

Runoff and soil erosion are responsible for about 83% of the land degradation worldwide. Many smallholder farmers in arid and semi-arid areas of Africa often use inexpensive indigenous soil and water conservation (ISWC) techniques to control runoff and erosion. This paper is a review of the ISWC methods and categorizes them into three: those suitable for semi-arid areas, those suitable for wet areas and those suitable for both semi-arid and wet areas. The usefulness of ISWC is generally appreciated but literature on the subject is scarce. A case study that investigated effects of 2-m spaced trash lines on runoff, erosion and crop yield in a cowpea – maize rotation is presented. Trash lines reduced runoff and soil loss and increased crop biomass yield three-fold. We conclude that ISWC like close-spaced trash lines are beneficial soil and water conservation methods and should be incorporated in future land resource conservation programmes

Sensitivity of selected chemical and biological soil quality parameters to tillage and rotational cover cropping at the Zanyokwe Irrigation Scheme, South Africa

Sustainable management of soils requires knowledge of the impact of agronomic practices such as tillage and crop rotation on soil quality. The objective of this study was to identify soil chemical and biological soil attributes with high sensitivity to tillage and rotational cover cropping practices on two sandy clay loam soils, a Bonheim at Burnshill and a Shortlands at Lenye, in the Zanyokwe Irrigation Scheme, Eastern Cape, South Africa. Treatments were no tillage, conventional tillage and three crop rotations involving maize–fallow–maize, maize–wheat–maize and maize–oat–maize. Principal component analysis was used to isolate the soil chemical and biological parameters that were most altered by tillage and crop rotations, while hierarchical cluster analysis was used to discriminate the crop rotation effects. Results showed that soil microbial biomass nitrogen, mineralisable nitrogen, and extractable phosphorus were the properties most altered by tillage and crop rotations. The soil properties that were most sensitive to tillage and crop rotations were those most influenced by organic matter inputs. The maize–wheat–maize and maize–oat– maize rotations were clustered together, indicating the similarity of their positive impact on soil quality.

Physical crust formation and steady-state infiltration rate in soils dominated by primary minerals in some South African ecotopes

Physical crust formation is affected by the inherent soil properties such as texture and mineralogy. However, inconsistencies exist in soil mineralogy effects of soil crusting. In addition, investigations of soils dominated by primary minerals such as quartz, especially in South Africa, are scant despite the abundance of such soils. Effects of texture and aggregate size on crust formation and steady-state infiltration rate (SSIR) in soils dominated by primary minerals were investigated. Soil aggregate sizes of <2, 2–3 and 3–5 mm were exposed to ∼60 mm h−1 simulated rainfall. Aggregate size significantly affected crust strength (P < 0.05). Physical crusts ∼0.2 to ∼0.8 mm thick with a surface layer of loose grains formed over a thin plasmic layer. Crusts with strengths between 0.25 and 3.42 × 10−4 kg m−2 developed on the <2 mm aggregates compared with <2.23 × 10−4 kg m−2 in the >2 mm aggregates. The reverse occurred in the Alice Jozini ecotope, which had a relatively low clay content of 120 g kg−1. The SSIR was between 1.24 and 3.60 mm h−1 in ecotopes dominated by primary minerals and clay content between 12% and 26%. In the Amatola Jozini ecotope, which was dominated by kaolinite and higher clay content, SSIR was 15.23 mm h−1. Consequently, physical crust formation decreased but SSIR increased with increase in aggregate size if there was a concomitant increase in the clay fraction, decrease in primary minerals and presence of kaolinite.

Aggregate breakdown mechanisms as affected by soil texture and organic matter in soils dominated by primary minerals

Soil aggregates breakdown mechanisms depend on soil properties such as texture, clay mineralogy and organic matter content. Little is known about the effect of soil properties on aggregate breakdown mechanisms in South African soils. The objective of this study was to establish the relationship between aggregate breakdown mechanisms and selected soil properties in some South African soils dominated by primary minerals. Soil samples with varying properties were collected from the surface 0–0.2 m from 14 ecotopes in Eastern Cape province. Aggregate stability was determined following the fast wetting (FW), slow wetting (SW) and wet stirring (WSt) methods. Soils with high quartz were the least stable due to its inability to bond with other clay minerals or soil organic matter (SOM). In contrast, soils with high kaolinite were the most stable. Slaking was the dominant aggregate breakdown mechanism in most of the studied soils. Aggregate stability was significantly correlated with particulate organic matter (POM) for FW (r2 = 0.74, P < 0.05) and SW (r2 = 0.64, P < 0.05). It was concluded that increasing POM content in soils through SOM inputs and farming methods that reduce POM mineralisation can reduce slaking during rapid wetting by raindrops.