Doctoral Degrees (Soil Science)

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Site and land use effects on some physical properties and the distribution of total carbon, aluminium, and iron within aggregates of some humic soils in KwaZulu-Natal, South Africa.
(2023) Mbanjwa, Vusumuzi Erick.; Muchaonyerwa, Pardon.; Hughes, Jeffrey Charles.
Abstract available in PDF.
Soil carbon and phosphorus fractions and microbial activity in sandy loam humic soil under contrasting sugarcane harvest systems.
(2022) Mkhonza, Nontokozo Pertunia.; Muchaonyerwa, Pardon.
Soils have the potential to store large quantities of organic carbon, with benefits of mitigating climate change and improved crop/pasture productivity. Humic soils, which are only known to occur in South Africa, are highly weathered soils with high acidity, low base status, > 1.8% soil organic carbon (SOC) and good internal drainage and the main land uses on these soils include forestry, grassland, maize and sugarcane production. Where sugarcane is produced, pre-harvest burning is practiced with the aim of removing excess trash, for easy harvesting and improving sucrose recovery, while policy shifts appear to be developing globally towards green cane production, in view of climate change. There is a paucity of research findings on the effects of green cane relative to pre-harvest burning on SOC, phosphorus and microbial activity in these acidic and carbon-rich humic soils. Therefore, the objective of this study was to determine the effects of green cane relative to pre-harvest burning on concentrations of stocks and fractions of SOC, P fractions, soil microbial biomass and activities of enzymes associated with cycling of carbon and P in sandy loam humic soils. The soils were analysed for SOC and its fractions, soil aggregates stability, microbial biomass carbon (MBC), β-glucosidase enzyme, charcoal C, P fractions, MBP, acid and alkaline phospho-monoesterase and organo-mineral complexes. Soil C stocks and soil microbial quotient (SMQ) were calculated from SOC concentrations and MBC. Green cane retention resulted in higher SOC content and stocks, MBC, β-glucosidase activity and SMQ when compared to pre-harvest burning in these humic soils. The carbon content in the macro-aggregates fraction constituted > 60% of total SOC making it the primary C storage fraction for both green cane and burnt cane. The aggregate stability (only in the top 10 cm) and SOC in macro- and micro-aggregate were higher, while mineral associated C in μSilt+μClay was lower under green cane than under burnt cane. Additional analysis of effects of sugarcane production relative to forest showed that soils under sugarcane had lower charcoal-C than forest only at Wartburg but not Eshowe and Eston, while the SOC, Fe and Fe+Al in Al/Fe-OM complexes were significantly higher under sugarcane than forest only at Eshowe but not Eston and Wartburg. Soil charcoal-C was significantly higher under pre-harvest burning than green cane, with no differences in SOC, Al and Fe in Al/Fe-OM complexes, between the production systems. Green cane reduced pH and increased available P, P fixation through precipitation with Al and Fe and as CDB-P when compared to burnt cane. The MBP in the top 20 cm, and activity of acid phospho-monoesterase was significantly higher, while that of alkaline phospho-monoesterase was lower under green cane when compared to burnt cane. The findings of this study imply that green cane production on sandy loam humic soils increase SOC storage, especially in macro-and micro-aggregates, microbial activity, and P availability, when compared to pre-harvest and that charcoal C and organo-mineral complexes, contribute to the high OC in these soils. The findings suggest that green cane production has a potential for sustainable sugarcane production when compared to burnt cane. Green cane production could contribute to lowering of greenhouse gas emissions when compared with the burnt cane.
Composting performance of vermiculite-cattle manure composts and their effects on selected soil properties and maize (zea mays L.) production on sandy loam soils in Zimbabwe.
(2021) Pisa, Charity.; Muchaonyerwa, Pardon.
Abstract available in PDF.
Effect of potassium, nitrogen and silicon fertilisation on sugarcane growth and quality, nutrient uptake dynamics and soil chemistry in two contrasting soils of KwaZulu-Natal, South Africa.
(2020) Rhodes, Ruth.; Hughes, Jeffrey Charles.; Miles, Neil.
Abstract available in pdf.
Soil carbon and phosphorus dynamics under various wheat-based conservation agriculture options.
(2020) Sosibo, Nondumiso Zanele.; Muchaonyerwa, Pardon.; Dube, Ernest.
Abstract available in pdf.
Tillage management impact on greenhouse gas emissions and soil health on a maize long-term trial in KwaZulu-Natal.
(2020) Vilakazi, Bonginkosi Samuel.; Zengeni, Rebecca.; Mafongoya, Paramu.
Abstract available in pdf.
Fertiliser value of biogas slurry for maize and dry bean production and its effect on soil quality and carbon dioxide emissions.
(2019) Mdlambuzi, Thandile.; Muchaonyerwa, Pardon.; Tsubo, Mitsuru.
Abstract available in pdf.
Comparative characteristics of biochar types from human faecal wastes and pine-bark and sorption of selected heavy metals from effluent and their mobility in an amended loam soil.
(2018) Koetlisi, Koetlisi Andreas.; Muchaonyerwa, Pardon.
Heavy metals from industrial effluents poses risks to the environment and human health. Pyrolysis of locally available organic wastes could address solid organic waste management and produce a biochar that could immobilise heavy metals in industrial effluents, at source, and in amended soils. Limited research work on characteristics and effectiveness, in addressing environmental challenges, of biochar from latrine waste, sewage sludge and pine bark which are ubiquitous organic wastes in South Africa. The aim of this study were to determine effects of pyrolysis temperature on yield, characteristics and sorption capacities of selected metals from solutions and effluents on biochar from latrine wastes, sewage sludge and pine-bark and on metal mobility in amended soil. The three materials were pyrolysed under limited oxygen at 350, 550 and 650°C. Proximate and ultimate analysis, surface area, porosity and functional groups were analysed on the biochar produced. Batch sorption studies were conducted to determine biochar sorption capacity with Cd, Zn, Cu and Cr in single metal solutions and for Zn, Cu and Cr from a multiple metal solution. A leaching column study was conducted using a loam soil amended with sewage sludge biochar at equivalent rates of 0, 50 and 100 kg ha-1 and the columns leached with industrial effluent while others were leached with distilled water, measuring pH, electrical conductivity, Zn, Cu and Cr in the leachate at each event and in three equal sections of the soil at the end of the experiment. A pot trial was conducted with spinach grown on a loam soil amended at 0, 25, 50 and 100t ha-1 of sewage sludge and latrine waste biochar pyrolysed at 350°C biochars and irrigated with 25% industrial effluent for some and tap water for others. Drymatter, tissue water content, Zn and Cu, and soil pH, EC, Zn and Cu were determined at the end of the trial. Latrine waste had higher biochar yield, ash content, surface area and pore volume, and lower fixed C and volatile matter than sewage sludge. Biochar yield, volatile matter, total C, N and H decreased with pyrolysis temperature, while ash content, surface area and porosity increased. Surface functional groups of the biochar also varied with feedstock and pyrolysis temperature. The Cd sorption capacities were higher for latrine waste biochar than from sewage sludge. Mixtures of pine bark biochar with latrine waste (1:1) or sewage sludge (1:3) biochar showed synergistic effects on Cd sorption. Sorption capacities of latrine waste, sewage sludge and pine- vi bark biochar (350°C) were, respectively, 312.5, 400 and 232.6 mg kg-1 for Zn, 102, 98.0 and 33.3 mg kg-1 for Cu, and 18.9, 13.8 and 67.1 mg kg-1 for Cr from industrial effluent. Conversely, sorption capacities biochar from latrine waste, sewage sludge and pine-bark, respectively, were 278, 227 and 357 mg Zn kg-1, 97.1, 137 and 21.3 mg Cu kg-1, and 122, 106.4 and 147.06 mg Cr kg-1 for single metal solutions. Addition of biochar did not affect shoot drymatter but affected root drymatter and tissue Zn and Cu, with higher Cu than Zn in the tissue. The additions of sewage sludge biochar to neutral, loam soil did not significantly reduce leaching of Cu and Zn from the applied effluent. Moreover, the application of biochar from both latrine waste and sewage sludge increased metal uptake by spinach in the same soil. The findings of this study imply that the characteristics of biochar from latrine waste, sewage sludge and pine bark are different and that faecal waste biochars show positive metal immobilization potential in batch equilibrium studies, but have little or negative effects when added to neutral soils even at extremely high application rates.
Application of soil indigenous knowledge in rural communities of eastern South Africa.
(2018) Buthelezi, Nkosinomusa Nomfundo.; Hughes, Jeffrey Charles.; Muchaonyerwa, Pardon.; Modi, Albert Thembinkosi.; Caister, Karen Fern.
This study investigated ethnopedological knowledge related to classification, fertility and non-agricultural uses of soil in four villages in KwaZulu-Natal and the Eastern Cape, South Africa. Ethnographic methods elicited general soil indigenous knowledge. Ethnopedologic techniques gathered understanding of soil taxonomy, mapping and fertility, and selection and use of healing, cosmetic and geophagic soils. Local assessments of soil fertility and mapping were compared to scientific approaches. Soil samples were analysed for physicochemical properties. Soils used for non-agricultural purposes were analysed by X-ray diffraction and X-ray fluorescence spectrometry. Local classifications were based on observable soil morphological properties. Soil maps produced by farmers in areas with distinct geomorphic units closely correlated with scientific maps; on a floodplain the correlation was poor. Farmers assessed soil fertility using both crop and soil variables. There was poor correlation between farmers’ fertility classes and laboratory data. Farmers understood soil-crop associations which formed the basis for their soil suitability assessment and have developed specific soil use and management practices. Two soil types were identified for non-agricultural uses. Ukhethe, used for agriculture, was also used for geophagy; ibomvu for sun protection, healing and cosmetics. Geophagic soils were mainly saprolite from Leptosols. They were mostly fine-grained, had bright Munsell hues, contained mica, kaolinite, quartz and iron oxides, and elements such as Cu, Zn, Co and Pb. Ibomvu occurred in Ferralsols and was red to dark-red. Despite low sun protection factors, critical wavelengths >370 nm, the presence of TiO2 and high Fe2O3 explained its sun protection ability. The soil was fine grained, had low pH and exchangeable bases, and contained kaolinite that possibly explained its healing role. These communities applied their pedological knowledge to soil use and management. There were diverse non-agricultural uses and possible land use conflicts where a soil has more than one use. Farmers classified soils at levels that could be incorporated as higher categories in the current South African system. Farmer fertility assessment could benefit from laboratory data. Soil suitability classification systems should be used to assess both agricultural and non- agricultural uses. Valuing all local uses of soil will ensure fair and relevant land use planning.
Long-term fertilizer and sugarcane residue management effects on structural stability of two soil types in South Africa.
(2017) Mthimkhulu, Sandile Siphesihle.; Titshall, Louis William.; Podwojewski, Pascal.; Van Antwerpen, Rianto.; Hughes, Jeffrey Colin.
Abstract available in PDF file.
Nutrient recovery from wastewater in intensive agricultural systems by duckweed and the value of the biomass as an organic fertilizer.
(2018) Chikuvire, Tichaedza John.; Muchaonyerwa, Pardon.; Zengeni, Rebecca.
Abstract available in PDF file.
An evaluation of the fertilizer requirements of Chicory (Cichorium intybus var. sativum) in KwaZulu-Natal, South Africa.
(2018) Gordon, Douglas Hamilton.; Hughes, Jeffrey Colin.; Manson, Alan David.
Abstract available in PDF file.
Potassium reserves and fixation capacity in soils of the South African sugar industry and potential for their inclusion in soil testing and fertilizer recommendations.
(2018) Elephant, Dimpho Elvis.; Muchaonyerwa, Pardon.; Miles, Neil.
Abstract available in PDF file.
Land use and land management impact on CO2 emissions from soils in selected smallholder farming systems.
(2016) Adam, El-Khatab Mohamed Abdalla.; Chivenge, Pauline.; Chaplot, Vincent A. M.; Everson, Colin.
Abstract available in PDF file.
Evaluation of extraction based fertilizer recommendations.
(2015) Nongqwenga, Nqaba.; Modi, Albert Thembinkosi.
There is a need to improve methods by which nitrogen, phosphorus and potassium are currently recommended. There is a considerable lack of mechanistical justifications for the methods used to recommend these nutrients. Lack of mechanistical justification can be attributed mainly to the disregard of nutrient (N, P and K) dynamics. Also the difficulty in incorporating these dynamics on fertilizer recommendation programs has compromised the mechanistical basis of extraction based approaches. The aim of the study was to evaluate these conventional (extraction based fertilizer recommendations) methods used to recommend these nutrients, by comparing their performance to the alternative approaches provided in this study. This evaluation was carried out through several studies, and a review of literature. From literature review it was apparent that there is indeed a need for revision of these methods. Their lack in mechanistical, technical and practical justification was considered and critically analyzed. It was proposed that alternative P and K recommendations can be achieved through quantity/intensity (Q/I) relations (amount of a respective nutrient in solution relative to the amount of nutrient adsorbed). It was also proposed that N recommendations can be improved by integrating mineralizable N. It was also concluded that these alternative approaches can routinely in a cost effective manner be determined. The first chapter evaluated P and K Q/I relations in several South African soils. Parameters of K dynamics were derived from activity ratio diagrams and these were used to explain K dynamics. Phosphorus sorption curves were linearized by Langmuir equation, and parameters derived therefrom were used to evaluate P dynamics. It was found that pH measured in water had a correlation coefficient (R2) of 0.71 with P sorption maxima. It was also found that electrical conductivity could account for 76% variance in K intensity parameter. It was suggested that these correlations could be exploited further to empirically model these crucial parameters. Thus, these correlations provide a possibility of determining these parameters routinely. Pot trials were also conducted to evaluate the crop response, when P or K was made with the alternative approaches using maize and potato as test crops. Conventional extraction approach recommended higher P rates, and the P uptake between the two methods was not significantly improved. The extraction based approach recommended lesser K rates and K uptake was significantly higher under the alternative approach. The impact of integrating mineralizable N on N recommendations was also evaluated under control conditions. It was found that although alternative N recommendation approach recommended lesser N rates the N uptake was not significantly reduced. In fact the non-significant trend was that N uptake was higher when N recommendations were made with an alternative approach. From these initial pot trials only one nutrient was allowed to vary and the rest were kept constant at optimum levels. The second set of pot trials were carried out (parallel to the previous one), and on this set, all three nutrients were allowed to vary per experimental units. On these NPK was recommended with alternative approach and compared to the conventional approach. The results obtained were similar to those obtained when N, P or K were allowed to vary individually. It was also suggested that total carbon can be used to assess the validity of these approaches. This was based on the consistent inverse correlation that was obtained between total carbon and P or K. Field trials were also conducted at Ukulinga research farm Pietermaritzburg and Wartburg, using maize and potato as test crops. The lack of concurrent response from nutrient uptake was also observed here similar to the observations already made in pot trials. These were characterized by conventional method recommending higher rates of N and N uptake not concurrent with the rates. It was also found that there was a poor correlation between applied fertilizer and extraction based intensity parameters, with R2 ranging between 0.005 – 0.011, compared to R2 of Q/I parameter which was 0.98 for both P and K. This poor correlation was evident between nutrient uptake and total biomass. Yield of both maize and potato at both sites was higher when recommendations were made by alternative approaches, and yield grade of potatoes was also improved when the recommendations were made by alternative approach. Total biomass of maize was also significantly improved when the recommendations were made by the alternative approach. Earlier, observation with regards to correlation of total carbon and nutrients was also observed under field conditions. This suggested that this is an important parameter to evaluate fertilizer recommendation program. It was concluded that recommending P and K with Q/I relations, and integrating mineralizable N on N recommendations is more mechanistically, technically, theoretically and practically justified compared to the conventional method.
Grassland degradation and rehabilitation of soil organic carbon and nitrogen stocks.
(2014) Dlamini, Phesheya.; Chaplot, Vincent A. M.; Chivenge, Pauline.
Land degradation is widely considered to adversely affect soil fertility, soil quality, constrain productivity, subsequently leading to a decline in soil organic carbon (SOC) and nutrients in soils, yet little is known about the stocks, environmental controls, destabilization mechanisms and carbon sequestration potential of degraded grassland soils. The aim of this dissertation was to evaluate (1) the impact of land degradation on SOC and nitrogen (N) stocks, distribution and SOC quality, to elucidate the environmental controls, in a communal rangeland with varying intensities of degradation, (2) to examine the rehabilitation potential of the same rangeland (3) to assess the spatial variability and replenishment potential of SOC and N stocks in a typically degraded grassland catchment. A meta-analysis was conducted to provide a quantitative review of the impact of land degradation on SOC stocks in grassland soils, worldwide. Subsequently, the impact of degradation on SOC and N stocks and organic matter quality was investigated in a communal rangeland in the KwaZulu-Natal province, South Africa with varying intensities of degradation. Thereafter, different rehabilitation techniques were applied in the same communal rangeland to replenish SOC and N stocks. Advantage was also taken of 23 ha degraded grassland catchment to assess the spatial variability, carbon replenishment potential of SOC and N and to elucidate the main environmental controls. Degradation resulted in a significant depletion of SOC stocks in grassland soils, both in the meta-analysis and field experiment. The meta-analysis indicated that the depletion of SOC stocks as a result of degradation was more pronounced in sandy acidic soils under dry climate than clayey soils under wet climate. The field experiment showed that degradation significantly depleted SOC stocks by 89% and N stocks by 76% in sandy acidic soils at the study site. The reduction of the stocks due to degradation was accompanied by an increase in soil bulk density, a decrease in soil aggregate stability and concomitant decrease of macro and micronutrients (e.g, Ca by 67%; Mn, 77%; Cu, 66% and Zn, 82%). SOC and N stocks decreased sigmodially with a linear decrease in grass aerial cover. After two years, the “Savory and fertilization techniques increased SOC stocks by 6.5% and 3.9%, respectively. At catchment level, degradation led to high spatial variability of SOC and N stocks controlled primarily by soil surface characteristics, including grass cover, soil surface crusting and secondarily by topography. The carbon replenishment potential of degraded grassland catchment was estimated to be 4.6 t C ha-1, with clay-rich Acrisols having a greater capacity to replenish SOC stocks than sandy Luvisols and Gleysols. In conclusion, the results of this dissertation indicate that degradation results in high depletion of SOC and N stocks. However, rehabilitation has the potential for carbon sequestration and can lead to more sustainable grassland ecosystems.
Characteristics of sesquioxidic soils.
(1974) Fey, Martin Venn.; Le Roux, J.
Sesquioxidic soil clays from Oxisols in South Africa, Australia and Brazil, and two clays from Andosols in Japan and New Zealand, were investigated by XRD, OTA, IR, EM and quantitative mineralogical analysis. The volcanic-ash soil clays are dominated by allophane; clays from Natal are dominated by kaolin (30 - 45%) and free iron oxides (20 - 25%), with smaller amounts of gibbsite (0 - 12%) and pedogenic chlorite (less than 20%); Oxisol clays from Australia and Brazil contain free iron oxides (40 - 50%), gibbsite and kaolin (both about 25%). Acid ammonium oxalate (pH 3) was found to be superior to currently popular alkaline reagents for extracting amorphous aluminosilicates and alumina from these clays. Boiling 0,5- NaOH dissolved large amounts of finely-divided kaolinite and halloysite, while hot 5% Na[2]CO[3] reaction was too slow (partial dissolution of synthetic amorphous aluminosilicates with one extraction) and insufficiently selective (gibbsite and kaolin of poor crystallinity dissolve to a variable extent). On the other hand, synthetic gels (molar Si0[2]/A1[2]O[3] ranging from 0,91 to 2,55) dissolved completely after 2h shaking in the dark with 0,2tM acid ammonium oxalate (0,2 ml/mg). Specificity of oxalate for natural allophane was indicated by removal of similar quantities of silica and alumina using different clay: solution ratios. Oxalate extraction data indicated that allophane is absent in Oxisol clays, which are characterized by small quantities of amorphous, A1-rich sesquioxide (1,5 to 7%), some of which may originate in interlayers of 2: l phyllosilicate structures. Allophane was determined quantitatively in volcanic-ash soil clays by allocating hydroxyl water content to oxalate-soluble silica plus alumina on the basis of an ignition weight loss/chemical composition function for synthetic amorphous aluminosilicates. Both Si02/A1[2]O[3] ratios and quantities of allophane were found to be lower than those obtained using boiling 0,5N NaOH, in agreement with the interpretation that the latter treatment attacks crystalline aluminosilicates. Parameters of chemical reactivity and distribution of electric charges following various chemical pretreatments of allophane were found to correspond closely to those predicted on the basis of synthetic gel behaviour. Results for Oxisol clays suggested that the role of amorphous (oxalate-soluble) alumina in governing physicochemical properties is generally subdorninant to that of the poorly-crystalline, A1-substituted iron oxide component which is removed by deferration with citrate-dithionite-bicarbonate reagent. Hysteretic pH-dependent net negative exchange charge was shown to arise from hysteresis of positive exchange charge, while CEC is fully reversible by titration with strong acid. A mechanism is postulated to account for this observation. Levels of silica in the soil solution of Natal Oxisols are higher than those of more strongly-weathered soils from Australia and Brazil, and may be sufficiently high to exert a favourable effect on plant-available P following phosphate fertilization. Although soluble silica levels are also relatively high in volcanic-ash soils, a similar effect is not likely to manifest itself significantly owing to the very high P adsorption capacity of allophane. A study of soil solution equilibria indicated that in terms of silica and aluminium hydroxide potentials, kaolinite is the most stable mineral in all the soils. Allophane persists as a partial metastable equilibrium state in volcanicash soils while gibbsite formation in Oxisols is contingent upon periodic, nonequilibrium leaching conditions. The role of clay mineral suite in governing levels of exchangeable aluminium in acid soils is examined. A revised model system for allophane is proposed in which tetrahedral substitution of Al for Si may reach a maximum of 1 : 1 in an aluminosilicate phase. Additional alumina takes the form of discreet amorphous or crystalline material. The composition of allophane corresponding to maximum A1 for Si substitution will depend upon the availability of basic cations for charge balancing during neogenesis of the amorphous aluminosilicate.
Factors affecting nitrogen utilization by sugarcane in South Africa.
(1972) Wood, Richard Anthony.; Orchard, Edwin Retief.
TIhe response of sugarcane to applied N in South Africa varies considerably from one soil to another, particularly in the plant crop. Responses to fertilizer N by ratoon cane are generally much greater than those given by plant cane. Where irrigation is practised yield response per unit of N is significantly higher than that obtained under rain grown conditions. Response of cane to N can be influenced by various factors, some of which are able to bring about differences in yield as great or greater than those obtained from the N fertilizer itself. These include seasonal effects, time and method of N application, the form of N applied, and the nature of the soil . The N cycle in relation to sugarcane has been examined, as several factors affecting response of cane to N are concerned with the transformations which N undergoes in the soil - plant system. The potential of different soil series within the sugar belt to mineralize N, greatly influences the response of plant cane to applied N. The N supplying power of sugar belt soils is also dependent upon how recently they were opened for cultivation, and the length of time they remain dry prior to replanting. However, accurate assessment of soil N available to cane remains difficult, and it is probable that N recommendations will continue to be made largely on an empirical basis of management and yield. Incorporation of cane trash in the soil, and the C/N ratio of cane roots may affect efficiency of N fertilizer usage by the crop, particularly in the sandier soils of the industry low in N, due to the immobilization of applied N. Apart from the soil pH effect as such, specific N carriers are able to influence r ates of nitrification and thus susceptibility to leaching, especially in the more weakly buffered soils which constitute over 30% of the industry. It appears likely that utilization of N by cane grown in these soils, could be enhanced by the use of the nitrification inhibitor N-Serve. Application of all the N to the furrow at time of planting can cause severe leaching losses even in heavily textured soils. Top-dressing some weeks after planting, results in more efficient recovery of fertilizer N. Even so, only 25%-30% of N applied in the widely used ammonium form is recovered by the above ground parts of the cane crop.
Soil acidity and liming in Natal.
(1970) Reeve, Neville George.; Sumner, Malcolm E.; Orchard, Edwin Retief.
1. Effects of Aluminium Toxicity and Phosphorus Fixation on Crop Growth on Oxisols in Natal Simple, routine methods for estimating exchangeable Al and P fixing capacity of acid soils are described. A glass-house study on eight Oxisols revealed marked growth response of 'trudan' (Sorghum sudanense) to amelioration by lime, gypsum and Ca silicate which is ascribed to elimination of A1 toxicity rather than to improved P availability. Growth response to amelia rants took place up to the point of elimination of exchangeable Al after which a significant reduction in yield occurred. P fixation is shown to be a major fertility limitation in the soils studied. Since no apparent relationship between P fixing capacity and exchangeable A1 existed and since lime did not decrease P fixation despite its ability to eliminate soluble A1, it is concluded that P fixation is an adsorption reaction rather than a precipitation reaction. Although the soils studied are all capable of fixing large quantities of P considerable variation exists between them. Fertilizer recommendations based only on an estimate of the available P in the soil per se could thus be in serious error. 2. Lime Requirements of Natal Oxisols based on Exchangeable Aluminium The exchangeable Al status of eight Natal Oxisols is a suitable criterion for the measurement of lime requirement defined as the amount of lime necessary for maximum crop production. The principal function of lime in these soils is to eliminate A1 toxicity; it has little or no effect on P availability. A "critical value" for exchangeable A1 below which 'trudan' did not respond to lime application was found. On the average the amount of lime necessary for maximum growth and exchangeable Al control was approximately one sixth the amount required to raise the soil pH to 6.5. 3. Cation Exchange Capacity and Exchangeable Aluminium in Natal Oxisols Positive charges in acid soils reduce CEC at low electrolyte concentration probably by double layer interaction. The resultant net CEC (determined by washing soil free of salt with water) is the effective CEC under field conditions and the difference between net CEC and exchangeable bases is accordingly a convenient measure of exchangeable A1. A reaction scheme is proposed which relates the large pool of non-exchangeable Al (extractable with N NH[4]0Ac-pH4) in these soils to the relatively small amounts of exchangeable AI; this reaction scheme is governed primarily by net CEC and exchangeable bases rather than by pH. 4. Amelioration of Subsoil Acidity in Natal Oxisols, The large pH dependent CEC in Natal Oxisols effectively limits the downward movement of lime. Although heavy fertilisation, particularly with acid forming nitrogenous fertilizers increases the rate of movement, relatively small amounts of Ca salts having little ability to neutralise subsoil exchangeable Al could be leached from limed topsoil. In contrast, bases equivalent to 80% of that applied leached rapidly from gypsum treated topsoil. Although gypsum did not eliminate subsoil exchangeable A1, it was considerably more efficient than lime in this respect. However, gypsum caused severe loss of exchangeable Mg which could have serious nutritional consequences if not corrected.
Soil physical factors affecting root growth and maize yield in four Rhodesian soils.
(1976) Rankin, James Malcolm.; Sumner, Malcolm E.
The platinum microelectrode technique for measuring oxygen flux in soils has been reviewed. Shortcomings in the existing technique and instrumentation have been discussed. The new instrumentation, electrode standardization and measurement techniques developed enable the method to be used with confidence in unsaturated soil systems. Measurements of oxygen flux index in four soil samples showed a very highly significant regression relationship between oxygen flux index and air space within the range 3 - 15% air space on each soil. There was no significant difference in the regression relation between soils. A field penetrometer, designed to measure the presence and strength of subsurface pans in field soils has been described. Measurements with the penetrometer on three depth of ploughing treatments (100, 230 and 355 mm) on tillage trials at four sites with different clay contents showed that hard layers were present on all the treatments. Except on the shallowest ploughing depth treatment on the fine-textured soil, where the pan was 225 mm below the nominal ploughing depth, the hard layers were present between a few mm and 150 mm below the nominal ploughing depth, and had strengths of between 16 and 24 bars. The theory and factors affecting measurement of soil strength with needle penetrometers have been investigated. The design and operation of a laboratory penetrometer used to measure soil strength under closely controlled laboratory conditions has been discussed. Physical factors likely to affect root growth, viz. soil texture, air space, bulk density, soil strength and available moisture, have been measured in a comprehensive range of undisturbed cores taken from the four tillage trials. High soil strength is considered as being the soil physical factor most likely to restrict root growth in these soils. Physical factors affecting soil strength have been investigated. Soil strength is shown to be highly dependent upon bulk density, matric potential and soil texture. The hard pans shown to exist in all the tillage trial soils exhibit many of the characteristics of tillage pans, but their existence cannot be attributed exclusively to the ploughing depth treatments imposed in the tillage trials. Rather, the pans have resulted from a combination of interacting factors, including the previous history of the soils, the imposed tillage treatments, crop, and climatic factors. A study of some of the data from the literature on root growth and soil strength shows that root growth is severely restricted by soil strengths of the order of 20 to 30 bars. In order to determine whether root growth was being restricted in the tillage trials, root profiles were extracted from one of the trials. These showed that the pans severely restricted root growth. Analysis of maize yield data from the tillage trials showed that on the three coarse-textured sites yield increased with increased depth of ploughing, and that there was a marked seasonal effect, ploughing depth having a relatively greater effect on maize yield in dry seasons than in wet. On the fine-textured site, however, where no pan existed near the surface in the shallow ploughed treatment, the ploughing depth effect was not significant, nor was there any marked seasonal effect of ploughing depth on maize yield. Evidence presented shows that the pans, by restricting root growth are reducing the amount of water available to the plant. This effect is greater in dry seasons, and in soils with low available water .

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