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The effect of different N management strategies on the grain yield, N uptake and use efficiency components of five commercial wheat cultivars were investigated for two consecutive years at Riet River and Loskop irrigation schemes. The cultivars ranged ingrowth period to initial flowering, from 95, 102, 106, 108 and 109 days for SST 822, Steenbras, SST 876, Olifants and Baviaans. Management strategies consisted of zero (NO) and recommended Napplication split-applied at different growth stages (N1 = 100% at planting; N2 = 54% at planting, 23% at stem elongation and 23% at flag leaf; N3 = 38% at planting, 44% at stem elongation and 18% at flag leaf; N4 = 100% at stem elongation in the first year, and 37% at planting and 63% at stem elongation in the second year). The lowest grain yield and N uptake resulted for N0, followed by N4 and then either N1, N2 and N3. The longer-growing Baviaans, Olifants and SST 876 (more than 106 days to flowering) responded more to N fertilization than the shorter growing Steenbras and SST 822 (less than 102 days to flowering). This also applied to grain yield, N uptake and N harvest index. The cultivars SST 822 and Steenbras had the lowest and SST 876 the highest N physiological efficiency (NPE). Application of N had limited effect on the agronomic (NAE) and recoveryefficiency (NRE) of the element. The results of this study showed that several factors must be considered in the selection of N efficient wheat cultivars.

Nitrogen is the most limiting nutrient for maize production under conventional and minimum tillage systems in western Ethiopia. Thus application of N is essential to sustain production in the region. However, very little is known about the efficiency of the fertilizer N applied. Experiments were therefore conducted to determine the integrated effects of tillage systems and N fertilization on the usage of applied N by maize from 2000 to 2004 at five sites. Three tillage systems (MTRR = minimum tillage with residue retention, MTRV = minimum tillage with residue removal and CT = conventional tillage) and three N levels (the recommended rate, 25% less and more than this rate) were combined in factorial arrangement with three replications. A significantly higher grain N content was recorded with MTRR than with MTRV and CT. The stover N content was not significantly affected by the three tillage systems. However, grain, stover and total biomass N uptake were consistently superior with MTRR compared to MTRV and CT. The agronomic (NAE), recovery (NRE) and physiological (NPE) efficient use of applied N by maize for the same tillage system were consistently higher at the lower N level range of 69 − 92 kg ha−1than at the higher N level range of 92 − 115 kg ha−1. At the lower N level range NAE and NRE were larger with CT than with the other two tillage systems. Both indices were higher with MTRR than with the other two tillage systems at the higher N level range. The NPE was not significantly affected by the tillage systems. However, the trend at both N level ranges was higher with MTRR than with MTRV and CT.

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.

Lachenaliabulbs used in the pot plant industry are produced in the nursery phase from bulblets propagated from leaf cuttings. Fertilisation during this phase of enlargement can ultimately influence the growth and flowering of pot plants through the nutritional status of the bulbs. The objective of this study was to determine the response of cultivars (Rupert and Ronina) in the pot plant phase to different nitrogen levels when grown from bulbs whereof the fertilisation history is known. During the nursery phase bulblets of cultivars Rupert and Ronina received three nitrogen levels (0, 250 and 520 kg N ha−1) combined with two different application times. The resulting bulbs were planted in the pot plant phase and five nitrogen levels (0, 70, 180, 330 and 520 kg N ha−1) applied. Ronina leaf area responded better than Rupert when fertilised in the pot plant phase. The leaf area was also promoted by nitrogen applied in the nursery phase. Nitrogen applied in the nursery and pot plant phases influenced the number of florets formed per inflorescence positively. Only high nitrogen levels applied in the nursery phase (250 and 520 kg N ha−1) increased the peduncle length of the plants. Peduncle diameter was increased with an increase in nitrogen levels in the nursery phase as well as in the pot plant phase. This trial showed that Lachenaliahas a low nitrogen requirement in the pot plant phase when grown from bulbs that were properly fertilised in the nursery phase but additional fertiliser in the pot plant phase improved the quality of the inflorescence.

In South Africa, the pedogenic relevance of soft plinthic B horizons in the border zone between arid and semi-arid bioclimates is often questioned because the prevailing climate is not conducive for fluctuating water tables as required for their formation. These horizons are therefore interpreted as being paleo features, a plausible possibility as the paleo climate of South Africa varied considerable from that experienced today. The genetic activity of the soft plinthic B horizon in the Bainsvlei soil form is questioned even more as it occurs under a red apedal B horizon which is regarded as freely drained. A micromorphological study was therefore carried out on a Bainsvlei Amalia fine sand soil profile located in an arid bioclimatic zone to determine redox activity in its soft plinthic B horizon. Undisturbed clods, impregnated blocks and optical thin sections were studied. Only a few Fe-Mn nodules of varying colour were present in the soft plinthic B horizon. However, mottles of biological and chemical origin with high and low chroma were prominent features. Grey and red mottles and to a lesser extent yellow and black mottles were randomly distributed in the soil matrix. Many of these mottles were concentric and their colours related closely to the position of the macropores displaying a juxtapositioned sequence from grey to yellow to red to black. In other instances the sequence was reversed. Both sequences are typical of redistribution of Fe-Mn under spatially varying redox conditions and hence plinthite formation. Another indication of active plinthite formation was the diffuse transitions between the mottles which increased with depth. Thus sufficient micromorphological evidence evolved to assume recent redox activity of low intensity in the soft plinthic B horizon of this particular Bainsvlei soil.

The benchmark soils of Lesotho are characterized by high P retention and this aspect should be accommodated in fertilization programmes. This together with long procedures for determination of P retention parameters and a lack of information on the relations between them and other soil properties motivated the study. The objective was to establish which properties influence P retention by the eleven benchmark soils of Lesotho and subsequently develop regression models for predicting P retention parameters from those properties that are determined in routine laboratory analyses. In an incubation study two P retention parameters were quantified with the extraction procedures of Bray and Olsen: (i) phosphorus retention index (PRI), which was the slope of retained P plotted against applied P and (ii) phosphorus retention capacity at an application level of 400 kg P ha–1(PRC400). Simple linear and non–linear regression models showed regardless of extraction procedure significant correlations (r2>0.60) between either PRI or PRC400 and the following soil properties: sample density, CEC, sand, clay, organic C, crystalline Fe oxides and amorphous Fe and Al oxides. When the data were fitted into a stepwise multiple regression analysis, the effect of organic C and CEC on P retention became insignificant while that of available P and exchangeable K became significant. The r2values for the multiple regression models were in the order of 0.93–0.94 for Bray or 0.96 for Olsen. Regression models excluding Fe and Al were also established to cater for situations where it may not be possible to determine those oxides and their r2values were in the order of 0.92–0.93 for Bray or 0.92 for Olsen. Hence any of these multiple regression models can be used to predict P retention in the benchmark soils of Lesotho.

The principle objective of this study was to establish P fertilizer guidelines for dryland maize on the South African Highveld according to the sufficiency concept of soil extractable P. Data sets from nine different P fertilizer trials at various localities in the Free State, Gauteng, Mpumalanga and North West provinces were used. Different P treatments were applied for all trials in order to establish differences in extractable soil P levels, which were expected to have corresponding effects on maize yield. Long-term rainfall varied from 765 mm per annum for the Dirkiesdorp trial in the east to 494 mm per annum for the Wolmaransstad trial in the west. The duration of trials varied between one and nine seasons. The clay content of the top 150 mm soil at these localities ranged between 8.4 and 47%. Extractable P threshold values with varying R2values were derived for all localities. These values were related to soil properties and it was shown that the degree of leaching and silt-plus-clay content were the parameters that explained most of the variation. However, it was decided only to explore relationships between threshold P values and silt-plus-clay contents in more detail. By excluding data from two localities of which the topsoil contained free lime, the R2values of the mentioned relationships improved substantially so that P threshold values could be derived from the silt-plus-clay content range of the other seven localities. The extractable soil P threshold concentrations based on Bray 1 for the top 150 mm soil layer, to obtain 90% relative yield varied from 33.5 mg kg−1at 13% silt-plus-clay to 14.6 mg kg−1at 60% silt-plus-clay. These P thresholds were much higher on the sandy soils than the value of 19 mg P kg−1(Bray 1) for 95% relative yield according to the ARC-Grain Crops Institute (1994) guidelines over all soils. This may not necessarily imply that overall more P fertilizers should be applied, since the corresponding soil sampling procedure also measures residual P from enriched zones over rows where P fertilizer was band-placed. Research results used to establish the ARC-Grain Crops Institute (1994) guidelines excluded sampling from enriched zones over rows.

Maize production in western Ethiopia is practiced mainly on Nitisols inherently low in N. Neither conventional nor minimum tilled maize reaches its full yield potential without fertilization. A large amount of applied N usually escapes from the soil-crop system and this may exert harmful effects on the environment in addition to yield losses. Hence, the objective was to quantify the fate of labelled urea applied to conventional and minimum tilled maize. The study was conducted in 2004 on selected plots of experiments started in 2000 at three sites. At each site, the experiment was laid out in a randomized complete block design with three replications. Three tillage systems (MTRR = minimum tillage with residue retention, MTRV = minimum tillage with residue removal and CT = conventional tillage) and three N fertilization levels (recommended rate of 92 kg N ha−1and 25% less and 25% more than this rate) were combined in complete factorial arrangement. Only the CT and MTRR plots fertilized at the recommended rate were used for this study. A micro plot of 2.4 m2was demarcated in the center of every selected 24 m2macro plot where labelled 15N @ 5 atom % urea was applied instead of unlabelled urea. The grain, stover and total biomass N derived from fertilizer was consistently higher for CT than MTRR. Conversely, the grain, stover and total biomass N derived from soil was consistently higher with MTRR than CT. Therefore, the fertilizer N recorded in the MTRR soils was higher than in the CT soils and mainly confined to the upper 450 mm. The fate of fertilizer N was in MTRR: 47% recovered by maize, 17% remained in the soil and 36% unaccounted for and in CT: 54% recovered by maize, 12% remained in the soil and 34% unaccounted for.

Nitrogen is the most limiting nutrient in the production of maize in western Ethiopia with its favourable climate for this crop. This is because Nitisols inherently low in N are used for maize production by resource-poor farmers who cannot afford fertilizer application. An option in combating this constraint is planting of maize genotypes that are efficient in N use. Experiments were therefore conducted to evaluate different maize genotypes for grain yield, N uptake and use efficiency at five sites. At each site two experiments, viz.one with open-pollinated genotypes and the other with hybrid genotypes were laid out in a randomized complete block design with three replications. In each of these experiments either five open-pollinated genotypes or five hybrid genotypes were combined in complete factorial arrangement with six N fertilization levels ranging from 0 to 230 kg ha−1at 46 kg intervals. The results indicated that only two genotypes qualified as N use efficient, viz.the open-pollinated Ecaval 1 and the hybrid CML373/CML202/CML384. On average they outyielded their respective local genotypes by 5.9% at a low N application and by 17.5% at a high N application. Both genotypes also showed consistently higher N agronomic efficiency, N recovery efficiency and N physiological efficiency at low and high N applications. The introduction of these two N use efficient genotypes into the farming systems of the resource- poor farmers could be beneficial to them.

The western part of Ethiopia has a high maize production potential as a result of favourable environmental conditions. However, maize production is constrained by non-sustainable cropping practices, particularly plough- or hoe-based cultivation, soil and water loss due to erosion, as well as N deficiency. Experiments were therefore conducted to determine the integrated effects of tillage system and N fertilization on the productivity of maize from 2000 to 2004 at five sites. Three tillage systems (MTRR = minimum tillage with residue retention, MTRV = minimum tillage with residue removal and CT = conventional tillage) and three N levels (the recommended rate and 25% less and 25% more than this rate) were combined in factorial arrangement with three replications. The recorded grain yield in the experiments ranged from 4649 to 8030 kg ha−1with an average of 6104 kg ha−1. Grain yield was positively and significantly correlated with several yield components, especially total biomass yield and thousand seed weight. During the initial two years of the experiments, there was no significant difference in grain yield between MTRR and MTRV and both were significantly superior to CT. However, during the final two years of the experiments, there was no significant difference between MTRV and CT and both were significantly inferior to MTRR. On average, MTRR increased grain yield by 6.6 and 12.2% compared to MTRV and CT, respectively. The application of N increased grain yield regardless of tillage system and the recommended fertilization rate of 92 kg N ha−1for conventional tilled maize was also found adequate for minimum tilled maize. These results proved that MTRR can be introduced successfully in western Ethiopia.

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.

During the nursery phase, Lachenaliaflower bulb producers plant bulblets propagated from leaf cuttings, for enlargement. They strive for a high yield of good quality bulbs that are of a marketable size within a year. In this regard nitrogen is of vital importance since it stimulates vegetative growth. Very little has been known about the nitrogen requirements of Lachenaliaplants that are cultivated in soil. Hence the response of two Lachenaliacultivars (Rupert and Ronina) to combinations of nine nitrogen levels (0, 30, 70, 120, 250, 330, 420 and 520 kg ha−1) and three application times (⅓ with planting and ⅔ ten weeks after planting; ⅓ with planting, ⅓ ten weeks after planting and ⅓ sixteen weeks after planting; and ¼ with planting, ¼ ten weeks after planting, ¼ sixteen weeks after planting and ¼ twenty one weeks after planting) was studied in a glasshouse using topsoil of a fine sandy loam Bainsvlei form. Nitrogen promoted the growth of Lachenaliaplants irrespective of cultivar. The largest leaf area for Rupert was measured between 250 and 420 kg N ha−1and for Ronina between 180 and 520 kg N ha−1. Both bulb circumference and firmness were influenced positive by higher nitrogen levels whereas nitrogen application time did not show such a clear trend. Bulb circumference was the largest where 330 kg N ha−1was applied and the firmest bulbs where 420 and 520 kg N ha−1was applied for Rupert and Ronina respectively. Nitrogen content of the bulbs increased with an increase in nitrogen levels. Bulbs that received their nitrogen in four equal applications had the highest nitrogen content. The glucose content decreased whereas the sucrose and starch content increased with higher nitrogen levels. Cultivar and nitrogen application times had variable effects on the carbohydrate content of the bulbs. The optimum nitrogen level varied between 250 and 330 kg N ha−1and the best response was obtained when this nitrogen was applied in four equal applications throughout the season.

The relevance of using redox morphology and irreversible hardened material to classify soft and hard plinthic soil forms is often questioned as the paleoclimate of South Africa varied significantly. Since the origin of the Soil Classification System of South Africa in 1965, enough soils have been classified to test the ability of the classification system to distinguish between different entities and the relevance of the criteria applied to classify plinthic soils. Data of 203 plinthic profiles, classified by various workers, were plotted in phase diagrams and interpreted. Results indicate that the Soil Classification System of South Africa is user friendly and can be applied with success. Soft and hard plinthic soils occur in different phases of environmental conditions. Both the dominant factors of soil formation currently controlling the soil water regime and the soil properties expected to relate to the conditions of soil formation differ between the plinthic soil forms. The soils, therefore, formed under conditions similar or related to present day conditions.

Traditional N fertilizer guidelines were based on relationships between N application rates and yield response. These relationships varied greatly according to residual inorganic N content in the soil. The objective of this study was to establish inorganic N threshold values for maize production on a specific soil that could also be extrapolated to similar conditions. During the 1993/94 season a site situated near Viljoenskroon (27°10'S, 26°55'E) on a sandy Clovelly soil form was selected for a N×P fertilizer trial. Five N application rates (0, 40, 80, 120 and 180 kg N ha−1) and five P application rates (0, 10, 20, 40 and 60 kg P ha−1) were factorially combined and replicated thrice according to a randomised block design. Treatments were repeated for five consecutive seasons on the same plots. Over seasons 100% relative yield was obtained at an inorganic N threshold value of 74 kg N ha−1, 90% at 56 kg N ha−1, 80% at 45 kg N ha−1, 70% at 35 kg N ha−1, 60% at 27 kg N ha-1and 50% at 20 kg N ha−1, measured approximately six weeks after planting in the 0–600 mm soil layer for the total soil volume. Values for soil volumes over rows were very similar. An average N requirement factor of 2.3 kg N applied per kg N measured was calculated for soil volumes over rows, as well as for total soil volumes, in the top 600 mm soil according to linear models. Thus by taking the inorganic N threshold values, the N requirement factor and the measured inorganic N level in the soil into account, an actual N recommendation can be calculated in order to obtain a specific relative yield between 50 and 100%. Farmers should for economic reasons be encouraged to manage inorganic N levels in the soil to obtain a certain percentage of the expected yield. Inorganic N threshold values can be extrapolated to similar soils varying in inorganic N content.

This study was conducted with the objective of determining and quantifying the capacity of Lesotho's benchmark soils to retain applied P. The soils were incubated with a range of P levels for a period of 42 days, whereafter P retention was determined. During incubation the soils were kept at room temperature and at a water content of 85% of porosity. Indices used to describe the P retention characteristics of the soils were: (i) percent P retention across all the P application levels; (ii) phosphorus retention index (PRI), which is the slope of retained P versus applied P; and (iii) phosphorus retention capacity at an application level of 400 kg P ha−1(P400). Two P extraction procedures, namely the Bray-1 and Olsen methods, were used. According to the Bray method, P retention ranged from 7 to 87% across all the P application levels, and from 1 to 83% at P400. Values of PRI varied from −0.063 to 0.237. The soil series were grouped in the following descending order of the P retention indices: Fusi, Machache, Sefikeng and Thabana > Khabos, Sephula and Tumo > Berea, Leribe, Matela and Rama. In the case of the Olsen method, P retention ranged from 53 to 83% across all P application levels, and from 47 to 81% at P400. Values of PRI varied from 0.108 to 0.232. The grouping of the soil series according to these P retention indices was slightly different from the Bray method, viz.Fusi, Machache, Sefikeng and Thabana > Khabos, Matela and Tumo > Berea, Leribe, Rama and Sephula. The conclusion, therefore, is that the Fusi, Machache, Sefikeng and Thabana series exhibit high P retention characteristics and the Berea, Leribe and Rama series exhibit low P retention characteristics, while the remaining soil series, viz.Khabos, Matela, Sephula and Tumo, had moderate P retention characteristics. This grouping of the benchmark soil series can facilitate P fertilization recommendations in Lesotho.

Geographic associations of duplex soils, like the Estcourt form, and plinthic soils, like the Avalon and Westleigh forms, such as are found in the Eastern Free State, are recognised worldwide. These soils share redox morphology. In duplex soils, the redox morphology is dominated by a contrast in clay content between the A or E horizon and the B horizon, indicating that degradation of silicate clay minerals dominates. In plinthic soils, mottling due to redistributed Fe-Mn dominates. Both features can be the result of ferrolysis. In order to research this relationship, an Estcourt profile with degrading peds was sampled and analysed. Undisturbed peds of the B- horizon were subdivided for analysis, together with disturbed samples of the A, E and C horizons. The spatial variation of the colours, Fe, Mn, clay content and mineralogy in the profile indicates that both ferrolysis and redistribution of Fe-Mn are active during short periods of reduction in the E and interpedal pores of the B horizon. Degradation of the silicate clay minerals results in ped degradation visible as rounded ped tops and grey material interfingering the B horizon. In this landscape, in soils of the Avalon and Westleigh forms, degraded peds are also present in the lower soft plinthic B horizons and underlying material with the degree of ped degradation visibly associated with the degree of redistribution of Fe-Mn in mottles and nodules. In this landscape, a transitional horizon with properties of both plinthic and degrading structured B horizons covers large areas. This indicates that plinthic and/or E horizons can develop from structured horizons under the current climate.

South African analytical laboratories use different extractants for P in soils of which the most important are Bray 1, Ambic 1, Bray 2, Truog and Olsen. The fertilizer industry prefers to use Bray 1 extractions for fertilization recommendations and the ARC-Grain Crops Institute, Ambic 1 extractions for calibration research. Therefore it became necessary to establish relationships between the P extracted by the two methods in specific soils and if possible to relate differences in these relationships to soil properties. Soil samples collected to a depth of 150 mm from phosphorus fertilization trials at 12 localities were used for this study to ensure a large gradient in extractable P. The clay content of the 150 mm topsoil at these localities ranged between 8.4 and 47.0%. More than 3000 soil samples were analyzed for extractable P using both the Ambic 1 and Bray 1 methods. Relationships between Ambic 1 and Bray 1 extractable P for each locality and for 12 localities in total were determined. In addition, multiple and simple regression analyses were carried out to establish relationships between the slopes of Ambic 1 versus Bray 1 relationships and some soil properties of the different localities. Excellent relationships between Ambic 1 and Bray 1 extractable P with Revalues ranging from 92 to 98% have been established at nine localities. At the remaining three localities the Revalues ranged from 75 to 88%. Although the relationship between Ambic 1 and Bray 1 extractable P over all 12 localities was statistically significant with a R2-value of 91 %, the slopes of the different localities differed significantly from each other. Most of the variation between these slopes was attributed to exchangeable calcium (R2= 80 and 83%), degree of leaching based on clay content (R2= 80 and 71%), pH(KCI) (R2= 70 and 69%), effective cation exchange capacity (R2= 68 and 74%), degree of leaching based on silt-plus-clay content (R2= 68 and 63%), pH (H2O) (R2= 62 and 68%), Mn- oxides (R2= 60 and 72%) and the exchangeable Fe content (R2= 60 and 54%) of the soils. It is suggested that at least some of these soil properties should be considered in the development of an universal equation for the conversion of extractable P values from one to the other.

Proper utilization of shallow water tables could substantially reduce irrigation water requirements in large agricultural areas. The objective was to develop and test a computer model for predicting the contribution of shallow water tables to root water uptake. The model uses a finite difference solution of the Richards' continuity equation, based on thermodynamic principles of water flow, to determine equilibrium conditions of matric potential in the soil profile. The model was tested using data from a lysimeter trial in Bloemfontein (South Africa). The field trial was carried out for different crops (maize, wheat, groundnuts and peas) and soils (sandy and sandy loam) by artificially keeping the water table depths constant (1 and 1.5 m) in lysimeter containers. The finite difference water redistribution model predicted crop growth, volumetric soil water content as well as the contribution of water tables to crop water uptake well, provided the soil hydraulic properties used as inputs were calibrated.

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 eight sites as a result of cultivation and irrigation. At another three sites only one of organic C or total N changed significantly and at the remaining three sites cultivation and irrigation had no significant effect. Neither cultivation and irrigation history, nor soil properties provided a clear explanation for these contrasting findings. The gain of soil organic matter at five sites from Ramah and two sites from Vaalharts could possibly be attributed to enhanced biomass turnover on these irrigated lands

Min is bekend oor die effek van boerderypraktyke soos bewerking, bemesting en bekalking op die fosforfraksies in gronde van die somerreënvalgebied (25°-30°S; 24°–30°O) in Suid-Afrika. Die doelstellings van die studie was om die effek van gewasverbouing op die totale (Pt), anorganiese (Pa) en organiese (Po) fosforfraksies van hierdie gronde te kwantifiseer en om vas te stel hoe die Pa- en Po-fraksies, wat ten opsigte van planttoeganklik- heid verskil, deur gewasverbouing beïnvloed word. Siegs die effekte van gewasverbouing op Pt, Paen Poword hier gerapporteer. Bewerkte en nabygeleë onversteurde bogronde (0–200 mm) is by 50 lokaliteite, wat verspreid in die ondersoekgebied voorkom, versamel. 'n Opeenvolgende ekstraheringsprosedure is gebruik om die Pa(NaHCO3-Pa, NaOH-Pa, HCI-Paen residuele-Pa) en Po(NaHCO3-Po, NaOH-Poen residuele-Po) in die monsters te fraksioneer vir kolorimetriese bepaling. Daarna is Pt, Paen Podeur sommering van die relevante fraksies bereken. Die Ptvan die bewerkte gronde was in 76% van die gevalle groter as die van nabygeleë onversteurde gronde. Dit is omdat die toename in Paweens bemesting groter was as die afname in Poweens mineralisasie. Gevolglik is die bydrae van Patot Ptgroter as die van Potot Ptin die bewerkte gronde, wat nie die geval in die onversteurde gronde was nie. Die korrelasies van organiese C, totale N en totale S met Poin die bewerkte gronde is oorwegend lare as in die onversteurde gronde. In beide die onversteurde en die bewerkte gronde het Pt, Paen Pogoed onderling gekorreleer. Klimaatsparameters en grondeienskappe het 'n kleiner persentasie van die variasie in die drie fosforfraksies verklaar.

The effect of maintaining veld in three conditions, viz. poor, moderate and good, for 15 years, on the organic carbon (C) and total nitrogen (N) content (as measures of organic matter content) of a fine sandy loam soil was investigated. Composite soil samples were collected and analysed to a depth of 300 mm at 50-mm depth intervals from plots with the imposed veld conditions. The organic C and total N content of the soil under the three veld conditions differed significantly (P<0.05). Both variables declined as the veld condition degraded. Total N differed less than organic C. The largest differences were measured in the 0-50 mm layer and the smallest in the 250-300 mm layer. Losses of soil organic matter can be attributed to reduced phytomass production, increased soil erosion and change in soil climate. It is suggested that the loss of soil organic matter from degraded veld may inhibit veld recovery.