摘要:

Four temperate grasses (Festuca arundinaceae, Festuca X Loliumhybrid,Lolium boucheanumandLolium multiflorum) were grown under irrigation at the Dohne Agricultural Centre. The object was to determine the monthly growth and nutritional value of the grasses. The effects of environmental features on the growth rate of the pastures were investigated. Polynomial regression of the environmental features against growth rate was used to create transformed values, which were then subjected to stepwise linear regression. In some cases it was possible to describe the growth rate, expressed as the increase in yield (g ha−1) per minute of day length, in terms of temperatures, minimum humidity and increasing day-length. The growth rate ofF. arundinaceaaereached a maximum at a mean average temperature of 18°C, and that ofL. boucheanumat 16°C.Festuca arundinaceae'sgrowth rate is depressed less by higher temperatures than that ofL. boucheanum.Seasonal growth patterns (t DM ha−1month−1) and fibre and N content are reported.

ISSN:0257-1862    

DOI:10.1080/02571862.1991.10634827

出版商:Taylor&Francis Group    

年代:1991

数据来源: Taylor

摘要:

A factorial crossing method was used to study the inheritance of and the relationship between characteristics which are related to drought resistance in grain sorghum seedlings. The potential for improving drought resistance in grain sorghum seedlings was verified by the high estimates of heritability and the predominance of additive genetic variation. Higher growth rate in grain sorghum seedlings not only provided a potential for increased root volume and hence the maintenance of water uptake, but also resulted in a relative decrease in moisture loss per leaf area.

ISSN:0257-1862    

DOI:10.1080/02571862.1991.10634828

出版商:Taylor&Francis Group    

年代:1991

数据来源: Taylor

摘要:

Three soybean (Glycine max(L.) Merr.) cultivars (Edgar, Duiker and Komati) were grown in pots together with morning glory (Ipomoea purpurea(L.) Roth.). Plants were exposed to three levels of bendioxide (0, 0.96 and 1.44 kg a.i. ha−1) in combination with four levels of 2,4-DB (0, 22, 34 and 45 g a.i. ha−1) to gauge the most effective herbicidal combination to control morning glory without affecting the growth of different soybean cultivars. All soybean cultivars responded similarly to the application of herbicide. The combination of both herbicides at the highest levels significantly reduced soybean leaf area and mass one week after application. Bendioxide applied alone at 1.44 kg ha−1, and the combination of bendioxide (0.96 kg ha−1) with 2,4-DB (22 g ha−1) reduced soybean leaf mass two weeks after application. The recovery of soybeans from all treatments after three weeks following application was rapid and complete. The addition of 2,4-DB to bendioxide enhanced the lethality of the herbicide to morning glory. Combinations of 0.96–1.44 kg ha−1bendioxide with 34–45 g ha−12,4-DB gave the best control of morning glory.

ISSN:0257-1862    

DOI:10.1080/02571862.1991.10634829

出版商:Taylor&Francis Group    

年代:1991

数据来源: Taylor

摘要:

Litter and topsoil were sampled in plantations ofPinus elliottiiandPinus taedaand in adjacent indigenous forest in three different locations in the southern Cape, to study some of the effects on the soil of the establishment of pine plantations in this region. Litter from the forest was less acid and contained less aluminium and more calcium, magnesium and potassium than litter from the plantations. At all three sites, topsoil under pines contained more organic aluminium and lower concentrations of basic cations. The pH was lower and aluminium levels were higher in the soil under pines at one of the sites. This could result in decreased fertility in later rotations. It is suggested that changes in the cycling process of mineral elements and in the decomposition dynamics of litter after the establishment of pine plantations have caused these changes in the soil.

ISSN:0257-1862    

DOI:10.1080/02571862.1991.10634830

出版商:Taylor&Francis Group    

年代:1991

数据来源: Taylor

摘要:

In a field trial conducted in a vineyard on a low-lying hydromorphic soil, it was established that ridging increased the depth from the soil surface to the water table. Ridging did not lower the level of the water table, but increased the soil depth above the water table by the same amount as the thickness of the additional topsoil layer. A good correlation was found between the depth to the water table and the water content of the overlying soil. The distribution of the soil water content with depth varied from a sigmoidal tendency to a virtually linear one as the depth to the water table increased. Ridges of varying width and height did not differ significantly in their ability to improve the internal drainage of the soil. However, during late summer, 600-mm high ridges lost more water through evaporation than 400-mm high ridges of the same width. The problem of higher evaporation caused by the increased soil surface of the higher ridges was overcome by applying supplementary irrigation. Unstable soil that landed on the ridges during the construction process resulted in unnecessary losses of irrigation water due to runoff.

ISSN:0257-1862    

DOI:10.1080/02571862.1991.10634831

出版商:Taylor&Francis Group    

年代:1991

数据来源: Taylor

摘要:

Waterlogged soil conditions during early spring can result in poor grapevine performance. In this study the effect of ridging as a method of soil preparation on the depth to the water table, soil air composition and vine reaction, as indicated by leaf water potential and stomatal resistance, was quantified. The soil and plant parameters were monitored throughout an entire growing season on 600-mm high ridges and flat control plots. Soil air analyses showed that under waterlogged conditions, oxygen in the soil air is to a large extent replaced by carbon dioxide. A linear relationship was found between oxygen and carbon dioxide concentrations, except during the phase of aeration directly following a period of continuous waterlogging. The production of ethylene, which is believed to be detrimental to vine growth, could not readily be traced under prevailing anoxic conditions. Ridging increased the depth to the water table, which resulted in higher oxygen and lower carbon dioxide concentrations in the soil air. Soil oxygen levels of below 16% may have resulted in the decrease in leaf water potential and increase in stomatal resistance noted for the control plots. These conditions of physiological stress prevailed until the water table dropped to a level below 600 mm. It is concluded that ridging, an alternative to deep soil preparation on waterlogged soils, produced a favourable environment for root growth on this marginal soil based on improved internal drainage and thus soil aeration.

ISSN:0257-1862    

DOI:10.1080/02571862.1991.10634832

出版商:Taylor&Francis Group    

年代:1991

数据来源: Taylor

摘要:

To avoid unnecessary crop failures it would be valuable to have a reliable threshold value of the root zone water content for wheat at planting (θrp) below which the risk is too high to plant. An attempt has been made to estimate this value for annual wheat on the Glen/Shorrocks crop ecotope. A simple procedure, based on measured values, was developed for predicting runoff, deep percolation and evaporation (and thus water storage) during the fallow season. This made it possible to estimateθrpvalues for each season. A simple stress-index yield prediction model, in whichθrpwas one of the inputs, was developed and calibrated against measured yields from this crop ecotope over five years. The calibrated model was then used withactualweather data (1922–1987) to predict yields for 66 seasons. Results showed that forθrpvalues of 300 mm or less, 320 mm and 340 mm or more, the chances of attaining a minimum target yield of 1 t ha−1are<50%, 62% and 74% or greater, respectively. Based on these results, 320 mm is considered to be a suitable threshold value ofθrpfor wheat on the Glen/Shorrocks crop ecotope with an effective soil depth of 1 800 mm. This value corresponds with 130 mm of potentially extractable water.

ISSN:0257-1862    

DOI:10.1080/02571862.1991.10634833

出版商:Taylor&Francis Group    

年代:1991

数据来源: Taylor

摘要:

Lime and phosphorus (P) requirements for the growth of temperate pasture species on acid, well-drained soils in the Natal Midlands were evaluated in factorial field trials. Species studied were white clover (Trifolium repens), tall fescue (Festuca arundinacea) and Italian ryegrass (Lolium multiflorum). All three species exhibited large yield responses to lime and P. In addition, evidence for both positive and negative lime-P interactions was present in dry matter production data. Positive interactions, exhibited during the establishment phase of growth of white clover and Italian ryegrass, appeared to be restricted to conditions of severe P deficiency. Yield data of all species contained evidence of negative interactions, which occurred at more favourable levels of P supply. Negative interactions reflect a decrease in response to P with increasing lime supply (to salt pH values of approximately 5.5). The latter interactions are of major importance from a practical standpoint since they imply a‘trade-off’between lime and P in pasture production. Consideration is given to the role of soluble Al in the development of lime-P interactions, as well as to factors possibly responsible for a noted lime-induced yield depression in the case of white clover.

ISSN:0257-1862    

DOI:10.1080/02571862.1991.10634834

出版商:Taylor&Francis Group    

年代:1991

数据来源: Taylor

摘要:

There is no single quantitative definition of a drought; what constitutes a drought depends both on water needs and on storage capabilities. In this paper afamilyof models is proposed which is sufficiently flexible to cover a range of different situations; the user may select some of the parameters to suit his particular application. The input to the model is a sequence of rainfall values which in turn may be generated by a stochastic model, thus enabling the simulation of a long sequence of rainfall data which allow the user to study the distribution of droughts of any given duration or severity. Several of illustrative examples are given.

ISSN:0257-1862    

DOI:10.1080/02571862.1991.10634835

出版商:Taylor&Francis Group    

年代:1991

数据来源: Taylor

摘要:

Die verloop van hidrolise is in vier gronde met verskillende urease-aktiwiteite, afkomstig van die sentrale besproeiingsgebiede, gemonitor na toediening van ureum teen die ekwivalent van 100 kg N ha−1. Gelyktydig met die toediening van ureum is sewe belowende inhibeerders ook toegedien teen 'n konsentrasie van 50 mg kg−1. Tydens inkubasie by 30°C is die ureumstikstof op gereelde tydsintervalle gemeet totdat die hidrolise van ureum voltooi was. Die data is gebruik om die urease-aktiwiteite van die verskillende behandelings by elke grond te bereken. Hidrokinoon het gemiddeld die grootste verlaging in urease-aktiwiteite van die vier gronde bewerkstellig (94%), gevolg deur p-benzokinoon (93%), fenielfosfordiamidaat (85%), katekol (75%), fenielkwikasetaat (70%), tioureum (30%) en ammoniumtiosulfaat (19%). Daardeur is die tydsduur vir volledige hidrolise van ureum aansienlik verleng.

ISSN:0257-1862    

DOI:10.1080/02571862.1991.10634836

出版商:Taylor&Francis Group    

年代:1991

数据来源: Taylor