Masters Degrees (Agronomy)

Permanent URI for this collection

https://scholar.sun.ac.za/handle/10019.1/521

Browse

Now showing 1 - 8 of 8

Influence of a coal derived potassium humate on the yield and nutrient content of tomato (Lycopersicum esculentum L.) under field conditions
(Stellenbosch : Stellenbosch University, 1997) Lecuona, Antonius Ignatius; Combrink, N. J. J.; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
ENGLISH ABSTRACT: The exact composition and physical properties of humic substances has not been clearly defined. humic substances are ubiquitous and are found in all soils. humic substance s are classified into humic acids, fulvic acids and humin on the basis of their solubility in water. The charateristics of humic substances also vary according to the extarction method. This makes the comparison between various types even more difficult. The importance of humic substances in plant nutrition has also been identified. Various elements are bund to humic substances which improve the soil nutritional status. Pesticides and herbicides are degraded or absorbed by humic substances which netralises these chemicals in the soil. It has been found that the relation between humic substances and microbial growth can have positive effect on plant growth. Although humic substances are resistant to microbial breakdown, there are some carbon compounds associated with humates that were readily decomposable. The most significant effects of humic substances have been found on root growth. It seems that roots react more readily that leaves and stems to applications of humic and fulvic acids. When humic acids are applied in low concentrations, its effect is more hormonal than nutritional. The humic substances with lower molecular mass have higher hormone-like activity then the high molecular humic subtances. A coal derived potassium humate was used in this study to determine the yield, quality and elemntal status of tomatoes under field condition. In the field study no significant differences in yield, quality or the elemental status of fruits and leaves were found. This was primarily ascribed to the high soil nutrient status and carbon content of the soil. The rate of potassium release from a coal derived potassium humate, a commercial potassium chloride fertilizer and soil alone was also determined under varying conditions. The rate of potassium release through diffusion in a soil with added potassium humate and without was higher than soils added with a commercial availbale potassium chloride. When the leachate of soil water was measured, the soil with the commercial pottasium chloride had the highest rate of potassium release. The release of potassium was also measured varying the tempreature and total amount of water. At lower temperatures more potassium was released than the higher temperatures and the lower water volumes released more potassium than the higher water volumes at all tested temperatures.
Invloed van bemesting op die opbrengs en kwaliteit van uie (Allium cepa L.)
(Stellenbosch : University of Stellenbosch, 1999-03) Langenhoven, Petrus; Combrink, N. J. J.; University of Stellenbosch. Faculty of Agrisciences. Dept. of Agronomy.
The Western Cape is one of the most important onion producing regions in South Africa. It is well suited for the production of intermediate daylength onion cultivars. Annually about 2500 ha are planted with a yield of about 120 000 tons. Only 9 000 tons was exported in 1998 and of these 9 000 tons 4.9% was rejected due to poor quality. In South Africa there are no guidelines for the production of intermediate daylength onions. It is very important to have fertiliser guidelines, because optimal yields of good quality can be achieved with a good fertiliser program. Produce with exceptional quality can boost exports and at the same time stabilize local markets. However fertilisers are very expensive and could be damaging to the environment ifused incorrectly. To produce fertiliser guidelines N, P and K field trials were planted at three differen~ localities (Koue Bokkeveld, Stellenbosch and Caledon). The localities were chosen according to the difference in climate and soil texture, and these are important onion growing areas. The minimum and maximum temperature for the Koue Bokkeveld, Stellenbosch and Caledon was 11123, 12/25 and 13/27 °C respectively. At all the localities 3 nitrogen-, 4 phosphorus- and 4 potassium levels were used. Treatments were factorially arranged in a randomised block design, with two replicates. At Stellenbosch a N fertiliser trial was planted with four N levels and four N application methods. The treatments were factorially arranged in a completely randomised block design, with three replicates. N, P and K was applied as limestone ammonium nitrate, single superphosphate and potassium sulphate. Marketable and unmarketable bulbs were quantified at harvest. Weight loss during storage was determined over a six month period and storage disorders were evaluated. It was clear that high N levels, especially on soil with a high potential for releasing N, had a negative effect on yield and keeping quality. At Caledon a significant reduction in yield took place with the highest N level. High N levels also had a significant effect on weight loss at Stellenbosch and Caledon. The same trend with high N levels occurred in the N fertiliser trial. Weight loss was increased with the late application ofN in the growing season. In spite of the fact that the P levels were high (58 - 66 mg.kg-I) in the different soils, the onions reacted very well to phosphorus fertiliser. As a result of the higher P levels there was more foliage in the leaf canopy and the bulbs were larger. This P reaction only occurred in the Koue Bokkeveld and at Caledon. At Stellenbosch yield was not improved with P fertilisation and bulbs were of poor quality. The highest P level resulted in the greatest weight loss in storage. K had an effect where the K status and clay content of the soil was low. High potassium levels improved yield and keeping quality remarkably in the Koue Bokkeveld. Some interactions also occurred. The percentage unmarketable bulbs were influenced by a P and K interaction. An increase in double bulbs was responsible for the increase in the percentage unmarketable bulbs. At the lowest P level, higher K levels decreased the percentage unmarketable bulbs. Alarming proportions were reached when high K levels were combined with the highest P level. The P and K interaction also had an effect on the percentage sprouting. K did not have any effect' on sprouting at low P levels. A combination of the highest P level and the lowest K level produced the highest percentage sprouting. The interaction between N and P, like the P and K interaction had an effect on the percentage sprouting. The highest N level in combination with the highest P level increased the percentage sprouting significantly. The N, P and K field trials showed that the optimum N level is less than 130 kg N.ha-l on a soil with 16 % clay. On a sandy soil it can be as high as 160 kg N.ha•l The optimum P level varied from 30 to 130 kg P.ha-l Where a poor P reaction was observed the Ca and S status in the soil was low. It is possible that the drastic P reaction with superphosphate (10.5 % P, 20.3 % Ca, 12.1 % S) in the Koue Bokkeveld can be a result of the application of Ca and S. On the shale soil at Caledon, with a K status of 288 mg.kg-\ the optimum K level is less than 75 kg K.ha-l On the sandy soil of the Koue Bokkeveld, with a K status of 43 mg.kg•l , the optimum K level was 200 kg K.ha•l . The N fertilisation trial was done on a soil with an optimum N level of 115 kg N.ha-l (10 % clay). It was demonstrated that the standard N application method (40 % N with planting and the rest divided in three equal applications of 20 % N on 2, 4 and 7 weeks after planting) is still an acceptable practice. Follow-up trials are being done to determine the exact optimum level of fertilisation.
Die invloed van boor, trosvibrasie en relatiewe humiditeit van die trosatmosfeer op die bestuiwing van kweekhuistamaties (Lycopersicon esculentum L.)
(Stellenbosch : Stellenbosch University, 2003-03) Smit, Johannes Nicolaas; Combrink, N. J. J.; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
ENGLISH ABSTRACT: In South Africa tomato production in greenhouses normally occurs out of season when conditions are unfavourable for production and for the pollination process. Pollination is of special importance and the backbone of production. High temperatures contribute to the formation of abnormalities in flower morphology such as splitting of the anther cone and style exsertion. Low temperatures inhibit growth of the anther cone and therefore the style is uncovered. The movement of pollen from the anther to the style is inhibited by the higher style position in relation to the anther cone. Under humid conditions the pollen tends to stick to the anther surface. Another factor contributing to the problem is the banning of European bumble bees in South Africa. Alternative methods of pollen transport have to be found. Truss vibration, honey bees and plant growth regulators (PGR) are the most popular alternatives. Truss vibration and the use of PGR's are labour intensive and honey bees tend to get disorientated inside a plastic greenhouse. During the first part of this study pollen from plants, grown at four different B-levels (0.02; 0.16; 0.32 and 0.64 mg rl) was germinated in vitro. It was kept at different temperatures and periods before incubating on different growth media. No proof was found that pollen from B-deficient plants germinated poorly. Germination of pollen decreased significantly after one week storage. Deterioration of pollen viability could be lowered by storage at 5°C. At least 10% sucrose is needed in the growth medium for in vitro germination but addition ofB had no positive effect. In a second phase of the study, the influence of the mentioned B application rates were tested. The experiment was done in a glasshouse where temperatures were mechanically regulated (22°C and 10°C day/night). Seedlings of the greenhouse tomato cultivar, Abigail, were planted in acid washed river sand. Plants were grown with the main stem trellised vertically and the side shoots removed weekly. Only the first, third and fourth cluster were used for this part of the study. The uptake of all the essential nutrient elements, fruit set, the relation of larger to smaller fruits, physical and chemical quality and fruit shelf life were evaluated. The application of B at higher rates increased the uptake of Ca and decreased K-uptake. Fruit set, fruit development, fruit color and shelf life were the best at a B-Ievel of 0.16 mg r'. At this rate the abortion of flowers was the least. The same plants were used for the second part of the study, using the third cluster. The influence of the same four B-Ievels, the relative humidity (RH) of the truss atmosphere and truss vibrations were tested. The second cluster was covered with a plastic bag. Dry {
Optimising water use efficiency for crop production
(Stellenbosch : Stellenbosch University, 2003-03) Sedibe, Moosa Mahmood; Combrink, N. J. J.; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
ENGLISH ABSTRACT: Poor water management and poor water use efficiency (WUE) have been identified as one of the major problems experienced by vegetable growers in most of the developing countries, including South Africa. This poor management and poor utilization of water have led to a drastic decline in the quality and quantity of available water. In South Africa agriculture uses about 50% of available water. Increasing water demand for domestic, industrial and mining uses, may decrease agriculture's share to less than the current 50%, henceforth, better utilization of this resource is imperative. Selection of a good irrigation system can limit water loss considerably. Some irrigation systems have a potential to save more water than others do. Since irrigation systems affect the WUE of crops, care should be taken when selecting an irrigation system under conditions of limited water quantity. Ebb-and- Flood watering systems have been introduced for effective sub-irrigation and nutrient delivery within closed systems. Such a system was adapted in South Africa, to develop a vegetable production unit for use by families in rural communities, while saving substantial amounts of water. A need to further improve the WUE of this system was subsequently identified. Two studies were conducted at the experimental farm of the University of Stellenbosch (Department of Agronomy). The first trial was conducted under controlled conditions in a glasshouse, and the second under open field conditions. In the first trial, Beta vulgaris (Swiss chard) and Amaranthus spp. ('Imbuya') were grown in two root media; gravel and pumice. In addition, an 'Ebb-and-Flood' and a 'Constant level' system were used with nutrient solutions at two electrical (EC) conductivity levels 1.80 and 3.60 mS cm-I. The results of this (2x2x2x2) factorial experiment indicated that a combination of the 'Ebb-and-Flood' system with gravel as a root medium produced the best results at a low EC, when 'imbuya' was used. A high total WUE was found with 'imbuya', (7.35 g L-I) at EC 1.80 mS cmicompared to a relatively low WUE of 5. 90 g L-I when the 3.60 mS cm-I nutrient solution was used. In the second trial, 'Imbuya's' foliage dry mass, leaf area and WUE was evaluated under field conditions at the Stellenbosch University experimental farm, during the summer of2002. The experimental farm (33°55'S, 18°52'E) is situated in the cooler coastal wine grape-producing region of South Africa with a relatively high annual winter rainfall. This trial was conducted on an alluvial soil, with clay content of 25% and a pH of 5.9 (KC!). A closed 'Ebb-and-Flood' system was compared with two open field irrigation systems ('Drip' and 'Flood') using nutrient solutions at two electrical conductivity levels (1.80 and 3.60 mS cm-i) in all three cases. Foliage dry mass, leaf area as well as WUE was best with 'Drip' irrigation, when a nutrient solution with an electrical conductivity of 3.60 mS cm-i was used. In spite of the fact that additional ground water was available for the soil grown 'Drip' and 'Flood' treatments, the 'Ebb-and-Flood' system outperformed the 'Flood' treatment, especially when the nutrient solution with an EC of 3.6 mS cm-i was used. Insufficient root aeration in the flooded soil could have been a contributing factor. The fact that the 'Ebb-and-Flood' and 'Drip' systems gave the best results when the high EC solution was used to fertigate the plants, may indicate that the plants could have hardened due to the mild EC stress, better preparing them to adapt to the extreme heat that was experienced in the field.
A study of greenhouse production techniques for evergreen disas
(Stellenbosch : University of Stellenbosch, 2005-03) Pienaar, D.; Combrink, N. J. J.
The seven evergreen Disa species are indigenous to South Africa. These orchids grow on mountain ranges subject to winter rainfall and are found on stream banks, around waterfalls and in other damp areas. Although the Disa genus accommodates more than 130 species, by far the most commonly grown is Disa uniflora and hybrids stemming from this species. Disas have great potential as cut flowers and pot plants, but production techniques need to be further investigated since cultivation methods vary greatly between hobbyists. This study evaluated the effect of N-source, shading, root medium temperature, electrical conductivity (EC), irrigation method, foliar feeding at different plant growth stages and substrate on the growth of evergreen Disa hybrids in a controlled environment. Results showed that Disa plants can be classified as being ammonium tolerant. Disa unidiorosa performed best with 40% of the applied N in the NH4 form, while D.kewensis was more tolerant towards a higher level of NH4 and grew best at 60% NH4. Shading levels (56% and 69%) were compared and did not differ regarding the growth of plants. A cooled root medium was found to have a negative effect on root growth and a positive effect on leaf length. High EC levels produced heavier mother plants with a bigger root:shoot ratio and a bigger stem diameter. Biomass accumulation was the best in plants receiving ‘Drip’ irrigation, compared to ‘Ebb-and-Flood’ irrigation treatments. Plants in the vegetative reproducing stage were more susceptible to leaf abscission and new leaves formed at a low rate compared to small- and potential flowering plants. Where foliar feeding is concerned plants seemed to benefit more by the presence of NH4NO3 than urea. There were no significant differences in root development between substrates in the ‘hardening-off’ phase. ‘Hydroton’ (clay pebbles) was not suitable as substrate for the cultivation of Disa plants. The growth and flowering properties of plants were optimal with sphagnum moss and peat but were negatively affected when the pH of acid peat:sand mixtures were increased. More research is needed before Disas can be cultivated on a commercial scale, while the effect of the treatments on flowering properties has to be investigated.
Verbeterde metodes vir miniknol produksie by aartappels (Solanum tuberosum L.)
(Stellenbosch : Stellenbosch University, 2001-12) Barnard, Ruben; Combrink, N. J. J.; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
ENGLISH ABSTRACT: Potatoes is the fourth most important food source in the world. To provide in the ever increasing demand by the growing world population for this staple food, alternative ways will have to be found to produce a cheaper and a healthier end product. The tubers mainly consist of carbohydrates and starch, although 9% of the dry weight of the tubers consist of proteins. In its natural state the potato tuber is low in sodium and high in potassium and vitamin C. These proteins can provide in all the amino-acid requirements of humans and make it an outstanding balanced diet. The most important factor by which the production and quality of potatoes can be increased, is to provide better propagation material to producers. Minituber production is a relatively new method of producing better quality propagation material for producers. It is, however, very expensive because of the high capital investment needed. By producing more tubers per plant, or more tubers per square meter, the unit price per tuber may be lowered. The system commonly used for minituber production is where in vitro plants are planted in a greenhouse, usually in a vermiculite/peat mixture as medium. As in a hydroponic system, nutrients are added to the irrigation water. As possible strategy to increase the production of mini-tubers, the effect of nutrition was investigated. No significant differences were found in tuber production with small deviations in phosphorous and calcium nutrition. Researchers who reported significant differences in tuber production with nutrition, either corrected extreme shortages or induced it. By subjecting the root and tuber medium of potato plants to a pH-shock (nutrient solution with a pH of 3.5 for 10 hours), large amounts of tubers were initiated. A new production technique was developed where plants were grown in a hydroponic way without any root medium. In this way the right sized tubers could be removed without damaging the plant. Potato plants do not initiate tubers easily where stolons are not subjected to mechanical resistance. This problem was solved with the help of a pHshock, where significantly more tubers were formed per plant. The action of the pH-shock could be explained by a temporary inaccessibility of nutrients to the plant. Both calcium and magnesium absorpsion are hampered at a low pH, but calcium is also poorly translocated in the plant. Considering that a nutrient solution without calcium produced the same results as the pH-shock treatment in this study, one may make the assumption that an induced calcium shortage may be responsible for the low pH effect. The same effect may even be induced by giving the plants clean water for some time. Such a treatment was, however, not tested but should receive attention in further studies. The correct timing for a low pH-shock may need more research. A precise number of days after transplanting can not be prescribed since many factors such as light intensity, daylength and cultivar difference affect plant growth and tuber initiation. Environmental conditions, the physiological age and the biomass of the plant may affect the timing of the shock. A low pH-shock could not at any stage induce tubers in potato plants where unfavorable environmental conditions occurred. It seems as if the plants need to be ready for tuber initiation before the pH-shock can be successfully applied. By removing/harvesting the tubers at the required weight, the burden on the plant may be relieved enabling more tubers to form. It was experienced that the reproductive system of the potato plant is complex with a number of factors affecting it. The new production system has potential, but more developmental work is still needed.
Die waarde van petioolsap analise in die optimalisering van N-voeding by aartappels (Solanum tuberosum L.) in sandgronde
(Stellenbosch : Stellenbosch University, 2000-04) Brink, Pieter Paul; Combrink, N. J. J.; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
ENGLISH ABSTRACT: Approximately 7000 ha of potatoes are cultivated under irrigation. The greatest part of the planting is entered under the South African Certification Scheme as seed potatoes, representing between 35 and 40 % of seed production in South Africa. The production of high-quality tubers is, therefore, in the interest of the entire country. Some production factors in the area, however, have a very negative influence on quality with a resultant increase in input costs. These factors include mainly low clay content of the soil «5 %), low organic matter content (% C = 0,2 to 0,4 %), a low pH value as well as a high level of chlorides (400 mg.kg") in the irrigation water. Nitrogen conversion as well as uptake can be influenced by these factors. There may also be negative influences on the environment if leaching of nitrates to subterranean water sources is involved, which can pose a threat to sensitive protected areas. Leaching of various nutritional elements occurs as a result of the low clay content of the soils. Over-irrigation or intense thundershowers could lead to the leaching of nearly all of the fertilisers from these soils. Various management aids are therefore needed to facilitate the decision-making process for corrective steps. Soil, leaf or total-plant analyses are possible, but it is a time-consuming process and the results could become available too late to be of any use. Consequently, the development of methods to determine the nutritional element status of plants on the land and the establishment of norms are both essential. A method which has been tried on a wide scale with varying results is the use of the nitrate content of the petiole, which is determined by means of a reflectoquant and nitrate strips. To evaluate the sensitivity of the method two sites in the Sandveld have been selected. A trial planting has been established at Wadrift and information was also obtained from a leaching trial at Sandberg. The cultivars Up-to-Date and Hertha were established in four nitrogen treatments (100, 200, 300 and 400 kg N.ha-1 ), and repeated over two seasons. At Sandberg irrigation was applied at three application levels and three fertigation divisions for one season. Drip irrigation was applied in both cases to enable the accurate distribution of water and fertilisers during the planting season. Petiole samples were taken weekly or every second week in all the trials. The first mature leaf was sampled (about 20 to 30 leaves per plot). During the initial planting at Wadrift nitrate N in petiole sap (measured by means of nitrate strips and a reflectoquant) was compared to nitrate N of dried petioles measured with an auto-analyser. In all test cases a good correlation was found. The readings from the auto-analyser were about 14.5 times higher than the readings from the petiole sap as a result of moisture loss during the drying process. A better correlation was obtained during the second season when the petiole sap was used for strips as well as the auto-analyser. The petiole nitrate content was increased by high N application rates and the 400 kg N.ha-1 treatment showed significant differences from the other N treatments. A substantially higher yield was obtained during the last season. Varying results were obtained regarding specific gravity for cultivars and nitrogen treatments. At Sandberg (1998) over-irrigation lowered the nitrate content of the petiole sap significantly. It was found that the optimum irrigation level and frequency, is two fertigations a week and kept at field water capacity. From the trial it was clear that the use of nitrogen strips in combination with the reflectoquant gave a good indication of the nitrate-N content of the petiole sap. It is, however, difficult to lay down a norm or critical range, as conditions during the growth period on sandy soil as well as the growth stage have a significant influence on the nitrate-No If yield is the only objective the 400 kg N.ha-1 curve can probably be used as an optimum nitrate-N norm for the particular cultivars. It is also not clear whether the nitrate N content of the petiole sap gives a good reflection of growth and yield potential.
Witloof chicory (Cichorium intybus L. var. foliosum) as a vegetable crop in South Africa
(Stellenbosch : Stellenbosch University, 1999-12) Konig, Roman; Combrink, N. J. J.; Stellenbosch University. Faculty of AgriSciences. Dept. of Agronomy.
ENGLISH SUMMARY: Witloof chicory (Cichorium intybus L. var. foliosum) is a new vegetable crop to South Africa. It is a typical Belgian product and English literature on production of this crop is scarce. The first aim of this study is to compile a summary of relevant literature in the form of a manual for quick reference and practical use concerning chicon cultivation in hydroculture. Endogenous gibberellic acid (GA) is known to play a pivotal role in the vernalization process of plants requiring a cold induction for flowering. The second goal of this study is to clarify the effect that an exogenous GA application has on the vernalization process of chicory roots grown for witloof production. Five cultivars of witloof chicory (Cichorium intybus L.) were treated by applying GA as a leaf and a root treatment, each at a high and a low concentration. After forcing the treated roots, the chicons were evaluated according to international quality standardization. Chicon quality was then used as an arbitrary indicator for completed root-vernalization. Irrespective of concentration, the GA leaf treatment showed no significant difference in chicon quality when compared to no treatment. A GA root treatment, on the other hand, had a strong retarding effect on chicon quality, again irrespective of the hormone concentration applied. In a follow-up trial to which a cold treatment was added, no significant difference in chicon quality was found between GA treatments and a cold induction treatment. The control plants, however, produced significantly more high quality chicons when compared to those plants that received GA treatments. It was concluded that GA had no positive effect on advancing root-vernalization. Further investigation then showed that root-vernalization may already occur during the vegetative growth period on the field as a result of high irradiance and high temperatures. In this case exogenous GA application would have no further effect.

Browse

Browsing Masters Degrees (Agronomy) by browse.metadata.advisor "Combrink, N. J. J."

Results Per Page

Sort Options