N-transfer from legume to grass when the two were intercropped in the field and the mechanisms of this transfer were studied. Studies involving either alfalfa-grasses or soybean-corn intercrops, were undertaken. Mycorrhizal and P-supplemented (to compensate for the lack of mycorrhizae) intercrops were compared. In these studies, the legume component of intercrops was labelled with $ sp{15}$N and any excess of the label was looked for in the associated grass plants. / There was no reversal of N transport at the legume-fungus interface. N-transfer from legume to grass must therefore, proceed via excretion of N by legume roots and subsequent uptake by grass roots. Soil microorganisms and proximity of plant root systems are important factors affecting N-transfer. Mycorrhizae enhance the transfer by increasing the absorptive efficiency of the receiver plants. / Mycorrhizal fungi enhance the competitiveness of the most mycotrophic component of the mixtures by either improving P uptake or the general nutrient balance of the plant. Mycorrhizal inoculation can decrease the level of P competition between corn and soybean by increasing the availability of P. / The observation that mycorrhizal plants differ in many regards from P-supplemented plants, emphasize the generally poor comprehension of the mycorrhizal symbiosis.

Thesis (doctoral)--Swedish University of Agricultural Sciences, 2000. / Includes bibliographical references.

Thesis (Ph.D.) - University of Queensland, 2003. / Includes bibliography.

Dissertation (PhD (Agric))--University of Stellenbosch, 2005. / Bibliography. Also available via the Internet.

No description available.

Grass clippings may be recycled onto nearby areas of turf as an alternative to disposal of clippings in landfills. In May, 1991, two field studies were initiated to evaluate grass clippings as a nitrogen (N) source for tall fescue (Festuca arundinacea, Schreb. Rebel’) turf, and to determine the maximum feasible loading rate of clippings onto tall fescue turf. Test plots in both studies received additional clippings from adjacent donor plots. In the first study, grass clippings alone were applied to 2.2 m² test plots at rates of 1 "plot equivalent harvest" (PEH), 2 PEH, and 4 PEH; where PEH is the amount of clippings harvested from 2.2 m² of donor plots. In the second study, test plots had clippings removed (Rem), clippings returned (Ret), Ret+1 PEH, and Ret+3 PEH. These four clipping treatments were arranged in a factorial combination with three fertilizer-N rates: low (73 kg N ha⁻¹ yr⁻¹), medium (147 kg N h⁻¹ yr⁻¹), and high (220 kg N h⁻¹ yr⁻¹). Donor plots received identical fertilizer-N applications as test plots. For both studies, N uptake generally showed a linear response to total N applied. Clipping-N produced roughly the same amount of growth and N uptake as the fertilizer-N, even as the rate of N uptake varied with time. In both studies, soil nitrate levels remained low until excessive clippings smothered the turf and caused a reduction in N uptake. Soil nitrate concentration tended to decrease with depth for all plots. Soil total Kjeldahl nitrogen (TKN) levels did not differ among treatments, suggesting that clipping-N was not being stored in the soil. At the end of both growing seasons, thatch accumulation was less than 12mm in all plots. In general, increased additions of clippings resulted in improved color, density, and growth. However, at the very high rates of added clippings, excess biomass caused turf thinning, chlorosis, and low growth. Thus, if grass clippings are applied at rates that are low enough to prevent these problems, they can be an effective N source for tall fescue. Furthermore, in order to dispose of the maximum amount of clippings, tall fescue should not be fertilized. / Master of Science

Research examining cover crop-corn systems focuses on corn planted into chemically desiccated cover crop residue using no-tillage practices. Two field studies including rye (Secale cereale L.), hairy vetch (Vicia villosa Roth), and rye+hairy vetch cover crops were established to determine the influence of cover crop management on N availability to corn (Zea mays L.). The presidedress soil nitrate test (PSNT) and the Nitrate Quick Test were utilized to determine their viability for making N fertilizer recommendations in cover cropcorn systems. Soil N concentrations peaked in the majority of cover crop treatments at the corn four to five leaf stage. Delaying cover crop desiccation from early flowering of rye (early) to early flowering of vetch (late) caused an increase in biomass accumulation in all cover crop treatments, but an increase in N accumulation only in vetch. Vetch provided more plant available N, less soil moisture conservation, and higher corn yields than cover crops including rye. Early desiccation enhanced plant available N, soil moisture, and corn yield. Mowing cover crop residue enhanced plant available N, soil moisture, and corn yield. The PSNT was a viable method of assessing corn N sufficiency in cover crop systems, but sampling to a depth of 15 cm was adequate. The Nitrate Quick Test was accurate in determining soil nitrate concentrations, but accuracy decreased as soil nitrate decreased. / Master of Science

Thesis (MScAgric)--University of Stellenbosch, 2000 / 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. / AFRIKAANSE OPSOMMING: In die Sandveld word bykans 7000ha aartappels onder besproeiing verbou. Die grootste deel van die aanplanting word as saadaartappels onder die Suid-Afrikaanse sertifiseringskema ingeskryf en verteenwoordig tussen 35 en 40% van Suid Afrika se saadproduksie. Die produksie van hoë kwaliteit knolle is dus van landsbelang. Somminge produksiefaktore van die area het egter 'n uiters negatiewe invloed op kwaliteit en het 'n meegaande verhoging van insetkostes tot gevolg. Faktore wat veral hieronder tel is die grond se lae klei inhoud «5%), lae organiese materiaal inhoud (%C = 0.2 tot 0.4%), lae pH asook hoë chloriede (400mg.kg-1 ) in die besproeiingswater. Hierdie faktore kan 'n invloed op die omsetting van stikstof en die opname daarvan hê. Daarmee saam kan dit 'n negatiewe effek op die omgewing hê indien loging van nitrate na ondergrondse waterbronne plaasvind wat sensitiewe bewaarareas kan bedreig. Weens die lae klei-inhoud van die gronde vind loging van verskeie voedingselemente plaas. Oorbesproeiing of hewige reënbuie kan tot gevolg hê dat bykans alle bemestingstowwe uit hierdie grond geloog word. Bestuurshulpmiddels is dus nodig om die besluitnemingsproses vir korrektiewe stappe te vergemaklik. Grond-, blaar- of hele plantontledings is moontlik maar is tydrowend en resultate daarvan kan soms te laat beskikbaar wees. Die ontwikkeling van metodes om die voedingselement inhoud van plante op die land te bepaal en die daarstelling van normes is dus noodsaaklik. 'n Metode wat reeds wyd deur ander navorsers met wisselende resultate beproef is, is die gebruik van nitraat inhoud van die petiool wat met 'n reflektoquant en nitraat strokies bepaal is. Om die sensitiwiteit van die metode te evalueer is twee proefpersele in die Sandveld gebruik. 'n Proef is by Wadrift geplant terwyl data ook van 'n logingsproef te Sandberg verkry is. By Wadrift is die kultivars Up-to-Date en Hertha by 4 N-behandelings (100, 200,300 en 400 kg N.ha-1) en vier herhalings vir twee seisoene aangeplant. By Sandberg is besproeiings teen drie toedingspeile met 3 sproeibemestingsverdelings vir een seisoen gedoen. In alle gevalle is van drupbesproeiing gebruik gemaak om akkurate plasing van water en bemestingstowwe oor die plantseisoen moontlik te maak. Petiool monsters is weekliks of tweeweekliks van alle behandelings geneem. Die eerste volwasse blaar is gemonster en daar is tussen 20 en 30 blare per perseel geneem. Tydens die eerste planting te Wadrift is nitraat-N in petioolsap, gemeet met nitraatstrokies en reflektoquant, vergelyk met nitraat-N van gedroogde petiole, gemeet met In outo-analiseerder. Hierdie waardes was goed gekorreleerd vir alle gevalle wat getoets is. Die outo-analiseerder se lesing was ±14.5 keer hoër as die petioolsap lesings weens die vog wat tydens droging verwyder is. Tydens die tweede seisoen is In beter korrelasie verkry waar petioolsap vir strokies asook vir die outo-analiseerder gebruik is. Petiool nitraat is deur hoë Ntoedieningspeile verhoog en die 400 kg N.ha-I behandeling het betekenisvol van die ander N-behandelings verskil en die laaste seisoen ook In betekenisvol hoër opbrengs gelewer. Wisselende resultate is t.o.v. soortlike gewig vir kultivars en stikstofbehandelings verkry. By Sandberg (1998) is gevind dat oorbesproeiing In betekenisvolle verlagende effek op die nitraat-N inhoud in die petioolsap het. Daar is getoon dat die optimum besproeiingspeil en frekwensie, veldwaterkapasiteit met twee sproeibemestings per week is. Uit die ondersoek is dit duidelik dat die nitraatstrokies in kombinasie met die reflektoquant In uiters bruikbare aanduiding van die nitraat-N in die petioolsap gee. Dit is egter moeilik om In norm of kritiese nitraat-N grens aan te dui, aangesien toestande tydens die groeiperiode op sandgrond en die groeistadium In beduidende effek op die nitraat-N het. Indien opbrengs as enigste doelwit gestel word, mag die 400 kg N.ha-I kurwe moontlik as aanduiding van 'n optimum nitraat-N norm vir die betrokke kultivars dien. Daar bestaan ook onduidelikheid of nitraat-N in die petioolsap wel In goeie refleksie van groei en opbrengspotensiaal is.

Dissertation (PhD)--Stellenbosch University, 2004. / ENGLISH ABSTRACT: Pine sawdust-shavings (Pinus spp.) is at present a very popular soilless substrate in South African greenhouses. Growers use fresh pine sawdust-shavings as a substrate, which is biologically highly unstable. The greenhouse industry is looking at alternative organic substrates such as coco peat, which already went through a decomposition process and is more stable. A biological inactive substrate such as sand was included to compare microorganism activity with organic substrates. The main objective of this study was to compare the growth, yield and quality of hydroponically grown tomatoes in response to different growth mediums in combination with nitrogen source, irrigation frequency, period of substrate use and liming. In general the drainage water pH declined with an increase in NlLt+-N in the nutrient solution. Low pH values in the drainage water, especially when coco peat was used, had a detrimental effect on marketable yield. The drainage water pH of pine sawdustshavings increased during the growing season when 100 % N03--N was used. Due to the higher cation exchange capacity of coco peat, the drainage water electrical conductivity tends to increase more rapidly than with pine sawdust-shavings, during conditions with high temperatures and when insufficient irrigation volumes per irrigation cycle is applied. As expected the drainage water N03--N content decreased as the NlLt+-N content increased in the nutrient solution. Pine sawdust-shavings recorded a much lower N03--N and NlLt+-N content than sand and coco peat and thus supports the hypothesis that microbiological activity is higher in pine sawdustshavings, especially in the second season of substrate use. Coco peat produced the highest number of marketable fruit and yield per plant, followed by pine sawdustshavings and sand in the first season of substrate use. The number of marketable fruit and yield decreased with an increase in NlLt+-N content in the nutrient solution during production in warmer, summer conditions. Contrary to these fmdings, production in cooler, winter conditions recorded high yields when only N03--N or 80% N03--N : 20% NRt +-N was applied. The unmarketable yield increased with an increase in NlLt+-N in the nutrient solution. Visual evaluations showed that blossom-end rot (BER) was the main contributor to unmarketable yield. Increasing levels ofN03--N as nitrogen source in the nutrient solution, reduced weight loss and increased the loss of fruit firmness of tomatoes during storage. Increasing levels of N03 --N also increased fruit pH and reduced total titratable acidity. Coco peat produced fruit with a higher pH than pine sawdust-shavings. An increase in irrigation frequency affected fruit firmness negatively when coco peat was used as substrate. Different irrigation and fertigation practices are needed for different growth mediums and management needs to be adapted according to the growing season (winter vs. summer). / AFRIKAANSE OPSOMMING: 'n Mengsel van dennesaagsels en -skaafsels (Pinus spp.) word tans deur Suid- Afrikaanse kweekbuisprodusente gebruik as grondlose groeimedium. Hierdie groeimedium word nie vooraf gekomposteer nie en is dus biologies onstabiel. Die kweekbuisindustrie ondersoek tans die gebruik van alternatiewe, gekomposteerde en stabiele organiese groeimediums soos kokosveen. 'n Biologies onaktiewe groeimedium soos sand is ook ingesluit om met organiese groeimediums te kan vergelyk. Die hoof doelwit van die studie was om plantontwikkeling, opbrengs en kwaliteit van hidroponies geproduseerde tamaties te evalueer in verskillende groeimediums en in kombinasie met stikstofbron-verhouding, periode van groeimedium gebruik, besproeiingsfrekwensie en bekalking. Oor die algemeen het die pH in die dreinaat gedurende die groeiseisoen toegeneem soos die NH/-N inhoud verhoog het in die voedingsoplossing. Lae pH waardes in die dreinaat, veral waar kokosveen gebruik was, het 'n nadelige effek op bemarkbare opbrengs gehad. Die pH in die dreinaat van dennesaagsels en -skaafsels het gedurende die groeiseisoen toegeneem met die gebruik van 100% NO)--N in die voedingsoplossing. Die elektriese geleiding in die dreinaat van kokosveen neem vinniger toe gedurende toestande waarin hoë temperature en onder besproeiing voorkom, as in dreinaat van dennesaagsels en -skaafsels. Die NO)--N inhoud in die dreinaat het soos verwag afgeneem soos die NRt+-N inhoud in die voedingsoplossing toegeneem het. 'n Baie laer NO)--N en NRt+-N inhoud is by dennesaagsels en -skaafsels aangeteken wat dus die hipotese ondersteun dat mikrobiologiese aktiwiteit, veral in die tweede seisoen van gebruik, hoër is in dennesaagsels en -skaafsels as in sand en kokosveen. Kokosveen het die hoogste aantal bemarkbare vrugte en massa per plant geproduseer, gevolg deur dennesaagsels en -skaafsels en sand. Die aantal bemarkbare vrugte en opbrengs het verlaag met 'n verhoging in NRt+-N in die voedingsoplossing gedurende warm, somer toestande. In teenstelling met vorige resultate is gevind dat 100% NO)-- N of 80% NO)--N : 20% NRt+-N hoë opbrengste gelewer het gedurende koeler, winter toestande. Die onbemarkbare opbrengs het verhoog met hoër NRt+-N vlakke. Visuele waarnemings het aangedui dat blom-end verrotting die grootste bydrae tot onbemarkbare opbrengs gelewer het. 'n Verhoging in NO)--N vlakke het massaverlies beperk en die verlies in fermheid verhoog gedurende opberging. Hoër NO)--N vlakke het ook die pH van vrugte verhoog en die totale titreerbare suur verlaag. Kokosveen het vrugte met 'n hoër pH as dennesaagsels en -skaafsels geproduseer. 'n Toename in besproeiingsfrekwensie het vrug fermheid negatief beïnvloed wanneer kokosveen as groeimedium gebruik was. Verskillende besproeiings- en voedingspraktyke word benodig vir verskillende groeimediums en bestuur van die groeimediums moet aangepas word by klimaatstoestande gedurende die spesifieke produksieseisoen.

No description available.