Die waarde van petioolsap analise in die optimalisering van N-voeding by aartappels (Solanum tuberosum L.) in sandgronde

[摘要] ENGLISH ABSTRACT:Approximately 7000 ha of potatoes are cultivated under irrigation. The greatest part ofthe 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 ofhigh-quality tubers is, therefore, in the interest of the entire country. Some productionfactors in the area, however, have a very negative influence on quality with a resultantincrease 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 levelof chlorides (400 mg.kg) in the irrigation water. Nitrogen conversion as well as uptakecan be influenced by these factors. There may also be negative influences on theenvironment if leaching of nitrates to subterranean water sources is involved, which canpose a threat to sensitive protected areas. Leaching of various nutritional elements occursas a result of the low clay content of the soils. Over-irrigation or intense thundershowerscould lead to the leaching of nearly all of the fertilisers from these soils. Variousmanagement aids are therefore needed to facilitate the decision-making process forcorrective steps. Soil, leaf or total-plant analyses are possible, but it is a time-consumingprocess 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 theland and the establishment of norms are both essential. A method which has been tried ona wide scale with varying results is the use of the nitrate content of the petiole, which isdetermined by means of a reflectoquant and nitrate strips. To evaluate the sensitivity ofthe method two sites in the Sandveld have been selected. A trial planting has beenestablished at Wadrift and information was also obtained from a leaching trial atSandberg. The cultivars Up-to-Date and Hertha were established in four nitrogentreatments (100, 200, 300 and 400 kg N.ha-1), and repeated over two seasons. AtSandberg irrigation was applied at three application levels and three fertigation divisionsfor one season. Drip irrigation was applied in both cases to enable the accuratedistribution of water and fertilisers during the planting season. Petiole samples weretaken 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 petiolesap (measured by means of nitrate strips and a reflectoquant) was compared to nitrate Nof dried petioles measured with an auto-analyser. In all test cases a good correlation wasfound. The readings from the auto-analyser were about 14.5 times higher than thereadings from the petiole sap as a result of moisture loss during the drying process. Abetter correlation was obtained during the second season when the petiole sap was usedfor strips as well as the auto-analyser. The petiole nitrate content was increased by high Napplication rates and the 400 kg N.ha-1 treatment showed significant differences from theother N treatments. A substantially higher yield was obtained during the last season.Varying results were obtained regarding specific gravity for cultivars and nitrogentreatments. At Sandberg (1998) over-irrigation lowered the nitrate content of the petiolesap significantly. It was found that the optimum irrigation level and frequency, is twofertigations a week and kept at field water capacity. From the trial it was clear that theuse of nitrogen strips in combination with the reflectoquant gave a good indication of thenitrate-N content of the petiole sap. It is, however, difficult to lay down a norm or criticalrange, as conditions during the growth period on sandy soil as well as the growth stagehave a significant influence on the nitrate-No If yield is the only objective the 400 kgN.ha-1 curve can probably be used as an optimum nitrate-N norm for the particularcultivars. It is also not clear whether the nitrate N content of the petiole sap gives a goodreflection of growth and yield potential.

[发布机构] Stellenbosch University