Melteras, Marie-Vianney (2007). SOIL FERTILITY as a FACTOR AFFECTING the PRODUCTION of YAMS (Dioscorea spp.) in VANUATU MPhil Thesis, School of Land and Food Sciences, University of Queensland.

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Author Melteras, Marie-Vianney
School, Centre or Institute School of Land and Food Sciences
Institution University of Queensland
Publication date 2007
Thesis type MPhil Thesis
Supervisor Dr Jane O'Sullivan
Subjects 0701 Agriculture, Land and Farm Management
Abstract/Summary Yam (Dioscoreae spp) is one of the major root crops cultivated in tropical regions, and particularly in the Pacific islands. Production of yam normally involves clearing a bush fallow and planting yam first followed by other root crops including sweet potato and cassava. Fallowing is the only practice used by farmers to sustain the production and soil fertility. The recent increase in population may impact on the soil fertility as the fallow periods have been reduced and farmers claim a decrease in yam production. The effects of fertilizer on yam production have been studied in West Africa and Caribbean but little has been done in the Pacific Region. Understanding the plant response to fertilizer would help farmers to improve production under more intensive farming conditions. The objectives of this study were: (i) to determine nutrient deficiencies in selected sites with pot experiments and (ii) use the result as a guide to test the response of yam to fertilizer application in the field. Pot and field experiments were conducted in Vanuatu. Leaf and soil samples were sent to the University of Queensland for chemical analysis. Soil from seven sites covering four different islands were collected for glasshouse experiments using sweet corn as the test plant. Nutrient omission pot experiments indicated that all seven soils were deficient in phosphorus (P) and bimass yields in six soils were < 50 % of the “all nutrients” control treatment (“All”) when no P was added. Nitrogen (N) was limiting to growth in four soils with yield in minus N treatment at between 54 % and 78 % of “All”. Potassium (K) was deficient at two sites, with yield in minus K treatment yields of approximately 70 % of “All”. Sulphur (S) was found to be limiting to growth at two sites and iron (Fe) deficiency was detected at one site. Single nutrient rate pot trials using six levels of supply of each deficient element were used to confirm the deficiency and estimated approximate fertilizer rates for field trials. Fourteen pot rate trials were conducted. In nine of these, the highest yield was obtained at the highest fertilizer rate, but in all except two cases, the next highest was not significantly lower, indicating that a response plateau had been reached. As most of the soils were known to be strongly P-fixing, three P-rate trials compared the response with fertilizer mixed throughout the pot to that were it was banded in a 2cm layer half-way down the pot. Banding resulted in a greater response at low fertilizer rates, but did not alter the optimum rate. Optimum N rates varied from 25 to 400 kg N/ha, P rates from 120 to 600 kg P/ha, and K from 150 to 200 kg/ha. The S rate trial did not detect a significant response to added sulphur. Field trials were conducted at six sites over three yam growing seasons with three yams species commonly grown in Vanuatu: D. alata, D. esculenta, and D. rotundata to test their response to fertilizer. Despite the deficiency found in pot experiments at each sites, the results of field experiments were not conclusive. Most of the trials showed no significant responses of yam to the application of fertilizer. The few significant results recorded did not generally follow a trend. There was one exception, in which a P rate trial at Valeteruru showed a significant and consistent response, with yield approximately 60 % higher with 240 kg P/ha than without P. Leaf analyses were conducted to assist evaluation of field trials at four sites. At Tagabe, all nutrients suspected of potential deficiency (N, P, K and S) were found to be adequately supplied. At Teouma N and K concentrations were below adequate range. At Valeteruru, the nutrient concentrations in D. nummularia cv Maro were in the adequate range. However, in the D. alata experiment, the concentrations were below adequate range. At Malo, only P was found to be deficient on the basis of pot experiments, but N, P and K were in the deficient range in leaves. In all sites except one, no change in nutrient concentrations was found in responses to fertilizer treatments. The exception was Tagabe, where N concentrations in both leaves and tubers of D. rotundata and D. alata increased with N fertilizer rate. In response to the poor field trial results, an experiment was conducted in the final season with D. esculenta to study the development of the plant root system. Ten plants were excavated each month from 2.5 to 12 months after planting. It indicated that yam developed an initially shallow root system which radiated over a wide area during the early stage of development. The root system of D. esculenta developed on the surface at 7-10 cm, descending to about 30 cm only after reaching near maximum length. Two months after planting, the shallow roots reached an average length of 123 cm. The longest root recorded was 436 cm at four months. At this time, 70 % of plants had at last one root exceeding 3 m in length. Tubers also appeared first at four months. Conducting field experiments with yam presents many challenges. Variability among plants is high, while the wide plant spacing (one per m2), limited size of farmers’ fields and very limited availability of uniform planting material necessitated small plot sizes, in most cases with 12 datum plants per plot. Variations in weight, moisture content and stage of dormancy of planting setts all influence plant vigour and yield. Steps were taken to unify setts size, stratify setts among plots, force breaking of dormancy, and reserve planting material from a single known source for next season’s trial. The placement of fertilizer was also varied in response to observations on early root growth pattern. However, these measures did not improve field trial results measurably. Given the nutrient deficiency status indicated by nutrient omission pot trials, the nonsignificant responses to fertilizer application might be the results of either: 1) poor access by roots to the nutrients applied as the spot placement was not placed where the feeder roots proliferate or 2) proliferation of roots across guard rows to access higher nutrient treatments in adjoining plots. A root development study showed that roots rapidly spread beyond the planting mound, and that most branching occurs some distance from the mound. The method of fertilizer application after crop establishment, by burying it around the base of mounds, may damage many primary roots, which lie close to the surface at that point. For future experiments, banding midway between yam rows may be preferable. To separate treatments adequately, it is suggest that installation of root barriers around the plots might be necessary. An alternative would be to align the treatments in the field from low to high treatments instead of using a randomized design. From there, even though the root system would cross to the next plot, it won’t get much advantage as the rate would be not much different from its own plot. The field experiment which showed the greatest response to P application was laid out in this way. While this study did not advance our knowledge as far as hoped with respect to nutritional limitations to yam production in Vanuatu, valuable information was generated on the morphology of yam and the requirements for yam field trials, which will assist further studies in this area.

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Created: Fri, 21 Nov 2008, 15:26:41 EST