Diagnosis and correction of nutritional disorders of yams

• Ministry of Agriculture and Forestry, Tonga
• Department of Agriculture Livestock and Horticulture, Vanuatu
• National Agricultural Research Institute, Papua New Guinea

Yams are staple foodstuffs in many developing tropical countries. Along with their importance in the diet, they also have great cultural significance in many Pacific nations. In addition, they provide income for semi-subsistence farmers, and export revenue for some countries. In 1995 the Pacific crop was around 288,000 tonnes, or around 42 kg per capita, making it the Pacific's third most important food crop. However, yam production in many Pacific nations has been falling, as intensified farming of other crops has taken over from smallholder production. Imported foodstuffs, often less nutritious than yams but cheaper, are starting to replace yams in the diet .This has serious health implications for the population.

There is still a preference for yams among the islanders, but consumers find the prices too high compared with imported foodstuffs, while farmers are discouraged from growing more than their own needs by a feeling that the prices are too low for what is a labour-intensive crop. Part of the reason for the rather high cost of yam production is the low yields obtained by farmers for the effort invested. This is caused mainly by soil nutrient deficiencies that are reducing both growth and tuber production by the plants.

This project provided information to help in the diagnosis of nutritional disorders affecting yam plants, and sought to develop feasible options for improving crop nutrition in affected areas.

Institutions in Queensland, Papua New Guinea, Tonga and Vanuatu participated, each taking responsibility for a different component of the project, according to local strengths. The team initially established diagnostic criteria using solution culture, employing the techniques already used successfully for sweetpotato in Project CS1/1991/001. Team members then undertook field surveys of yam nutrition. The next task was to characterise yam nutrient requirements at selected sites, using pot experiments to determine which nutrients were inadequate for optimal growth of yams in particular soils.

Field trials in Tonga and PNG then tested the yield responses of yams to inorganic and organic soil treatments. In particular, a range of legumes was tested to establish which was most effective in improving nitrogen nutrition. There was also a socio-economic analysis of the various treatment options. Further field trials tested the effects of the plant's nutritional status on the quality of the yams harvested - the physical and storage characteristics as well as nutritional value and resistance to disease. This clarified the relationships between specific nutrient deficiencies, disease tolerance and root characteristics.

The final part of the project involved an overall assessment of yam production systems in terms of the costs incurred, the various constraints on production and the economic gains for typical growers. This information gave the team a basis for delivering to growers the best management techniques for dealing with the specific nutrient problems of each area.

In laboratory trials at the University of Queensland the researchers produced deficiency symptoms of most of the nutrient elements of interest and established critical concentration values for the macronutrients (nitrogen, phosphorus and potassium) in leaf tissue of two yam species. This was an essential prerequisite to the study of deficiencies in the field.
Surveys of yam-growing areas were undertaken in PNG, Tonga and Vanuatu. The results of the project work throughout the partner countries, using pot experiments to characterise yam nutrient requirements at their selected sites, suggested nitrogen (N), phosphorus (P) and potassium (K) (and sulfur (S) in some places) were major limiting nutrients-findings that agreed with earlier data gathered for sweet potato and taro.
Field trials in Tonga and PNG tested a range of legumes to see whether they could improve nitrogen nutrition of the yams. They focused on the use of legumes as green manures, or growing a legume crop as fallow to increase the nitrogen supply in the soil and to recycle phosphorus and potassium. Good results were recorded for Tonga, especially in the area of phosphorus fertiliser field trials and legume fallow species in rotation with yams. The green manure trials found that mucuna (velvet bean) was able to improve phosphorus nutrition of the yam crop.
In PNG there was found to be a growing tension between shorter fallows and/or competition for fertile land for cash crops, and a general decline in soil fertility with a resultant decline in yam production. But this was reassurance that the project was effectively targeting the right issues in PNG, and hence there was significant scope for impact.
The team introduced a novel agroforestry system into PNG, using the leguminous tree Gliricidia sepium (gliricidia) as improved fallow and live stake for yam at several locations. Gliricidia poles are planted on a 2 x 2 m grid, with each tree supporting four yam vines. Data from four trial sites harvested in 2003 showed no difference in yield attributable to the staking system, while NPK fertiliser increased yield by 50% (from 18.6 to 27.9 t/ha) on two Bogia District sites but had no effect at two Markham Valley sites.
Preliminary results indicate that, when regularly pruned, the trees do not compete to the detriment of the crop, and can reduce management inputs for weeding and staking. The system may alleviate many of the problems associated with shortened fallows, including weed intensity, decline in soil nutrient availability and organic matter content, and shortage of staking materials. Further benefits to farmers include softer soil texture, enabling the yams to be harvested more easily, and shading of workers at planting and harvest.
In Vanuatu, research at Vanuatu Agricultural Research and Training Centre (VARTC) initially revealed little response by yams to fertiliser in many of the field trials. Therefore the scientists decided to study more closely the growth and development of D. esculenta by investigating its rooting structures. Examination of the roots and recording of root length and depth yielded data that called to question the efficacy of placing fertiliser in the mound, leaving researchers to ask just where the fertiliser should be placed for optimum effect.
A small trial in Brisbane addressed these questions. Strontium was applied to trace root activity and determine the ability of roots to reach a certain location. The experiment showed that yam roots may reach horizontally for at least 5.5 metres and go to depths exceeding 40 cm. A significant observation was that fertiliser placed in the planting hole under the set of plants was rapidly accessed by the plant but that uptake was greater when fertiliser was placed in a ring around the set, within the mound. These finding have great promise for future trials.