Research, Development and Extension

Research, Development and Extension Needs for Overcoming Nutritional Limitations of Root Crops in the Pacific: a Workshop Summary
J.N. O'Sullivan1 and F.P.C. Blarney1
Abstract
Needs for continued work on the nutritional requirements of root crops were identified, particularly the diagnosis, prognosis and correetion of nutrition disorders within the farming systems in which the crops are produced. Further work is needed also on the extension of research results, focusing on identifying and correcting local problems using means acceptable to farmers. Workshop participants recognised that good progress had been made in identifying visible symptoms of nutrition disorders of sweet potato (Ipomoea batatas (L.) Lam) and laro (Colocasia escuiellfa (L.) Schott), and recommended that this information be made available to agriculturalists of the region as soon as possible. Other crops on which similar research should be conducted include yam (Dioscorea spp.) and Xanthosoma sagittiJolium (L.) Schott, and perhaps Alocasia macrorrhiza (L.) Schott and Cyrtosperma chamissonis (Schott) Merr. Overall, it was concluded that the identification and correction of nutrition disorders of root crops is an important factor contributing to sustainable farming systems in the Pacific.
THIS report summarises the views and observations of workshop participants on the priorities for future research, development and extension in root crop nutrition, particularly in relation to ACIAR Project 9101. The discussion was divided into four focus areas, each of which was introduced by a discussion leader followed by general contributions. (Discussion leaders were: Professor Jack Loneragan, Dr Malcolm Hazelman, Dr Steve Rogers and Mr Siva Halavatou; Dr Ramon de la Pefia chaired the session.) The importance of root crops in the region, as staple food sources, as major commodities for local commerce, and as export crops was reflected in the high priority given by Pacific country participants to the completion, distribution and implementation of the project findings. Knowledge of nutrient requirements and management of sweet potato (Ipomoea batatas (1..) Lam) and taro (C%casia esculenta (1..) Schott) still lags far behind that of crops such as maize (Zea mays 1..) and even cassava (Manihot esculenta Crantz), while the nutrition of yam (Dioscorea spp.) has received very little attention. This discussion was intended to highlight the priorities, preferred processes for and perceived barriers to further development of tropical root crop nutrition.
Diagnosis and Prognosis of Nutrition Disorders
Considerable progress has been made in the area of diagnosis of nutrition disorders in tara and sweet potato, particularly in the use of visible symptoms to characterise problems. The main refinement required here is to verify the symptoms observed on
I Department of Agriculture, The University of Queensland, Brisbane, Qld. 4072 Australia
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glasshouse-grown plants with those found in the field. Field validation is also required in the area of plant tissue analysis. Records of tissue nutrient concentrations at various times during the growth of high-yielding crops would provide useful additional data. Quick tests which can provide diagnostic answers in the field are highly desirable, especially for situations where laboratory analyses are not readily available. Leaf painting has been used successfully to demonstrate defieiencies of iron (Fe), manganese (Mn) and zinc (Zn) in sweet potato, and Fe in taro. Refinement is needed in the concentration of salts and wetting agents for elements other than Fe in taro. The area of sap testing has not been successfully developed in sweet potato or taro. A range of indicator test strips is available commercially for testing water for various nutrients. However, preliminary attempts to use these strips for analysis of taro and sweet potato sap were complicated by the heavy staining of the indicator strips by the sap. This has also been the experience with taro in Hawaii. Further work employing flocculants or the use of macerated plant tissue may overcome this problem. The use of fertilizer strips to confirm field diagnoses, or to differentiate between a small number of possible or co-existing deficiencies, is a simple and informative qualitative technique which could be used more widely. For example, phosphorus (P), sulfur (S) and nitrogen (N) deficiencies were identified as potential limitations in a commercial taro nursery on Tongatapu. To evaluate the importance of these disorders, parallel strips of urea and ammonium sulphate, crossed by a perpendicular strip of triple-superphosphate, were recommended. Colour-illustrated booklets containing diagnostic information for sweet potato and taro are being prepared, and are seen as a valuable resource for research and extension personnel throughout the region. The inclusion of diagnostic keys and instructions on the collection and preservation of leaf tissue for analysis, and on the use of leaf painting techniques and quick tests, where available, would greatly increase the utility of these publications. Where possible, information assisting the reader to distinguish between symptoms of nutrition disorders and those of plant pathogens and of herbicide injury would be helpful. Prognostic tests, allowing the prediction of yield responses to soil amendments, are far less advanced than are the diagnostic plant tests, and pose different problems in their development. Both plant and soil tests need to be calibrated against crop yields. The development of prognostic soil tests is in progress in Tonga, and may provide a basis for extension of procedures to other regions. The role of vesicular arbuscular mycorrhiza (VAM) may complicate prognosis for P, and possibly also for Zn and copper (Cu).
Correction of Nutrition Disorders
The addition of nutrients to the soil, whether as chemical fertilizer, or in organic material, is essential for sustainable agriculture. Currently, there is little use of fertilizers by South Pacific farmers on root crops, even when they are grown as a commercial crop, and when the same farmer may be willing to use fertilizers on other 'cash crops'. There is a perception that traditional crops require only traditional management. Where fertilizer recommendations have been made, they are often inappropriate NPK mixes, extrapolated from responses of other crops in other areas, or based on the availability of a particular commercial mix. There is a great need for properly calibrated soil tests to allow appropriate recommendations to be made. These must bc cheap and quick, for effective amendments to be made. Where micronutrient deficiencies exist, they need to be recognised to allow efficient management of macronutrients. Often locally available materials (steel cans for Fe, galvanised iron pieces for Zn) can provide a simple and cost-effective source of micronutrients. Some low analysis fertilizers contain useful amounts of micronutrients as contaminants. It is important for farm advisers to be aware of such additional sources of micronutrients. Organic materials, whether grown on the site or gathered from other areas, may be able to supply sufficient quantities of nutrients for the root crop, and may represent
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efficient use of local resources. The opportunity to take advantage of market premiums for organically-grown produce provides a further incentive to develop organic nutrient sources. In some areas, waste materials such as chicken manure and sugarcane pulp may be cheaply available, but supply of such materials will be localised. Some commercially available organic fertilizers represent poor value in terms of the nutrients they supply, compared with chcmical fertilizers. Bringing fresh plant material from other areas is labour-intensive, and the nutrient export from the source area should also be considered. Growth of organic mulching material on the crop site, either as a green manure rotation or intercropped planting, may increasc the availability of nutrients to the crop, but cannot add nutrients to the system other than N, and will not be effective in areas where, for example, a micronutrient deficiency remains uncorrected. Attention is needed to the area of fertilizer use efficiency. Identifying which nutrients are limiting is the first step. Within ACIAR Project 9101, progress is being made in identifying nutrient limitations of root crops on some major soil types in the region. Maximum benefit requires optimisation of the form of nutrient source, time of application and placement. Soils with high P-fixing capacity need to be recognised and managed appropriately. The widespread deficiency of P in Tongan soils was overlooked for many years, as fertilizer trials had never employed rates of P high enough to obtain a yield response. The probability of water stress or disease incidence reducing the ability of the crop to utilise added nutrients is an important economic consideration in many areas. Organic materials, in addition to their nutrient contributions, may be effective in increasing the availability of P fertilizers in high P-fixing soils, and in conserving soil moisture. Organic matter may also ameliorate problems relating to low soil pH. While trials have tended to compare the separate effects of organic and chemical soil amendments, there is scope for better integration of these resources. With the introduction of new management practices, environmental impacts such as increased nitrate or P levels in water supplies need to be anticipated and monitored, bearing in mind other sources of contamination such as sewage and detergents.
Other Crops
Yam is a crop of great cultural significance in the region, and a major food source in many areas. It is recognised by farmers to have a high nutrient requirement, being planted first in the cropping cycle. However, little work has been done on the nutritional requirements of yam. The value of a systema

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As fertilizer trials had never employed rates of P high enough to obtain a yield response. The probability of water stress. Or disease incidence reducing the ability of the crop to utilise added nutrients is an important economic consideration. In many areas. Organic materials in addition, to their, nutrient contributions