In Asian countries where fertilizer is relatively cheap in relation to the value of the crop, farmers tend to apply too much fertilizer. This is particularly true of vegetables, since they are a high-value crop. For the farmer, this represents a type of insurance against crop losses due to nutrient deficiencies. However, overuse of fertilizer is not only wasteful, but is damaging to both crop quality and the environment.
Furthermore, vegetables tend to be a short-term crop which are grown as part of a multiple cropping system. In intensive tropical systems with vegetables, and also in temperate systems where protective structures are used, it is common for a number of crops to be grown each year on the same piece of land.
This means that the nutrient supply of each crop is affected by the fertilizer which was applied to previous crops. Efficient fertilizer use means matching the supply of nutrients to those required by the crop. Rather than managing the nutrients for separate successive crops, vegetable production involves managing the nutrient supply and requirements of the total cropping system.
There seem to be two main patterns of vegetable production in Asia. One is intensive vegetable production, in areas either specially suited to vegetables or near large population centers. The second is a vegetable/cereal cropping system, in which vegetables are usually the subsidiary crop. Combined vegetable/cereal systems can in turn be divided into vegetable production on raised beds in paddy fields after rice, and vegetables grown as an intercrop or relay crop with corn, wheat etc. in rainfed uplands.
Sustainable vegetable production for each of these two systems will take a different course, but in each case there must be a positive nutrient balance. Since N fertilizer rates for vegetables are generally fairly high, there is usually a positive N balance in cereal/vegetable systems. However, this may not be true of all nutrients. For example, boron deficiency is fairly common in the vegetable production area around Chiang Mai, Thailand.
Intensive vegetable production involves a large volume of output _ 30 mt/ha/crop of Chinese cabbage would be quite a normal yield. Vegetable-producing soils are constantly being mined of nutrients with every harvest. This massive removal must be compensated by a large volume of inputs, either of chemical fertilizer, organic materials, or a mixture of both. While farmers attempt to compensate for nutrients removed by applying the same level of nutrients in fertilizer, it is difficult for them to achieve a proper nutrient balance, especially if they use only chemical fertilizers.
In many other countries, there is a gap between cereal and vegetable yields from farmers' fields and those from research stations. Often in the case of cereals, the gap is caused by underuse of fertilizer, while in the case of vegetables, it may be due to fertilizer overuse. Many soils used for vegetables show accumulation of nutrients over time, rather than depletion. Farmers in Sri lanka even try to reclaim the soil of vegetable fields by removing the topsoil, and adding fresh soil brought in from uncultivated land.
Soil testing is necessary to identify nutrient deficiencies. There is a need for an expanded soil testing service, and for simple and quick testing procedures. In areas where rural transport is difficult, a mobile service which collected samples for testing might be very useful.
Applied compost and other organic materials are known to have a beneficial effect on soil productivity, and on the control of pests and diseases. Vegetables tend to receive higher applications of organic fertilizer than any other type of crop, the main limits being the cost and availability of compost.
Not only do organic materials benefit the soil and crop but, as one paper presentation pointed out, composting should be seen as part of solid waste management. In most industrialized countries, it is becoming increasingly difficult and expensive to find landfill sites. In the United States in 1993, only 3.1% of municipal solid wastes were composted. The majority of wastes (62.3%) were used as land fill, with the remainder being incinerated or recycled. Raising the proportion of composted wastes would make a large savings in landfill costs. However, effective recycling of municipal wastes needs some preliminary sorting at a household level, so that wastes are already sorted into organic materials, plastic etc. at the point of collection.
The point was also made that organic materials do not necessarily always give higher yields than synthetic ones. Experiments with green pepper in the United States showed that yields were higher when polyethylene mulch was used, compared to organic mulches. However, the fruit were larger when they were grown with organic mulch. Interestingly, the same effect was seen in the following crop of squash, in that the squash were larger in the organic mulch plot, although the overall yield was not any higher. While organic mulches can sometimes give yields comparable to those from plastic ones, this depends on the type and rate of organic mulch used, its effect on soil microorganisms, and the type of vegetable being grown.
In some long-term fertilizer experiments at five sites in Taiwan where vegetables had been grown for at least ten years, chemical fertilizers have been applied according to conventional recommendations, together with a small percentage of organic fertilizer. Crops showed a marked response to fertilization, to the extent that if fertilizer was discontinued for only one crop, there was a fall in yield of 20-35%. There was also a fall in the quality of the vegetables. Since large quantities of irrigation water were used in this system, good fertilizer management depended on suitable water management, to minimize leaching.
There was a linear and highly significant relationship between the plant uptake of N, P and K, and the crop yield. There was also a linear and significant relationship between the N content of the plant, and the eventual yield. However, there was a negative relationship between the yield and the P content of the plant, while there was no significant relationship between yield and the plant K content. This perhaps reflects the high levels of available P and K in these soils, all of which had been used for long-term vegetable production.
There was also a correlation between N levels and nitrate levels in the plant. Thus, any reduction in nitrogen applications to reduce nitrate levels tended to result in a fall in crop yield.
There is a considerable difference between different types of vegetable, in terms of their efficiency of nitrogen use. Data presented at the seminar showed that spinach and leek, for example, both use nitrogen inefficiently. Such crops may have nitrogen leaching losses of more than 200 kg N/ha, compared to losses of 30-40 kg N/ha for cabbage. Reducing the rate of losses for crops such as leek would mean a great increase in fertilizer efficiency.
A further interesting point made is that some crop combinations make more efficient use of fertilizer than others. For example, combinations of rice-green pepper and rice-tomato are extremely inefficient in their use of N, with an N recovery rate of only 27%. In contrast, a combination of rice-mungbean-corn showed highly efficient N use, with a recovery rate of 56%. Another efficient combination, in terms of N use, was rice-garlic-sweet potato.
This shows the importance of treating the fertilizer requirements of the total cropping system, rather than of the separate crop components. The relative fertilizer efficiency of crop combinations needs to be taken into account when fertilizer recommendations are being made.
Appropriate crop combinations sometimes allow a vegetable crop to be grown in a cereal-based system without any additional fertilizer. In India before the 1960s, when indigenous cereal varieties were grown, little fertilizer was used, and legume pulses were grown in rotation or as intercrops. With the introduction of high-yielding varieties and chemical fertilizers, cereals began to dominate the cropping system at the expense of legumes. Nowadays, there is a return to something like the traditional pattern, since short-duration varieties make it possible to grow mungbean as an intercrop or as an interim crop. Mungbean yields are about 0.2-1.0 mt/ha, and the production potential of the following crop is sustained or even improved, especially if the mungbean straw is incorporated into the soil. If the previous crop has been given the recommended fertilizer applications, no more fertilizer is needed for the mungbean. Thus the production potential of the farm is increased, at a relatively low cost.
It is generally agreed today that an ideal agricultural system must be not only productive but sustainable. Some types of vegetable production are presumably more sustainable than others, but to define which these are, we need indicators of sustainability. It is still diffiocult to define these indicators, especially in quantitative terms. General indicators of sustainability have been suggested, including productivity (yield), stability (yield over time), protection of the resource base, viability (whether a system is economically profitable) and social acceptability (to both farmers and the general public). The problem is to apply such criteria to give quantified results.
It is clear that many improvements are possible in fertilizer efficiency. These include improved water management, efficient crop combinations, and improved formulations such as slow-release fertilizers to synchronize nutrient availability with crop needs. The timing of fertilizer applications is also important, not only to minimize leaching losses, but also to maximize yield. Vegetables are very sensitive to nutrient deficiencies during the early growth period. Unlike cereals, any deficiency suffered by a vegetable crop early on is not compensated during later growth stages.
Because of the high levels of nutrients removed with the crop, and because vegetables are a high-value crop, vegetables often receive very heavy fertilizer applications. Unbalanced fertilizer use and micronutrient deficiencies are common problems. There is a widespread need among the region's vegetable farms for more soil testing, including perhaps a mobile service specially adapted to the needs of small-scale farmers.
Planning and implementation of research should follow an integrated approach, with input from economists, plant pathologists and soil scientists as well as agronomists. Farmers should also be included, not just to evaluate new technology, but also to contribute from their own experience and traditional knowledge. Models are likely to play an increasingly important role, not just in research but also in extension with the development of DSS (Decision Support System) models.
Sustainable agriculture must take into account the long-term impact of agricultural production on environment. However, environmentally friendly methods may involve some sacrifice of yield, in which case there is the question, Who should pay the cost of this? Fertilizer recommendations for vegetables based on maximum yield usually involve some cost to the environment. Conversely, environmentally optimum application rates can result in a substantial loss of yield.
Location: Tainan, Taiwan ROC
Date: November 4-10, 1996
No. Participating Countries: 14
(Bangladesh, Denmark, Hong Kong, India, Japan, Korea, Malaysia, Netherlands, Philippines, Sri Lanka, Taiwan ROC, Thailand, United Kingdom, USA)
No. Papers: 22
No. Participants: 40 plus observers
Co-sponsor: Asian Vegetable Research and Development Center (AVRDC)
List of papers
1. Vegetables, nutrient rates and management
- R.A. Morris
2. Soil as a filter for nutrients and chemicals: sustainability aspects
- R. Lefroy, and E.T. Craswell
3. Nutrient requirements of solanaceous vegetable crops- D.M. Hegde
4. Nutrient content and uptake by some fruit vegetables in Malaysia
- P. Vimala, N.H. Yeong, and H. Salbiah
5. Nutrient management practices for intensive vegetable cultivation systems in Bangladesh
- A.K.M. Hossain
6. Maintenance of nutritional balances for vegetable production in the montane agroecosystems of Northern Thailand
- B. Rerkasem, K. Rerkasem, and R. Noppakoonwong
7. Effect of mungbean as inter and interim crop on soil and crop productivity in different cropping systems in India
- O.P. Meelu and Yadvinder-Singh
8. Improving productivity of vegetable cultivation on Ultisols of Sri Lanka
- S. Maraikar, J.D.H. Wijewardena, and S.L. Amarasir
i9. Nutrient balance in intensive cereal-vegetable cropping in subtropical Asia
- A.S.R. Juo and Z.O. Wang
10. Phosphorus and potassium nutrient management for vegetable soils in Shanghai and Guangdong, PRC
- N.H. Yao, Z. Dingguo, and J. Wang
11. Nutrient management in red pepper production in Korea
- C.W. Hong, H.K. Kwak, J.H. Yoon, Y.H. Park, and D.K. Lim
12. Analysis of fertilizer responses and efficiencies in vegetable production in the Hsilo area (Taiwan)- S. Lian, C.H. Wang, and Y.C. Lee
13. N balance and N fertilizer management in different cropping sequences - T. Koyama
14. Nutrient balances in rice-vegetable systems of the Northern Philippines
- J.K. Ladha
15. Contributions of N from waste applications to vegetables - R.A. Morris and C.H. Ma
16. Contributions of P and K from waste applications to vegetables
- R.A. Morris and C.H. Ma
17. Utilization of organic waste composts in vegetable crop production systems
- P.J. Stoffella, Y. Li, N.E. Roe, M. Ozores-Hampton, and D.A. Graetz
18. Nutrient uptake by vegetable crops and residual soil nutrients from different systems of field management
- Y.P. Wang and C.C. Chao
19. The rational fertilization of cabbage grown in the Tachi reservoir catchment
- Y.M. Huang, J.C. Lu, and Y.P. Wang
20. Sustainable nitrogen management in intensive vegetable production
- J. Neeteson and A. Whitmore
21. A user-friendly decision support system for adjusting N fertilizer requirements to local condi
Figure 1 Reclaiming the Soil by Replacing It by Hand, in a Vegetable Field Where Too Much Fertilizer Has Been Applied.
Figure 2 Edible Chrysanthemum, a Common Asian Vegetable
Figure 3 Bitter Melon, Eaten Cooked As a Vegetable. the Melons Are Protected by Paper Bags for Higher Quality.