- Sustainable Agriculture for the Asian and Pacific Region
- Soil Management for Sustainable Agriculture in Korea
Over the last three decades, Korea has experienced dramatic changes in farm practices as a result of government-supported programs for the development and dissemination of improved agricultural technology. The aim of this program was to achieve self-sufficiency in staple foods and to increase farm incomes. Since the 1960s, Korean agriculture has done a tremendous job of producing enough rice for self-sufficiency, and providing consumers with food of high quality at a reasonable price. Nowadays, Korean farmers rely more on chemical fertilizers and pesticides than on the traditional renewable resources drawn from the farm itself. As in other industrialized countries, the pattern of modern agriculture in Korea has aroused public concern over environmental problems such as contamination of water by agricultural chemicals, pesticide residues in food, growing resistance to pesticides among insects and pests, loss of genetic diversity, loss of natural soil productivity, and aggravated salinity.
The increased inputs of modern agriculture are largely of artificial origin, and may have a negative impact on the environment. Chemical fertilizer applied to soils can provide crops with specific ingredient elements, but not with all the essential elements they need. The other essential nutrients for plant growth must be supplied from other sources, that is, from the soil. Crops can not take up all the nutrients added as fertilizer. Thus, farming practices which use heavy applications of chemical fertilizer may cause some elements in the soil to be depleted and others to be deposited in excess, resulting in a worsening of the soil's nutrient balance and reduced soil productivity. Some of the surplus chemicals may degrade the soil ecosystem and act as pollutants.
"Sustainable agriculture" is a topic which has received considerable attention in recent years from environmentalists, agriculturalists, and consumers. Sustainable agriculture has been given a number of different definitions, but the term implies three basic values: sustainable agriculture is ecologically sound, economically viable, and socially just and humane (Aiken 1983, Dahlberg 1986, Keeny 1990, O'Connell 1991).
In Korea, sustainable agriculture has received little attention, mainly because farming has been focussed on maximizing yield. It was not until 1990 that the term "sustainable agriculture" was publicly discussed for the first time in Korea, in a paper presented at a major national symposium (Yoo 1990). The senior author (Yoo 1991) in the following year stressed again the concept of sustainable agriculture at the Symposium on Conservation of the Agricultural Environment, held in 1991 by the Korean Society of Environmental Agriculture.
In terms of agricultural technology, the major components of sustainable agriculture are cultural practices and plant breeding, soil and water management, pest and weed control, and integrated plant-animal production and nutrient cycling. Soil is the key natural resource in agricultural production. This paper discusses current problems of Korean soils associated with agricultural productivity, and a soil management strategy for sustainable agriculture.
Until the 1920s, Korean farmers made little use of chemical fertilizer. The consumption of chemical fertilizer in 1925 was only 21,000 mt, so farming had to depend mainly on natural soil fertility and organic manure. However at this time, rice yields from paddy fields were less than 1.5 mt/ha. By 1937, 570,000 mt of chemical fertilizer were being applied, and average national rice yields had increased to 2 mt/ha. The recommended application rates for chemical fertilizers at this time were 26 kg/ha of nitrogen, 34 kg/ha of phosphorus and 39 kg/ha of potassium ( Table 1). By the early 1960's, Korea still suffered from a food deficit, and the government began a program to boost agricultural production. Farmers began to make widespread use of agricultural chemicals, as fertilizers and for pest management and weed control. In the 1970s, high yielding rice varieties bred by crossing Japonica and Indica types were disseminated throughout Korea, and as a result average rice yields soared to 4.5 mt/ha. Small-scale farmers began to mechanize their farm operations, and heavy inputs of chemical fertilizers and pesticides became common. By the mid 1970s, self sufficiency in rice, the staple crop, was achieved, and Korea even recorded a surplus in rice production during the 1980s.
The recommended levels of fertilizer for different crops were based on a large number of field trials, in which the application rate which gave the maximum yield was taken as the optimum level. In Table 2 we can compare the total amount of fertilizers recommended by the Rural Development Administration (RDA) with the actual fertilizer consumption by farmers. There is little difference in the amounts of phosphorus and potassium recommended and those purchased, but the amount of nitrogen fertilizer purchased by farmers exceeded the recommended amount by more than 90%. It is clear that too much nitrogen fertilizer is being applied.
At present, farming in Korea uses high inputs of fertilizers, chemicals and machinery. These high inputs mainly originate from petrochemical energy, which is nonrenewable. A comparison of inputs, in terms of energy demand, of conventional and highly mechanized rice production, is shown in Table 3. Tillers with a capacity of 8 HP are commonly used for plowing in conventional farming, while heavy machinery such as large tractors for plowing, transplanters, power threshers and power sprayers, are used in the highly mechanized farm operations. The energy input calculation shown in Table 3 is based on Pimentel and Pimentel (1986), and is only a rough approximation, but we can note that the energy input for nitrogen fertilizer alone is one third of the total energy input. The use of livestock manure in integrated crop livestock farming can reduce the relative energy input from nitrogen to one-fifth of the total.
Since one of the principal constraints to plant growth is a deficit of available nutrients in the soil, farmers tend to use more fertilizers to get higher yields. In a natural ecosystem, the 16 elements essential for plant growth are kept in balance, the amounts of required by the plants matched by those supplied naturally by the soil, including nutrient recycling. Agricultural practices, however, have changed this balance, and fewer nutrients are being recycled as the harvested plant parts are removed from the field. The result is a loss of soil productivity.
The Rural Development Administration has carried out a large-scale soil testing program since it was established in 1962. Under this program, an enormous number of soil samples have been analyzed. Table 4 shows a summary of the results of the tests of paddy soils. Most soils in Korea have a low pH and organic matter content, and a low level of phosphorus (P) potassium (K) and Calcium (Ca). There has been a steady decline in the organic matter and magnesium content since 1936, while the phosphorus content has increased. The organic matter content was 3.3% between 1936 and 1946, at a time when soil fertility was still being maintained with organic manure. In the 1960's, the organic matter content of the soil fell to 2.6%, and simultaneously the average pH rose and the potassium and calcium content also increased. This marks the period at which chemical fertilizers came into widespread use as intensive farming began.
As Table 5 shows the phosphorus content in upland soils used for grain crops and vegetables is more than double the recommended level. In greenhouse soils where vegetables are intensively grown, phosphorus exceeds the recommended level by ten times. Data on the levels of base saturation show that these greenhouse soils were already saturated with bases by 1989.
An excessive use of chemical fertilizers certainly causes economic loss to the farmer, and gives rise to salt accumulation which may bring about a deterioration in the soil environment (Yoo et al. 1974).
The water-soluble and mobile constituents in the soil may be leached out of the root zone, or removed in runoff to pollute the water system. The nutrients most closely associated with water pollution are inorganic nitrogen and phosphorus from agricultural land. Fig. 1 and Fig. 2 show the seasonal changes in the nitrate and phosphate concentrations in water used for irrigation.
It is clear that even the percolated water contains a high concentration of both N and P. This means that a considerable amount of applied fertilizer is being leached out of the root zone (Cho et al. 1989).
Yoon and Yoo (in Yoo 1991) analyzed nitrate levels in the soil profile after nitrogen fertilizer in the form of urea had been applied to grassland. The nitrogen application rate was 140 kg/ha. Although this rate was only one-half of the recommended level, a large amount of nitrate moved down into the subsoil ( Fig. 3). The nitrate concentration (as measured by a lysimeter) in the leachate under fallow conditions was much higher than when the field was under grass ( Fig. 4). The results clearly showed that large amounts of nitrate could leach out of the root zone, particularly if the soil had no crop cover, to become a potential pollutant of groundwater.
Using the USLE formula, Jung (1976) estimated that potential soil loss from erosion from steep slopeland (22.5% slope) could be as high as 485 mt/ha year, but suggested that this could be reduced to less than 13 mt/ha year with proper soil conservation measures. Jung et al. (1985) conducted a 2 x 10m 2 lysimeter study on a sandy loam soil with 20% of slope. Measured soil loss from 1977 to 1982, using different cropping systems, ranged from 0.1 mt/ha year from the plot planted in grass to 226 mt/ha year from the clean tilled plot ( Table 6).
A large amount of nutrients can be lost from surface soil as the topsoil erodes. Measurements in a corn field found that 15.5 kg of nitrogen and 10 kg of phosphorus were washed away by runoff water with topsoil when 21.5 mt of topsoil were eroded in a year ( Table 7). If the loss of fixed forms of these elements in the soil particles and organic matter lost were included, this amount would be even higher. For example, a fertile soil contains 2 - 4 kg of nitrogen in one metric ton of soil. Therefore, the 21.5 mt of eroded soil particles might remove 43 to 86 kg of nitrogen, which corresponds to half the amount applied in the form of chemical fertilizer in one year. No-tillage reduced soil losses by 62% and nutrient losses by 32%.
In traditional farming, most animal wastes were returned to cropland as fertilizer. One cow produces about 30 kg of wastes every day, while a pig produces 6 kg. The total annual production of animal wastes in Korea is estimated to be 37 million mt ( Table 8). This is enough to cover Korea's whole area of agricultural land at a rate of 16.9 mt/ha, and could substitute for 44% of the nitrogen which was applied in the form of chemical fertilizer in 1990, as well as 67% of the phosphorus, and 70% of the potassium. If we assume that the maximum rate at which animal wastes should be applied is 50 mt/ha, the amount produced in Korea would cover 750 thousand ha. However most livestock wastes are not used for agriculture, but are discharged into streams and rivers. The efficient recycling of animal wastes in Korea is one of the most urgent problems in order to protect water quality.
Improving soil quality for better plant growth has long been a primary objective of soil science, but many problems of soil quality remain. Loss of soil quality can result from the mismanagement of soil resources, in the absence of information on how to manage it properly. A soil management strategy for sustainable agriculture must be based on maintaining soil quality in the long term ( Table 9).
Soil properties have changed as a result of intensive cropping, monoculture, and the heavy use of agrochemicals. The present recommended rates for fertilizers in Korea were set when soil fertility was rather low, so it is not appropriate to apply these to the soils of today or in the future. The accumulation of some nutrient elements in soil is already evident, and it is time to re-evaluate soil fertility.
Nitrogen is the most important plant nutrient. Plants absorb nitrogen at different rates, according to their growth stages. If there is an excess of soil nitrogen, the surplus will be lost by denitrification and/or by leaching. A correct assessment of the amount of available nitrogen, based on soil testing, is therefore highly desirable.
The composition of complex commercial fertilizers should be reexamined and adjusted. The repeated use of chemical fertilizers which contain exactly the same constituents will accelerate the accumulation of unused elements.
A long-term dependence on chemical fertilizers for macro nutrients may possibly result in the depletion of micronutrients, leading to hidden problems of micronutrient deficiency. It is necessary to pay more attention to micro elements.
The excessive use of chemical fertilizers may result in a build-up of dangerous residues. For example, fused phophate and silicate fertilizers may contain heavy metals such as copper, cadmium or even uranium. Although these do not appear to be serious soil contaminants at present, there should be careful long-term monitoring of the mass balance and behavior of these elements. The mass balance of carbon, water, and the many gases involved in agriculture and the wider ecosystem, also deserve more attention. For example, the methane and nitrous oxide produced in agricultural production will affect air quality in the same way as those produced by the industrial sector.
The maintenance of soil organic matter has been a key point in soil management for generations. Organic matter is the principal reservoir of nitrogen and other nutrients. It increases the soil buffering capacity, helps maintain a good soil texture and protect soil from erosion, and maintains a healthy community of soil microorganisms (Cho 1986). Although organic matter should not be considered a panacea in modern agriculture, the maintenance of a high soil organic matter content is always desirable, and appropriate management of the soil organic matter is critical in achieving profitable, sustainable and environmentally friendly agriculture (Hoeft and Nafziger 1988, Darst and Murphy 1989). Crop residues should be returned to the soil where they are cut, possibly after composting. Mechanization will aid in the transport of heavy, bulky organic materials such as rice straw. It must be emphasized that modern organic farming does not represent a retreat to the past, but is an improved agricultural system. More research on the role of organic farming is required if we are to reduce the level of inputs needed to maintain the agricultural ecosystem. Selection of proper crops and cropping systems will minimize nutrient losses while increasing the level of soil organic matter.
Conservation tillage is a form of low-input agriculture, in that it requires a lower input of energy and labor, and minimizes disturbance to the soil. However, whether reduced tillage gives good results partly depends on the soil type. It is not suited to poorly drained soils, or soils compacted by heavy machinery. Reduced or no-tillage may also need heavier applications of herbicides to control weeds. The process of tilling the soil rapidly incorporates organic materials into the soil matrix, while no-tillage leaves organic materials undisturbed and stratified in the topsoil. Runoff water, drying and wetting, and freezing and thawing, will have different effects on soil properties and soil microorganisms (Stinner and House 1989). The advantages and disadvantages of no-tillage should be examined according to the particular local situation.
The use of animal wastes from livestock farming is desirable, but at present in Korea there are various difficulties in doing this. The few larger livestock farms tend to be located near urban areas, or in hilly areas far from the croplands where the organic resources are needed. More than two-thirds of Korea's livestock farms are rather small, and farmers tend to stock them at a very high density in order to reduce capital input and management costs. These small farms generally dispose of animal wastes without giving them any pretreatment. The Ministry of the Environment does regulate the disposal of animal wastes by law (Kim 1991), and requires livestock farms to have a treatment system for animal wastes, but these regulations apply only to large farms. Treatment systems suitable for small farms should be developed, and farmers provided with the necessary financial assistance to install them. Efficient waste management would not only cut down on pollution, but provide crop farmers with a useful source of cheap organic fertilizers.
The area of uncultivated marginal land in rural areas is increasing. These uncultivated slopelands are highly susceptible to erosion if left without a good vegetation cover. Proper measures to control soil erosion in such areas should be developed and put into practice.
Over the past three decades, modern agricultural technology in Korea has been very successful in increasing productivity. Nowadays, however, we are faced with difficult environmental problems that arise, not only from industrialization, but from agriculture itself. Agricultural scientists are urgently required to provide economically and socially acceptable alternatives to help solve these problems.
Sustainable agriculture may be defined as an agricultural system which gives farmers a profitable livelihood while conserving agricultural resources and environmental quality. It makes efficient use of resources produced on the farm, reducing the need for commercially produced inputs. Good soil management is a core component.
Soil testing should be the first step in managing soil for sustainable agriculture, so we can know what the problems are now, and what problems are likely to arise in the future. Soil testing over the past 30 years has shown that most soils in Korea are acidic, and have a low organic matter content and too low a level of many available nutrients for good plant growth. Continuous applications of chemical fertilizer containing the major nutrient elements have resulted in both the accumulation of unused constituents, and the hidden depletion of minor elements. Excessive use of chemical fertilizers has become a cause of environmental pollution, and is partly a result of the fact that the present recommended levels of fertilizers were determined at a time when soil fertility was quite different from what it is now. A reexamination of both soil fertility and how this is measured would be a useful preparation for more efficient fertilizer management. The recycling of nutrient and chemical components on agricultural land should be evaluated.
Large amounts of Korea's fertile surface soils are being lost as a result of erosion. In recent years the area of fallow land has been increasing, as farmers leave for the cities, and these abandoned fields are very vulnerable to erosion. Proper management practices and measures to control soil erosion are necessary.
A large amount of organic materials from livestock farms is being discharged untreated into water courses rather than applied to agricultural land. At present it is not practical for small-scale increase livestock farmers to treat their farm wastes and convert them into organic fertilizer. Not only technological support is needed for such farmers, but also some financial support to subsidize the cost of treatment facilities. The advantages and disadvantages of conservation tillage and low-input agriculture should be carefully and systematically tested in a range of sites, so as to develop the best combinations of current and traditional farm practices and modern technology.
Dr. Hsieh was interested in the marked increase of nitrate leaching in Korea over recent years, and asked whether Korea shared Taiwan's problem of a high nitrate level in some crops, especially vegetables. Dr. Yoo agreed that there was some evidence of this, especially in vegetable crops, although the problem was probably not as serious in the cooler climate of Korea.
Dr. Umali, referring to Dr. Yoo's report of fertilizer losses in the field and his data that only 35% of applied potassium was absorbed by the crop, asked whether losses were reduced if the soil had a high organic matter content, so that there was e.g. less leaching or more absorption by the plant. Dr. Yoo thought it was likely that organic matter had both these effects, and pointed out that since 1960 in Korea, an increasing level of nitrogen fertilizer had been applied while the organic matter content of the soil had decreased. This had meant a fall in the CEC, so that the rate of absorption was also reduced. Most farmers in Korea were not able to get as much organic matter for their farms as they would like: livestock and crops were produced in different areas, which made recycling difficult. However, if farmers were able to apply as much organic matter as they needed, they would be able to improve the absorption capacity of the soil.
Dr. Reganold commented that one problem in using organic matter with chemical pesticides is that leaching studies of pesticide residues had found that leaching of pesticides is less when organic matter is applied, and there is also greater absorption of pesticides by the plant. Not only the presence of organic matter affects the rates at which pesticides are leached or absorped, but even the type of organic material applied.
Figure 1 Nitrate Levels in Water in Reservoirs and Paddy Fields, Korea
Figure 2 Phosphate Levels in Water in Reservoirs and Paddy Fields, Korea
Figure 3 Seasonal Changes in Mineral Nitrogen in Grassland Soil
Figure 4 Changes in Nitrate N Concentration in Leachate
Table 1 Recommended Application Rate of Fertilizers for Paddy Rice
Table 2 Comparison of the Amounts of Fertilizer Recommended and Applied, Korea 1990
Table 3 Comparison of Energy Input and Output for Production in a 0.1 Ha Paddy Field Farmed by Conventional and Highly Mechanized Techniques
Table 4 Changes in the Chemical Properties of Paddy Soils, 1936-1988
Table 5 Changes in the Chemical Properties of Upland Soils
Table 6 Soil Loss from a Sandy Loam with 20% Slope under Different Cropping Systems, 1977 to 1982
Table 7 Nutrient Losses from Corn Field, 1979-80
Table 8 Annual Production of Livestock Wastes in Korea
Table 9 Soil Management Problems and Research Needs for Sustainable Agriculture
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