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SOIL MANAGEMENT FOR SUSTAINABLE AGRICULTUREIN KOREA

Sun-Ho Yoo and Yeong-Sang Jung*

College of Agriculture and Life ScienceSeoul National University,

Suwon, Korea

*College of Agricultural SciencesKangwon National University

Chunchun, Korea

ABSTRACT

This paper discusses the effect on Korea's soils of modern farming practices, particularlyheavy applications of chemical fertilizers. It outlines the problems which are beginning to be-come serious, and suggests ways in which the situation might be improved. Soil managementpractices for sustainable agriculture are discussed, and some suggestions are made on the focusof future research.

INTRODUCTION

Over the last three decades, Korea hasexperienced dramatic changes in farm practices as aresult of government-supported programs for thedevelopment and dissemination of improved agricul-tural technology. The aim of this program was toachieve self-sufficiency in staple foods and to in-crease farm incomes. Since the 1960s, Koreanagriculture has done a tremendous job of producingenough rice for self-sufficiency, and providing con-sumers with food of high quality at a reasonableprice. Nowadays, Korean farmers rely more onchemical fertilizers and pesticides than on the tradi-tional renewable resources drawn from the farmitself. As in other industrialized countries, the pat-tern of modern agriculture in Korea has arousedpublic concern over environmental problems such ascontamination of water by agricultural chemicals,pesticide residues in food, growing resistance topesticides among insects and pests, loss of geneticdiversity, loss of natural soil productivity, and aggra-vated salinity.

The increased inputs of modern agricultureare largely of artificial origin, and may have a nega-tive impact on the environment. Chemical fertilizer

applied to soils can provide crops with specificingredient elements, but not with all the essentialelements they need. The other essential nutrients forplant growth must be supplied from other sources,that is, from the soil. Crops can not take up all thenutrients added as fertilizer. Thus, farming practiceswhich use heavy applications of chemical fertilizermay cause some elements in the soil to be depletedand others to be deposited in excess, resulting in aworsening of the soil’s nutrient balance and reducedsoil productivity. Some of the surplus chemicals maydegrade the soil ecosystem and act as pollutants.

“Sustainable agriculture” is a topic whichhas received considerable attention in recent yearsfrom environmentalists, agriculturalists, and con-sumers. Sustainable agriculture has been given anumber of different definitions, but the term impliesthree basic values: sustainable agriculture is ecologi-cally sound, economically viable, and socially justand humane (Aiken 1983, Dahlberg 1986, Keeny1990, O’Connell 1991).

In Korea, sustainable agriculture has re-ceived little attention, mainly because farming hasbeen focussed on maximizing yield. It was not until1990 that the term “sustainable agriculture” waspublicly discussed for the first time in Korea, in a

Keywords: Soils, Korea, erosion, fertilizer, organic matter, soil management, sustainableagriculture.

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paper presented at a major national symposium (Yoo1990). The senior author (Yoo 1991) in the follow-ing year stressed again the concept of sustainableagriculture at the Symposium on Conservation of theAgricultural Environment, held in 1991 by the Ko-rean Society of Environmental Agriculture.

In terms of agricultural technology, themajor components of sustainable agriculture arecultural practices and plant breeding, soil and watermanagement, pest and weed control, and integratedplant-animal production and nutrient cycling. Soil isthe key natural resource in agricultural production.This paper discusses current problems of Koreansoils associated with agricultural productivity, and asoil management strategy for sustainable agricul-ture.

Changes in Soil Management in KoreanAgriculture

Until the 1920s, Korean farmers made littleuse of chemical fertilizer. The consumption ofchemical fertilizer in 1925 was only 21,000 mt, sofarming had to depend mainly on natural soil fertilityand organic manure. However at this time, riceyields from paddy fields were less than 1.5 mt/ha. By1937, 570,000 mt of chemical fertilizer were beingapplied, and average national rice yields had in-creased to 2 mt/ha. The recommended applicationrates for chemical fertilizers at this time were 26 kg/ha of nitrogen, 34 kg/ha of phosphorus and 39 kg/haof potassium (Table 1). By the early 1960’s, Koreastill suffered from a food deficit, and the governmentbegan a program to boost agricultural production.

Farmers began to make widespread use of agricul-tural chemicals, as fertilizers and for pest manage-ment and weed control. In the 1970s, high yieldingrice varieties bred by crossing Japonica and Indicatypes were disseminated throughout Korea, and as aresult average rice yields soared to 4.5 mt/ha. Small-scale farmers began to mechanize their farm opera-tions, and heavy inputs of chemical fertilizers andpesticides became common. By the mid 1970s, selfsufficiency in rice, the staple crop, was achieved, andKorea even recorded a surplus in rice productionduring the 1980s.

The recommended levels of fertilizer fordifferent crops were based on a large number of fieldtrials, in which the application rate which gave themaximum yield was taken as the optimum level. InTable 2 we can compare the total amount of fertiliz-ers recommended by the Rural Development Admin-istration (RDA) with the actual fertilizer consump-tion by farmers. There is little difference in theamounts of phosphorus and potassium recommendedand those purchased, but the amount of nitrogenfertilizer purchased by farmers exceeded the recom-mended amount by more than 90%. It is clear thattoo much nitrogen fertilizer is being applied.

At present, farming in Korea uses highinputs of fertilizers, chemicals and machinery. Thesehigh inputs mainly originate from petrochemicalenergy, which is nonrenewable. A comparison ofinputs, in terms of energy demand, of conventionaland highly mechanized rice production, is shown inTable 3. Tillers with a capacity of 8 HP are com-monly used for plowing in conventional farming,while heavy machinery such as large tractors for

Table 1. Recommended application rate of fertilizers for paddy rice

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plowing, transplanters, power threshers and powersprayers, are used in the highly mechanized farmoperations. The energy input calculation shown in

Table 3 is based on Pimentel and Pimentel (1986),and is only a rough approximation, but we can notethat the energy input for nitrogen fertilizer alone is

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 fieldfarmed by conventional and highly mechanized techniques

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one third of the total energy input. The use oflivestock manure in integrated crop livestock farm-ing can reduce the relative energy input from nitro-gen to one-fifth of the total.

SOIL FERTILITY STATUS AND SOIL MANAGE-MENT PROBLEMS

Soil Nutrient Balance

Since one of the principal constraints toplant growth is a deficit of available nutrients in thesoil, farmers tend to use more fertilizers to get higheryields. In a natural ecosystem, the 16 elementsessential for plant growth are kept in balance, theamounts of required by the plants matched by thosesupplied naturally by the soil, including nutrientrecycling. Agricultural practices, however, havechanged this balance, and fewer nutrients are beingrecycled as the harvested plant parts are removedfrom the field. The result is a loss of soil productiv-ity.

The Rural Development Administration hascarried out a large-scale soil testing program since itwas established in 1962. Under this program, anenormous number of soil samples have been ana-lyzed. Table 4 shows a summary of the results of thetests of paddy soils. Most soils in Korea have a lowpH and organic matter content, and a low level ofphosphorus (P) potassium (K) and Calcium (Ca).There has been a steady decline in the organic matterand magnesium content since 1936, while the phos-phorus content has increased. The organic matter

content was 3.3% between 1936 and 1946, at a timewhen soil fertility was still being maintained withorganic manure. In the 1960’s, the organic mattercontent of the soil fell to 2.6%, and simultaneouslythe average pH rose and the potassium and calciumcontent also increased. This marks the period atwhich chemical fertilizers came into widespread useas intensive farming began.

As Table 5 shows the phosphorus contentin upland soils used for grain crops and vegetables ismore than double the recommended level. In green-house soils where vegetables are intensively grown,phosphorus exceeds the recommended level by tentimes. Data on the levels of base saturation show thatthese greenhouse soils were already saturated withbases by 1989.

An excessive use of chemical fertilizerscertainly causes economic loss to the farmer, andgives rise to salt accumulation which may bringabout a deterioration in the soil environment (Yoo etal. 1974).

The water-soluble and mobile constituentsin the soil may be leached out of the root zone, orremoved in runoff to pollute the water system. Thenutrients most closely associated with water pollu-tion are inorganic nitrogen and phosphorus fromagricultural land. Fig. 1 and Fig. 2 show the seasonalchanges in the nitrate and phosphate concentrationsin water used for irrigation.

It is clear that even the percolated watercontains a high concentration of both N and P. Thismeans that a considerable amount of applied fertil-izer is being leached out of the root zone (Cho et al.1989).

Table 4. Changes in the chemical properties of paddy soils, 1936­1988

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Table 5. Changes in the chemical properties of upland soils

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Fig. 1. Nitrate levels in water in reservoirs and paddy fields, Korea

Fig. 2. Phosphate levels in water in reservoirs and paddy fields, Korea

Source: Kim et al. (1989)

Source: Kim et al. (1989)

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Yoon and Yoo (in Yoo 1991) analyzednitrate levels in the soil profile after nitrogen fertil-izer in the form of urea had been applied to grassland.The nitrogen application rate was 140 kg/ha. Al-though this rate was only one-half of the recom-mended level, a large amount of nitrate moved downinto the subsoil (Fig. 3). The nitrate concentration(as measured by a lysimeter) in the leachate underfallow conditions was much higher than when thefield was under grass (Fig. 4). The results clearlyshowed that large amounts of nitrate could leach outof the root zone, particularly if the soil had no cropcover, to become a potential pollutant of groundwa-ter.

Erosion

Using the USLE formula, Jung (1976) esti-mated that potential soil loss from erosion fromsteep slopeland (22.5% slope) could be as high as485 mt/ha year, but suggested that this could bereduced to less than 13 mt/ha year with proper soilconservation measures. Jung et al. (1985) con-ducted a 2 x 10m2 lysimeter study on a sandy loamsoil with 20% of slope. Measured soil loss from 1977

to 1982, using different cropping systems, rangedfrom 0.1 mt/ha year from the plot planted in grass to226 mt/ha year from the clean tilled plot (Table 6).

A large amount of nutrients can be lost fromsurface soil as the topsoil erodes. Measurements ina corn field found that 15.5 kg of nitrogen and 10 kgof phosphorus were washed away by runoff waterwith topsoil when 21.5 mt of topsoil were eroded ina year (Table 7). If the loss of fixed forms of theseelements in the soil particles and organic matter lostwere included, this amount would be even higher.For example, a fertile soil contains 2 - 4 kg ofnitrogen in one metric ton of soil. Therefore, the21.5 mt of eroded soil particles might remove 43 to86 kg of nitrogen, which corresponds to half theamount applied in the form of chemical fertilizer inone year. No-tillage reduced soil losses by 62% andnutrient losses by 32%.

In traditional farming, most animal wasteswere returned to cropland as fertilizer. One cowproduces about 30 kg of wastes every day, while apig produces 6 kg. The total annual production ofanimal wastes in Korea is estimated to be 37 millionmt (Table 8). This is enough to cover Korea’s wholearea of agricultural land at a rate of 16.9 mt/ha, and

Fig. 3. Seasonal changes in mineral nitrogen in grassland soil

Source: Yoo (1991)

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Fig. 4. Changes in nitrate N concentration in leachate(Yoo 1991)

Table 6. Soil loss from a sandy loam with20% slope under differentcropping systems, 1977 to 1982

Table 7. Nutrient losses from corn field,1979­80

could substitute for 44% of the nitrogen which wasapplied in the form of chemical fertilizer in 1990, aswell as 67% of the phosphorus, and 70% of thepotassium. If we assume that the maximum rate atwhich animal wastes should be applied is 50 mt/ha,the amount produced in Korea would cover 750

thousand ha. However most livestock wastes are notused for agriculture, but are discharged into streamsand rivers. The efficient recycling of animal wastesin Korea is one of the most urgent problems in orderto protect water quality.

Source: Jung et al. 1985

Source: RDA 1989

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Table 8. Annual production of livestock wastes in Korea

SOIL MANAGEMENT FORSUSTAINABLE AGRICULTURE

Maintaining Soil Quality

Improving soil quality for better plantgrowth has long been a primary objective of soilscience, but many problems of soil quality remain.Loss of soil quality can result from the mismanage-ment of soil resources, in the absence of informationon how to manage it properly. A soil managementstrategy for sustainable agriculture must be based onmaintaining soil quality in the long term (Table 9).

Soil properties have changed as a result ofintensive cropping, monoculture, and the heavy useof agrochemicals. The present recommended ratesfor fertilizers in Korea were set when soil fertilitywas rather low, so it is not appropriate to apply theseto the soils of today or in the future. The accumula-tion of some nutrient elements in soil is alreadyevident, and it is time to re-evaluate soil fertility.

Nitrogen is the most important plant nutri-ent. Plants absorb nitrogen at different rates, accord-ing to their growth stages. If there is an excess of soilnitrogen, the surplus will be lost by denitrificationand/or by leaching. A correct assessment of theamount of available nitrogen, based on soil testing, istherefore highly desirable.

The composition of complex commercialfertilizers should be reexamined and adjusted. Therepeated use of chemical fertilizers which containexactly the same constituents will accelerate theaccumulation of unused elements.

A long-term dependence on chemical fertil-izers for macro nutrients may possibly result in thedepletion of micronutrients, leading to hidden prob-lems of micronutrient deficiency. It is necessary topay more attention to micro elements.

The excessive use of chemical fertilizers

may result in a build-up of dangerous residues. Forexample, fused phophate and silicate fertilizers maycontain heavy metals such as copper, cadmium oreven uranium. Although these do not appear to beserious soil contaminants at present, there should becareful long-term monitoring of the mass balanceand behavior of these elements. The mass balance ofcarbon, water, and the many gases involved in agri-culture and the wider ecosystem, also deserve moreattention. For example, the methane and nitrousoxide produced in agricultural production will affectair quality in the same way as those produced by theindustrial sector.

Use of Organic Matter

The maintenance of soil organic matter hasbeen a key point in soil management for generations.Organic matter is the principal reservoir of nitrogenand other nutrients. It increases the soil bufferingcapacity, helps maintain a good soil texture andprotect soil from erosion, and maintains a healthycommunity of soil microorganisms (Cho 1986).Although organic matter should not be considered apanacea in modern agriculture, the maintenance of ahigh soil organic matter content is always desirable,and appropriate management of the soil organicmatter is critical in achieving profitable, sustainableand environmentally friendly agriculture (Hoeft andNafziger 1988, Darst and Murphy 1989). Cropresidues should be returned to the soil where they arecut, possibly after composting. Mechanization willaid in the transport of heavy, bulky organic materialssuch as rice straw. It must be emphasized thatmodern organic farming does not represent a retreatto the past, but is an improved agricultural system.More research on the role of organic farming isrequired if we are to reduce the level of inputs neededto maintain the agricultural ecosystem. Selection of

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Table 9. Soil management problems and research needs for sustainable agriculture

proper crops and cropping systems will minimizenutrient losses while increasing the level of soilorganic matter.

Reduced Tillage

Conservation tillage is a form of low-inputagriculture, in that it requires a lower input of energyand labor, and minimizes disturbance to the soil.However, whether reduced tillage gives good resultspartly depends on the soil type. It is not suited topoorly drained soils, or soils compacted by heavymachinery. Reduced or no-tillage may also needheavier applications of herbicides to control weeds.The process of tilling the soil rapidly incorporatesorganic materials into the soil matrix, while no-tillage leaves organic materials undisturbed and strati-fied in the topsoil. Runoff water, drying and wetting,and freezing and thawing, will have different effectson soil properties and soil microorganisms (Stinnerand House 1989). The advantages and disadvan-tages of no-tillage should be examined according tothe particular local situation.

Use of Livestock Wastes

The use of animal wastes from livestockfarming is desirable, but at present in Korea there arevarious difficulties in doing this. The few largerlivestock farms tend to be located near urban areas,or in hilly areas far from the croplands where theorganic resources are needed. More than two-thirdsof Korea’s livestock farms are rather small, andfarmers tend to stock them at a very high density inorder to reduce capital input and management costs.These small farms generally dispose of animal wasteswithout giving them any pretreatment. The Ministryof the Environment does regulate the disposal ofanimal wastes by law (Kim 1991), and requireslivestock farms to have a treatment system for animalwastes, but these regulations apply only to largefarms. Treatment systems suitable for small farmsshould be developed, and farmers provided with thenecessary financial assistance to install them. Effi-cient waste management would not only cut down onpollution, but provide crop farmers with a usefulsource of cheap organic fertilizers.

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Protection of Non-cultivated Land

The area of uncultivated marginal land inrural areas is increasing. These uncultivatedslopelands are highly susceptible to erosion if leftwithout a good vegetation cover. Proper measuresto control soil erosion in such areas should bedeveloped and put into practice.

CONCLUSION

Over the past three decades, modern agri-cultural technology in Korea has been very success-ful in increasing productivity. Nowadays, however,we are faced with difficult environmental problemsthat arise, not only from industrialization, but fromagriculture itself. Agricultural scientists are urgentlyrequired to provide economically and socially ac-ceptable alternatives to help solve these problems.

Sustainable agriculture may be defined asan agricultural system which gives farmers a profit-able livelihood while conserving agricultural re-sources and environmental quality. It makes effi-cient use of resources produced on the farm, reduc-ing the need for commercially produced inputs.Good soil management is a core component.

Soil testing should be the first step in man-aging soil for sustainable agriculture, so we canknow what the problems are now, and what prob-lems are likely to arise in the future. Soil testing overthe past 30 years has shown that most soils in Koreaare acidic, and have a low organic matter content andtoo low a level of many available nutrients for goodplant growth. Continuous applications of chemicalfertilizer containing the major nutrient elements haveresulted in both the accumulation of unused con-stituents, and the hidden depletion of minor ele-ments. Excessive use of chemical fertilizers hasbecome a cause of environmental pollution, and ispartly a result of the fact that the present recom-mended levels of fertilizers were determined at a timewhen soil fertility was quite different from what it isnow. A reexamination of both soil fertility and howthis is measured would be a useful preparation formore efficient fertilizer management. The recyclingof nutrient and chemical components on agriculturalland should be evaluated.

Large amounts of Korea’s fertile surfacesoils are being lost as a result of erosion. In recentyears the area of fallow land has been increasing, asfarmers leave for the cities, and these abandonedfields are very vulnerable to erosion. Proper man-agement practices and measures to control soil ero-sion are necessary.

A large amount of organic materials fromlivestock farms is being discharged untreated intowater courses rather than applied to agriculturalland. At present it is not practical for small-scaleincrease livestock farmers to treat their farm wastesand convert them into organic fertilizer. Not onlytechnological support is needed for such farmers, butalso some financial support to subsidize the cost oftreatment facilities. The advantages and disadvan-tages of conservation tillage and low-input agricul-ture should be carefully and systematically tested ina range of sites, so as to develop the best combina-tions of current and traditional farm practices andmodern technology.

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DISCUSSION

Dr. Hsieh was interested in the marked increase of nitrate leaching in Korea over recent years, andasked 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 wasprobably 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% ofapplied potassium was absorbed by the crop, asked whether losses were reduced if the soil had a high organicmatter content, so that there was e.g. less leaching or more absorption by the plant. Dr. Yoo thought it waslikely that organic matter had both these effects, and pointed out that since 1960 in Korea, an increasing levelof nitrogen fertilizer had been applied while the organic matter content of the soil had decreased. This had meanta fall in the CEC, so that the rate of absorption was also reduced. Most farmers in Korea were not able to getas 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 thatleaching 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 affectsthe rates at which pesticides are leached or absorped, but even the type of organic material applied.