Paper Number 17

Soil fertility status and crop nutrient management practice in Democratic People’s Republic of Korea*

* This country report has not been formally edited and the designations and terminology used are those of the author.

Pak Pyong Hyu
Soil Science Institute, AAS
and
Ryang Yong Nam
Soil Science Research Institute (SSRI), Pyongyang

1. Background

DPR Korea is 80 percent mountainous and home to 22 million people. Arable land suitable for agricultural purposes is limited, with notable low to average level of soil fertility. The soil organic matter content is also relatively very low compared with European countries and majority of arable land has low available nutrient content due to long cultivation.

Most part of dry fields consists of the soil generated from granite and more than 50 percent is composed of steep slope resulting to severe soil erosion. Annual average precipitation is 1 000-1 200 mm, and during July-August when rainfall is concentrated, 30 soil per is washed out in sloping land. Consequently, soil thickness of dry field is shallow with an average figure of 30-50 cm.

In the last 10 years, supply of nutrient by chemical fertilizer and organic manure was not enough to compensate for the amount removed by grain output, so that nutrient balance in soil was severely affected. This resulted to lower yield, while the utilization proportion of chemical fertilizer was also not increased.

2. The state of soil fertility

According to soil test data in cultivated land covering the period of 1982-1999 (Table 1), the average organic matter content of the whole land decreased from 1.8 percent to 1.6 percent, while that of fields dedicated to orchard and mulberry also decreased at 1.6 percent to 1.5 percent and 1.7 percent to 1.6 percent, respectively.

Table 1. The decrease of average soil OM content during 1982-1999 (percent)

In this period, the average organic matter content of paddy field in each province decreased from 2.0 percent to 1.7 percent in Chagang province, from 1.9 percent to 1.7 percent in north Pyongan province, from 1.7 percent to 1.6 percent in south Pyongan province, south Hwanghae province and north Hwanghae province and the similar trend appeared in dry field.

Similarly, the available nitrogen and phosphorous content also decreased in cultivated land as shown in Table 2.

Soil fertility of dry field dropped dramatically due to poor soil erosion prevention measures. According to a soil investigation of a nationwide scope, the average soil thickness of dry field fell by 4.9 cm in 1999 compared with 1977, and soil loss in sloping field above 15 degrees amounts to 40-100 t per ha. Notably, the average yield of maize in the field with low soil erosion was 4.6 t per ha, but in the field with moderate soil erosion was 3.2 t, and that in severely eroded field was 2.8 t.

If fertile soil particles are leached and soil become shallow, root development of crop is limited and the water holding capacity of soil and the soil nutrients decrease. Nutrient components are dissolved or lost in the adsorbed state in soil.

Table 2. The decrease of soil available N content during 1982-1999 (mg/100 g soil)

Table 3. The decrease of soil available P content during 1982-1999 (mg/100 g soil)

Table 4. Soil loss in sloping field (t/ha year)

Table 5. The decrease of yield due to soil erosion (t/ha)

2.1 Crop nutrient management in relation to crop cultivation type

2.1.1 Organic and mneral nutrient management in double cropping condition

Double cropping requires more nutrients compared with monoculture. In case balanced nutrient supply is not ensured, the soil fertility drops rapidly by produced total grain removing.

Table 6 shows the change of soil fertility in double cropping field where wheat was cultivated as forecrop and maize as second crop during a 3-year period.

2.1.2 Second crop maize was transplanted on July 23

As seen in the table, under condition in which wheat was cultivated as forecrop and maize as second crop and N alone was applied (80 kg), the total N content was lowered by 0.01 percent, organic matter content by 0.05 percent, available N by 0.3 mg/100 g soil, available P by 1.8 mg, and available K by 1 mg. But in the case of applying 10 t/ha of farmyard manure, the whole fertility parameters improved.

In the case of wheat-maize double cropping system, the application of the total amount of straw of the forecrop (wheat) to the soil resulted to an increase in soil fertility. Similar tendency appeared in other double cropping condition where potato is cultivated as first crop and maize as second crop.

Table 8. The change of several agrochemical properties

* Application of 10 t/ha of farmyard manure to monoculture plot of maize and forecrop potato.

Consideration of soil fertility in double cropping is necessary, but it is of equal importance to take into account the correct balance between amount of removed nutrients through grain and straw from soil, and the amount of applied nutrients from straw, organic material. This will eventually establish nutrient management system towards sustainable soil fertility.

In this regard, nutrient balance was studied using application of straw in double cropping system involving wheat-maize, barley-maize and wheat-soybean (Table 9). It clearly showed that applying or incorporating straw of the previous crop significantly aided in attaining nutrient balance. Relative to this, the effect of applying organic manure on soil organic matter was also studied and provided similar positive results (Table 10).

Table 9. Nutrient balance in relation to crop cultivation type in double cropping field

Precipitation per ha – 1 000 mm (N 0.062 percent)
Irrigation water – 0.25 jungmi (N 0.19 percent, P₂O₅ 0.02 percent, K₂O 0.31 percent)
Farmyard manure 10 t (N 0.5 percent, P₂O₅ 0.36 percent, K₂O 0.2 percent)
Combined fertilizer – 500 kg/ha (N 20 percent, P₂O₅ 17 percent, K₂O 17 percent)

Table 10. Influence of application of organic manure on soil fertility in potato field

2.1.3 Improvement of fertilization based on soil test data and fertilizer response in monoculture condition

In the last 10 years, the affectivity and efficiency of fertilizer application decreased attributed to the destruction of balance of soil nutrients. Furthermore, the application rate of fertilizer is not decided in conformity with the crop response to chemical fertilizer depending on soil fertility level and soil types. Instead, the application rate per ha is determined uniformly in a given area soley considering crop varieties.

Table 11 shows the results of an evaluation of the effect of N, P, and K fertilizers in south Pyongan province, the main grain production region in DPRK, indicating that an average of almost 1 tonne of yield is lost annually by applying N fertilizer alone.

Table 11. Crop response to fertilizer by paddy soil types in south Pyongan province

Experiment results mentioned that based on soil analysis data on similar kind of soil involving nitrogen, phosphorus, and potassium fertilizers, different relative yield was attained. Fertilizer trials with five types of paddy soil and three types of non-paddy soil across the whole south Pyongan province were used to estimate relationship between soil analysis data and relative yield percent. According to soil data, the established critical available nutrient value has been re-evaluated based on the yield response. Table 12 shows that fertilizer dose of nitrogen, phosphorus, potassium for maximum crop yield production was different in paddy field, and same trend was also observed in non-paddy (Table 13).

New calibration based on critical value of available N, P₂O₅, K₂O differs from old calibration that has been used throughout the country in previous years.

Table 12. Fertilizer response point for maximum yield based on soil analysis data (paddy)

* Fertilizer dose for maximum yield produced.

Table 13. Fertilizer response point for maximum yield based on soil analysis data (non-paddy)

In the old calibration for paddy and non-paddy, available N content is «Low» in range of <5 mg, «Medium» in range of 5~10 mg, «High» in range of >10 mg. As for available P₂O₅ content, it is «Low» in range of <2.5 mg, «Medium» in range of 2.5~5 mg, «High» in range of >5 mg. Finally, available K₂O content is «Low» in range of <5 mg, «Medium» in range of 5~10 mg, and «High» in range of >10 mg. However, new estimates by fertilizer response yield based on soil tested data resulted to different calibrations both for paddy and non-paddy.

For paddy, the new calibrations are as follows: in terms of N, «Low» in range of <4 mg, «Medium» in range of 4.1~8 mg, «High» in range of >8 mg; available P₂O₅ content is «Low» in range of <3 mg, «Medium» in range of 3.1~6 mg, «High» in range of >6 mg; and available K₂O content is «Low» in range of <5 mg, «Medium» in range of 5~10 mg, «High» in range of >10 mg.

For non-paddy, the new calibrations are as follows: in terms of N, «Low» in range of <4 mg, «Medium» in range of 4.1~8 mg, «High» in range of >8 mg; available P₂O₅ content is same as old one and K₂O content is «Low» in range of <6 mg, «Medium» in range of 6.1~8 mg, «High» in range of >8 mg.

According to the contents of available nutrient elements made by new critical value, fertilizer application dose with soil types are shown in Tables 14 and 15. According to the refined calibration unit in paddy and non-paddy with soil types, fertilizer application dose has been controlled and tested for each production area.

On the other hand, refined fertilizer application method based on the soil tested data and crop response shows more yield and 30~50 percent less of fertilizer supply compared to the previous calibration as presented in Table 16.

Table 14. Fertilizer application dose control based on new calibration unit with soil types (paddy)

Table 15. Fertilizer application dose control based on new calibration unit with soil types (non-paddy)

Table 16. Production experiment data for the refined fertilizer application in paddy and non-paddy (2005)