INTRODUCTION

The rice consumption per capita has decreased from 136.4 kg (1970s) to 61.9 kg (2016) due to changes in people's diet patterns (Kim and Cho, 2004). Rice stockpile was increased over 500,000 tons in 2013. In the past, Korea focused more on increasing rice production for selfsufficiency of food, and rice self-sufficiency rate has increased to 101% in 2015. However, due to increase in rice stock amount, the government decided to decrease rice cultivation area of t he whole country. It is expected to reduce 80,000 hectares by 2018 and reduce the stock of rice (MAFRA, 2015). Therefore, it is necessary to improve the self-sufficiency rate of the upland crops by increasing the cultivation area of the upland crops (Kim and Lee, 1994; Song et al., 2005). The self-sufficiency rate of upland crops is 13.0% (2015, MAFRA) in Korea, which is considerably lower than the self-sufficiency rate of rice, and imports of upland crops from foreign countries are inevitable, and the amount of it is steadily increasing. The reason for the difficulty to grow upland grains in the rice paddy field is that the yield is low compared to the upland field cultivation due to poor drainage in rainy season and bad soil environment for upland crops (Kono et al., 1987; Seo et al., 2012). Therefore, this study conducted waxy maize as a rice substitute crop.

The organic farming areas have been expanded worldwide since the late 1990s (Granatstein, 2002). The difficulty of organic farming is to control disease, pests and weeds. Rice paddy-upland rotation is a form of farmland use technology that cultivates rice and upland crops cyclically on the same land. It has many advantages such as increase of permeability in soil, formation of the soil pores, and decomposition of organic matter (Ahn et al., 1992; Huang and Ding, 1995; Kim et al. 1995a; Kim et al., 1995b).

In order to succeed to cultivate of upland crops in rice paddy field, it is necessary to investigate the yearly variation of growth and yield of crops and yearly changes in soil characteristics in rotated upland field from rice paddy field.

In Asian countries, continuous rice planting has had a negative impact on soil properties, such as reduced soil nitrogen supply and organic carbon content (Cassman, et al., 1995; Dobermann et. al., 2000). Rice paddy-upland crop rotations have been recommended and used to improve soil quality and reduce input (Yamaguchi et al., 2009; Witt et al., 2000). Rice paddy soils show a large difference from upland soils in physical, chemical, and biological properties (Kirchhof et al., 2000). Furthermore, because of long-term submergence and mineral fertilizer application, rice paddy soils experience degradation of soil quality, such as breakdown of stable aggregation and deterioration of soil organic matter, which negatively affects agricultural sustainability (Boparai et al., 1992; Mohanty et al., 2004).

Therefore, this study was conducted for three main purposes. The first was to investigate yearly changes of growth and yield for waxy maize in the organic farming waxy maize at rice paddy-upland rotation system, the second was to investigate yearly changes of soil physical and chemical characteristics of rice paddy field soil in organic farming waxy corn at rice paddy-upland rotation system, and the third was to select the suitable varieties for rice paddy-upland rotation.

MATERIALS AND METHODS

Yearly Variation of Growth and Yield

This study was conducted in a field of silty clay loam located in the middle district of Gyeonggi-do, Korea (37°00'40.7"N,127°11'35.0"E). We used the upland field rotated from matured rice paddy field, which have been used as a rice paddy field long time, for three years from 2015 to 2017. The test varieties were that 8 varieties of waxy maize of Mibaek 2, Ilmichal, Daehakchal, Chalok 4, Miheukchal, Eolrukchal 1, Heukjinjuchal, Heugjeom 2. Seeding and transplanting date were shown in Table 1. Planting space of waxy maize was 70 cm (ridge) × 10 cm (plant). 300 kg of organic compost per 10a was fertilized as basal fertilization about one month before transplanting in every year. Plants were grown in ridge covered with vinyl. Plot design was randomized block deign with three replications. Tasseling and silking date were determined at when 50% of them were tasseled and silked. Characteristics of growth and yield such as culm length, ear length, and ears per hill were investigated. The harvest of waxy maize was carried out from 21 to 22 days after silking. 1000 grain weight was weighed after dry in heated-air dryer for 72 hours at 60?. Fresh ear yield per 10a was evaluated by four replications of 1 m2 in each treatment. Other cultural and analyzing methods were based on the standards of RDA in Korea. Statistical analysis of the data was conducted by use of SAS 9.2.

Changes of Soil Characteristics

Soil sample was taken about mid-May in each year and analyzed for soil physical characteristics. Soil bulk density was determined by using a soil core method (Blake and Hartge, 1986a). Three replications of the core samples were taken from each plot from arable soil. depths. The samples were dug out with taking care not to damage them. Both ends of the core cylinders were closed with caps and placed in polyethylene bags after tightening them thoroughly. The cores were weighed and oven-dried at 105? for 24 hrs and then the dried samples were reweighed in the laboratory. Also, their diameters and lengths were measured to determine the bulk density and water content of the soil as well for three phases of the soil. In addition, soil particle density was determined by the Pycnometer method (Blake and Hartge, 1986b) to estimate soil porosity. Soil porosity was determined from the results of soil bulk density and particle density using a following equation: P = (1 –ρs/ρp) × 100, where P is the soil porosity (%), ρs is the soil bulk density (Mg m^-3) and ρp is the soil particle density (Mg m^-3). The water content was calculated from the mass of water in the soil sample using a following equation: W= (mass of wetting soil –mass of drying soil)/ core volume × 100, where W is water content (%). Soil hardness was expressed as a force divided by the vertical projection of the cone area.

Soil sample for chemical characteristics was taken about mid-May in each three year and analyzed in accordance with the standards of survey analysis research of National Institute of Agricultural Science and Technology (NIAST, 2000). For the determination of soil pH, air-dried soil samples were mixed with deionized water with a ratio of 1:5 (soil: water) and the pH of clear supernatant was measured by pH electrode (Orion VERSA STAR, Thermo fisher scientific, US), and then, soil electric conductivity (EC) was measured by electric conductivity meter (Cm^-30R, DKKTOA, Japan). Soil available phosphate was determined according to the method of Lancaster. For determination of soil exchangeable cations (K, Ca, and Mg), air-dried soil samples were extracted with 1 N ammonium acetate for 30 minutes, and concentration of the extract was determined by SAS (Analyst 400, Perkin Elmer, USA). Statistical analysis of the data was conducted by use of SAS 9.2.

RESULTS AND DISCUSSION

Yearly Variation of Growth and Yield

The three-year average of days to tasseling among 8 varieties showed 66.1 days in the first year, 67.6 days in the second year and 68.6 days in the year (Table 2). There was no difference in the days from seeding to tasseling between three years. The three-year average of days to silking among 8 varieties showed 70.8 days in the first year, 71.1 days in the second year and 70.6 days in the third year. In the two-cropping (double cropping) test, Jung et al. (2012) reported that days from to silking was getting faster as the seeding was later. However, it was not easy to compare them with the results of this test, since there are few tests of waxy maize for rice paddy-upland rotation.

There was significant difference in term of plant height and ear height. The difference in the plant height between cultivars are considered to be their characteristic of cultivars. There is a trend of reduction for plant height by 30cm with the time of upland cultivation increasing year by year. Plant height in the three-year average of 8 varieties showed 260.0 cm in the first year, 236.8 cm in the second year and 201.6 cm in the third year. Among the three-year average of 8 varieties, Miheukchal and Ilmichal were the highest plant height and they were 254.1 cm and 252.2 cm respectively. Huh et al. (2003) stated that the average plant height for the test of three different cultivations in the Chalok 1 was 171.4 cm, while the average plant height for this test was 242.6 cm. Kim et al. (2017) stated that the plant height in the first cultivation was 204.0 cm, while it was 222.0 cm in the second cultivation, which was 12 cm shorter than the first. But it is not the same variety and difficult to compare. Plant height can affect the lodging resistance of maize, which can be used as an index of lodging resistance (Lan and Chu. 2005). Variety of Mibaek 2 was thought to be a desirable for lodging resistance. Plant height was big difference among three years, showing the reducing trend by 30 cm as the number of years of upland cultivation increased. This result is consistent with the previous research of Ji et al. (2006).

Ear height also showed significant difference between varieties and years. Ear height of the first year showed the highest (133.1 cm), while that of the third year was 95.4 cm. Ear height decreased by 18 to 20 cm, as the number of years of cultivation of rotated upland increased. Miheukchal showed the highest ear height (138.5 cm), while Mibaek 2 was the lowest (94.4 cm) among the three-year average of 8 varieties. Huh et al. (2003) stated that the average height of ears for Chalok 1 was 71.4 cm. Although it was different variety to compare with the ear height of Chalok 4 in this test, Chalok 4 was 118.8 cm, which was 47.4 cm higher ear height. The reason for the decrease in the ear height after rotation of the rice paddy field to the upland is that the nutrients are gradually depleted in the soil over the course of the year.

Culm diameter was also recognized as a significant difference between varieties and years. Culm diameter of the first, second, third year in the average of 8 varieties showed 24.3 cm, 22.3 cm, and 22.6 cm respectively. As the number of years of cultivation of rotated upland increased, it also decreased in order of year 1, year 2 and year 3. Chalok 4 showed the highest culm diameter (23.5 mm), while Daehakchal was the lowest (20.2 mm) among the three-year average of 8 varieties.

In the number of leaves, although there was no significant difference between years, it was between varieties. Eolrukchal 1 showed the highest number of leaves (14.2 leaves), while Daehakchal was the lowest (11.4 leaves) among the three-year average of 8 varieties.

Ear length showed no significant difference between years. But it was significant difference in the second year of cultivation. The average of ear length in the second year of cultivation was 17.9 cm. Mibaek 2 showed the highest ear length (20.6 cm), while Heugjeom 2 was the lowest (15.4 cm) in the second cultivation rotated from rice paddy field. Kim et al. (2010) tested using Chalok 4 to compare between the prior and second cultivation, and found that the average ear length was 19.1 cm. Ear length of this test was 1.2 cm shorter compared to result of Kim et al. (2010). Perhaps this result was due to organic farming with rotated rice paddy field for waxy maize.

In the three-seeding date average of ear length for Chalok 4, Jeong et al. (2012) showed it as 20.7 cm, which was 2.5 cm longer than the average for three years of Chalok 4 in this test. In the three-seeding date average of 15 varieties including Kangpyeongok, the average of ear length was 19.1 cm in the seeding date on June 25 (Kim et al., 2017). In this test, the three-year average of ear length of eight varieties was 17.8 cm, which was 18.3 cm in the first year and 17.9 cm in the second year, and 17.3 cm in the third year rotated from rice paddy field. It had been reduced as the cultivation number of years increased. These results of short ear length compared the other’s results were believed to be for the reason that this test were cultivated on rotated upland from rice-paddy field and was cultivated as organic agriculture.

Although there was not significant difference between years, it was significant difference between varieties in term of the average ear weight of each year. Ilmichal showed the highest ear weight (158.7 g), Heukjinjuchal was the lowest (108.6 g) in the three-year average of 8 varieties. In ear diameter, although there was non-significant difference between years, it was between varieties. Chalok 4 showed the highest ear diameter (43.9 mm), while Daehakchal was the lowest (34.9 mm) among the three-year averages of 8 varieties. Although the variety used was different, Lee et al.(2011) reported that the ear diameter of Daehakchal Gold 1 was the highest with 46.8 mm, which was 12 mm thicker than the average three-year of 34.9 mm in this test. In the three-seeding date average of 15 varieties including Kangpyeongok, the average of ear diameter was 48.0 mm and ear weight was 342 g (Kim et al., 2017). Though different varieties between tests, the ear diameter and ear weight in the three-year average of 8 varieties were 40.6 mm and 133.0 g respectively, of which both were low compared to the result of Kim et al. (2017). Direct comparisons were difficult, but ear weight and ear diameter were lower than the results of other researchers. I think this results were due to organic farming as well as rotated upland.

Although there was no significant difference between years, it was between varieties in ear length ratio. Heukjeom 2 showed the highest ear length ratio (91.4%), while Eolrukchal 1 was the lowest (80.1%) among the three-year average of 8 varieties.

The highest yearly yield was obtained in the first year of 949.6 kg, the second highest was in the third year of 680.6 kg 10a^-1, while the second year was the lowest (675.4 kg) among the three-year average of 8 varieties. Waxy maize yield tended to decrease with increasing number of years rotated upland from paddy field. This was probably due to the higher water content in soil and high nutrient content in the first year. Ji et al. (2006) reported that the yield of maize in rotated upland was higher than that of continuous upland field, because rice paddy soils have adequate soil moisture for growing maize so that the amount of water needed for growth was constantly supplied. It was also reported that rice paddy soils with good drainage are more advantageous for growing upland crops than those grown in continuous upland field soil. Among 8 varieties of waxy maize, both varieties of Chalok 4 and Ilmichal showed the highest yield with about 900 kg 10a^-1 in the three-year average of 8 varieties (Table 2). Yu et al. (2017) reported that variety of Chalok 4 was the highest yield of 789.0 kg 10a^-1 in the rotated upland from rice paddy field. This result was in agreement with the this test result. Huh et al (2003) stated that the average-three cultural methods of Chalok 1 was 675.5 kg 10a^-1. Although it was difficult to compare different variety with the Chalok 4 in this test, the three-year average of Chalok 4 variety was 891.6 kg 10a^-1, which was 216.1 kg 10a^-1 higher yield.

In conclusion, the yield of waxy maize decreased with the increasing number of years of the rotated upland field. Among 8 varieties, Ilmichal and Chalok 4 showed higher yield compared to other varieties in the three-year average yields. The reason for the decrease in the ear height after rotation of the rice paddy field to the upland is that the nutrients are gradually depleted in the soil. Nutrients are only fertilized by compost.

Changes of Soil Characteristics

In this experiment, solid and liquid phase of the soil were significant difference between first and third years. Compared with the first year, the ratio of solid phase and liquid phase of the soil tended to decrease by 1.7% and 2.6%. On the other hand, a increased 4.5% of gases phase in the third year compared to the first year. It was seen that the soil pore was increased as the number of years of rotated upland was increased. Porosity rate of the third year (53.4%) was 2.7% higher than that of the first year (50.7%).

In organic rice paddy-upland rotation, bulk density of the third year was a little lower than that of the first year, but there was no significant difference (Table 3). Witt et al. (2000) reported that traditionally, the main patterns of rice and upland crop in China are rice-wheat, rice-oilseed, ricemilk vetch, and rice-ryegrass. He stated that the bulk density was significantly greater in RC (rice-Chinese milk vetch) and ROF (Rice-oilseed rape with fresh straw return) compared to the other treatments (Chen et al., 2012).

Average soil pH of three years was 6.5, which means slightly acid, but there was no significant difference between years. EC of the soil showed no significant difference between three years.

Soil organic matter content was significant different between three years. Average soil organic matter of was 12.0 g kg-1 in the first year, 10.0 g kg-1 in the second year, and 9.5 g kg-1 in the third year rotated from rice paddy field. Soil organic matter in the three-year average was 9.5 g kg-1, which was 2.5% lower than that of the first year. The soil organic matter content decreased with the increase of the number of years of rotated upland cultivation.

There was a significant difference in exchangeable K, Mg concentration between rotated years. The average exchangeable K concentration in soil was the highest in the second year by 0.37 cmol+ kg-1. The first year of Mg concentration showed the highest (2.8 cmol+ kg-1) and the lowest was in the second year (2.3 cmol+ kg-1)(Table 4). In rice paddy-upland rotation, Chen et al. (2012) reported that significant differences in soil chemicals (i.e., soil pH, total nitrogen, CEC, soil organic matter) and soil bulk density was significantly difference between cropping seasons within the year (rice and upland crops season), Double cropping of rice-Chinese milk vetch and rice-rapeseed rotations improved the soil quality to some extent, which might result in the greatest yield performance in rice-Chinese milk vetch rotations among the tested rotations.

In conclusion, as the number of years of rotated upland cultivation from rice paddy field was increased, the gases phase of the soil tended to increase and the rate of soil porosity also increased. Soil organic content tended to decrease. In addition, exchangeable cations of K content tended to increase, and the Ca and Mg content decreased as the number of years of upland cultivation rotated from rice paddy field was increased.

However, it was difficult to compare them with each other over the three years, because of lack of study on the changes in soil's physical and chemical characteristics. Further researches are expected in the future.

CONCLUSION

In the three-year average yields of Ilmichal and Chalok 4 were 898.1 kg 10a^-1 and 891.6 kg 10a^-1 respectively and the yield of Chalok 4 was greater than the other 7 varieties. We compared and selected the two best waxy maize varieties of Chalok 4 and Ilmichal for rice paddy-upland rotation. When we look at the yearly variation for waxy maize, waxy maize yield was the highest in the first year and decreased year by year. Therefore, it would be better to restore upland field to rice paddy fields after the first year.

In the soil physical characteristics, the three phases of soil remained unchanged, but the rate of soil porosity increased, as the number of years of upland cultivation rotated from rice paddy field was increased. Soil organic content was decreased, the K content tended to increase, and the Mg content decreased as the number of years of upland cultivation rotated from rice paddy field was increased in the soil chemical characteristics. The increase and decrease of these nutrients are thought to be due to the absorption rate of the crops and supply to crops.

적 요

본 시험은 답전윤환에 있어서 논으로부터 밭으로 전환된 포 장에서 년차간 풋옥수수의 생육 및 수량특성을 조사는 물론, 년차간 토양의 물리화학적 특성 변화를 분석하였다. 이의 결 과를 토대로 답전윤환에 적합한 풋옥수수 품종 선발 및 풋옥 수수 답전윤환 생산체계 수립을 위한 기초자료를 수립코자 수 행한 시험결과는 다음과 같다.