Improving the Establishment and Yield of Soybean through Planting Depth and Land Preparation Methods in Northern Ghana

Two field experiments, organized in randomized complete block design, were conducted in 2005 and 2006 farming seasons at Nyankpala in the Northern Region of Ghana to investigate the effects of planting depth (1, 2, 4, 6 and 8 cm) and land preparation methods (flat field, mounding and ridging) on crop establishment (percentage number of established plants) and grain yield of soybean (Glycine max cv. Anidaso). In the 2005 farming season results indicated that crop establishment increased from planting depth of 1 cm (97.75%), reached a peak at 4 cm (98.82%) and declined through 6– 8 cm (94.15%) under flat field condition. Nodule formation was high between planting depth of 2 and 4 cm. Generally, the number of pods and grain yield decreased with increasing plant depth. The grain yield values for 1, 2, 4, 6 and 8 cm depths were 4.25, 4.22, 3.87, 3.26 and 3.12 t/ha, respectively. In the 2006 farming season, at 4 cm planting depth ridging gave the highest crop establishment of 97.77%. The highest number of pods was recorded for mound (197 per plant) followed by ridge (189 per plant) and flat (143 per plant). This pod formation characteristic reflected in the grain yield of 6.14 t/ha, 5.95 t/ha and 4.07 t/ha for mound, ridge and flat, respectively. The study revealed that for good crop establishment and optimum grain yield of soybean in northern Ghana planting could be done on mounds or ridges at planting depth between 1 and 4 cm. Ridging may be preferred because it can easily be prepared mechanically using a tractor to save time and labour. Introduction In Ghana, soybean is cultivated mainly in the Northern, Upper West, Upper East, Central and Volta regions. Among these geographical regions, the largest production occurs in northern Ghana, which lies within the Guinea savanna and Sahel agro-ecological zones. The average yield for northern Ghana (Northern, Upper West and Upper East regions) was about 2.5 tonnes/ha on the farmers’ field (Awuku, 1991) compared to that of USA which was 4.6 tonnes/ha (Richard et al., 1984). One of the factors contributing to low crop yields in northern Ghana is poor soil fertility, mainly due to low content of organic matter and nitrogen (Adu, 1969). Besides, one of the consequences of poor soil fertility in northern Ghana in the past few decades has been the falling agricultural productivity index (Dogbe, 1998). Crop establishment is another problem and is often cited as a production problem for soybean in both arid and semi-arid areas of Central and West Africa (ICRISAT, 1984). Farmers are frequently obliged to resow several times to attain an acceptable stand of plants. However, resowing increases labour requirement for crop production, and the crops from late sowing often yield less than those sown with the first rains (Charreau, 1974). Thus, poor and irregular stand or crop establishment remains the major problem with farmers who cultivate on flat fields. Johnson & Wax (1979) documented that cultural variables such as seedbed conditions, seed quality, and minor differences in planting depth are often the major variables causing stand problems. In the USA, the North Dakota State University (NDSU, 2008) reported that soybean should be planted at a depth of 2.5– 3.75 cm and never deeper than 5 cm. The National Soybean Research Laboratory (NSRL, 2008) of the USA also reported that the typical planting depths of soybean are 2.5–3.75 cm, but if the soil is low in moisture or sandy, one should plant at 5 cm deep; and seldom should soybeans be planted deeper than 6.25 cm. In northern Ghana, attempts have been made by researchers to improve the characteristic growth and yield of soybean. For instance, chemical fertilizers have been applied with the aim of increasing crop establishment and yield of soybean (Asante, 1999). In another research work the use of spacing of 5 and 10 cm between plants and within row, respectively, and 60 and 75 cm had been used to improve establishment of soybean (Kaleem, 1986). Besides, the application of organic matter and inoculation with Bradyrhizobium japonicum were also used to enhance yield of soybean West African Journal of Applied Ecology, vol. 14, 2008 2 (Lawson & Quainoo, 2002). However, little attempts have been made using planting depth and land preparation methods to improve establishment and yield of soybean in northern Ghana. It is against this background that the present study was conducted with the aim to improve the crop establishment and yield of soybean through planting depth and land preparation methods in northern Ghana. Materials and methods Experimental site The experiment was conducted at the Faculty of Agriculture Farm, University for Development Studies at Nyankpala in the Northern Region of Ghana. The area lies within the interior Guinea savanna of Ghana, which falls on latitude 9O25’141”, longitude 0O58’142” and at an altitude of 183 m above sea level. The area is characterized with natural vegetation dominated by grasses with few shrubs. The area has an average minimum temperate of 25 OC and maximum average temperature of 35 OC, with total annual monomial rainfall of about 1022 mm. The soil was derived from Voltaian sandstone and classified as Nyankpala series (Plinthic Acrisol; FAO, 1988). Plant culture The experimental field was ploughed, harrowed and demarcated into plots in which each plot measured 2 m × 4 m. The plots were laid in four blocks with 1 m between blocks and 0.5 m between plots. During the 2005 farming season the test crop, soybean (Glycine max cv. Anidaso), was sown at planting depths of 1, 2, 4, 6 and 8 cm at two seeds per hill by the dibbling method at a planting distance of 60 cm × 5 cm under flat field condition. During the 2006 farming season, the soybean was sown at 60 cm × 5 cm on mounds (diameter 60 cm and height 30 cm), ridges (height 10 cm) and flat field at a planting depth of 4 cm. The treatments were laid out in a randomized complete block design (RCBD). The plants were grown in the field to maturity period under rain fed condition without fertilization. Data collection and analysis The parameters taken were crop emergence (percentage number of emerged seedlings) crop establishment (percentage number of established plants at flowering), nodule number, dry matter (nodule, shoot and root biomass), number of pods and grain yield. All data collected were subjected to analysis of variance (ANOVA) using the Genstat package (Lawes Agric. Trust, 2002). Crop emergence and establishment data were transformed as described by Gomez & Gomez (1976) before subjected to ANOVA. Treatment means were considered to be significantly different at P ≤ 0.05. Results Crop emergence Results have shown that planting between 1 and 4 cm gave high crop emergence values in the range of 94.14–96.60% (Fig. 1). There was a sharp decrease in emergence below 4 cm depth. The least crop emergence was observed for seeds sown at a depth of 8 cm with a low crop emergence value of 19.58%. Mounding and ridging the field significantly (P < 0.05) increased crop emergence by 3.95 and 10.61%, respectively, when compared to the conventional sowing on flat land (Fig. 2). Ridging gave the highest crop emergence, and it was significantly (P < 0.05) different from mounding.