Effect of EM on the Growth and Yield of Sweet Potato in Wet and Dry Seasons

Sweet potato (Ipomoea batatas L.) is a tropical tuber crop, primarily cultivated by small landholders in Asia. The crop is a versatile component of the agricultural systems; it has the capacity to produce a significant quantity of carbohydrates, and the protein and vitamin content enhances its value. Thus, it is cultivated in both wet and dry seasons, although yields are generally lower under dry conditions. Experiments were conducted with a common short season variety of sweet potato using effective microorganisms (EM) as a microbial inoculant to study the effect on crop production during a wet and dry season, Effective microorganisms are reported to improve soil characteristics and to provide the required growth resources as well as a sustainable environment. Treatments over a wet and a dry season at the same location included (a) chemical fertilizer, (Z:) chemical fertilizer with organic amendments, (c) these same treatments with EM, and (d) control treatment with no additives. The data included growth and development of the crop and yield, and water holding capacity of the soil for both seasons. The results showed that EM increased yields of sweet potato, especially in the dry season when the numbers of tubers, and bulking rates were generally lower. The study also indicated that EM maintained the water holding capacity of the soil to a greater extent during the dry season. Introduction Sweet potato (Ipomoea batatas L.) is a popular tropical tuber species, primarily grown in the developing world by small landholders. Its primary use is as a food crop; some of the surplus is sold along with other produce from home gardens (Horton et al., 1983). The crop is popular and widespread because of its relatively short season in comparison with most other tropical tuber crops and its adaptability to a wider range of environments (Horton, 1988). The sweet potato requires adequate soil moisture for high yields (Onwueme, 1977). Thus, it is best adapted to regions with well distributed rainfall or with irrigation facilities because of the moisture requirement for tuber initiation and development (Martin, 1988). However, sweet potato is cultivated in Sri Lanka only as a rainfed crop under most agricultural systems, and yields vary widely with location. Hence, a primary factor for improving yields of this popular tuber crop is modification of the soil environment to retain a greater proportion of the available moisture for plant use, especially since drought has been identified as the most serious abiotic factor affecting yield (Ekanayake et al., 1988). Organic matter has been a primary source of nutrients for small farm agriculture (Harwood, 1990). Addition of plant or animal wastes to the soil provides nutrients and improvements to the rhizosphere in terms of physical properties such as water holding capacity and texture (Allison, 1973). Thus, traditional agriculture of the tropics has relied on organic amendments to maintain yields, especially in dry seasons. This phenomenon has been adequately demonstrated in many studies (Wilson, 1983; Sangakkara and Ratnayake, 1988) in rainfed environments. The value of organic amendments cannot be assessed by plant nutrient analysis alone, since nutrient availability to crops is of greater importance. In addition, the rate of decomposition and release of plant nutrients in an available form determines the usefulness of organic amendments. It is affected by the carbon : nitrogen (C:N) ratio; high values can delay microbial decomposition of organic matter. In addition, organic matter may also immobilize soil nutrients, especially nitrogen, causing temporary deficiencies to the crop (Tomar and Soper, 1981). Recent research illustrates an alternative method for enhancing the value of organic amendments. Reports from Japan indicate that effective microorganisms (EM) have the ability to enhance the decomposition of organic materials and to improve physical, chemical and biological properties of soils (Higa, 1988). In addition, research using hydroponic cultures have indicated the ability of EM to increase root growth of selected vegetables (Hornick and Parr, 1990, Personal Communication). Thus, a study was undertaken to determine the effect of EM on growth and yield of sweet potato in both wet and dry seasons with addition of low C:N ratio organic amendments. The selection of organic materials was based on other information, i.e., the increased benefit of using low C:N organic matter with EM solutions for increasing the yields of food crops (Sangakkara and Higa,