Essay: Soil in Southeast China

CHAPTER ONE
1. INTRODUCTION
1.1 Background to the problem
There are large areas of red soil distributed in Southeast China. Approximately, the covering area of red soil is 2.04 million km2, about 20% of the total land of this country. So, red soil is an important resource for the exploitation and utilization of agriculture in the tropics and subtropics of China (Cao and Zhu, 1999). These soils are highly weathered and inherently infertile. They are acidic, nutrient-deficient, and poor in organic matter (Huang et al. 2010). Soil physical nature has high clay content, more macropores like sandy soil and many micropores within the aggregates, low available water and high unavailable water content, high soil bulk density, high soil penetration resistance, which increases as the water content decreases, high saturated hydraulic conductivity, but rapid decrease of unsaturated hydraulic conductivity with the decrease of soil water content, usually increase the soil drought in the root layer. This makes the red soil with low water supply ability liable to drought (Cao and Zhu, 1999; Lin et al. 2012; Yang et al. 2013).
The hydrothermal conditions in red soil region are favorable, with an annual precipitation usually more than the annual evaporation rate. But the distribution of rainfall in a year is uneven due to the effect of Southeast Asia monsoon weather. The rainfall from July to September is 18.8% of the annual precipitation and the evaporation from June to September is 53.5% of the total annual evaporation rate and this period with high evaporation in a red soil region (Cao and Zhu, 1999). There are always low temperatures in spring, and severe drought and high temperature in summer and early autumn. During the wet season of spring to early summer, soil erosion is caused by intense rainfall, which largely contributes to the degradation of soil properties (Lin et al. 2012). Thus, hydrothermal conditions become one of limiting factors for crop growth in red soil region.
Fertilizers have great potential in agriculture because of their positive effects on soil properties, plant growth, and the environment, in addition to being one of the most common materials for rehabilitation practices to improve soil properties and stimulating plant growth. Organic manure and inorganic fertilizer are the most common materials applied in agricultural management to improve soil quality and crop productivity (Zhang and Xu, 2005). Conservation of the quantity and quality of soil organic matter is considered a central component of sustainable soil management and maintenance of soil quality (Li, 1983). Many studies have shown that balanced application of inorganic fertilizers or organic manure can increase soil organic carbon and maintain soil productivity (Sun and Huang, 2011; Chen et al. 2009).
Application of organic manure to the soil can increase soil organic matter. Increase in soil organic matter is adsorption properties of soils and, thus; their soil properties (Zhang et al. 2006). Although an increase in organic matter is usually beneficial for improving the soil properties, these effects may vary according to local climates and soil types (Haynes and Naidu, 1998).