**2. The nature and properties of red soils**

### **2.1. Distribution of red soils in southern China**

Red soils are mainly distributed in the tropical and subtropical zone all over the world, which occupy around 6.4 × 109 ha, accounting to about 45% of the world land area. In addition, most of the red soils are distributed in the developing countries. The population living in the red soil regions is about 2.5 billion, roughly 48% of the global population [10]. Various types of yellow or red soils collectively known as red soil series, such as laterite, laterite red soil, red soil, and yellow soil (equivalent to ferrisol or ultisol [11]), are widespread in southern China. Scattered patches of similar soil are also seen in south central China. The total area of red soil in China is about 2.18 × 108 ha, covering about 21.8% of the total land area, or 28% of the farmland area. However, the population in the red soil series region accounts for 40% of the overall national population [10, 12] due to its warm and humid climate.

#### **2.2. Basic properties of red soils**

are also characterized by low pH, cation exchange capacity (CEC), and fertility. Red soil also has low concentrations of P in soil solution and results in frequent P deficiency of plants [1]. The improvements of soil fertility especially plant available P level, soil pH, soil structure, and waterholding capacity in the red soil region are always a major challenge. The application of chemical fertilizer alone, and that combined with organic fertilizers, such as crop residues and farm‐ yard manure, are two common approaches to improving soil quality for grain production [2– 5]. Rice (*Oryzasativa* L.) is the main cereal crop in the red soil region. Returning rice straw containing C, N, P, K and microelements to the soil (both used as a surface cover and incorpo‐ rated) has been extensively shown to increase organic matter and nutrient contents resulting in improved soil physical, chemical and biological characteristics. Crop residue is returning not only can increase crop yields, but it also can enhance soils' resistance to wind and water erosion [6, 7]. Animal manures, such as pig manure with ample N, P and K, are valuable resources to

Therefore, rice straw and fresh pig manure are commonly used as organic amendments in the red soil region of China. When combined with chemical fertilizers, those organic amendments could effectively regulate soil physicochemical properties and soil fertility. Many researchers have proven that long-term application of organic-inorganic fertilizers significantly increased soil water supply capacity, promoted soil nutrient recycle and distribution, improved soil aggregate structure [2]. Therefore, in this chapter, the changes or fluctuations of soil acidity, soil organic matter, soil NPK fertility, and soil aggregate structure due to the combined application of chemical fertilizer and rice straw or pig manure in a long-term (1988–2009) experiment were summarized. Lessons learned can be used to improve nutrient management so that crop yields are optimized and the impact of food production on the environment is

Red soils are mainly distributed in the tropical and subtropical zone all over the world, which occupy around 6.4 × 109 ha, accounting to about 45% of the world land area. In addition, most of the red soils are distributed in the developing countries. The population living in the red soil regions is about 2.5 billion, roughly 48% of the global population [10]. Various types of yellow or red soils collectively known as red soil series, such as laterite, laterite red soil, red soil, and yellow soil (equivalent to ferrisol or ultisol [11]), are widespread in southern China. Scattered patches of similar soil are also seen in south central China. The total area of red soil

farmland area. However, the population in the red soil series region accounts for 40% of the

overall national population [10, 12] due to its warm and humid climate.

ha, covering about 21.8% of the total land area, or 28% of the

supply the needed plant nutrients and organic matters [2, 4, 5, 8, 9].

190 Organic Fertilizers - From Basic Concepts to Applied Outcomes

**2. The nature and properties of red soils**

**2.1. Distribution of red soils in southern China**

in China is about 2.18 × 108

minimized.

Rich in iron-aluminum (hydr)oxides with strong fixation capacity of phosphate, low pH, and organic matter content, and poor nutrient availability are the main yield-limiting factors for the red soils. According to the results of the second national soil census data, the fertility of major red soils was moderate or poor (**Tables 1** and **2**). Soil phosphorus (P) availability of red soils was considered seriously deficient. These serious P-deficient red soils cover most of the farmland in the region. Soil potassium (K) of the red soils is not as bad as P by 26.3 and 13.6% of the red soil area considered moderate and seriously deficient, respectively. Soil nitrogen (N) status of the red soils is situated between the P and the K with most in the moderate and serious deficient category [12, 13].


**Table 1.** Evaluation standard of soil fertility status of the hilly upland red soil in southern China [12].


\* Capital letter in the bracket represents soil with different fertility level. A, fertile; B, mild deficiency; C,: moderate deficiency; D, severe deficiency.

**Table 2.** Average content of various soil nutrients and their corresponding soil fertility level in the upland red soils from Hunan, Jiangxi, and Zhejiang provinces, China [12].
