**3. Long-term application of fertilizers**

#### **3.1. Commonly used organic fertilizers in the red soil region**

Commonly used organic fertilizers in the red soil region come mainly from green manure, farmyard manure, and crop residues (returned and left in place). In this area, rice and rapeseed are the main field crops covering 55 and 11% of the total cropped area, respectively. The amount of rice straw and rapeseed stalk account for 70–75% and 8.5–11% of the total crop residues in this region [14]. Double rice (two rice crops per year) and winter rapeseed is the main cropping pattern producing 9500–1200 kg hm2 straw yearly, but the rapeseed only yield about 1660–6900 kg hm2 residue yearly. Dry land crop straw is mainly used as livestock feed and cooking fuel, and only small portion is returned to the field. Radish (*Raphanus sativus*) and milk vetch (*Astragalus sinicus* L.) are also used as the main winter green manure which produced about 10,500–15,000 kg hm2 fresh biomass yearly. Pig and cattle manure were the main livestock manures in the region [14]. Crop, especially rice, above and below ground residue is another important source of soil organic matter. The nutrient contents of major organic materials used in the typical red soil region, Yujiang Country, Jiangxi Province, China, are presented in **Table 3**


**Table 3.** Nutrient contents of major organic materials used at the Yujiang Country, Jiangxi Province, a typical red soil region [14].

#### **3.2. Long-term fertilization experiment on a typical red soil**

The Red Soil Ecological Experiment Station, constructed in 1985 by the Institute of Soil Science, Chinese Academy of Sciences (ISSAS), is located in Liujia Zhan, Yujiang County, Jiangxi Province, China (28°15′20″ N, 116°55′30″ E). It is one of the key laboratories for studying the ecology of red soils in China. Red Soil Ecological Experiment Station is dedicated to finding solutions to ecologically sound and environmentally friendly use of the red soil resources. It primarily explores (1) the structure, function, and productivity of red soil agricultural ecosystems; (2) the characteristics of material cycling and energy transformation between red soils and the environment; and (3) the relationships among regional resources, the environ‐ ment, and the economy, in order to provide a scientific basis for sustainable development of the region.

In order to understand the changes of soil physicochemical properties, soil fertility, and the sustainable utilization of red soil as affected by the combined application of organic–inorganic fertilizers and the response of the cultivated crops to those changes and variations, a long-term fertilization experiment was established at the station in 1988. The experiment had a com‐ pletely randomized block design with five fertilizer treatments and three replications. Each plot was 33.3 m2 . In this chapter, three relevant treatments were chosen for discussion. Those treatments were: NPK (control, CK), CK plus rice (*Oryza sativa* L.) straw (RS) and CK plus pig manure (PM). The crops grown during the experiment before 1995 included peanut (*Arachis hypogaea* L.) in Season 1 (April–August) and cole (*Brassica napus* L. *var. napus*) in Season 2 (September–December). Since 1995, peanut was planted in Season 1 and the land was followed in Season 2. The sources of chemical fertilizers for the experiment were urea for N, KCl for K, and Ca-Mg phosphate for P. The amounts of chemical fertilizers applied to the plots every year were as follows: N, 60.0 kg hm−2; P2O5, 19.65 kg hm−2; K, 58.85 kg hm−2. At the same time, 3000 kg hm−2 of rice straw (dry weight) and 30,000 kg hm−2 of fresh pig manure were added to the Treatment RS and PM in addition to chemical fertilizers, respectively. Nutrient contents of the rice straw and pig manure on dry weight basis were as follows: organic carbon, 376.0 and 265 g kg−1; total N, 6.7 and 36.5 g kg−1; total P, 2.0 and 23.0 g kg−1; Total K, 23.0 and 52.0 g kg−1; C/N ratio, 56.1 and 7.26, respectively.

The test site is in the typical middle subtropical region, with a mean annual rainfall of 1785 mm (ranging 1040–2550 mm) during the past 22 years, a mean annual temperature of 17.8°C (ranging 16.1–18.9°C), and the frost-free days per year ranged from 240 to 300 days. The soil at the site is a typical udic ferrosol [11] with kaolinite red clay parent material of Quaternary age. Before the long-term experiment, the land was a gently sloping hill covered with herba‐ ceous vegetation with no history of crop production, and partial chemical and physical characteristics of the soils tested in 1988 were as follows: pH, 4.65; soil organic carbon, 3.71 g kg−1; TN, 0.34 g kg−1; TP 0.53 g kg−1; 10.6 g kg−1; C/N ratio, 10.9.
