**4.1. Soil acidity**

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

192 Organic Fertilizers - From Basic Concepts to Applied Outcomes

produced about 10,500–15,000 kg hm2

are presented in **Table 3**

region [14].

**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

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,

**Table 3.** Nutrient contents of major organic materials used at the Yujiang Country, Jiangxi Province, 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

**Type C Total N C/N ratio Total P Total potassium g kg−1 g kg−1**

Cattle dung 404.1 18.82 21.5 7.19 4.37 Pig dung 368.4 24.36 15.1 9.36 2.80 Milk vetch 440.2 30.82 14.3 3.65 26.02 Radish 429.6 23.41 18.4 2.82 18.12 Peanut straw 435.6 12.48 34.9 0.84 12.61 Cole stalk 467.4 5.22 89.7 0.46 15.72 Rice straw 432.4 10.89 39.7 0.55 19.95 Wheat straw 457.5 6.12 74.8 0.66 7.90 Rice root 306.2 8.38 36.5 1.23 3.57 Wheat root 329.6 6.34 52.1 0.89 4.05

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

fresh biomass yearly. Pig and cattle manure were the

Soil pH had a significant increasing trend during the long-term organic–inorganic fertilization (**Figure 1**) from 1988 to 1995, but it gradually decreased since then. There was a significant negative linear correlation between soil pH and cultivation time after the seventh year of the experiment. This suggests that initial fertilization raised pH in the very acidic soil to near neutral, but continuous fertilization led to further soil acidification. The mechanism of pH changes remains to be explored. The CK treatment decreased soil pH by 0.07 yearly, while addition of rice straw and pig manure decreased the pH by 0.05 and 0.04 yearly, respectively (**Figure 1**). At the rate of pH decline, soil acidity of the upland red soil would reach the original soil condition in 1988 after 30 (CK and RS) or 48 (PM) years of cultivation. In addition, soil pH of RS treatment decreased faster than that of the PM treatment after the seventh year.

**Figure 1.** Changes of soil pH in the upland red soil as affected by the long-term application of organic–inorganic fertil‐ izers (1988–2009).

#### **4.2. Soil organic matter**

Compared with the CK, addition of rice straw and pig manure significantly increased soil organic matter contents in the upland red soil during the long-term fertilization (**Figure 2**) study. The rice straw treatment had a higher accumulation rate of organic matter (0.34 g kg−1 yearly) than the pig manure treatment (0.14 g kg−1) probably due to the higher amount of C and C/N ratio of the former.

**Figure 2.** Changes of soil organic matter in the upland red soil as affected by the long-term application of organic– inorganic fertilizers (1988–2009).

#### **4.3. Soil nitrogen, phosphorus, potassium**

changes remains to be explored. The CK treatment decreased soil pH by 0.07 yearly, while addition of rice straw and pig manure decreased the pH by 0.05 and 0.04 yearly, respectively (**Figure 1**). At the rate of pH decline, soil acidity of the upland red soil would reach the original soil condition in 1988 after 30 (CK and RS) or 48 (PM) years of cultivation. In addition, soil pH

**Figure 1.** Changes of soil pH in the upland red soil as affected by the long-term application of organic–inorganic fertil‐

Compared with the CK, addition of rice straw and pig manure significantly increased soil organic matter contents in the upland red soil during the long-term fertilization (**Figure 2**) study. The rice straw treatment had a higher accumulation rate of organic matter (0.34 g kg−1 yearly) than the pig manure treatment (0.14 g kg−1) probably due to the higher amount of C

**Figure 2.** Changes of soil organic matter in the upland red soil as affected by the long-term application of organic–

izers (1988–2009).

**4.2. Soil organic matter**

and C/N ratio of the former.

inorganic fertilizers (1988–2009).

of RS treatment decreased faster than that of the PM treatment after the seventh year.

194 Organic Fertilizers - From Basic Concepts to Applied Outcomes

Compared with the initial value in 1988, soil total nitrogen (TN) showed a significant increasing trend during the long-term fertilization (**Figure 3**). Both RS and PM treatments significantly increased soil TN content compared to the CK, but PM treatment was more effective in the accumulation of TN in the upland red soil.

**Figure 3.** Changes of soil total nitrogen in the upland red soil as affected by the long-term application of organic–inor‐ ganic fertilizers (1988–2009).

The upland red soil in this study is a typical acidic soil and characterized by its strong P-fixation capacity and low P availability due to the high content of aluminum and/or iron oxides, which can convert P in the soil solution to water-insoluble Fe-Al-P. Compared with the CK, rice straw addition was not significant in improving soil total phosphorus (TP) and Olsen-P, but the addition of pig manure did significantly increase soil TP and Olsen-P by 28.4–116.7% and 292.6–731.8%, respectively (**Figure 4**). There is a significant positive correlation between soil TP content and the year of cultivation (*r* = 0.843, *p* < 0.05). Based on this relationship, the soil TP content in the PM treatment was increased by 30 mg kg−1 yearly.

**Figure 4.** Changes of soil total phosphorus (P) and Olsen-P in the upland red soils as affected by the long-term applica‐ tion of organic–inorganic fertilizers (1988–2009).

During the long-term fertilization study, soil potassium (K) in the upland red soil significantly decreased at the rate of 50 or 60 mg kg−1 yearly and there was no significant difference among the three treatments (**Figure 5**). Compared with their initial values in 1988, soil total K was reduced by 1.9–12.0%, 2.8–9.3% and 0.9–14.0% in the CK, RS, and PM, respectively (**Figure 5**). The K reduction is probably due to plant uptake and removal or leaching losses.

**Figure 5.** Changes of soil total potassium (K) in the upland red soil as affected by the long-term application of organic– inorganic fertilizers (1988–2009).

#### **4.4. Soil aggregate structure**

Soil aggregate formation and stability are the results of complex interactions among biological, chemical, and physical processes in the soil, which are also important determinants of soil fertility and productivity. Distribution or/and redistribution of various sized soil aggregates in the upland red soil could be significantly affected by the long-term application of organic– inorganic fertilizers (**Figure 6**, [15]). The distribution of dry-sieved aggregates (DSA) in the upland red soil showed similar trends among CK, RS, and PM in the order of (>5) >1–0.25 > 5– 2 > 0.25–0.05 > 2–1 > (<0.053 mm), whereas the distribution of water-stable aggregates (WSA) was in the order of 0.25–0.053 > 1–0.25 > (>5)> (<0.053) > 5–2> 2–1 mm (**Figure 5**). Compared with the CK, addition of rice straw and pig manure significantly increased the proportion of DSA in 1–0.25, 0.25–0.053, and <0.053 mm fractions. No change in the 2–1 mm fraction, however, was detected. Addition of rice straw significantly decreased the proportion of WSA in the >5 mm fraction and increased the proportion of WSA in 2–1 and 1–0.25 mm fractions but did not affect the 1–0.25 mm aggregate fraction (**Figure 6**). Evidently, there was a significant difference between the effect of rice straw addition and pig manure amendment on aggregate size distribution in the upland red soil.

Overall, 86.9–93.8% of DSA and 47.4–50.0% of WSA were in the macro-aggregate (>0.25 mm) fraction in the upland red soil after 22 years of combined application of NPK fertilizer and organic amendments (**Figure 6**). Compared with the CK treatment, addition of rice straw and pig manure increased the proportion of the >0.25 DSA fraction by 4.9 and 7.9%, respectively; and only the addition of pig manure significantly increased WSA by 5.9% in the >0.25 mm fraction (**Figure 7**).

During the long-term fertilization study, soil potassium (K) in the upland red soil significantly decreased at the rate of 50 or 60 mg kg−1 yearly and there was no significant difference among the three treatments (**Figure 5**). Compared with their initial values in 1988, soil total K was reduced by 1.9–12.0%, 2.8–9.3% and 0.9–14.0% in the CK, RS, and PM, respectively (**Figure 5**).

**Figure 5.** Changes of soil total potassium (K) in the upland red soil as affected by the long-term application of organic–

Soil aggregate formation and stability are the results of complex interactions among biological, chemical, and physical processes in the soil, which are also important determinants of soil fertility and productivity. Distribution or/and redistribution of various sized soil aggregates in the upland red soil could be significantly affected by the long-term application of organic– inorganic fertilizers (**Figure 6**, [15]). The distribution of dry-sieved aggregates (DSA) in the upland red soil showed similar trends among CK, RS, and PM in the order of (>5) >1–0.25 > 5– 2 > 0.25–0.05 > 2–1 > (<0.053 mm), whereas the distribution of water-stable aggregates (WSA) was in the order of 0.25–0.053 > 1–0.25 > (>5)> (<0.053) > 5–2> 2–1 mm (**Figure 5**). Compared with the CK, addition of rice straw and pig manure significantly increased the proportion of DSA in 1–0.25, 0.25–0.053, and <0.053 mm fractions. No change in the 2–1 mm fraction, however, was detected. Addition of rice straw significantly decreased the proportion of WSA in the >5 mm fraction and increased the proportion of WSA in 2–1 and 1–0.25 mm fractions but did not affect the 1–0.25 mm aggregate fraction (**Figure 6**). Evidently, there was a significant difference between the effect of rice straw addition and pig manure amendment on aggregate

Overall, 86.9–93.8% of DSA and 47.4–50.0% of WSA were in the macro-aggregate (>0.25 mm) fraction in the upland red soil after 22 years of combined application of NPK fertilizer and organic amendments (**Figure 6**). Compared with the CK treatment, addition of rice straw and pig manure increased the proportion of the >0.25 DSA fraction by 4.9 and 7.9%, respectively;

inorganic fertilizers (1988–2009).

**4.4. Soil aggregate structure**

size distribution in the upland red soil.

The K reduction is probably due to plant uptake and removal or leaching losses.

196 Organic Fertilizers - From Basic Concepts to Applied Outcomes

**Figure 6.** Distribution of dry-sieved aggregates (DA) and water-stable aggregates in the upland red soil impacted by long-term fertilization (1988–2009). Same lowercase letters indicate no significant difference at *p* < 0.05 between differ‐ ent fertilizer treatments.

**Figure 7.** Proportion of macro-aggregate (>0.25 mm) in the upland red soil impacted by the long-term fertilization treatment (1988–2009). Same lowercase letters indicate no significant difference at *p* < 0.05 between different fertilizer treatments.
