Alkali Soil Formation Characterization and Its Distribution in China

Alkali soil is formed when soil colloidal materials are inundated for a long period with groundwater whose chemical composition is predominantly sodium carbonate or sodium bicarbonate. Fig. 10.1 shows the concept of alkali soil formation. These sodium salts in the groundwater are produced when carbon dioxide in soil or atmosphere is absorbed by sodium hydroxide, which is hydrolyzed from sodium ions. With ionic absorption on the surface of soil colloids, predominantly by sodium ions, soil colloidal materials become dispersed, forming very compact, hard structures of high pH, beyond 9, as they dry. The layer cohered from sodium colloids is called a natric horizon, and will become a soil so undesirable as a medium for plants that almost all of crops even weeds cannot grow at the presence of the horizon. Alkalinization of soils once formed change to much poorer soils for vegetation than the case of saliniza-tion because soil condition changes to impaired aeration, restricted rooting depth, interference in nutrient uptake and metabolism, corrosion of root surface and sodium toxicity. The existing hard horizon with prismatic structure formed by clay and humus coating on the ped surfaces makes also very difficult to leach excess salts from soil.

Alkali soils distribution in China are found in the arid and semi-arid regions of Inner-Mongolia, Jillian, Heilonjiang, Liaoning, Xinjiang-Uygur Autonomous, Heibei, Gansu, Shaanxi and Qinghai provinces and especially the distribution in the northeast and north China of latitude 40 degree north is intensified. And its acreage is estimated more than

100,000 km2.4 According to the criteria of saline soils,1 the saline soils are classified to three types due to TSS (Total Soluble Salts), pH and ESP (Exchangeable Sodium Percentage) as shown Table 10.1. And Tables 10.2 and 10.3 give the chemical properties of non-alkali soils (Haplic Solonchak) and alkali soils (Alkali Solonchak) distributed in northeast China. The most characteristic difference between non-alkali soil and alkali soil is found in the content of calcium, magnesium, and carbonate and bicarbonate ions in the solution extracted from soil by distilled water. In northeast China, the distribution of alkali or alkali-saline soil is also related to the topographical characteristics of the land; they usually are found in the areas with high ground-water tables, or in low lying ground. These areas are favorable for forming sodium carbonate or bicarbonate, because the sodium ion is more mobile than other cations, and because carbonate and bicarbonate originate from microbial decomposition of the organic matter which predominates in wet locations.

Corn cultivation was originally very popular among Chinese farmers because of it produces a large biomass as well as seeds. However, its recent crop cultivation in upland soils of China has been more popular than before. The reason lies in the fact that the consumption of meat in China has been increased largely in the past decade owing to Chinese economic growth, and that corn cultivation has been stimulated by subsidy from the Chinese government. As water consumption of corn cultivation is estimated at 7 to 8 times that of wheat,5 a large amount of irrigation water has applied to cornfields during the vegetative stage in the summer season. The observed data from groundwater levels of wells in the Kangpin district, Liaoning province, which lies in the middle of the corn cultivation area, show that the groundwater level changed from 4 to 12 m in the last eight years.6 The large, rapid declines of groundwater levels are also found in many upland areas not only in northeast China but also in China as a whole; this fact suggests that there are still high possibilities of expanding soil alkalinization.

Saline Alkali Soil China
Fig. 10.1. Concept of alkali soil formation.

Table 10.1. The classification of saline soils

EC (dS/M)

pH

(%)

Saline

>4

<8.5

<15

soil

Alkali

<4

>8.5

>15

soil

Saline-

>4

>8.5

>15

alkali soil

EC: electric conductivity; ESP: exchangeable sodium percentage.

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  • ari
    How alkali soil formed?
    1 year ago

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