Problems South-China farmers face in a carbonate-rock terrane

China’s decision makers sometimes draw attention to the tropical/subtropical parts of their country as being key to increasing China’s food production.  Some claim that a farmer may be able to harvest three or perhaps four crops/year.  Certainly, the warm temperatures and high annual rainfall in South China seem to support such statements.  But farmers, agronomists, and other agricultural specialists understand that the rainfall and temperature are only part of what is needed in geological areas of carbonate rocks to attain such levels of production on a continued basis.

The focus here is to illustrate how South China’s agricultural promise, or lack thereof, is linked to carbonate-rock geology.   The provinces under discussion include Guizhou, Guangxi, and Yunnan and parts of certain others.

Soils formed from carbonate rocks originate from the chemical and physical weathering.  Limestone is rich in the mineral calcite (CaCO3), sometimes reaching as much as 95 percent.   Dolostone contains 90 to 100 percent of the mineral dolomite [CaMg(CO3)2].   Silica (SiO2) and alumina (Al2O3) generally constitute only 1 to 8 percent of limestone and dolostone (Pettijohn, 1957).

Because normal rainfall is slightly acidic, it is able to dissolve carbonate rocks.  Rain water flows over rock surfaces, along rock joints and bedding planes and, in time, produces open channels and caves.  When the rock openings become too large to support themselves, they collapse producing sink holes, a feature typical of karst topography.  Through this natural process, the general land surface becomes lower.  Where the carbonate rocks are thick, as in certain areas of south China, collapse leaves behind steep-sided towers of partly weathered carbonate rock protruding from a flat topography.   The landscape of Zhaoqing in Guangdong Province typifies one such case (Fig. 1).

Figure 1. Karst topography, Zhaoqing, Guangdong Province, China, (photo by W.E. Parham, 2005)

Even during periods of high rainfall, the open drainage-channels in carbonate rocks allows the water to drain rapidly downward to the water table, leaving the soil and vegetation with little water.   As a result, the farmer is often faced with too little water to support crop growth.  To obtain needed water, farmers in many cases must construct high-cost wells to bring a sufficient amount of water to the surface for the crops.  This is time consuming and costly.  Where farmers apply commercial fertilizer,  animal-waste fertilizer and pesticides to their land, rainwater can carry the chemicals downward rapidly through rock openings to the water table below.  Consequently, the rapid downward movement of fertilizer- and pesticide-laden water results in a drinking-water hazard to the farmers, their families, and the community.  Human wastes follow the same pathways.

Soils formed from carbonate-rock weathering contain small amounts of  silicate minerals, the residue of minerals that are slow to dissolve.  In spite of the surrounding hills and outcrops of carbonate rocks, the resulting soils are acid (Tang, 2002) and low in fertility, and predominately contain the clay minerals kaolinite and halloysite (Singer, 1992).  In some cases, fine-grained sediment may be washed in from surrounding the area to add to the silicate-mineral residue of local soils (Tang, 2002).

The weathering process commonly produces a sharp boundary between soil and limestone (Institute of Soil Science, 1990). The thin soils themselves can be swept off the carbonate-rock surface into openings and local depressions during heavy summer monsoon rains, leaving bare rock exposed.  What commonly remains are agricultural sites of poor quality, with a sparse cover of shrubs and patches of grass growing among rock outcrops (Cao et al., 2004).  Destruction of the vegetative cover leads to rocky desertification (Wang et al., 2004) because  soil forms more slowly than its rate of erosion (Cao et al., 2004).  During the dry season, springs and streams dry up rapidly.  Raising crops in such an environment is difficult, and farmers avoid such sites where possible. [For a description of farmer economic, social and political problems in south China karst lands, see (Huntoon, 1992), and  (Jiang et al., 2007).

Restoration of south China’s degraded limestone/dolostone karst terrane is not an easy task.  Even though some small-scale restoration efforts exist, large-scale remediation has not been achieved (Wang, 2004).  Complicating the problem further, are the agricultural challenges presented by the  deeply weathered granitic terranes lying geographically immediately to the east and west (Parham et al., 1993).

Walter E. Parham, Ph.D., May 2012

Literature cited:

Cao, J. et al., 2004, Karst ecosystem of Guangxi Zhuang Autonomous Region constrained by geological setting: Relationship between carbonate rock exposure and vegetation coverage, http://geomorf.wnoz.us.edu.pl/KARST/Karst&Cryokarst_paper%2013.pdf

Huntoon, P.W., 1992, Hydrogeologic characteristics and deforestation of the stone forest karst of south China, Ground Water, v. 30, no. 2, p. 167-176.

Institute of Soil Science, Academia Sinica, 1990, Soils of China, Science Press, Beijing, China, 873 p.

Jiang, Z.C., Yuan, D.X., and Cao, J.H., 2007, (abst.) Fragility and geological background of rocky desertification environment in the karst areas of southwest China, Internat. Conf. on karst hydrogeology and ecosystems, Western Kentucky University in Bowling Green, USA, p. 41-42, http://digital.lib.usf.edu:8080/fedora/get/usfldc:K26-00036-ICKHE_2007/DOCUMENT

Parham, W.E., Durana, P.J., and Hess, A.L., 1993, Degraded tropical lands of China: problems and oppotunities, (in) Improving degraded lands: promising experiences from south China, eds. W.E. Parham, P.J. Durana, and A.L. Hess, Bishop Mus. Bull. in Botany 3, Bishop Museum Press, Honolulul, Hawaii, 243 p.

Pettijohn, F.J., 1957, Sedimentary rocks, Harper and Brothers, New York, 718 p.

Singer, A., 1993, (abst.) Weathering patterns in representative soils of Guangxi Province, south-east China, as as indicated by detailed clay mineralogy, European Jour. Soil Sci., v. 44, p. 173-188., http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2389.1993.tb00443.x/abstract

Tang. T., 2002, Surface sediment characteristics and tower karst dissolution, Guilin, southern China, Geomorphology, v. 49, p. 231-254.

Wang, S.J.,  Liu, Q.M., and Zhang, D.F., 2004, (abst.) Karst rocky desertification in southwestern China: geomorphology, landuse, impact and rehabilitation, Land degradation and development, v. 15, is. 2, p. 115-121. doi: 10.1002/ldr.592., http://onlinelibrary.wiley.com/doi/10.1002/ldr.592/abstract

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