CHAPTER VII. 



THE SOIL IN RELATION TO PLANT GROWTH. 



WE are now in a position to summarise the effect of the various soil 

 conditions on the growth of plants. The soil serves several functions ; 

 it affords a more or less continuous supply of food and water, it 

 regulates the temperature and provides anchorage for the roots with- 

 out interfering too much with the air supply. The extent to which any 

 of these things is done depends on the nature of the particles and 

 therefore varies from soil to soil. As the requirements of plants also 

 vary in degree, the different soil types have come to possess their own 

 flora made up of those plants that tolerate the conditions better than 

 other possible competitors, and on cultivation they show different 

 agricultural characteristics. 



In discussing the relationship between plant growth and the soil it 

 is necessary to take into account not only the intrinsic properties of 

 the soil due to the nature of its constituent parts, but the extrinsic 

 properties impressed by topographical and climatic factors. A certain 

 indefiniteness thus becomes unavoidable, because none of the latter 

 can be expressed in exact measurements. This point is well illustrated 

 by the water supply. The amount of water in the soil at any time 

 is the balance of gains over losses. The gains depend on the rain- 

 fall (i.e., climate), and on the amounts of water derived by drainage 

 from higher land (i.e., topographical position), or by surface tension from 

 the subsoil ; the losses depend on the extent to which the subsoil facili- 

 tates drainage and on the rate of evaporation, which in turn depends 

 on the temperature, the exposure of the soil, the velocity of the wind 

 and the mode of cultivation. Five factors have therefore to be con- 

 sidered: (i) the nature of the soil particles, (2) the amount and dis- 

 tribution of the rainfall, (3) the position of the soil in relation to the 

 land round about it, its aspect, shade, and any other factors affecting 

 its relative temperature and water supply, (4) the depth of the soil, (5) 

 the nature of the subsoil, especially its perviousness to air, water and 

 plant roots. Any of these factors may, within certain limits, dominate 



the rest and profoundly affect the flora and the agricultural value ; thus 



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