t 53 ] 



acid. Plants generally grow in any soil which contains a 

 sufficient proportion of these. The presence of an excess of 

 certain salts or of some substances poisonous to plants may 

 render the soil sterile in spite of the presence in sufficient 

 quantities of all the essential constituents. Nearly every 

 soil contains all the essential constituents for the growth of 

 vegetation, and even the well water or drainage water per- 

 colating through soils contains all the essential constituents 

 for the growth of vegetation, so much so, that water-culture 

 with such well or drainage water alone has been successful 

 with reference to a good many plants including oats. It is 

 from solutions that plants can absorb food. The solubility is 

 helped by the organic acids and the carbon-dioxide excreted 

 by the rootlets. Soil digested in water ought to part with 

 i part of solid for every 1000 parts of water for plants to 

 make proper use of the solid. If over 2 parts of solid are 

 dissolved in 1000 parts of water the rootlets cannot make 

 proper use of the food, nor if less than -5 part in 1000 parts. 

 A soil can be too rich in soluble plant foods or too poor, as 

 the solubility is required to be in a certain dilution. A soil 

 becomes too rich if in the dry season it is manured with 

 fresh urine which contains nearly 2 per cent of urea, a sub- 

 stance which can be directly used by plants as food. But 

 a 2 per cent solution even of a valuable plant-food is at least 

 10 times too rich. This accounts for Bengal cultivators re- 

 garding urine as injurious to crops though it is really more 

 valuable in the fresh state than cowdung. Diluted with ten 

 times as much water urine proves a most excellent fertilizer 

 of soils. As nearly all soils contain all constituents of plant 

 food, the chemical classification of soils is based not on 

 absolute but only on relative grounds. 



60. Schubler's classification is based on a consideration of 

 only four of the proximate constituents of soils, viz., Humus, 

 Lime, Clay and Sand. It takes no note of the proportion 

 of N, P 2 O 5 and K 3 O which are the important constituents 



