v.] CHEMICAL CONSTITUENTS OF SOILS 93 



But though all soils are built up of sand and clay, 

 humus and carbonate of lime, these materials only 

 give rise to its physical structure ; they account for 

 the manner in which the soil works and its relations to 

 water, but in themselves they have nothing to do with 

 feeding the plant. In order to get information on that 

 aspect of the soil it is necessary to try a new experi- 

 ment Take a series of funnels, plugged as before 

 loosely with cotton-wool, and pack each with a different 

 sample of soil, then pour on to each a small quantity of 

 hot water and let it percolate through, an operation 

 which will be hastened if each funnel can be led into a 

 flask communicating with a filter pump. Take the 

 liquid which comes through, filter if necessary, and 

 evaporate it to dryness in a clean basin — a small 

 residue will be left representing the material in the soil 

 which was soluble in water. One test only need be 

 applied to it; pour on to the residue a solution of 

 diphenylamine in sulphuric acid and an intense blue 

 colour will appear, indicating the presence of nitrates in 

 the extract from the soil. In this nitrate, which is 

 present in all soils, we have a substance which, as we 

 have learnt before, will serve as food for plants, supply- 

 ing them with the combined nitrogen that is necessary 

 for their existence. Ammonia, the other nitrogen 

 compound which plants can utilise, may also be detected 

 in soil, but in such small quantities that a delicate test is 

 necessary. The small proportion is due to the fact that 

 ammonia in the soil is constantly being converted into 

 nitrates by a process which will be explained later. 

 Indeed the nitrates themselves are only present in the 

 soil in very small amounts, from two to twenty parts 

 per million of a dry soil ; how minute a fraction of the 

 whole soil does the soluble portion constitute, may be 

 guessed from the residue lefx in the dishes after evapora- 



