204 THE BOOK OF NATURE STUDY 



times called which constitutes the chief reserve of fertility in the 

 soil. 



It has been demonstrated, however, in previous chapters that 

 the plant's roots can only take in nutriment which is dissolved in 

 the water present in the soil, yet none of the substances hitherto 

 described as making up the soil the sand, the clay, the chalk, 

 the humus are capable of dissolving in water. As a matter of 

 fact, the actual substances which go to feed the plant exist in 

 comparatively small quantities in the soil, and at any time only 

 traces of them are present in the soil water, though they may be 

 constantly renewed as they are removed by the plant. Take the 

 small quantities of water which have percolated through soil 

 in a previous experiment, filter if need be, and evaporate them 

 carefully to dryness in a clean porcelain basin a very small 

 quantity of saline residue will be left in the basin, but it repre- 

 sents the nutrient materials which were immediately available for 

 plants living in those soils. It would be going beyond our present 

 object to examine this residue in any detail, but one of the con- 

 stituents is of so much importance that it cannot be entirely 

 passed over. It is well known that nitre in some form or other- 

 either the saltpetre that is extracted from Indian soils, the nitrate 

 of soda which comes from Chile, or the nitrate of lime which can 

 sometimes be scraped off old walls of buildings is in certain 

 places a product of the soil, and can be extracted from it on a 

 commercial scale. To a small extent one of these nitrates 

 is present in all fertile soils. The most sensitive test to apply 

 is a solution of di-phenylamine in sulphuric acid, and if a little 

 of this be poured on to the dry soil residue in the porcelain basin 

 it will assume an intense blue colour, just the same colour as 

 will be obtained by pouring the solution on a tiny crystal of 

 nitre in another basin (di-phenylamine must only be used as a 

 test with dry or nearly dry substances). This nitrate represents 

 the final soluble state of the nitrogenous humus previously re- 

 cognised in the soil; it is in this form the plants supply 

 themselves with the nitrogen they want. At first sight it is not 

 exactly easy to understand how the dark carbonaceous matter 

 of the soil, though it does contain nitrogen, can ever pass into 

 substances like nitre, and indeed in the laboratory the task of 



