l62 CHEMISTRY AND CROPS. 



wards neutral saline solutions, and the fact that normal vegeta- 

 tion thrives but poorly on soils that are only slightly acidic, there 

 is littlf need to call upon plant roots to secrete acids in order to 

 extract their daily wants from Mother Earth. 



How profound this action of water on the soil particles is 

 may be gathered from the extensive accumulations of soluble 

 salts in soils that are not subjected to natural leaching as, for 

 instance, those of arid countries. In irrigation areas soils that 

 have shown no signs of "alkali" at the commencement of irriga- 

 tion operations have had to be abandoned in a few years owing 

 to excessive saline accumulations, which it is impossible to 

 account for solely by the composition of the water used. Iii 

 America not less than lo per cent, of irrigated lands have had 

 to be abandoned for this reasoh. 



Now, what is this accumulation of alkali? It is the accu- 

 mulation near the surface, owing to faulty methods of irrigation, 

 of soluble salts that have been liberated from the insoluble soil 

 particles by the hydrolytic and chemical action of the irrigation 

 waters. Therefore, it would appear that the great solvent agency 

 at work in the soil is water, which in turn is aided in its opera- 

 tions by the atmospheric agencies and by soil organisms. Some 

 such considerations as these possibly induce King and others 

 to attempt to measure soil fertility by extractions with pure water. 

 King's experiments have certainly produced very promising re- 

 sults ; his method may be said to be fundamental except in so 

 far as it does not represent all the conditions that obtain in 

 the soil, notably the influence of time, the action of soil organ- 

 isms, the varying concentration of the soil solution, together 

 with its effect on the zeolitic plant food reserves, and the con- 

 tinual removal of salts from the sphere of action by plant roots. 

 The heating of the soil as practised by King is quite an 

 arbitrary operation ; but it probably compensates in some measure 

 for those natural conditions which it is impossible to imitate 

 successfully in the laboratory. 



We have now passed over the achievements of chemistry as 

 regards the measurement of the immediate fertility of soils. To 

 sum up, it may be said that we cannot yet claim to have reached 

 the long-desired goal ; but we are just about to get there. Above 

 everything rises this fact, that given a soil whose physical con- 

 ditions are such as will not interfere with the proper development 

 of the plant, the chemist is able at the present time to determine 

 with considerable accuracy its immediate productive capacity, 

 whether it belong to the virgin class or that class which has 

 been subjected to the operation of cultivation for an indefinite 

 number of years. This is no slight achievement. 



Let us now turn to» the second problem mentioned above, 

 viz. — the ascertainment of the future behaviour of a virgin soil 

 for a number of years to come, or, in other words, the determi- 

 nation of the permanent productivity of soils. It is obvious that 

 the methods used for the ascertainment of the immediate outlook 

 will not suflfice. The methods adopted must be such as will 

 extract from the soil the total quantity of plant food materials 

 that may be expected to come into solution within a given inter- 



