CHEMISTRY AND CROPS. l6l 



that it was by their solvent action on the soil particles that the 

 plant obtained its food in solution. Dyer estimated the averag"e 

 acidity of the sap of roothairs as being equivalent to i per cent, 

 citric acid, and from this determined the strength of his solvent. 

 Dyer's method has come to be looked upon by many as a funda- 

 mental one, and by such people the amounts of plant food in- 

 gredients extracted by it are called the "available" amount. 

 For my part, I consider the method one of great utility ; but 

 fiind myself quite unable to urge that it is in any sense a funda- 

 mental one. That it cannot be fundamental is evident from 

 many facts. For example, the minimum limits assigned by Dyer 

 are yet much in excess of the actual requirements of any crop ; 

 the limits are too high for soils of warmer climates ; the method 

 gives very unreliable results with ferruginous soils and with cal- 

 careous ones, unless a sufficient extra quantity of citric acid is 

 used to neutralise the lime and magnesium carbonates. This 

 means if the method is fundamental, that when grown on calcare- 

 ous soils a plant is able to secrete additional quantities of acid. 

 But it would appear from Ingle's investigations that Transvaal 

 soils are generally deficient in lime carbonate, and, further, that 

 Dyer's limits for these soils are too high. This mean5 if the 

 method is fundamental, that an absence of acid consuming sub- 

 stances diminishes the activity of the acid solvent, which is 

 absurd. Therefore, the method is not fundamental. Its great 

 utility can be accounted for without making any extravagant 

 assumptions ; one only needs to assume that Dyer's solvent has 

 in very many cases a power closely related to the solvent 

 agencies of the soil. That Dyer's method is not a fundamental 

 one could also be inferred from the fact that deci-nomial nitric 

 acid has been found by some chemists — notably French and 

 American — to be a more reliable solvent than citric acid ; whereas 

 nothing could possibly be more reliable than that which is fun- 

 damental. 



I do not wish to make extravagant claims for chemistry, 

 and therefore do not claim that we have yet come across a 

 fundamental method of determining the immediate productivity 

 of long cultivated soils ; but the making of such a statement in 

 no wise detracts from the great value of such methods as have 

 been elaborated, and have been used with undoubted success in 

 every part of the world. Our knowledge of the methods by 

 which plant food materials are rendered soluble in the soil has of 

 recent years been very much enlarged. The statement that plant 

 roots secrete acids other than carbonic acid goes no longer un- 

 challenged, and it appears to be very probable that there are no 

 acid secretions other than the one mentioned, provided the plants 

 are grown under normal conditions. Even if other acids were 

 secreted, it seems to me that they must play quite a subordinate 

 part in the formation and solution of plant food materials. When 

 one calls to mind the well-established possibility of materially 

 increasing the store of available plant food by means of a bare 

 fallow ; the considerable solvent action of carbonated water not 

 only on lime and magnesium carbonates, but also on undecom- 

 posed rock minerals of all kinds ; the instability of zeolites to- 



