8 SOIL CONDITIONS AND PLANT GROWTH 



1795, adds alkaline phosphates to the list of nutritive salts, 

 but he attaches chief importance to humus as plant food. 

 The " oxygenation " process going on in the soil makes the 

 organic matter insoluble and therefore useless for the plant ; 

 lime, "alkalis and other saline substances" dissolve it and 

 change it to plant food ; hence these substances should be used 

 alternately with dung as manure. Manures were thus divided, 

 as by Wallerius, into two classes : those that afford plant food, 

 and those that have some indirect effect. 



Throughout this period it was believed that plants could 

 generate alkalis. "Alkalies," wrote Kirwan in 1796, " seem 

 to be the product of the vegetable process, for either none, or 

 scarce any, is found in the soils, or in rain water." In like 

 manner Lampadius thought he had proved that plants could 

 generate silica. The theory that plants agreed in all essentials 

 with animals was still accepted by many men of science ; some 

 interesting developments were made by Erasmus Darwin in 

 1803 (77> 



Between 1770 and 1800 work was done on the effects of 

 vegetation on air that was destined to revolutionise the ideas 

 of the function of plants in the economy of Nature, but its 

 agricultural significance was not recognised until later. In 

 1771 Priestley (230), knowing that the atmosphere becomes 

 vitiated by animal respiration, combustion, putrefaction, etc., 

 and realising that some natural purification must go on, or life 

 would not longer be possible, was led to try the effect of sprigs 

 of living mint on vitiated air. He found that the mint made 

 the air purer, and concludes " that plants, instead of affecting 

 the air in the same manner with animal respiration, reverse the 

 effects of breathing, and tend to keep the atmosphere pure and 

 wholesome, when it is become noxious in consequence of 

 animals either living, or breathing, or dying, and putrefying in 

 it ". But he had not yet discovered oxygen, and so could not 

 give precision to his discovery : and when, later on, he did 

 discover oxygen and learn how to estimate it, he unfortunately 

 failed to confirm his earlier results because he overlooked a 

 vital factor, the necessity of light. He was therefore unable to 



