HISTORICAL AND INTRODUCTORY 7 



fertility on exposure to air/ which therefore supplies another 

 food. Home made pot experiments to ascertain the effect of 

 various substances on plant growth. " The more they {i.e. 

 farmers) know of the effects of different bodies on plants, the 

 greater chance they have to discover the nourishment of plants, 

 at least this is the only road." Saltpetre, Epsom salt, vitriol- 

 ated tartar {i.e. potassium sulphate) all lead to increased plant 

 growth, yet they are three distinct salts. Olive oil was also 

 useful. It is thus clear that plant food is not one thing only, 

 but several ; he enumerates six : air, water, earth, salts of 

 different kinds, oil, and fire in a fixed state. As further proof 

 he shows that " all vegetables and vegetable juices afford those 

 very principles, and no other, by all the chymical experi- 

 ments which have yet been made on them with or without 

 fire ". 



The book is a great advance on anything that had gone 

 before it, not only because it recognises that plant nutrition 

 depends on several factors, but because it indicates so clearly 

 the two methods to be followed in studying the problem — pot 

 cultures and plant analysis. Subsequent investigators, Wal- 

 lerius (293), the Earl of Dundonald (90), and Kirwan (149) 

 added new details but no new principles. The problem indeed 

 was carried as far as was possible until further advances were 

 made in plant physiology and in chemistry. The writers just 

 mentioned are, however, too important to be passed over com- 

 pletely. Wallerius, in 1761, professor of chemistry at Upsala, 

 after analysing plants to discover the materials on which they 

 live, and arguing that Nutritio non fieri potest a rebus hetero- 

 genets, sed homogeneis, concludes that humus, being homogeneis, 

 is the source of their food — the nutritiva — while the other soil 

 constituents are instrumentalia, making the proper food mix- 

 ture, dissolving and attenuating it, till it can enter the plant 

 root. Thus chalk and probably salts help in dissolving the 

 " fatness " of the humus. Clay helps to retain the " fatness " 

 and prevent it being washed away by rain : sand keeps the 

 soil open and pervious to air. The Earl of Dundonald, in 



' Recorded by most early writers, e.g. Evelyn (Terra, 1674) (96). 



