4 SOIL CONDITIONS AND PLANT GROWTH 



contribute in some manner to the increase of plants, but it is disputed 

 which of them is that very increase or food : (i) nitre, (2) water, (3) air, 

 (4) fire, (5) earth". 



The Search for Plant Nutrients. 

 I. The Phlogistic Period, 1750-1800. 



Great interest was taken in agriculture in this country during the 

 latter half of the eighteenth century. " The farming tribe," writes 

 Arthur Young during this period, " is now made up of all ranks, from 

 a duke to an apprentice." Many experiments were conducted, facts 

 were accumulated, books written, and societies formed for promoting 

 agriculture. The Edinburgh Society, established in 1755 for the im- 

 provement of arts and manufactures, induced Francis Home (138) " to 

 try how far chymistry will go in settling the principles of agriculture ". 

 The whole art of agriculture, he says, centres in one point : the nourish- 

 ing of plants. Investigation of fertile soils showed that they contain 

 oil, which is therefore a food of plants. But when a soil has been 

 exhausted by cropping, it recovers its fertility on exposure to air, 1 

 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, vitriolated 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 fol- 

 lowed in studying the problem pot cultures and plant analysis. 

 Subsequent investigators, Wallerius (292), the Earl of Dundonald (90), 

 and Kirwan (149) added new details but no new principles. The prob- 

 lem indeed was carried as far as was possible until further advances 

 were made in plant physiology and in chemistry. The writers just 



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



