6 SOIL CONDITIONS AND PLANT GROWTH 



same kind of food ; it was incorrect to assert, as some had 

 done, that different kinds of plants fed as differently as 

 horses and dogs, each taking its appropriate food and no 

 other. Plants will take in anything that comes their way, 

 good or bad. A rotation of crops is not a necessity, but only 

 a convenience. Conversely, any soil will nourish any plant 

 if the temperature and water supply are properly regulated. 

 Hoeing increased the surface of the soil or the " pasture of 

 the plant," and also enabled the soil better to absorb the 

 nutritious vapours condensed from the air. Dung acted in 

 the same way, but was more costly and less efficient. 



So much were Tull's writings esteemed, Cobbett tells us, 

 that they were " plundered by English writers not a few and 

 by Scotch in whole bandittis ". 



The position at the end of this period cannot better be 

 summed up than in Tull's own words : " It is agreed that all 

 the following materials contribute in some manner to the in- 

 crease 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, 1 750-1 800. 



Great interest was taken in agriculture in this country dur- 

 ing 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 experi- 

 ments were conducted, facts were accumulated, books written,^ 

 and societies formed for promoting agriculture. The Edin- 

 burgh Society, established in 1755 for the improvement of arts 

 and manufactures, induced Francis Home (137) "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 

 nourishing 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 



