104 BIOLOGICAL PROBLEMS 



culating clay. It facilitates cultivation operations for 

 all crops, and increases yields of some but not all- 

 barley, for instance, responds, while potatoes and 

 oats generally do not. 



When this is done, cultivation proper can begin : 

 this constitutes "the daily round, the common task", 

 of the husbandman's life. It has been practised for 

 ages, and has reached a high pitch of perfection by 

 purely empirical means. 



The skilful cultivator can reduce a sticky soil to 

 a nice crumbly condition for sowing, he can break 

 down clods to granules of the proper size, he can 

 increase or decrease the water-supply and the air- 

 supply to the roots of the plant, and appreciably 

 modify the soil temperature. The methods adopted 

 were at the beginning of the war under investigation 

 in our laboratory by Mr. B. A. Keen. It cannot be 

 said that we know much about the principles under- 

 lying them, or that they themselves have greatly im- 

 proved during the last fifty years; they have, how- 

 ever, become cheaper. In spite of shortage of skilled 

 labour, cultivation before the war on well-managed 

 farms was still just as good as ever. The work 

 was done by machine rather than by hand, but the 

 machine was being steadily improved so that it 

 should work as well as the hand. 



Having improved the environment by these three 

 methods, the fourth means of increasing the growth 

 of crops is to increase the amount of plant nutrients 

 in the soil. The old method of procedure was to add 

 farmyard manure, and this still remains the most 

 popular. The advance of agricultural chemistry has 

 shown that the nutrient materials can also be added 

 as inorganic salts, and great quantities of these are 

 used every year. The salts are of three kinds, sup- 

 plying respectively nitrogen, phosphoric acid, and 



