158 CHEMICAL COMPOSITION OF THE SOIL [CHAP. 



fertility of the soil is to be maintained. As we shall see 

 later, we cannot wholly gauge the requirements of a 

 given crop by first ascertaining what it takes away from 

 the soil, but we can sum up the effect of a given rotation 

 and learn whether it is likely to leave the land richer or 

 poorer. We may consider arable land first, and assume 

 that when farmed on the Norfolk four-course rotation 

 it is of such fertility as to yield, on the average, 4J 

 quarters of wheat and 5 quarters of barley, 2 tons of 

 clover hay and 20 tons of roots. We may also assume 

 that only corn and meat are sold off the farm, the straw 

 being trampled down, and some of it fed with the roots 

 in order to make the dung which comes back. When 

 farmyard manure is made in the yards we may expect, 

 for reasons which will be set out later, that only one-half 

 of the nitrogen contained in the food will find its way 

 back to the land in the dung, though when roots are fed 

 off by sheep folded on the land about nine-tenths 

 will be returned. About three-quarters of the phos- 

 phoric acid and practically the whole of the potash may 

 be expected to come back in the dung. On this basis 

 we should get figures somewhat as shown in Table 

 XVI, — the annual loss to the land per acre would 

 amount to about 31 lb. of nitrogen, 9 lb. of phos- 

 phoric acid, and 5 lb. of potash. 



As regards phosphoric acid and potash, there can be 

 no possible compensating agents at work other than 

 feeding stuffs or fertilisers brought on to the farm from 

 some external source ; it will, therefore, be necessary to 

 use about 240 lb. per acre of superphosphate (33 per 

 cent, acid phosphate) once during the rotation, say for 

 the swedes, in order to maintain the fertility of the land. 

 The draft on the potash is not so great, and on the 

 stronger soils we may expect that the weathering of soil 

 particles will render available a sufficient proportion of 



