n6 BIOLOGICAL PROBLEMS 



of accumulated fertility on our grass-land that is 

 standing us in such good stead in our day of trouble. 

 Lincolnshire potato growers in days of peace were 

 willing to pay ^15 per acre for the privilege of break- 

 ing up grass-land. In the north of England and in 

 Scotland it has been customary to leave the arable 

 land for a period of years in grass, with a view to 

 building up this reserve of fertility. In the south 

 of England the grass is only left for one year, but 

 another method is locally adopted which achieves 

 the same purpose ; crops are grown for the express 

 purpose of ploughing them in or allowing them 

 to be eaten on the land. This practice might with 

 advantage be much more widely used. One great 

 difficulty is to find suitable crops that will germinate 

 and grow in the rather dry soil conditions usually 

 prevailing after the main crop has been gathered. 

 This problem I hope our breeders will be able to 

 solve. But there is another and more formidable 

 difficulty. When vegetable matter is added to the 

 soil there is not only a great increase in the organisms 

 decomposing organic matter and liberating from it 

 the elements of fertility which we may regard as 

 beneficial inhabitants of the soil- -but there is a 

 marked increase in certain pests, such as leather 

 jackets, wireworms, &c. The benefit we hope to 

 derive from the liquidation of our capital in plough- 

 ing up grass-land in our present emergency is seri- 

 ously threatened by the depredations of these pests, 

 not only in the first year but in the second and third. 

 No cultivation method can be relied upon to repress 

 them entirely; the only reliable treatment is to find 

 some efficient soil insecticide. This problem is being 

 seriously attacked in the Rothamsted Laboratory: it 

 is very similar to another which has caused us a great 

 deal of trouble and has never yet been solved the 



