CH. v] Spring Cultivations 85 



The most difficult case arises when the winter is mild 

 and wet and the spring wpt so that the soil never dries 

 for the preliminary breaking down into small lumps. 

 The heavier the soil the worse the trouble : and seasons 

 of this kind have sometimes proved disastrous. No way 

 round the difficulty is yet known, and this is one reason 

 why heavy soils are often not brought into arable 

 cultivation. 



The disk harrow is very useful in spring in mitigating 

 these effects and in putting right the bad work some- 

 times done by motor or tractor ploughs. The tilth pro- 

 duced may look better than it really is: "forced" tilths 

 are not much in favour with farmers. 



Rolling. It has already been stated that frost puffs 

 up and lightens the soil and where the soil is already 

 rather porous the effect may be actually harmful to the 

 plant. It is essential that sufficiently close contact 

 should be maintained between the soil and the root, 

 otherwise the plant does not obtain the proper supply 

 of water and nutrient salts ; wherever the winter frosts 

 have so puffed up the soil as to reduce this contact too 

 much it is necessary to compact the soil again by rolling. 

 These effects are produced by the furrow presser used 

 for cereal crops on light chalky soils, and for wheat after 

 a ley on many other soils. 



Another factor comes into play on grass land. Earth- 

 worms burrow in the soil and throw out material from 

 below on to the surface. These casts accumulate steadily 

 at a rate calculated by Darwin to be about one inch in 

 ten years. The worms honeycomb the ground with 

 their burrows, and this action, though necessary to the 

 plant, becomes after a certain stage harmful and injures 

 the grass while allowing moss to grow. 



