1922.] 



( rREBN Man i RING 



105 



decrease of 400.000 as compared with 1914, and of nearly 

 800,000 compared with 1919, while sheep decreased by nearly 

 4 million between 1914 and 1921. So much as regards a dwindl- 

 ing supply. As to demand, this, so far from dwindling, has 

 increased, for although the number of acres of land under the 

 plough in Great Britain has been steadily falling since 1918. in 

 1921 it still showed an increase of 800,000 compared with 1914, 

 and the more land there is under the plough, the greater is the 

 need for organic matter. 



The Value of Farmyard Manure and the Need for Organic 

 Matter in the Soil. — The shortage of farmyard manure and the 

 causes of that shortage have thus been discussed in order to 

 show that the situation is one which is not likely to improve 

 in the future, but rather is likely to become more serious. Now 

 although every practical man admits the value of farmyard 

 manure, and knows that in order to maintain the fertility of 

 his soil and to keep it in good heart, a plentiful supply of 

 organic matter is indispensable, there is very little certainty 

 as to the mode of action of that organic matter. We know that 

 the main requirements of a crop for mineral substances and 

 nitrogen can be completely satisfied by artificial fertilisers, so 

 that it is unlikely that the unique properties of farmyard manure 

 reside intrinsically in the mineral substances and nitrogen it 

 contains. There is indeed the possibility that certain of the 

 rarer elements, such as boron, present in farmyard manure and 

 usually absent from artificials, may play a part in soil fertility 

 — and this question is under investigation at Rothamsted 

 at the present time — but it is practically certain that the 

 superiority of dung is mainly due to the organic, humus-form- 

 ing material in it. As to the exact nature of " humus " we 

 still know little, and the term, although commonly used, is 

 only one of the many convenient labels which scientists, no 

 less than other mortals, use to hide their ignorance. Humus may 

 be regarded as pre-eminently the characteristic constituent of 

 a fertile soil, in which it exists as a gelatinous brown or black 

 material. The influence of humus on the growth of crops is 

 mainly indirect : if is intimately related to the life of the complex 

 soil population of micro-organisms, and it lias important effects 

 on the tilth, moisture relations, and other physical properties 

 of the soil. Tt affects plant growth by so modifying the pro- 

 perties of the soil as to secure a well-regulated supply of the 

 soluble mineral and nitrogenous substances absorbed by the 

 plant roots, and of the water which serves as the vehicle bv 



