IX] CONCLUSION 125 



The soil conditions fall mainly into four groups. 



(1) The physical constitution of the soil determines 

 the movement of water, of air, and of the plant roots. 



(2) The chemical composition shows the amount of 

 food materials present, and whether there are any 

 detrimental substances in the soil. (3) The micro- 

 organisms of the soil are very mixed, some of them 

 work up certain of the food materials into forms 

 suitable to the plant and are therefore eminently 

 useful, others are detrimental in various ways. 

 (4) Extrinsic factors such as climate, situation of 

 the soil, nature of the subsoil, etc., play an important 

 part. Any of these groups may determine the fertility 

 of a particular soil. To a certain extent, however, all 

 of them are under control. The physical constitution 

 is altered and made more favourable to plant growth 

 by adequate cultivation, by addition of organic matter 

 (e.g. farmyard manure) and if necessary of lime, chalk, 

 marl, or clay. The chemical composition may be 

 entirely altered so far as food constituents are con- 

 cerned by adding appropriate fertilisers. Control of 

 the micro-organisms of the soil is as yet in its infancy 

 although a beginning has been made. The extrinsic 

 factors are naturally less easy to change, but the 

 subsoil may be altered by breaking any pan or rock 

 layer, and drainage may be effected by suitable means. 



Soils do not fall into sharply defined classes but 

 form a perfect gi*adation from intractable clays 



