22 FIELD CROP PRODUCTION 



organic matter; also, the decaying roots render the soil 

 looser and more friable. 



(5) A rotation alternates deep and shallow rooted 

 crops. Crops that send their roots down deeply help 

 to prevent a compact condition of the subsoil and so 

 maintain better drainage and a better moisture reservoir. 

 Also, by feeding on deeper layers of soil the total supply 

 of plant food is more economically utilized. It is, there- 

 fore, advantageous to both the shallow and deeply rooted 

 crops that they should occasionally alternate with one 

 another. 



(6) Influence of toxic substances. It is probably true 

 that many or all crops excrete or leave in the soil certain 

 organic compounds which are more or less injurious to 

 succeeding crops of the same kind, but are less harmful or 

 perhaps not at all so to other crops. In certain instances, 

 however, a crop is thought to exert a more toxic effect 

 on another growing with it than on itself. The Duke 

 of Bedford and Spencer U. Pickering, working at the 

 Woburn Experimental Fruit Farm near Bedford, England, 

 report experiments which seem to prove that grass has a 

 very toxic effect on fruit trees. They demonstrate that 

 this effect is entirely separate from that of robbing the 

 trees of available plant food and moisture. The United 

 States Bureau of Soils has investigated the subject of 

 toxic compounds in the soil to an elaborate extent. It 

 has shown clearly that when the water extract from a 

 poor soil is shaken with some insoluble absorbing material 

 such as finely powdered charcoal and then filtered, the 

 filtrate will grow better plants (in aqueous solution) 

 than the original extract. Other experiments show that 

 the extract from certain poor soils will not grow plants so 

 well for the first few weeks as distilled water. Wheat 



