THE SOIL IN RELATION TO PLANT GROWTH 317 



the common crops, oats, barley, wheat, etc. ; unfortunately 

 these can only be discovered by direct field trials, and even 

 then the results only hold so long as similar conditions 

 prevail and may often be reversed in a different climate or 

 season. 



Still more subtle differences may be observed : one and the 

 same variety of a crop will acquire one habit of growth on one 

 soil and a different habit on another. Wheat growing under 

 the best soil conditions will produce stiff straw and ears well 

 set with corn, so that a crop of 50 or 60 bushels per acre may 

 be raised without difficulty ; on soil rather different in type, 

 and especially under somewhat different climatic conditions, 

 only 3a or 40 bushels can be raised, because the ears are less 

 thickly set and the straw is too weak to carry a heavier crop, 

 becoming '* laid " directly an attempt is made to increase pro- 

 duction by increased manuring.^ Whether some unknown 

 nutrient is absent from these soils, or whether the adjustment 

 of the air and water supply is wrong, is not known ; but the 

 limitation of yield arising from this unsuitability of soil con- 

 ditions is one of the most serious problems of our time. 

 Another instance may be given : in Romney Marsh pastures 

 commonly occur carrying a vegetation of rye grass and white 

 clover, with crested dog's-tail and agrostis, easily capable of 

 fattening sheep in summer without any other food. All 

 round these pastures are others, with the same type of vegetation, 

 but the plants grow more slowly, produce more stem and less 

 leaf, are less nutritious and incapable of fattening sheep. The 

 soils are identical in mechanical analysis and in general water 

 and temperature relationships, although certain differences 

 have been detected (i2if). Again: grass grown on Lower 

 Lias pastures in Somersetshire and Warwickshire causes acute 

 diarrhoea (" scouring ") in cattle, whilst grass on adjoining 

 alluvial pastures does not (io6). Harmful effects of a wholly 

 different nature are recorded from certain Swiss pastures.^ 



1 Further illustrations are given by the author in Science Progress, 1910, v., 

 286. 



* yahrb. Schweiz., 1898, 104-5. 



