128 SOIL CONDITIONS AND PLANT GROWTH 



both require deep soils, and only seem to find their most favourable 

 circumstances in a restricted class of soils: the fruit soils generally 

 contain rather more sand and less silt than the hop soils. But the 

 fruits differ among themselves ; the best nursery stock is raised on soils 

 of the potato class, where the conditions are for some unknown reason 

 very favourable to fibrous root development ; strawberries prefer the 

 lighter and apples the heavier kinds of fruit soil. Even different 

 varieties of the same plant show distinct preferences for one class ot 

 soil over another : the finest varieties of hops are found only on the 

 typical hop soils, and have to be replaced by coarser varieties directly 

 it is desired to grow hops on heavier soils. Preferences for certain 

 soil conditions are also shown by varieties of 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 fifty or sixty bushels per acre may be raised without diffi- 

 culty ; on soil rather different in type, and especially under somewhat 

 different climatic conditions, only thirty or forty 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 production 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 conditions 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 relation- 

 ships, although certain differences have been detected (123). Again: 

 grass grown on Lower Lias pastures in Somersetshire causes acute 

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



' Further illustrations are given by the author in Science Progress, 1910, v., a86. 



