THE SOIL IN RELATION TO PLANT GROWTH 127 



logged or to allow plants to become parched in very dry weather than 

 clays. The soil decompositions proceed normally, rapidly producing 

 plant food, with little tendency to " sour " ^ or other abnormal con- 

 ditions so long as sufficient calcium carbonate is present. In con- 

 sequence most plants will grow on loams, even some of those supposed 

 to be specially associated with some other soil type. Thus, where a 

 chalk and a loam soil meet, it is not uncommon to find the chalk plants, 

 e.g. traveller's joy {Clematis Vitalba), guelder rose, etc., wandering on 

 to the loam and it is much more difficult to find the line of separation 

 of the soils than where the chalk abuts on to a sand or a clay. For 

 the same reason loams allow of very wide choice in the systems of 

 husbandry, and, as they become very fertile under good management, 

 they are usually in this country all cultivated. Closer observation 

 over a limited area shows, however, that a given class of loam is 

 more suited to one crop than to another; the ecologist recognises 

 differences in the sub-associations or facies, and the practical man 

 will distinguish between a potato soil, a barley soil, a wheat soil, 

 etc. ; distinctions due no doubt to water and air relationships, and 

 arising from differences in the compound particles. Unfortunately no 

 method of investigating the compound particles has yet been devised, 

 and a study of the ultimate particles by a mechanical analysis is alone 

 possible. But even though the differences are thus attenuated they 

 can still be traced, as shown by the analyses in Table LI. of soils in 

 Kent, Surrey, and Sussex, known to be well suited to the parti- 

 cular crops. 



Table LI. — Mechanical Analyses of Soils Well Suited to Certaik Crops in 

 THE South-eastern Counties; Limits op Variation. Hall and Russell (123). 



Low amounts of clay and fine silt, and high -amounts of coarse 

 sand whenever the clay begins to approach 1 2 per cent, characterise 

 the potato soils ; these are the most porous of the series, allowing free 

 drainage and aeration. Barley soils on the whole are heavier and 

 other analyses show they may be much shallower. Fruit and hops 



' See p. 62. 



