THE SOIL IN RELATION TO PLANT GROWTH 315 



dressings of clay (as in Lincolnshire) or to warping (in the 

 Fens). 



Barren conditions also result when, by reason of a thin 

 parting of clay or its low situation, water cannot run away but 

 accumulates and forms a marsh. Reclamation in such cases is 

 possible as soon as a way out has been found for the water. 1 



Loams. As the proportion of fine material in the soil 

 increases and that of coarse material falls off, a gradual change 

 in the character of the soil sets in, till finally, but without any 

 sharp transition, a new type is reached known as a loam. The 

 increase of fine material somewhat retards the movements 

 both of air and of water, so that loams are characterised by a 

 more uniform water content throughout the mass than sands. 

 On the other hand, loams show less tendency to become 

 water-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" 2 or other abnormal conditions so long as sufficient 

 calcium carbonate is present. In consequence, 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 



1 For a fuller discussion of land reclamation see E. J. Russell, Journ. Roy. 

 Ag. Soc., 1919, 80, 112-132. 



2 See p. 118. 



