J8 THE MECHANICAL ANALYSIS OF SOILS [chap. 



clay is present. The soil, however, is kept very close by 

 the lack of coarse sand and of any of the still coarser 

 gravel and stones, the absence of carbonate of lime also 

 makes it stickier and more difficult to work. If a good 

 tilth is obtained, as for instance a seed bed for roots, and 

 heavy rain follows, these soils are particularly liable to 

 run together and set on drying to a glazed caked surface, 

 very inimical to germination. When well supplied with 

 lime and organic matter, these soils are fertile and carry 

 magnificent crops ; but they are rather late and expensive 

 to work, so that they have in great measure been laid 

 down to grass. They carry good grass when well 

 treated, and particularly when dressed with lime and 

 basic slag. 



Soil No. 6 is taken from the Broadbalk Wheat Field 

 at Rothamsted : it is a heavy loam, stubborn and 

 intractable to work, which would lie very wet were not 

 the land naturally under-drained by the chalk rock at a 

 depth of ten or twelve feet below. The surface soil also 

 contains a large number of flint stones, not shown in 

 the analysis, and these help to keep the soil more open 

 and assist the drainage. Heavy as it is, the soil is not 

 a true clay ; it is the silt and fine sand fractions which 

 predominate, and to these must be attributed the 

 tendency of the soil to run and dry with a caked surface, 

 if much rain falls after a fine tilth has been attained. 

 In the soil but not the subsoil there is a fair proportion 

 of calcium carbonate, of artificial origin, and this con- 

 tributes greatly to the workability of the soil, for it has 

 been found unprofitable to retain some of the fields, 

 in which the calcium carbonate is absent, under 

 arable cultivation. Land of this class is still largely 

 under the plough, and is good wheat, mangold, 

 and bean land, but is too heavy for barley or turnips. 

 An occasional bare fallow is desirable to clean the land 



