x.] CLA y SOILS 287 



with manures containing this substance. Superphos- 

 phate may be used with advantage wherever there is 

 enough calcium carbonate in the soil, but basic slag 

 is the typical phosphatic manure for the strong soils 

 which retain sufficient water to render the phosphates 

 active. While supplying phosphoric acid, it also 

 contains free lime in a fine state of subdivision, and 

 so liberates in a soluble state the reserves of nitrogen 

 and potash in the soils. It should, however, not be 

 forgotten that as the basic slag only supplies one 

 element of plant food, the phosphoric acid, the soil 

 may easily be exhausted by continual cropping and 

 manuring with basic slag alone. 



Potash is always present in large amounts in clay 

 soils, 05 per cent, soluble in strong hydrochloric acid 

 is often to be found, while the proportion which can 

 be extracted after completely breaking up the soil 

 with hydrofluoric acid may rise to 2 per cent. Clay 

 soils are late, and their crops grow sltfwly and ripen 

 tardily except in specially dry seasons, when the clay 

 shrinks so much as to cut off all access of moisture 

 from the subsoil, and prematurely ends the period of 

 growth ; on the other hand, the quality of crops grown 

 on the clay is often high. The typical crops of strong 

 land are wheat, beans, and mangolds; owing to the 

 closeness of the texture of the soils, weeds are much 

 less in evidence on the clays than elsewhere, though 

 some few are exceedingly troublesome. 



On the poorer pastures, the spiny form of 

 rest harrow (Ononis arvensis), the wild teazel 

 (Dipsacus sylvestris)) Ranunculus arvensis, and Genista 

 tinctoria, are characteristic and often troublesome 

 weeds ; on cultivated land the " black-bent " grass 

 (Alopecurus agrestis), and field mint (Mentha arvensis) 

 are difficult to deal with. Other plants characteristic 



