v.] PHOSPHATES APPROPRIATE TO SOILS 151 



unit for unit, is not much less than that of the phos- 

 phoric acid in superphosphate, especially when it has 

 been put on early and has had plenty of time to 

 saturate the soil water and to be disseminated within 

 the soil by solution and reprecipitation. 



Moreover, the lime contained in the basic slag is 

 itself of considerable value ; it supplies what is often a 

 much-needed base, and on old grass land in particular 

 its effect in bringing the soil potash into solution and 

 in promoting the oxidation of the nitrogenous reserves 

 in the soil is very marked ; on tillage land also the 

 lime is of assistance in improving the texture of the 

 soil. When such soils are poor in carbonate of 

 lime there is always some danger of "finger-and-toe" 

 in the turnips, and if once this disease has appeared 

 superphosphate should no longer be used, but basic 

 slag should take its place. The spread of the disease 

 is promoted by any acid manure like superphosphate ; 

 the free lime of basic slag, on the contrary, tends to 

 render the soil unfit for the survival of the spores. Thus 

 the choice between superphosphate and basic slag 

 should in the main be determined by the soil ; the 

 more calcareous and loamy the soil, the more effective 

 is superphosphate, but heavy soils and land poor in 

 carbonate of lime respond better to basic slag, and on 

 wet sour soils no other phosphatic manure should 

 be used. 



In this country there is rather a prejudice against 

 the use of basic slag on the lighter soils — the sands, and 

 gravels, which are yet too poor in carbonate of lime to 

 be fitted for superphosphate. They are generally 

 regarded as too dry to allow the basic slag to be effec- 

 tive, but in view of the value that basic slag has been 

 found to possess on the light sandy soils of Eastern 

 Germany, where, too, the rainfall is less than that of 



