I50 PHOSPHATIC FERTILISERS [chap. 



disseminated through the soil. For the turnip crop there 

 mav perhaps be some advantage in drilling the manure 

 with the seed, so important is it to have the young 

 roots stimulated by an abundance of phosphoric acid 

 close at hand, but with other crops much of the benefit 

 of phosphatic manures is often lost because they arc 

 applied when the land has already begun to run short of 

 water. Fine grinding and early application are the two 

 great factors in making phosphatic manures available. 



The essential condition that should dictate the choice 

 of superphosphate as a fertiliser, is the presence of 

 sufficient carbonate of lime in the soil to ensure the 

 precipitation of the soluble phosphoric acid as a calcium 

 compound. On acid soils, on some clays, and on many 

 sands and gravels, there is such a deficiency of carbonate 

 of lime that the phosphoric acid becomes precipitated 

 as iron or aluminium phosphate, which possess a much 

 lower solubility in the soil water and are therefore 

 less available to the plant But on the great majority 

 of our British soils experience has shown that the 

 extra price of the unit of phosphoric acid in its soluble 

 form in superphosphate is more than justified by its 

 superior cfTectiveness, which is due to the rapidity with 

 which it becomes disseminated in a finely divided 

 cijiidition in the soil immediately near the roots of the 

 crop. 



On many of the heavier clays, which are in general 

 deficient in phosphates, though superphosphate is a 

 valuable fertiliser, especially if lime has been applied to 

 the soil previously, basic slag is really the more effective 

 manure when quantities costing the same money are 

 compared. In the first place, basic slag is so much 

 cheaper that nearly twice as much phosphoric acid can 

 be bought at the same cost, and on heavy soils well 

 provided with moisture or on peaty soils its effectiveness, 



