4 INTRODUCTION 



acid is more effective than superphosphate, a result which was sub- 

 sequently confirmed by the trials at Sevington, Cransley, Hatly and 

 Yeldham<28). At Sevington where the soil is well supplied with calcium 

 carbonate (32) the returns for the two types of phosphates are for 

 practical purposes identical, there being only a difference of 3 Ibs. 

 live weight gain in favour of slag over a period of nine years. 



The superior results from basic slag at the remaining centres is 

 probably due to the fact that on ' sour ' soils, and on soils where the 

 calcium carbonate content is not high, as at Cockle Park (0-59 % 

 CaCO 3 ), a certain proportion of the phosphoric acid in superphosphate 

 is retained by the soil in the form of somewhat insoluble phosphates 

 of iron and aluminium. With repeated dressings of superphosphate 

 increasingly large proportions of the phosphoric acid revert to such 

 insoluble forms. These experiments may therefore be said to have 

 established the fact that insoluble basic phosphates have a distinct 

 function in agriculture, and that under certain soil conditions they 

 are to be preferred to the water soluble phosphates in superphosphate. 



As a consequence of the Cockle Park experiments basic slag is 

 used for the manuring of grass-land almost to the exclusion of other 

 types of phosphatic fertilisers. Nor has its use been confined to 

 grass-land, where perhaps rapidity of action is not of primary im- 

 portance. In the south of Essex, basic slag is used on the arable land 

 almost to the exclusion of superphosphate, and many of the most 

 progessive farmers have attributed their success to the use of basic 

 slag instead of superphosphate on their heavy clay soils, which are 

 either devoid of calcium carbonate or have only a very poor supply. 



Some idea of the extent to which basic slag has been appreciated, 

 and the lessons which Cockle Park taught assimilated, may be ob- 

 tained from the following figures (Table II) showing the production 

 and consumption of basic slag during the period 1903-1920. 



BASIC OPEN HEARTH SLAG 



Unfortunately for agriculture important changes in the manu- 

 facture of steel have been taking place during the past few years. 

 Economic conditions and to a certain extent the working out of the 

 higher grade ores have made the basic Bessemer process uneconomical, 

 and it has been replaced by the basic open hearth process. In this 

 process iron-ore and lime are charged on to a basic hearth heated 

 by producer gas, and the molten metal poured over the heated lime 



