THE ESSEX EXPERIMENTS 45 



giving returns comparable to those obtained by the use of the old 

 basic Bessemer slag. The explanation probably rests on the fact that 

 rainfall is the most important limiting factor on this type of soil. (It 

 will be observed that the hay crop varies from 14-3 cwts. per acre 

 in the dry season of 1919 to 51-2 cwts. during the favourable season 

 of 1916.) Shortage of phosphate is possibly the second limiting factor, 

 and the original dressing applied is more than is essential. 



CONCLUSIONS DRAWN FROM THE FIELD EXPERIMENTS 



With two exceptions the field experiments show a marked response 

 to phosphates. The failure at Hassobury is probably partly due to 

 the fact that the soil is comparatively rich in phosphoric acid, and 

 partly to the fact that it is very much poorer in potash than the soil 

 at those centres where a response to phosphates was secured. The 

 failure at Farnham is not due to the soil being well supplied, in 

 phosphoric acid, but to a deficiency in some other factor. 



If the centres where a definite response has been secured are con- 

 sidered, it is quite apparent that good results can be expected on 

 both the London clay and boulder clay soils from the various types 

 of rock phosphates, and that, considered over a period of four or 

 five years, it is reasonable to expect these phosphates to give results 

 approximately equivalent to those secured from the high citric soluble 

 types of basic slag. Seasonal differences have, however, been apparent 

 which suggest that the rock phosphates require a considerably higher 

 rainfall to produce the maximum effect than is the case with the high 

 soluble slags. These differences are clearly apparent at Martin's 

 Hearne, Latchingdon and Lambourne End. The seasons 1917 and 



1919 were dry, or comparatively so, whilst those of 1916, 1918, and 



1920 were moist. If the results for the two dry years on the high 

 soluble slag plot and the Gafsa rock phosphate plot, and the corre- 

 sponding results from the moist season, are compared as is done in 

 Table XX the influence of the season on the availability of the rock 

 phosphates will be seen to be very pronounced. 



During dry seasons high soluble basic slag gives considerably better 

 results both at Martin's Hearne and at Latchingdon. Latchingdon 

 is in the eastern and drier portion of the county, whilst Martin's 

 Hearne is in the western and moister section, and it is of interest to 

 note that, as might be expected, the advantage of the high soluble 

 slag over the Gafsa rock phosphate is greater at Latchingdon than 

 at Martin's Hearne. 



