1922.] Experiments with PiiosruATES and Slags. 



On the average of 5 years the Gafsa phosphate has proved 

 just as effective as the high soluble slag, and, although not quite 

 so good, Algerian and Egyptian phosphates follow closely 

 behind. 



During the season of 1919 clover did not make its appear- 

 ance on any of the plots. The rock phosphate and basic slag 

 plots could, however, be clearly distinguished from the un- 

 treated, even at a distance, by their healthier green colour. In 

 1920 clover was present in the hay crop on the basic slag and 

 rock phosphate plots to the extent of from 27-35 per cent, of 

 the crop by weight. 



Homdon-on-the-Hill: London Clay.— The soil is a heavy 

 London Clay containing a small reserve of calcium carbonate 

 (0.25 per cent.) — the residuum of the heavy dressings of past 

 times. The soil is exceedingly heavy and impervious and is 

 known in Essex as " three horse land." It is always put up 

 into 7 ft. 6 in. stetches, to secure the maximum amount of 

 drainage. The experimental field, like all fields whether grass 

 or arable on this type of soil, lies cold and wet during the 

 autumn in spite of the fact that the annual rainfall is only 

 20 in. — the lowest in the country. The summer is equally 

 tryin^x — the dry and hot weather which is usually experienced 

 in Essex in June and the latter part of May " caps " or bakes 

 the soil. It is but seldom that the crop of hay exceeds 10 cwt. 

 to the acre and it is only too frequently left uncut altogether. 



The experimental field was laid down to grass in 1890, and 

 until the experiments started had received no manurial treat- 

 ment of any description. 



In this experiment an attempt was made to ascertain whether 

 better effects could be obtained from rock phosphates by finer 

 .orindinp-. With this object in view the Florida Pebble, Alge- 

 rian, Gafsa, and Tunisian phosphates used were specially 

 ofround. 



All the phosphates were passed through a Griffin Mill for 

 " coarse grinding," the mill being adjusted to give the standard 

 usual for the manufacture of superphosphate (90 per cent, to 

 pass a 60 sieve). In actual fact about 80 per cent, of the 

 material will pass a "100" sieve, i.e., a sieve with 10,000 

 holes to the square inch. For fine grinding the mill was closed 

 down so that the output per hour was reduced by one-half. A 

 much finer product was obtained, but owing to the " woolly " 

 nature of the rock j^hosphates it has not been practicable to 

 distinguish satisfactorily by moans of sieves between the 

 " fine " and the " coarse " grinding. 



