PLANT FOOD IN SOILS. 



203 



condition soluble in dilute citric acid. The last-named fact 

 is one of much practical importance. 



The great difference between the supply of phosphoric 

 acid on plots 1 A and 2 A is thus plainly shown by the 

 analyses, and most conspicuously by the results obtained on 

 extraction with dilute citric acid. 



The facts just mentioned must suffice as an example of the 

 results obtained by the application of Dr. Dyer's method to 

 soils known to be respectively poor and rich in available 

 phosphoric acid ; we will next give an example of the results 

 he has obtained in his determinations of potash. 



POTASH IN ROTHAMSTED BARLEY SOILS, 1889, 

 IN POUNDS PER ACRE IN FIRST NINE INCHES. 



Plots 2 A and 4 A have each received during thirty- 

 eight years the same annual dressing of ammonium salts 

 and superphosphate, but 4 A has received in addition 

 throughout the whole period a mixture of potassium, 

 sodium, and magnesium sulphate. For many years the 

 want of potash was not felt on plot 2 A, although none was 

 applied, but in later years a falling off in the quantity of 

 straw has become apparent, as will be seen in the above 

 table ; moreover, the composition of the ash of the straw 

 shows that very little potash is at the disposal of the crop 

 on this plot. Notwithstanding, therefore, that the soil still 

 contains the enormous quantity of 36,376 lbs. of potash in 

 the first nine inches (1*439 per cent, of the dry soil), ex- 

 haustion of available potash has clearly commenced, and 

 we find accordingly that the treatment with a one per cent, 

 solution of citric acid only succeeds in dissolving fifty-seven 

 lbs. of potash. This is certainly a most striking result. 



