26 



PLANT PRODUCTS 



manner. Broadcasting these very fine powders is trouble- 

 some in windy weather. Whether these phosphatic manures 

 happen to be soluble in water or not, they very quickly 

 become insoluble in the soil. The soluble compounds attack 

 the lime and ferric hydrate in the soil and form compounds 

 insoluble in water. There is, therefore, no appreciable loss 

 to phosphatic fertilizers through drainage. At Rothamsted, 

 all the phosphates added during the preceding fifty-five 

 years is accounted for in Table 3 : — 



Table 3. 

 Phosphorus Balance Sheet, Hall and Amos. 



It will be noticed that the very large amount expected to 

 be left in the soil, estimating the difference between what was 

 supplied and what was found in the crop, was almost exactly 

 equal to the amount actually found in the soil. In two cases 

 a little too much, and in two cases just too little, quite as 

 close an agreement as one could possibly expect. These 

 results, however, refer to a soil which, whilst very typical, 

 is poorer than some agricultural soils. From soils that are 

 very rich in phosphates, such as some garden soils, drainage 

 does remove appreciable quantities of phosphate. The 

 phosphates in the soil, whether natural or added by fertilizers, 

 are attacked by the carbonic acid in the soil, and thereby 

 rendered slightly soluble. The root hairs of a plant are 

 probably permeable to such a solution of phosphates in 

 water containing carbonic acid. When such solutions have 

 entered the root, the acid in the root will take up the phos- 

 phate itself, and leave the carbonic acid free. The carbonic 



