1905.] 



On the Retention of Bases by the Soil. 



17 



drawn as to whether the loss of calcium carbonate fluctuates with the rainfall 

 and percolation during each period. But the magnitude of the annual loss is 

 somewhat surprising; assuming it to be only 800 lbs. per acre, then the 

 Broadbalk Field must have contained at least 70 tons per acre of chalk at the 

 beginning of the nineteenth century, and still contains so much that it will 

 not be exhausted by the end of the present century. As, also, the rate of 

 loss will probably fall with each reduction in the quantity present, dissolution 

 being proportional to the surface exposed, the period of complete exhaustion 

 will be considerably postponed. Since much of the value of the land agricul- 

 turally, both in its fertility and in its ease of working, depends on the 

 presence of calcium carbonate, it is clear that for the last 100 years the 

 agricultural community have in this respect been living upon the capital 

 accumulated by their forefathers, and are taking no steps to replace the 

 inevitable depletion of this capital. 



C. Effect of Manures upon the loss of Calcium Carbonate. 



Most of the manured plots under investigation receive a dressing of 

 " mineral manures " in addition to the varying amounts and compounds of 

 nitrogen. This mineral manure consists of 3-| cwt. per acre of superphosphate 

 containing 17 per cent, of soluble phosphoric acid (equivalent to 37 per cent, 

 of calcium phosphate " made soluble "), 200 lbs. of potassium sulphate, and 

 100 lbs. each of magnesium and sodium sulphates. Of these substances the 

 superphosphate reacts immediately with the calcium carbonate of the soil, the 

 sparingly-soluble di-caleiuni hydrogen phosphate being precipitated wherever 

 the superphosphate solution comes in contact with a particle of chalk in the 

 soil in accordance with the equation 



CaKtPaOg + CaC0 3 = Ca 2 H 2 P 2 8 + C0 2 + H 2 0. 



To complete this reaction, the 3£ cwt. of superphosphate would require 

 about 47 lbs. of calcium carbonate, but so small an annual loss would hardly 

 be perceptible in the analyses. 



The neutral sulphates of potassium, sodium, and magnesium should occasion 

 no loss, for though they react with calcium carbonate, the resultant alkaline 

 carbonate is retained by the soil and would be estimated as calcium carbonate 

 by the method of analysis adopted, which is based upon the carbon-dioxide 

 evolved on treating the soil with acid. The action of the plant also, 

 discussed later iu this paper, would probably result in the reconversion of the 

 sodium and potassium carbonates into calcium carbonate. The action of the 

 mineral manures, as seen in the analyses of the soil, has not occasioned 

 sufficient loss of calcium carbonate to be apparent within the limits of accuracy 

 vol. lxxvii. — B. C 



