AVAILABILITY OF PLANT NUTBIBNTS 



236. Calcium carbonate. — A determination of the 

 amount of calcium present as a carbonate is important 

 as an aid to the interpretation of an analysis of the soil. 

 Lime not so combined is generally in the form of a sili- 

 cate, or possibly a phosphate. If there is a large quantity 

 of calcium carbonate in a soil, the potash, phosphoric 

 acid, and nitrogen are likely to be more readily soluble, 

 and smaller quantities are sufficient for crop growth, than 

 if the calcium is not found in this form. The effect of 

 the carbonate of lime on the nitrogen ^ compounds is to 

 furnish a base for the acids produced in the formation of 

 nitrates, and its presence promotes this process. It 

 probably replaces potassium in certain compounds where 

 otherwise it would be secured with more difficulty. It 

 insures the presence of some phosphates of lime, in which 

 form phosphorus is more soluble than when combined 

 with iron. The form of the manures to be used on the 

 soil will also depend in large measure on the presence 

 or the absence of calcium carbonate. For example, 

 where calcium carbonate is deficient, steamed bone or 

 Thomas slag are likely to be more profitable than super- 

 phosphate, and nitrate of soda than sulfate of ammonium. 

 Finally, the absence of calcium carbonate indicates the 

 need of liming, and if the analyses show a considerable 

 quantity of potash and phosphoric acid, but practice 

 shows these materials to be somewhat deficient, it is 

 probable that liming will be very benefiei'al, and that 

 manures carrying these substances will not be so essential 

 as the chemical analysis would indicate. It must be 

 stated, however, that there are cases for which these de- 

 ductions do not hold, owing to the intervention of other 

 factors. 



^ Not determined in tlie hydroehlorie acid extract. 



