CARBONATE AND PHOSPHATE OF LIME. 87 



the rain-fall is assimilated, thirty-five pounds in addition would 

 have to be supplied to meet the wants of the wheat-field. 



Carbon, the agent so largely consumed by plants, fortunately 

 costs us nothing. The farmer need not trouble himself concern- 

 ing this important element in plant-food, for the atmosphere 

 furnishes an abundant supply for all our wants. It is supplied 

 in the form of carbonic acid, and we do not know that it can be 

 assimilated through any other carbon compound. A carbonate, 

 unless it be of potash or soda, is practically valueless to the 

 farmer. Carbonate of lime, in any form, cannot be regarded as 

 a fertilizing substance having a commercial value. Very strenu- 

 ous attempts have been made to induce farmers to purchase 

 ground clam and oyster shells, the vendors alleging that they 

 were equal to ground bones in fertilizing value ; but this is a 

 fraud of a serious nature. Clam shells are composed of car- 

 bonate of lime, while bones are made up of the phosphate of 

 lime — quite a different substance, chemically and agriculturally 

 considered. The shells are composed of carbonic acid and lime, 

 the bones of phosphoric acid and lime, — the former acid having 

 no money value, the latter having a high value. 



Calcic carbonates should not be confounded with sulphate of 

 lime, which is plaster or gypsum. In this substance sulphuric 

 acid or oil of vitrol is in combination with the lime, in place of 

 carbonic acid, and a very different chemical and fertilizing 

 agent is supplied. It has high value as an application to some 

 fields, although its action is not well understood. The experi- 

 ments which the writer has made with plaster go to prove that 

 its good effects are due rather to the acid than the lime. It has 

 the power of fixing the ammonia of the atmosphere and form- 

 ing sulphate of ammonia, which is a salt of much value. In 

 applying gypsum to soils, it must be remembered that but a 

 small quantity can be made available in a season, as it requires 

 nearly five hundred pounds of water to bring one pound of it 

 into solution. Half a ton is a sufficient dressing for an acre of 

 ground. 



The element hydrogen is freely supplied to plants by dew, 

 mist and rain, and therefore is costless to the husbandman. It 

 is only through water that hydrogen can be presented to the 

 plant, but this is by no means its only important office. It 

 enters the plant as water, and it is through its agency that all 



