THE ACTION OF GYPSUM IS COMPLEX. 327 



The pure gypsum The gypsum water which 



water had been in contact 

 contained in 1 litre with the earth- 

 Sulphuric acid 1-1 70 grammes. 1-180 grammes. 



Lime 0'820 " 0-736 



Magnesia 0-074 



Besides the magnesia, a certain amount of potash also 

 seems to be dissolved out of the earth by aid of the 

 oypsum. 



Out of 1000 grammes of earth from a wheat-field, 

 there was dissolved by 



3 litres of pure water. 3 litres of gypsum- water. 

 Potash 24-3 milligr. 43 '6 milligr. 



These experiments show that the action of gypsum 

 is very complex, and that it promotes the distribution 

 of both magnesia and potash in the ground. This much 

 is certain, that gypsum exercises a chemical action upon 

 the soil, which extends to any depth of it, and that in 

 consequence of the chemical and mechanical modifica- 

 tion of the earth particles of certain nutritive elements 

 become accessible to, and available for, the clover plant, 

 which were not so before. 



The cause of the action of a manuring agent is usually 

 sought for in the composition of the plant, but I do not 

 think that this is always to be relied upon. The compo- 

 sition of the seed of plants of wheat, for instance, is so 

 constant, or varies so little, that it is quite impossible 

 to infer from the results of the analysis of the seeds 

 whether the soil on which they grow abounded or was 

 deficient in phosphoric acid, nitrogen, potash, &c. The 

 abundance or deficiency of food in a field exercises an 

 influence upon the number and weight of the seeds, 

 but not upon the relative proportion of their compo- 

 nent elements. Thus, for instance, Pincus found a some- 

 what larger percentage of magnesia in the unmanured 

 clover than in the plants manured with the sulphates ; 

 but taking the magnesia of the whole crop, the quantity 

 of this substance was much larger in the latter than in 

 the former. 



