No. 63.] .89 



ral fertilizers. Much variety of opinion has been entertained re- 



_, spectinsc the manner in which it exerts its influence or pro- 



Gypsum or r & r 



Plaster, duces its effects on plants; and these opinions can scarcely 

 be said to be harmonized, even at the present time. Davy was in- 

 clined to consider it a direct food for the plant, as it is found, 1o some 

 extent, in those plants on which it exerts the most power. Chaptal 

 referred its power to its stimulating agency on plants, produced by 

 its action when dissolved in water. Liebig ascribes its value to its 

 giving a fixed condition to the nitrogen or ammonia which is brought 

 into the soil, and which is indispensable for the nutrition of plants. 

 Dana, to the action of the lime and acid of which the gypsum is com- 

 posed, on the organic matter and silicates of the soil. He says — " It 

 seems almost incredible that so minute a portion of a mineral can act 

 at all; yet how beautifully is the result explained by the principle 

 that plants decompose first this salt; the lime, for plaster is a sul- 

 phate of lime, then acts on geine, which is thus rendered soluble; 

 while the acid, the oil of vitriol or sulphuric acid, immediately acts 

 on silicates." It seems very probable that no single one of these sup- 

 positions will be found able to account in full for the action of plaster. 

 That of Dr. Dana appears to approach as nearly to a solution as any of 

 them, if we extend his term silicates so as to embrace those combi- 

 nations formed by the union of the acid of the gypsum with ammo- 

 nia, after its separation from the lime. If the action of plaster was 

 due to its fixing ammonia alone, then it ought to be equally efficient 

 at all times and places, which it certainly is not; or if it acted directly 

 as nutriment, then its action would be as constant as that of rotted 

 manure or compost, which farmers well know is not the case. Plas- 

 ter does not act as usefully in the vicinity of the sea, as in the inte- 

 rior; and on heavy wet soils, is scarcely felt at all. Light sandy soils, 

 or loamy ones, are those on which plaster acts the most sensibly; and 

 clover, lucerne, potatoes, cabbages, and the leguminous plants, such 

 as peas, vetches, &c., are the vegetables on which it exerts the most 

 powerful influence. It is much valued as a dressing for wheat, not 

 so much, perhaps, for its direct action on that plant, although that is 

 not trifling, as for its effect in securing and promoting the growth of 

 the clover and other grass seeds, usually, in wheat countries, sown with 

 this crop. So marked is the influence it exerts in this respect, that 

 plaster, clover, and wheat, are always associated in the mind of the 

 most successful wheat growers; and its use is the most extensive in 

 [Senate No. 63.] M 



