FAEMEES' IXSTITUTES. 261 



twenty tous of plaster, we should hardly expect that the addition of fifty or one 

 hundred pounds would produce any marked effect. 



It is sometimes observed in this country that after plaster has been uesd many 

 years, and especially if used in large doses, it ceases to be beneficial. The 

 explanation may be, that the soil then contains so much of the material that 

 further additions are not for the time needed. The explanation I have seen in the 

 newspapers, that "the plaster was too old — had lost strength by keeping," has 

 no foundation in fact, because plaster is a fixed and changeless substance, and 

 does not lose strength by keeping any more than does common salt. Old plas- 

 ter is just as strong as new. 



WHY DOES PLASTER BENEFIT CEOPS? 



1. Because it contains two substances necessary for the growth of crops, viz. : 

 sulphur and lime. Different crops contain different proportions of these mate- 

 rials, and here we find one reason wdiy plaster is more beneficial for one crop 

 than another. A clover crop of four tons contains more than forty-nine pounds 

 of sulphate of lime or plaster ; while a wheat crop of twenty-five bushels (with 

 the straw) contains less than seven pounds ; while fifty bushels corn contain less 

 than one pound of plaster, and Avitli three tons of stalks the crop removes less 

 than tiiirty pounds of plaster, 



2. Licbig claimed that the chief benefit from plaster was from its combining 

 with the carbonate of ammonia of the air and of rain water, forming sulpliate 

 of ammonia, which is not volatile at ordinary temperatures, while the carbonate 

 is volatile. In this Avay plaster has a power of fixing the atmospheric ammonia. 

 This is the explanation we almost always find in the books, but it is not very 

 satisfactory when closely examined. The amount of carbonate of ammonia 

 brought down by tlie rain varies from seven to twelve pounds to the aci'e ; only 

 a part of this would be volatilized under ordinary circumstances, so that the 

 power of plaster to fix volatile atmospheric ammonia would seem to be small at 

 the best under ordinary conditions. 



3. A large number of investigators liave conclusively shown that a solution of 

 the sulphate of lime has a jDCCuliar disintegrating influence on the compound 

 silicates of the soil, setting free potash and magnesia for the use of the plant. 

 This result has been found so uniformly by different investigators in different 

 countries, that there could be no doubt on the subject. By a dressing of plaster, 

 therefore, we not only afford the j^lant a supply of sulphur and lime, but we also 

 afford it the means of obtaining from the soil an increased suj^ply of potash and 

 magnesia. Both of these substances are essential to plant growth, but potash 

 is one of the most important and abundant constituents of the ash of cultivated 

 plants, and without this substance no plant can grow. The influence of potash 

 on plant growth is well seen when wood ashes are used as a manure, and you 

 are all aware how much the vigor of the plant is increased thereby. When we 

 use wood ashes we directly increase the amount of potash in the soil ; when we 

 use plaster Ave render active the potash which was locked up in the soil in the 

 form of a compound silicate, and thus indirectly supply the plant with this 

 important material. By this power of rendering potash and magnesia soluble 

 and active in the soil, we in reality furnish the plant with four very essential 

 constituents of plant growth by the use of plaster, viz, : sulphur, lime, potash, 

 and magnesia. 



4. Plaster has a peculiar action on the vegetable matter of soils. All observ- 

 ant farmers have noticed that plaster does not benefit the crop when the soil is 



