SUMMARY 255 



unreasonable extent, and also will injure germinating seeds 

 if applied too near to seeding. 



190. Use of Magnesian Lime. — Some limestones contain 

 magnesium, and small amounts do no harm. Large amounts 

 up to 45 per cent, (dolomite) are questionable. When burned, 

 such a lime slakes with difficulty, and may cause serious 

 harm to crops if the soil to which it is applied contains an 

 undue proportion of magnesium. Just what this proportion 

 should be varies with the crop and depends of course also 

 on the relative amounts of calcium and magnesium that are 

 dissolved in the soil moisture. On the other hand, magnesian 

 lime does no harm on soils not so well supplied with mag- 

 nesium. To be safe it is better to use a grade of lime high in 

 calcium carbonate. 



191. Calcium Sulphate, Gypsum, Land Plaster, CaS0 4 .2H 2 0. 

 — This material is used to some extent on soils as an amend- 

 ment. It frees potassium and phosphorus from insoluble 

 compounds, and is said to hasten the decomposition of 

 organic matter, but it has no neutralizing effect and is not 

 of much value. The other compounds of clacium have all 

 these effects plus the neutralizing effect. 



192. Waste Lime. — Lime is used in purifying coal gas and 

 in the manufacture of sugar from beets. Gas-lime should 

 be exposed to the air for some time before applying to the 

 soil, or should be added to the soil a long time before seeding 

 because it contains sulphides and sulphites from sulphur 

 compounds absorbed from the gas. These compounds 

 change to sulphates on exposure to the air and are thus 

 rendered harmless to plants. 



Lime is a waste product in the manufacture of acetylene 

 gas, and should be exposed to the air before use to allow 

 traces of acetylene to escape, which is harmful to seeds. 



Lime from these processes is usually a mixture of hydroxide 

 and carbonate and is valuable for agricultural purposes if 

 it can be obtained cheap and if the content of calcium 

 oxide is known. 



193. Summary. — Calcium compounds as a rule are con- 

 sidered soil amendments, that is, their effect is on the soil 

 and not on the plant directly. But calcium is a necessary 



