66 Report of the Department of Agronomy of the 



as it is exposed to the air. The figures above, then, are changed 

 somewhat in actual practice. For practical purposes it may be 

 remembered that 1200 pounds of freshly-burned lime, 1500 pounds 

 of hydrated lime and 2000 pounds of ground limestone all contain 

 approximately the same amount of actual lime, and are capable of 

 neutralizing the same amount of acidity when applied to the soil, 

 provided all forms are of good grade. Bulk lump lime offered to 

 the agricultural trade at $4 to $5 per ton is not as good a grade as 

 that considered in these figures. In fact, it often contains the ashes 

 from the coal used in burning, may contain some unburned core, 

 and may be partly air-slaked. Ground limestone (or its equivalent, 

 " air-slaked lime,"), quicklime and hydrated lime are the only three 

 forms that can properly be called lime or that can be used for neutral- 

 izing acidity in the soil. Marl is the same in composition as ground 

 limestone and when it contains the same percentage of carbonates is 

 of equal value. " Agricultural lime," " Land lime," etc., are only 

 trade names given to any one of the three forms or a mixture of 

 thera. 



Magnesium limestones are such as have a part of the calcium 

 carbonate replaced by magnesium carbonate (MgC0 3 ). Magnesium 

 carbonate is also efficient for neutralizing soil acidity, in fact a 

 given weight of it will neutralize somewhat more acidity than the 

 same weight of calcium carbonate, due to a difference in the mole- 

 cular weights of the two compounds. One hundred pounds of magne- 

 sium carbonate is chemically equivalent to 119 pounds of calcium 

 carbonate. Limestone carrying calcium and magnesium carbonates 

 in proportion to their molecular weights (100:84) are known as 

 dolomites. A limestone having 75 per ct. calcium carbonate and 

 20 per ct. magnesium carbonate is equal in neutralizing power to 

 one having 98.8 per ct. calcium carbonate. Owing to these facts 

 and also to the fact that magnesium as well as calcium is an essential 

 element o r plant food, magnesium limestones sometimes give some- 

 what better results than straight calcium stones. However, magne- 

 sium salts in excessive quantities often have a toxic effect on plants, 

 and so on some soils already containing large amounts of magnesium 

 the application of magnesium limestones may not give as good 

 results as straight calcium stones. 



COMPARATIVE EXPERIMENTS WITH GROUND LIMESTONE AND BURNED 



LIME. 



Because of the caustic properties and finely powdered condition 

 of burned lime, the theory has often been advanced that it is better 

 to use than the carbonate. There is no evidence, however, from any 

 carefully conducted field test to show that this is ever the case. 

 On the other hand seven years of comparative trials in the field at 

 the Ohio station, eleven years at the Maryland station and twenty- 



