59 RELATION OF LIME AND MAGNESIA TO PLANT GROWTH. 
The experiments herein reported show that where the lime and mag- 
nesia are in a wholly soluble condition, the plants germinated quickest 
and made the more rapid growth where the lime was in slight excess 
over the magnesia. In actual practice the case can not be governed 
so closely, for then we are dealing with a very complex combination— 
the soil. Moreover, it is hard to determine at each stage the form in 
which the lime and magnesia exists in the soil. Again, different plants 
are variously affected by an excess of magnesia in soils. In practice, 
therefore, it is difficult to lay down hard and fast rules for liming the 
soils for physiological results. 
As borne out by experiments, gypsum appears to be, all things con- 
sidered, the most available form of lime to apply in overcoming the 
noxious influence of an excess of magnesia. An excess of gypsum is 
little to be feared, as the plant seems to be able to use magnesia if 
present in sufficient amount for its direct needs, whether gypsum be 
present in large amount or small. On the other hand, a lack of lime 
in a soluble form is more to be guarded against, for in this case the 
magnesia, if in a certain excess, will be assimilated to the detriment of 
the plant. Magnesium carbonate we found might be ina slight excess 
over calcium sulphate, and normal healthy growth of the plant be 
made. However, this excess should be small, not greater, with cereals, 
than 2 to 1. With cowpeas, as shown, a ratio of MgO as carbonate 
0.68 to CaO as sulphate 0.2 per cent, while not toxic to the extent of 
killing the plant, was so injurious as to prevent profitable growth. In 
these cases the solubility of the salts in the soil were not determined. 
In liming, therefore, for any purpose, it is advisable to know the lime 
and magnesia content of the soil, both the soluble and total, as well 
as the content in the fertilizer applied. Underliming is more to be 
guarded against than overliming, care being taken that magnesian 
>) 
limestone is not applied where an excess of magnesia is already present. 
SUMMARY. 
Soil analyses show that lime and magnesia are widely distributed in 
soils and generally in sufficient quantities for the direct needs of plants. 
They are not always in the best proportions to each other, from a 
physiological standpoint, for favoring plant growth. 
Magnesia ina soil in great excess over lime in a finely divided or 
soluble condition is noxious to the growth of plants. With a great 
excess of lime over magnesia the physiological action of the plant is 
hindered and it exhibits phenomena of starvation. An excess of lime 
counteracts the poisonous effects of magnesia, while the more favor- 
able proportion of the two bases obviates the poor nutrition of the 
plant. 
The best proportion of soluble lime to soluble magnesia for the 
germination and growth of plants is about molecular weight 5 to 4, or 
actual weight 7 to 4. 
