74 SOIL CONDITIONS AND PLANT GROWTH . 



smaller amounts, however, appeared to cause an increased 

 growth. 



CcBsium salts are less harmful (290), 



Calcium is an essential plant food, the function of which 

 was first carefully studied by von Raumer (234), but has not 

 yet been satisfactorily cleared up. Little has been inferred 

 from the fact that, like potassium, it occurs more in the leaf 

 than in the seed. It certainly gives tone and vigour to the 

 plant ; gypsum is used in alkali regions to counteract the 

 harmful effects of excessive amounts of saline matter in the 

 soil. It also appears to stimulate root production : if calcium 

 is withheld from water cultures the size of the root is much 

 reduced. Maquenne and Demoussy ^ show that the amount 

 present in the seed of the pea is insufficient for root develop- 

 ment, which therefore ceased on the third or fourth day of 

 germination in pure water. Addition even of traces of calcium 

 sulphate was followed by further root growth: O'OI mg. of 

 calcium sulphate per seed, representing an addition of calcium 

 equal to 1/40,000 the weight of the dry seed, led to formation 

 of root hairs and a 40 per cent, increase in root length. 



The close relationship between calcium and nitrogen 

 content suggests that the calcium may be associated with 

 protein metabolism, perhaps combining with the acids to 

 which such metabolism gives rise. Plants fall into two groups 

 so far as calcium is concerned : — 



{a) Those (including calcifuges) with low content of 

 calcium and low calcium-nitrogen ratio ; (^) those (including 

 calcicolous plants) in which these quantities are high.^ 



Barium and strontium, cannot replace calcium in the 

 nutrition of plants. McHague (187) has shown that the 

 carbonates are toxic, though in the presence of calcium car- 

 bonate they cause an increase in plant growth, strontium being 

 more effective than barium. In Voelcker's experiments (290) 

 the addition to the soil of even O'l per cent, of strontium 

 sulphate, hydrate, or carbonate was without effect, but the 



1 Cornet. Rend., 1917, 164, 979-985; and 165, 45-51. 

 * F. W. Parker and E. Truog {Soil Sci., 1920, 10, 49). 



