382 



SOILS. 



That magnesia is a very important plant-food ingredient is 

 apparent from its invariable and rather abundant presence in 

 the seeds of plants, where it takes precedence of lime. Its func- 

 tions in plant nutrition have been specially investigated by O. 

 Loew, 1 particularly with respect to its relations to lime. As 

 already stated in connection with the soil-forming properties 

 of magnesian minerals (see chapter 2), soils containing large 

 proportions of magnesia generally are found to be unthrifty, 

 the lands so constituted being frequently designated as " bar- 

 rens." Loew finds that certain proportions of lime to magnesia 

 must be preserved if production is to be satisfactory, the pro- 

 portion varying with different plants, some of which (c. g. 

 oats) will do well when the proportion of lime to magnesia is 

 as i :i, while others require, that that ratio should be as 2 or 3 

 is to i, to secure the best results. In general it is best that 

 lime should exceed magnesia in amount. 



Loew explains the injurious action of magnesium salts thus : The 

 calcium nucleo-proteids of the organic structures are transformed in 

 presence of soluble salts of magnesium into magnesium compounds, 

 while the calcium of the former enters into combination with the acid 

 of the magnesium salt. By this transformation the capacity for im- 

 bibition will change, which must result in a fatal disturbance of functions. 

 The presence of soluble lime-salts will prevent that interchange. Thus 

 certain alg?e perished in a solution containing i per 1000 of magnesium 

 nitrate, but remained alive when .3 per 1000 of calcium nitrate was 

 added. 



Magnesia seems to be specially concerned in the transfer of 

 phosphoric acid through the plant tissues, in the form of 

 dimagnesic-hydric phosphate, which is rather soluble in the 

 acid juices of plants. It is probable that, apart from the re- 

 lations just referred to, such excess of lime as is known to pro- 

 duce chlorosis in plants interferes with the transfer of the mag- 

 nesic phosphate. Some plants, as already stated, dispose of an 

 excess of lime by depositing it in the form of oxalate, while 

 others (such as the stone crops) excrete it on the surface of 



1 Bull. No. 18, Div. Vegetable Physiology and Plant Pathology; Bull. No. i 

 Bureau of Plant Industry, U. S. Dept. of Agr.; Bull. College of Agriculture, Tokyo 

 Vol. 4, No. 5. 



