IN SAND 49 



ferent salts. This subject is reserved for Chapter VIII 

 and will not be discussed here. 



A series of experiments was made by Marchal (20) to 

 discover the effect of salts on the bacterial activities of 

 the nodules of pea roots. He found alkaline nitrates in 

 concentrations of 100 parts per million checked the tu- 

 bercle production in water cultures. Ammonium salts 

 were injurious in concentrations of 500 parts per million. 

 Potassium salts at 5000 parts per million and sodium 

 salts at 3333 parts per million tended to retard symbiosis, 

 but calcium and magnesium salts favored it. 



Soil Results. - - Soil studies of alkali have been found 

 to show less variation for like treatment than solution 

 studies. Some of the other disadvantages of solution 

 studies of the effect of alkali on the higher plants are that 

 the seed in germination tests and the root system are placed 

 in an unnatural environment, the air circulation being 

 eliminated and the normal resistance of the soil being 

 changed. Studies of plants in solutions compared with 

 similar soil cultures have shown that physiological dis- 

 turbances are more likely to occur in solutions than in 

 soils; the root-hairs are less numerous and the roots grow 

 longer and thinner in the solution than in the soil. In- 

 dividuals show much more variation due to unfavorable 

 causes in the solutions than in the soils even where the 

 soil consists of sand containing practically no nourishment. 



In Sand. - - The physical conditions under which the 

 plants grow seem to have some influence on their natural 

 development. The author (10) found that whereas wheat 

 seedlings produced about a half normal crop of dry matter 

 in a sand containing 1000 parts per million of sodium 

 chloride in solution cultures, more than half a normal 

 crop was obtained when the concentration was over 10,000 



