THE REQUIREMENTS OF PLANTS 



43 



plant nutrient ; it thus delays the setting in of potash starvation, but 

 will not keep it off altogether. Hellriegel (131) found that sodium 

 salts always gave increases in crop even when potassium salts were 

 present in quantity. 



Table XIX.— Effect of Sodium Salts with Small and with Large Amounts of 

 Potassium Salts on the Growth of Barley. Hellriegel (131). 



Breazeale (52) has more recently obtained similar results in water 

 cultures. It is well ascertained in farming practice that sodium salts 

 can be used with great effect as manures wherever there is any 

 deficiency of potash in the soil. 



Lithium, salts, on the other hand, have a toxic action on plants. 

 Gaunersdorper's older experiments (loo) have been confirmed by 

 J. A. Voelcker (290), who found that amounts of the chloride, sul- 

 phate, or nitrate, corresponding to -00375 per cent, of the metal were 

 distinctly injurious to wheat ; smaller amounts, however, appeared to 

 cause an increased growth. 



Caesium salts are less harmful (189, 290). 



Calcium is an essential plant food, the function of which was first 

 carefully studied by von Raumer (233), but has not yet been satis- 

 factorily cleared up. Nothing can be 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. 



Its most remarkable effects are seen in water cultures. Curiously 

 enough, single salts of potassium, magnesium, sodium, etc., are toxic 

 to plants, while a mixture of salts is not. Calcium salts are by much 

 the most powerful reducers of this toxic effect. Thus Kearney and 

 Cameron (147) found that a root of Lupinus albus was just killed 

 when immersed in -00125 N magnesium sulphate solution (7 parts per 

 100,000), but the effect was modified by added salts, as shown in 

 Table XX. :— 



A * 



