186 HOW CROPS GROW. 



trol and great variety of alteration, and is, therefore, oi 

 the utmost value in vegetable physiology. It has taught 

 important facts which no other means of study could re- 

 veal, and promises to enrich our knowledge in a still 

 more eminent degree. 



Potassium, Calcium, and Magnesium as soluble 

 Salts, Phosphorus as Phosphates and Sulphur as 

 Sulphates, are absolutely necessary for the life of- 

 Agricultural Plants, as is demonstrated by all the ex- 

 periments hitherto made for studying their influence. 



It is impossible to recount here in detail the evidence 

 to this effect that is furnished by the investigations of 

 Salm-Horstmar, Sachs, Knop, Nobbe, Birner and Luca- 

 nus, and others ( Vs. St., VIII, p. 128-161). 



Some of the experimental proof of this statement is 

 strikingly exhibited by the figures on Plate I, copied 

 from Kobbe, showing results of the water-culture of 

 buckwheat in normal nutritive solutions and in solutions 

 variously deficient. 



Is Sodium Essential for Agricultural Plants? 

 This question has occasioned much discussion. A glance 

 at the table of ash-analyses (pp. 164-170) will show that 

 the range of variation is very great as regards this alkali- 

 metal. The older analysts often reported a considerable 

 proportion of sodium oxide, even 20% or more, in the ash 

 of seeds and grains. In most of the analyses, however, 

 sodium oxide is given in much smaller quantity. The 

 average in the ashes of the grains is less than 3 per cent, 

 and in not a few of the analyses it is entirely wanting. 



In the older analyses of other classes of agricultural 

 plants, especially in root crops, similarly great variations 

 occur. Some uncertainty exists as to these older data, for 

 the reason that the estimation of sodium by the processes 

 customarily employed is liable to great inaccuracy, espe- 

 cially with the inexperienced analyst. On the one hand, 

 it is not or was not easy to detect, much less to estimate, 



