THE NATURE OF SEEDS 349 



b. The Nutrient Salts. Of those substances that must 

 be placed in the water of water cultures, you have just 

 previously noted (page 344) that nitrogen is one ; there 

 must be present some soluble compound of whose mole- 

 cule nitrogen is a part. Other elements besides nitrogen 

 which are used in the manufacture of protoplasm are sul- 

 phur and phosphorus. They, like nitrogen, must be pre- 

 sented to the plant in the form of available compounds; 

 that is, as a part of molecules which, by their solubility 

 for one thing, are available for plant use. 



You noted (page 344) that the principal kinds of nitrogen- 

 containing compounds in which the nitrogen is available 

 for plant use are compounds of the class called nitrates. 

 Nitrates are compounds of nitrogen, oxygen, and of one other 

 of various elements of a group called bases. Calcium, mag- 

 nesium, potassium, and sodium are the bases of those nitrates 

 which are of common occurrence in the soil, and these other 

 elements are taken into the plant along with the nitrogen. 



Nitrates belong to that chemical class of substances 

 known as salts, and it is common to speak of the " nutrient 

 salts of the soil," meaning those salts which are necessary 

 to plant life. It is nitrogen- and phosphorus-containing 

 salts that the fertilizers used by farmers principally supply. 

 What general use the plant may make of the calcium, 

 magnesium, potassium, and sodium salts, as well as using 

 them as incidental to getting nitrogen, is not well under- 

 stood. It appears, however, that for some plants at leagt 

 they are quite necessary for their own sake as well as for 

 the nitrogen which their compounds contain. 



Iron also is necessary to the plant. You have noted 

 (page 232) that in the absence of iron chlorophyll is not 

 manufactured. 



