26 PHILIPPINE AGRICULTURE 



be full of starch. Iron is a part of the chlorophyll; so 

 plants without iron become white, and die because they 

 have no chlorophyll to absorb the light. One use of 

 potassium is to keep plants stiff and fresh by helping 

 them to hold water; wilted plants cannot grow. 



How Plants obtain the Elements. Plants can take up 

 none of these elements, except, perhaps, oxygen, by them- 

 selves, but must receive them in combinations. The 

 most important of these combinations is water, from 

 which plants obtain hydrogen, and perhaps oxygen. 

 Carbon is obtained from the carbon dioxid in the air. 



The other combinations which supply plants with food 

 are called salts. The salts dissolve in water, and plants 

 take them up with the water in which they are dissolved. 

 Although nitrogen is a gas, making up a large part of 

 the air, plants cannot use it as a food in this form; their 

 nitrogen must be in the form of salts, dissolved in water, 

 which the roots can get. The water dissolves all these 

 salts from the soil. Therefore, soil which is without any 

 of them cannot support plants. 



Elements usually Abundant. Except, rarely, in the case 

 of iron, without which the younger leaves are white instead 

 of green, it is impossible to know from a plant's appear- 

 ance that it is in especial need of any one food. The 

 only way to determine this is by experiments, as described 

 on page 29. But most of the necessary elements are so 

 abundant everywhere in the soil that plants never suffer 

 for them. 



Magnesium, iron, and sulphur are not present in large 



