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exceedingly small quantities. While the plant is growing a con- 

 stant stream of water flows up through it and evaporates at its 

 leaves. For every pound of growth in dry matter made by the 

 plant, from 300 to 800 pounds of water flow up through it. 



The plant food substances dissolved in the soil water may be 

 divided into two classes according to their ultimate source. 



MINERAL PLANT FOOD. 



Plants in their growth make use of thirteen chemical elements, 

 nine of which they secure directly from the soil. These are called 

 the mineral plant foods ; they are phosphorus, potassium, calcium, 

 magnesium, sodium, iron, silicon, chlorin, and sulphur. We have 

 already seen that the soil consists mainly of small particles of 

 rock. The rock particles are of many kinds, but nearly all kinds 

 contain more or less potassium, calcium, phosphoric acid, &c. 

 Every year the soil water dissolves off a thin surface layer from 

 each particle. Plants appropriate this water and thus secure 

 mineral plant food. 



Many generations of plants have thus been collecting their 

 small toll of food from the soil and storing it up in their tissues. 

 The amount of plant food made ready for plant use during 

 each growing season through the slow solution of the mineral par- 

 ticles of the soil is doubtless supplemented to a considerable 

 degree by the same kinds of materials set free from the organic 

 matter also found in the soil — that is, the mineral matter originally 

 secured from the dissolving minerals, but built into plants during 

 some former season, may again be used by other plants when the 

 old matter is given an opportunity to decay in the soil. These 

 foods derived directly from the mineral matter of the soil and in- 

 directly from it through the growth, death, decay, and return of 

 former crops are also supplemented in many cases by the ap- 

 plication of mineral matter in the form of commercial fertilizers. 



NITROGEN COMPOUNDS. 



In addition to the nine elements already mentioned, the growing 

 plant requires four other elements, as follows : hydrogen, which 

 it secures from water (water is a compound of hydrogen and oxy- 

 gen): oxygen, which it secures partly from water and partly from 

 the air ; carbon, which is secured from carbonic-acid gas in the 

 air ; and nitrogen. 



Nitrogen is in many respects the most important of all the plant- 

 food elements. It is not found in appreciable quantities in the 

 rock particles of the soil. Ordinary plants depend for their 

 nitrogen entirely on decaying organic matter. As decay proceeds 

 nitrates are formed from the nitrogen contained in organic matter. 

 The nitrates are exceedingly soluble, and unless soon made use of 

 by growing crops they are washed out of the soil. Nitrogen is 

 therefore usually the first element to become exhausted in the soil_ 



Fortunately, there are certain species of bacteria that can use 

 atmospheric nitrogen, of which there is an inexhaustible supply. 

 One family of plants — the legumes — has learned to exchange work 



