THE IRRIGATION AGE. 



335 



continual decomposition and re-composition. In its 

 fluid state it finds its way into every vessel and every 

 tissue. In this situation the water yields its oxygen to 

 one portion of the plant and its hydrogen to another 

 portion, wherever either is needed, and, in like manner, 

 the oxygen and the hydrogen resume their combination 

 as water and cling together until a new chemical change 

 is needed. To comprehend this better the reader ha^ 

 only to observe the effects of water on his own system, 

 for, as between plants and animals, the transmutations 

 of oxygen and hydrogen, conveyed into the system by 

 means of water, are practically identical. 



We shall have more to say upon this subject in the 

 chapter on the advantages of irrigation. 



' CARBONIC ACID. 



Carbonic acid, as has been said, is the gas 

 from burned charcoal, or carbon. It has an acid 

 taste and smell, is soluble in water, and reddens vege- 

 table blues. Water dissolves more than its own bulk 

 of this gas. It is one-half heavier than atmospheric 

 air, and is deadly in its effects. Yet it is the principal 

 food of plants, being absorbed by the leaves and roots 

 in large quantities, hence its presence in the atmos- 

 phere is necessary to plant growth, though the pro- 

 portion is small. 



Carbonic acid unites with potash, soda and lime,, 

 forming compounds known as "carbonates." Thus 

 pearlash is carbonate of potash; the common soda of 

 the shops is carbonate of soda, and limestone, or chalk, 

 is carbonate of lime. The common carbonate of lime, 

 in its various forms of chalk, limestone, or marble, is 

 insoluble in pure water, but it dissolves readily in 

 water containing carbonic acid. We know that water 

 absorbs a quantity of carbonic acid from the atmos- 

 phere, and hence as it trickles through the soils con- 

 taining limestone, etc., it dissolves a portion of the 

 earth and carries it in its progress to the roots of the 

 plants, where the earthy solution is used directly or in- 

 directly to promote vegetable growth. 



As to its absorption by water, a reference to a 

 common glass of soda water will be sufficient to make 

 this clear. 



Some plants manufacture their own acids out of 

 the carbonic acid distinctive acids for instance, ox- 

 alic acid, which is found in the leaves and stems of the 

 common sorrel (oxalis). It is an acid not found in the 

 soil and may be obtained from sugar, starch and even 

 from wood by various chemical processes, principally 

 by the use of nitric acid. To detail all the uses to which 

 carbonic acid may be put would be going deep into 

 chemistry, which is beyond the scope of this book. 

 However, vegetable acids will be referred to in the next 

 chapter. 



AMMONIA. 



Ammonia is a compound of hydrogen and nitrogen, 

 and performs a very important part in the process of 

 vegetation. It promotes not only the rapidity and lux- 

 uriance of vegetation, but exercises a powerful control 

 over the functions of vegetable life. It possesses sev- 

 eral special properties which bear upon the preparation 

 of plant food. 



First It has a powerful affinity for acid sub- 

 stances, and unites with them in the soil, forming saline 

 compounds or "salts," which are more or less essential 

 to vegetable life. 



Second It possesses a very strong affinity for 



RODNEY H. YALE, BEATRICE, NEB. 



the acids of potash, soda, lime and magnesia. When 

 mixed with these acids the acid in the salt of am- 

 monia (sal ammoniac) for instance, is taken up by 

 the potash, etc., and the ammonia is set free in a 

 gaseous state. This is the effect of lime dressing on 

 a soil rich in animal and vegetable matter; it de- 

 composes the salts, particularly those of ammonia. 



Third The salts which ammonia forms with the 

 acids are all very soluble in water, and thus ammonia 

 is brought down to the roots of plants for their use. 



Fourth. In the state of carbonate it decomposes 

 gypsum, forming carbonate of lime (chalk) and sul- 

 phate of ammonia, both of which are peculiarly favor- 

 able to vegetation. 



Fifth The presence of ammonia in a soil con- 

 taining animal and vegetable matter in a decaying 

 state causes this matter to attract oxygen from the 

 air with great rapidity and in abundance, the result 

 being that organic acid compounds are formed which 

 combine with the ammonia to form ammoniacal salts. 

 On the decomposition of these latter salts by the action 

 of lime or other of the affinities above mentioned, the 

 organic acids separated from them are always further 

 advanced toward the state in which they become fit 

 for plant foods. 



Sixth The most important property of ammonia 

 is the ease with which its salts undergo decomposition, 



