THE PHYSIOLOGICAL ROLE OF WATER IN PLANTS. 19 



enabled to produce more perfect seed. This is also the case with trees 

 in a held, whicli are well known to injure plants cultivated about them. 

 Weeds injure cultivated plants in the same way. In view of these 

 facts it would be interesting to investigate from a practical standpoint 

 the elfect on the total product of S(>wing crops together and cultivating 

 one crop between the rows of another. An attem})t should be made to 

 determine by experiment the proportional reduction in the yield of one 

 grouj) of plants growing in the same soil with another, as, for exami)]e, 

 legumes cultivated with cereals or between rows of potatoes. 



Opinions are very diverse as to the cause of the efficiency of water 

 employed in irrigation.' Some claim that the fertilizing action is due 

 entirely to nuiterials held in solution, and that water for irrigation 

 should be turbid and impure, while others maintain that clear water 

 produces the best results. It is known that the (piautity of carbonic 

 acid contained in water and its temperature modify the fertilizing action 

 to a very great degree on different soils. 



Prof. Ronna^ has shown that upon clay soils abundant rains which 

 thoroughly saturate and flood the soil may advantageously take the 

 place of fertilizers. In dry seasons fertilizers remain without effect in 

 the soil, and in most seasc:is no fertilizer is able to supply the fertiliz- 

 ing effect of rain. Water dissolves the elements of the soil which are 

 necessary to the growth of plants. In well-manured, high, sandy soils 

 the abundant spring rains wash out the soluble materials, such as 

 nitrate of soda and guano. Pure water may be beneficial on some soils 

 but injurious to others. Water charged with fertilizing elements is a 

 valuable agent in fertility, but it must not be forgotten that on well- 

 manure<l soil it may carry off' more than it brings to the soil. This is the 

 case when sewage waters are applied in excessive amounts. Yoelcker 

 has called attention to the presence of nitric acid in many waters 

 suitable for irrigation. He insists on the necessity of studying the 

 natural causes of loss through filtration of the fertilizing materials. 

 In intensive cultivation the art consists in preserving the fertilizing 

 materials as much as possible at the surface, where they provide for tlie 

 needs of the i)lant at the beginning of its growth. Voelcker concludes 

 that in order that irrigation with sewage water may be profitable on 

 sandy meadows it will be necessary to em])loy about 20,000 cubic meters 

 of water ])er hectare in 4 or 5 separate applications. Plants which 

 grow rapidly make use of the fertilizers about them, but, on the other 

 hand, cereals and truck crops are not able to receive large quantities 

 of water throughout tlie entire year. Green plants produced by sewage 

 water are not as nutritious as those grown on meadows irrigated with 

 pure water. ' 



' On irrigation : A lecture l)y Prof. Voelcker (Jour. Roy. Agl. 8oc. of England, 1867, 

 p.4fi4). 



"A. Ronna, (;himio api)li(in('(', ;\ ragricnltdre. 



^Tiiird re])()rt of the coiHiiiiusion for iiKiuiry into tlic bent mode ol' distributing the 

 Bew.'tge of towiiH, p. IS. London, 1865. 



