56 MISC. PUBLICATION 3 69, U. S. DEFT. OF AGRICULTURE 



considered. It seems quite safe to conclude, however, that the plant 

 composition is modified by both the climate and the soil in which it 

 is growing, and that both of these factors are closely inter-related, 

 often one modifying the effect of the other. Thus the assimilation 

 of phosphorus by the plant growing in calcareous soils may be less in 

 dry than in wet years, whereas entirely contrary results may be ob- 

 tained in plants growing in siliceous soils. The period of plant 

 growth at which high rainfall occurs is of paramount importance in 

 the assimilation of nutrients by the plant as well as in the growth 

 of the plant, and, for this reason, crops growing in the same soil type 

 separated by a sufficient distance so as not to receive the same amount 

 of rainfall at the same time in any one year may be quite different 

 in their chemical composition. 



IRRIGATION 



The discussion under climate indicates that the composition of the 

 plant, under some conditions at least, is closely related to the moisture 

 or rainfall available during certain times of the growing period. 

 Irrigation is a climatic factor, inasmuch as moisture is supplied 

 artificially instead of naturally. 



In 1900, von Daszewski and Tollens (137) studied the effect of 

 irrigation in Germany on the composition of potatoes. They con- 

 cluded that although yields were increased with irrigation, no change 

 in the phosphorus and potassium content of the plant occurred. 

 Stahl-Schroder (586) reported that increasing the water in a soil from 

 35 up to 95 percent of its water capacity resulted in a large increase 

 in ash, silica, and phosphorus in the grain of oats. The nitrogen con- 

 tent decreased to a minimum at 90-percent water capacity, but in- 

 creased markedly at 95 percent. 



Irrigation in varying amounts had no effect on the nitrogen content 

 of the corn grain, according to Harris and Pittman (241) % m Utah. 

 The same observation held for phosphorus, calcium, and magnesium, 

 although a large variation in the mineral nutrients was observed in 

 the crops grown in different years, but with the same amount of irri- 

 gation. The opposite effect was reported by Greaves and Carter (221) 

 in 1923, in their experiments with wheat, oats, and barley. These 

 authors found that phosphorus, potassium, calcium, and magnesium 

 in the grain were all increased as a result of irrigation. Greaves and 

 Nelson (224) showed that while the iron, chlorine, and sulfur contents 

 of wheat were increased by irrigation, these elements in oats and 

 barley either underwent no change or actually decreased. Further 

 work of Greaves and Nelson (223), in 1925, using corn, confirms the 

 work of Harris and Pittman, who used the same crop. 



The addition of irrigation water to potatoes grown in Georgia had 

 no influence on the iodine content of that crop, according to the experi- 

 ments of Holley, Pickett, and Brown (274), reported in 1935. 



The effect of irrigation, like climate, varies with other factors such 

 as the species of plant, the soil, or the amount of water added. The 

 data are too few and too narrow in scope to justify any general con- 

 clusions regarding this factor. 



