102 PHYSIOLOGY OF NUTRITION 



not been in alfalfa culture, had no injurious effect upon the growth of this 

 plant. 



Experiments have also been made to determine the effects of various plant 

 substances upon plant growth. Such substances are sometimes injurious and 

 sometimes beneficial. Watering with a 3-per cent, solution of nicotin, for 

 instance, produces good growth in tobacco, and is likewise beneficial to potatoes. 1 



[§7. Oxygen Absorption from the Soil. — Besides water and the substances 

 considered above, the soil supplies the ordinary plant with a little oxygen, and 

 some oxygen supply from the soil is essential to the healthy growth of the roots 

 of most plants. Soil aeration is generally necessary for good plant growth. 

 Hutchins has developed methods for studying soils in this regard.' — Ed.] 



Summary 



I. Cultures in Artificial Media. — The essential chemical elements for plants in 

 general are: Carbon, hydrogen, oxygen, nitrogen, sulphur, phosphorus, potassium, 

 calcium, magnesium, and iron (C, H, O, N, S, P, K, Ca, Mg, Fe). Carbon and oxygen 

 enter ordinary green plants mainly as carbon dioxide (C0 2 ), while hydrogen and oxygen 

 enter mainly as water (H 2 0). As already seen, these two compounds are decomposed 

 in chlorophyll-bearing cells, by the action of sunlight, forming carbohydrates ([CH 2 0]„) 

 and free oxygen. From carbohydrates and other substances the plant cells form the 

 many different organic compounds found in the plant body. Tissues without chloro- 

 phyll must absorb their carbohydrates (and often many other organic compounds, from 

 their surroundings, including the green tissues of the same plant. As has also been 

 seen, nitrogen enters the ordinary plant mainly as nitrates (sometimes as nitrites, 

 ammonium salts, or organic nitrogenous substances), and these become combined with 

 carbohydrates, etc., to form many of the most complex substances occurring in plants. 

 When plants are completely burned all of the carbon, hydrogen, oxygen, and nitro- 

 gen are givqn off as gases, but there remain small amounts of many other essential 

 and non-essential elements in the form of incombustible ash. The total ash of ordinary 

 plants constitutes only about 5 per cent, of the total dry weight, or about 0.02 per 

 cent, of the green weight. The other essential elements (S, P, K, Ca, Mg, Fe) of the 

 ash are absorbed by ordinary plants from the soil, just as are water and nitrates, and 

 the supply is in the form of inorganic salts: mainly nitrates (N0 3 ), sulphates (S0 4 ), 

 and phosphates (P0 4 ), of potassium, calcium, magnesium, and iron. 



The elements absorbed through the roots may be studied by artificially controlled 

 cultures in water solutions or in pure quartz sand, etc., the latter of course containing 

 water solution in its interstices. Numerous different solutions have been tested by 

 many workers. A very good medium for solution culture may be prepared with cal- 

 cium nitrate [Ca(N0 3 ) 2 ], mono- or di-potassium phosphate, (KH 2 P0 4 or K 2 HP0 4 ), 

 and magnesium sulphate (MgS0 4 ), about seven thousandths of a gram-molecule (the 

 molecular weight expressed in grams) of each salt, all dissolved together in a liter of 

 water, with addition of a very small amount (about 3 mg.) of an iron salt such as 



1 Otto, R., and Kooper, W. D., Untersuchungen uber der Einfluss giftiger, alkaloidfuhrender Losungen 

 auf Boden und Pflanzen. Landw. Jahrb. 39 : 397-407. ioio. 



( Hutchins, Lee M., and Burton E. Livingston, Oxygen-supplying power of the soil, as 

 indicated by color changes in alkaline pyrogallol solution. Jour. Agric. Res. 25: 133-140. 

 1923. Hutchins, Lee M., Studies on the oxygen-supplying power of the soil together 

 with quantitative observations on the oxygen-supplying power requisite for seed 

 germination. Plant Physiol. 1 : 95-15°- 1926. Cannon, W. A., (1925) [See p. 6]. 



