Oct. IS, 1919 Rate of A hsorption of Soil Constituents 7 1 



The concentration of the water extracts of normal soils under crop 

 and producing good yields varies greatly during the growing season {8). 

 With barley and numerous other plants such concentrations may be- 

 come relatively low after the plant becomes well established. Nitrates 

 in particular tend toward a very low level in cropped soils at the height 

 of the growing season even if present in considerable quantities at the 

 time of planting the crop. Since the variation of the freezing-point 

 depressions appears to accord closely with the water extracts (2), it 

 seems clear that the normal habitat of annual land plants includes a 

 soil solution which may be of relatively high concentration at the begin- 

 ning of the growing season but which inevitably falls off at a certain 

 stage of development if the growth is at all prolific. In the light of the 

 results reported above, this diminution of total and nitrate concentra- 

 tion, which we believe to be general in cropped soils, doubtless has an 

 important and probably a favorable effect on crop production. This is 

 confirmed with respect to nitrogen by the abundant evidence we have 

 that applications of nitrates late in the season delay the maturation of 

 grain crops. We interpret these facts to mean that the mutual relations 

 of soils and plants are such that it is generally desirable to have the 

 large amounts of solutes incidental to relatively high concentrations in 

 the soil solution at the commencement of the plant's growth cycle but 

 that it is unnecessary and may be undesirable to maintain this condi- 

 tion during certain later stages of growth. This conclusion has been 

 successfully utilized in this laboratory (j) in formulating water-culture 

 experiments with barley, but may not apply to all crops. 



It would seem that studies of the absorption of other plants grown on 

 natural soils, particularly those yielding good crops, have important 

 applications in investigations for determining the conditions for optimum 

 growth by means of sand and water cultures; for while the amount of a 

 given constituent absorbed does not necessarily indicate the quantity 

 essential to proper development, fluctuations in the rate of absorption 

 may be expected to reflect the nutritional peculiarities of the crop and 

 serve as a guide in regulating the concentrations and amounts of solutes 

 at successive stages of growth. 



SUMMARY 



(i) The composition of barley grown on two different soils was studied 

 at successive stages of growth. 



(2) In spite of differences in the character of the soils and the yields 

 obtained, a striking similarity was observed in the growth cycles and in 

 the successive changes in the rates of absorption of the plants. 



(3) Attention is called to remarkable losses of potassium and nitrogen 

 from the plant at an early stage of development, which are succeeded 

 by renewed absorption at a later period. 



