574 



SCIENCE 



[N. S. Vol. XXXVI. No. 931 



is a poison: sulphates or nitrates are as a 

 rule much better for the plant than the 

 corresponding sulphites or nitrites. Most 

 of the energy obtainable by oxidation of 

 the latter substances is not available to 

 the plant. 



Thorough oxidation of the soil is one of 

 the principal benefits of tillage and is a 

 prime requisite for soil fertility: wherever 

 it is interfered with by excess of water 

 disastrous results follow, as may be seen in 

 an extreme form in bogs. Investigations 

 are being made on the oxygen content of 

 bog waters and it is to be hoped that they 

 may be extended to soil waters generally. 

 Quantitative methods of estimating the 

 rate of oxidation in soils under natural 

 conditions are much to be desired. 



The excretion of oxidizing and reducing 

 catalyzers by living roots has recently been 

 described and the statement has been made 

 that the ability to oxidize such substances 

 as gum guaiae (and to decompose hydro- 

 gen peroxide) is characteristic of good soil. 



The study of catalyzers in the soil is a 

 field in which we should have long since 

 made a serious beginning. As I have just 

 spoken of oxidation, an example from that 

 field may be chosen to illustrate the point. 

 It is well known that certain salts of man- 

 ganese act as catalyzers which enormously 

 increase oxidation, so much so that the 

 minute traces of manganese which dissolve 

 out from the glass vessels used in the ex- 

 periment may affect the result. It is there- 

 fore possible that salts of manganese in the 

 soil may affect fertility, and we may sup- 

 pose that salts of iron and other substances 

 may act in a similar way. 



Another important field of study is 

 found in the relation between those ions 

 which are selected by the plant and those 

 which it leaves in the solution. When 

 the plant takes the K from a solution 

 and leaves the CI behind we have an illus- 



tration of this. The separation of posi- 

 tive from negative ions in this manner 

 results in electric stresses which tend to 

 bring the processes to a standstill. This 

 may be overcome in two ways: first, the 

 plant may give up to the solution some 

 other positive ion in place of the K it 

 takes in. It is evident that if such a 

 process is taking place our usual view of 

 the process of absorption neglects one of 

 the most important factors of the process, 

 and thus we should speak of an exchange 

 rather than of absorption of ions. 



On the other hand, we may find that 

 along with every K ion the plant absorbs 

 an OH ion derived from hydrolytie decom- 

 position. If the plant should take the 

 NO3 ion and leave the Na we might find 

 that along with every NO3 ion the plant 

 absorbed an H ion produced by hydrolytie 

 decomposition. This amounts to saying 

 that in all eases when the plant selects one 

 ion and leaves the other it takes up the 

 absorbed substance in the form of a very 

 dilute acid or alkali. It is hoped that 

 some investigations which are now going 

 on may throw some light on this subject. 



A further question concerns the manner 

 in which ions which have been absorbed 

 by the protoplasm are prevented from dif- 

 fusing out again. It seems necessary to 

 assume that only those ions which unite 

 into undissociable compounds can be re- 

 tained in the protoplasm. In this way we 

 may explain how potassium, which pre- 

 sumably forms such compounds, may accu- 

 mulate in the plant to a greater extent 

 than sodium. 



It may be pointed out that the chemical 

 effects of soil substances on the plant may 

 be of at least four kinds : 



1. Toxic Action. — A large number of 

 substances found in soils are toxic in suffi- 

 ciently high concentration. This is true 

 of both organic and inorganic substances. 



