SOILS. 



147 



tion under the influence of micro-organisms, or 

 of other lower forms, either within the soil itself, 

 or in symbiotic growth with a higher plant. 



Of all the recent results bearing upon the 

 subject, those of Hellriegel and Wilfarth with 

 certain Leguminous plants seem to be by far the 

 most definite and significant; pointing to the 

 conclusion that, although the higher chloro- 

 phyllous plants may not directly utilize free 

 nitrogen, some of them, at any rate, may acquire 

 nitrogen brought into combination under the 

 influence of lower organisms; the development 

 of which is, apparently, in some cases a coinci- 

 dent of the growth of the higher plant whose 

 nutrition they are to serve. 



As to the explanation of the fixation of free 

 nitrogen, the facts at command do not favour 

 the conclusion that under the influence of sym- 

 biosis the higher plant itself was enabled to fix 

 the free nitrogen of the air by its leaves. Nor 

 does the evidence point to the conclusion that 

 the nodule-bacteria became distributed through 

 the soil and there fixed free nitrogen, the com- 

 pounds of nitrogen so produced being taken up 

 by the higher plant. It seems more consistent, 

 both with experimental results and with general 

 ideas, to suppose that the nodule-bacteria fixes 

 free nitrogen within the plant, and that the 

 higher plant absorbs the nitrogenous compounds 

 produced. In other words, there is no evidence 

 that the chlorophyllous plant itself fixes free 

 nitrogen, or that the fixation takes place within 

 the soil, but it is more probable that the lower 

 organisms fix the free nitrogen. If this should 

 eventually be established, we have to recognize 

 a new power of living organisms — that of as- 

 similating an elementary substance. But this 

 would only be an extension of the fact that 

 lower organisms are capable of performing assi- 

 milation work which the higher plant cannot 

 accomplish ; whilst it would be a further instance 

 of lower organisms serving the higher. 



Finally, it may be observed that Loew has 

 suggested that the vegetable cell, with its active 

 protoplasm, if in an alkaline condition, might 

 fix free nitrogen with the formation of am- 

 monium nitrate. 



As to the importance of the fixation of the 

 nitrogen of the air for horticulture, and for 

 vegetation generally, there is much yet to learn. 

 It is obvious that different Papilionacece growing 

 under the same external conditions manifest 

 very different susceptibility to, or power to take 

 advantage of, the symbiosis. 



The fact, as shown by Prof. Nobbe, that 

 Papilionaceous shrubs and trees — the follow- 



ing being those experimented with: Robinia 

 Pseudacacia (locust tree), Cytisus Laburnum (la- 

 burnum), and Gleditschia triacantha (honey locust), 

 as well as herbaceous plants — are susceptible to 

 the symbiosis, and under its influence may gain 

 much nitrogen, is of interest from a scientific 

 point of view as serving to explain the source 

 of some of the combined nitrogen accumulated 

 through ages on the surface of the globe ; and 

 also from a practical point of view, since, espe- 

 cially in tropical countries, such plants yield 

 many important food materials, as well as other 

 industrial products. 



Exhaustion of Plant-food in Soils. — It needs to 

 be said, perhaps, that the word "exhaustion", 

 as used in horticulture, has no very precise 

 meaning. It is a term based on money values, 

 rather than on scientific conceptions. It is true 

 enough that, in many situations, land may be 

 utterly ruined by improper or careless cultiva- 

 tion, for the richest soils in the world, if badly 

 used, will produce but comparatively few profit- 

 able crops, after which they become less and 

 less productive, until in the end the gardens 

 are left barren or to weeds. Strictly speaking, 

 a soil is exhausted, as regards any particular 

 crop, whenever the cost of cultivation comes to 

 as much as the crop is worth. 



The word "condition" is in very common 

 use amongst cultivators of the soil. It is said 

 that a piece of land or a garden is "in con- 

 dition" or "out of condition", or in "high con- 

 dition" or in "low condition". These terms 

 are well understood to imply certain states of 

 fertility. The word "condition" thus refers 

 to those elements of fertility in a soil which, 

 whether they have been accumulated by natural 

 processes or by the art of the gardener, are 

 capable of being turned to account in the growth 

 of plants and crops within a limited period of 

 time, and which by such growth are soon ex- 

 hausted. "Condition" is, therefore, something 

 altogether distinct from the natural or inherent 

 fertility of the soil. A soil may be naturally 

 very fertile, but at the same time very much 

 out of condition; or it may be naturally very 

 poor, but in very high condition. 



The proportion of plant-food present in soils 

 is very small, even in gardens where the soil is 

 extremely fertile, the bulk of the soil serving 

 chiefly as a support and as a sponge to hold 

 water for the use of plants. But in addition to 

 all this, it must be remembered that the roots 

 of plants, or rather matters exuded by the roots, 

 play a very important part in dissolving sub- 

 stances out of the soil which mere water would 



