August, 1908.] 



143 



Scientific Agriculture' 



absorption into the system of the plant ? 

 The extreme insolubility of nitrogen 

 shows that the agency of water is not 

 accountable for the absorption of this 

 element even when it is remembered 

 that, in order to produce one pound of 

 dry vegetation, the enormous quantity 

 of four hundred pounds of water is 

 necessary.* 



In order to satisfy the difficult question 

 of the assimilation of nitrogen by 

 plants, many theories have at times 

 been considered. Of these, digestion by 

 means of the sap juice cannot be held to 

 be the cause of the absorption of insolu- 

 ble elements, for such a process would 

 necessitate the ' breaking up ' of nitrogen 

 into an assimilable form— a process at 

 variance with the elemently constructive 

 function of sap. It is now, however, 

 generally held that the appropriation of 

 nitrogen by vegetation is due to the 

 agency of bacteria, which infest all 

 fertile soils and exist upon most plants. 

 By means of these minute organisms, 

 the insoluble nitrogen is, as it were, 

 decomposed or predigested, and rendered 

 assimilable as plant food. 



In the sequence of vegetation growing 

 upon soil undergoing the process of 

 weathering from primitive rocks, until 

 late years, the licnens were considered 

 as appearing first. The action of these 

 simple structures after many gener- 

 ations prepared the way for mosses, 

 which again rendered conditions suitable 

 for plants of a higher order. 



It is now known, however, that pre- 

 ceding the lichens are the minute bac- 

 teria to which reference has been made, 

 whose function is to render the organic 

 constituents ot the soil capable of 

 supporting plant food. The origin of 

 the bacteria themselves is a more pro- 

 found question, which has not been 

 solved, but their presence to the agri- 

 culturist appears to be as necessary as 

 the working of the yeast plant is to the 

 brewer. 



This renders the distinction between a 

 sterile and a fertile soil better under- 

 stood, for of two soils almost similar in 

 chemical and physical characteristics, one 

 may be extremely fertile and the other 

 non-productive. The question of the 

 presence of suitable bacteria in a soil is 

 therefore a very important one, and 



* The same problems affect the absorption by 

 the plant of the extremely insoluble elements 

 phosphorus and silica. The latter element, inert 

 alike to the action of water and all acids (except 

 hydrofluoric) is first 1 broken up ' and rendered 

 assimilable by the plant root, and then reconverted 

 within the system of some plants where it exists as 

 minute crystals, 



ranks equally with those affecting its 

 chemical constituents and physical pro- 

 perties. 



The action of the beneficial organisms 

 referred to, as regards their function of 

 supplying plant nutrition is principally 

 confined to the nitrogenous matter 

 already in the soil, although these 

 organisms may possess to a small degree 

 the power of tapping the air reservoir 

 itself and obtaining a supply of nitrogen 

 direct therefrom. 



The conversion of the nitrogenous con- 

 stituents of the soil into nitrate 

 assimilable by plants is known as nitri- 

 fication. In order to bring about this 

 little-understood process three conditions 

 are necessary, viz., the presence of 

 bacteria, a supply of oxygen, and a 

 salifiable base such as lime, soda or 

 potash. Nitrification takes place under 

 favourable circumstances in all fertile 

 soils, and as the bacteria are most active 

 at a temperature of from 75 to 100 

 degrees, F., it is between these thermal 

 points that plant growth is most 

 vigorous. At temperatures much below 

 that indicated the work of the bacteria 

 is retarded, and at a certain degree 

 of coldness their operation ceases. 

 The economic use, from the point 

 of plant life of these beneficial organ- 

 isms, therefore, is their conversion 

 of the nitrogenous organic matter and 

 ammonia compounds of the soil and, to 

 a less extent of the air, into soluble plant 

 food. This is affected by combination 

 with such bases as lime and potash 

 with which nitrates are formed. From 

 these latter compounds plants derive 

 most of their nitrogen, and in order to 

 support plant life, it is necessary that 

 such soluble nitrates be present. 



When the natural supply of nitrates 

 in the soil becomes exhausted, it is neces- 

 sary that these be removed artificially. 

 This opens up the important subject of 

 plant fertilization, which is every year 

 forcing itself more and more to the 

 notice of agriculturists. " As the avail- 



* An additional vagary on the part of the element 

 nitrogen also tends to emphasize the importance of 

 the diminishing available supply of this important 

 vegetable food, for it does not follow the natural 

 cycle pursued by most other substances in their 

 appropriation by the organic world. As a rule, the 

 constituents of the primitive rocks crumble into 

 soil and after assimilation by vegetables and animals, 

 are returned again to the soil. With regard to 

 nitrogen, however, such a cycle is not completed, 

 for this element, instead of returning to the soil 

 passes off into the atmosphere. There is therefore 

 a continual process in operation for the liberation 

 of nitrogen from combination with other elements, 

 and each generation of life depletes the earth of its 

 supply and diminishes the potontiality of our 

 planet for sustaining organic life, 



