170 LECTURES ON 



carbonic acid resulting from its ceaseless oxydation is of vast import- 

 ance, both as a supply of this form of plant food, in more 'abundant 

 measure than the atmosphere alone could yield, and as the most pow- 

 erful means of maintaining the requisite store of solved saline and 

 earthy food in the soil. 



The general statement that humus, or, in other words, condensed atmo- 

 spheric plant food, is needful in the soil, requires some qualification. 

 It is not essential to all, even, of the so-called higher orders of plants, 

 or, indeed, to all agricultural plants. The cactus has its home on 

 the most naked arid sands. Pines and firs flourish in soil equally 

 destitute of humus. Buckwheat commonly grows on light, poor soils; 

 and it is asserted that in Peru and Chili, maize prospers in soils free 

 from humus, if started by a little guano, and afterward supplied with 

 water. We may, however, safely assert, that in temperate climates, 

 for the usual course of crops, a soil to be productive, in a practical 

 sense, must either contain originally, or have added to it, nitrogen 

 and carbon in assimilable form. Natural growth, in soil, destitute of 

 atmospheric ingredients, either of those plants just mentioned, whose 

 proper habitat is such a soil, or of the grains and common agricul- 

 tural plants, is, other things being equal, invariably too slow for the 

 purposes of agriculture. Not, indeed, for all purposes of agricul- 

 ture, for in what is called agriculture many very inferior crops 

 are annually reaped; but for the general purposes of a culture which 

 seeks to be in a high degree remunerative, the telluric elements are 

 insufficient. 



The same holds true of the atmospheric as of the earthy ingre- 

 dients of soil in respect of varying quantity and different assimila- 

 bility. 



In the poorest sand, analysis reveals the presence of nitrogen, 

 often one hundred times as much as is needed by the largest grain 

 crop; while in good soil the quantity of this element may amount to 

 from one to two thousandths of fhe entire weight. Of this nitrogen, 

 a portion exists as ammonia, another as nitric acid, but another and 

 far larger share of it, is in a form that is insoluble in water and una- 

 vailable to the plant. 



In a rich garden soil that had been cultivated for many years, 

 Boussingault found in 100 parts — 



Nitrogen 0. 261 



Ammonia 0.0022 Containing nitrogen 0.00181 



Nitric acid 0.00034 Containing nitrogen • • • • 0.00009 



By actual trial with this soil, the same distinguished experimenter 

 found that only the small amount of nitrogen existing as ammonia 

 and nitric acid was of present use to vegetation; the remainder, 

 96-100 of the whole, being for the time quite inert. 



The inert nitrogen appears to exist chiefly in the humus of the 

 soil, in a form analogous to that assumed by the same element in 

 bituminous or anthracite coal. It is, however, most probable not 

 utterly unassimilable; but, as the carbon and hydrogen which are 

 combined with it oxydize, it appears in the form of nitric acid, 



