106 SOME OF THE CHABACTEBISTICS OF SOILS. Dec, 



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Thus carbon, and oxygen, and watery vapour too, which can penetrate 

 the delicate outer membranes, are all the substances the plant is able 

 to obtain by means of its leaves and other exposed parts. The rest of 

 its food it must take up by way of its roots. The greater volume of 

 the water it requires is taken up through those channels and it carries 

 with it what it is able to dissolve from the soil. But the active or 

 absorbent parts of the roots have the property of so acting on some 

 matters required by the plants which are insoluble in water that they 

 can pass through the absorbing cells and vessels into the sap or general 

 circulating medium of the organism. The potash, lime, soda, and 

 magnesia required are taken up by the roots in the forms of the nitrates, 

 sulphates, phosphates, carbonates, &c., of these bodies, and at the 

 same time the requisite nitrogen, sulphur, phosphorus, and additional 

 carbon to what it can otherwise obtain, passes into the sap. The 

 plant's supplies of nitrogen, phosphorus, ';^and potash, and perhaps lime 

 as well, though to a less extent, are the most precarious, because these 

 are the scarcest of the essential substances of plant food in soils. 

 Enough of the other substances required by the plant are contained 

 in all ordinary soils, and the agriculturist need not trouble himself 

 over them. Of the four mentioned, nitrogen is the most valuable, it 

 being the least available ; in fact, the amount of nitrogen available as 

 plant food in any soil is the measure of its fertility. The element is 

 abundant enougli, for our surrounding atmosphere is composed four- 

 fifths of free nitrogen, but then it is one of the most inert of all the 

 elements, and least inclined to combine with others. Under certain 

 natural conditions, combination with others is brought about. The 

 intense heat of lightning flashes causes union between the nitrogen 

 and other elements in the atmosphere. Thus it comes that ammonia 

 and nitric acid can be detected in the air. These are extremely soluble 

 in water, and rain carries them to the soil. From this source aloue each 

 acre of soil gets annually something like 7h lbs, of nitrogen. All organic 

 matter — remains of animal and plant life, —contains a large percentage 

 of it, and this finding its way to the soil increases the latter's supply of 

 nitrogen. Coal, peat, deadwood, leaves, dung, &c., contain it in abund- 

 ance. But it is only when nitrogen is in combination as nitric acid that 

 plants can assimilate it to themselves, and, as we have already said, by- 

 way of their roots. Plants cannot make use of the insoluble compounds 

 containing nitrogen in the soil, but by a wonderful provision of 

 Nature, the nitrogen of those bodies is made available to them. This 

 is accomplished through the means of a minute fungoid organism, 

 whose economy in life seems to be that of attacking the nitrogen, of 

 insoluble bodies, and of ammonia as well, and bringing it into 

 combination as nitric acid. Given a due temperature, a fair amount 

 of moisture, and sufficient organic or other matter containing nitrogen, 



