9 



ground, but have vital functions to perform which require 

 the soil to be presented to them in a suitable mechanical 

 condition. They have to respire. If the soil around them 

 be so compact as to exclude air, or if it be drenched with 

 water so as to drive the air out of its interstices and keep 

 it out, the roots will infallibly die. In fact they can no 

 more do without the small quantity of air they need than 

 animals can do without the enormous supplies necessary to 

 their more active respiration. The difference is one of 

 degree only. The respiratory process is the same, con- 

 sisting in the intake of atmospheric oxygen, its use to 

 oxidize carbon compounds, and the subsequent output of 

 carbonic acid. But because vegetable respiration, even 

 from the leaves, is very sluggish, and masked from 

 observation by the much more active work of assimi- 

 lating carbon to build up the tissues, it is apt to 

 be overlooked. And from this ignoring of a vital 

 necessity spring some of the worst errors in cultural 

 practice. We have enumerated certain mineral soil-con- 

 stituents which must be present amidst the vast bulk of 

 earth the planter has to deal with. It follows from the 

 above that besides these, and in far greater bulk, there 

 must be present air and water. The only way in which 

 these two elements, a gas and a fluid, can be secured for 

 the service of the roots is by breaking up the soil into a 

 more or less powdery, porous, open condition by mechani- 

 cal means. In the interstices of the separated particles of 

 earth there enters an abundant modicum of air, nearly 

 equal in cubic measurement to the bulk of the solid matter 

 of the soil. One may almost predict the degree of success 

 of any crop by the degree to which atmospheric air has been 

 mixed up with the earth it is to grow in. The traditions 

 of agriculture unconsciously point to the same truth. To 

 till land is to mix the top stratum with air. Coarse plough- 

 ing of wet lands is leaving the great slab-like clods lying 

 loose to dry out their water and absorb air instead. 

 Harrowing among other ends, mixes air very completely 

 with the loose tilth. But the most thorough and effective 

 addition of this necessary ingredient is obtained by the 

 process of trenching. By trenching almost every cubic inch 

 of the soil is dissociated, large cavities full of air are left 



