AGRICULTURAL PROGRESS AND EXPERIMENTS. 575 
releasing fresh plant-food elements from in- 
soluble combinations and combining with them, 
forms double humates again, to be broken up 
anew, and have their plant-food elements again 
appropriated by plants. Humic acid in this 
way acts as a carrier to crop-plants of some of 
the most valuable plant-foods from potential 
forms, and on this account is a very important 
ingredient of soils; and, provided they do not 
contain enough of it to make them sour, con- 
tributes greatly to their fertility. There are, 
again, reasons of another character for dead 
vegetable matters forming an important consti- 
tuent of soils. Their presence in it is of great 
importance to its physical condition. In the 
partially-decayed form of humus they increase 
its water-retaining qualities; they also modify 
its texture, and make it to be more easily put 
into a condition of good tilth. 
Vegetable substances can be added con- 
veniently and with ease to soils which are defi- 
cient in them. We can supply them as green 
manure, or ploughing in the parts of crop- 
plants which are not required for other uses. 
Weeds usually provide us with them in con- 
siderable quantity; and in this respect do us 
yeoman service. In our climate, stubbles are 
too valuable to be burnt off, and ought to be 
ploughed in; although the slowness with which 
they usually decay makes them to be much 
inferior to more succulent substances. The close 
dependence of the soil for its fertility on the dead 
vegetable matter it contains points to the proba- 
bility that the increase of the chemical and bac- 
terial activities, upon which depends the increase 
of its fertility that we are seeking, will be obtained 
more than in any other way by our directing 
attention to this one of its constituents; and an 
examination into the effects which follow from 
the manner of its disposition in the soil may lead 
to additional light being thrown on this sub- 
ject. 
©) It may be hae we shall learn how, either by par- 
ticular methods of treatment or by the addition 
of certain substances, to give direction to, as 
well as increase the chemical activity amongst, 
