This search for fungi in excrement is necessarily incomplete. In Mr. Murray's list it is evident that the greatest number of species has been found in the dung of animals which are domesticated and common, and which offer facilities to the fungologist. The numbers are startling, but when we consider that the dung of every living thing which crawls or burrows, or swims or flies, has properties which are peculiar to it, and which fit it to become the nidus of some peculiar fungoid or bacterial growth, the part played by fungi in the distribution and circulation of organic matter cannot be over-estimated.
The facts which have been recounted, and which seem to show that fungi and bacteria are necessary for the growth and development of even the highest plants and animals, and that fungi and animals are equally necessary for the dissolution of organic matter, point to the conclusion that the correlation of the biological forces in this world is no less exact than the correlation of the physical forces. The uniform composition of the atmosphere, except under special and local conditions, is a fact which tends in the same direction.
While it is impossible to over-estimate the debt which agriculture owes to chemistry, we have, nevertheless, learnt from the bacteriologist that there are biological problems underlying the question of fertility, and that a mere chemical estimation of the constituents of organic manure is insufficient, by itself, to fix its manurial value. It is by the agency of bacteria that organic matter is changed into nitrates and other soluble salts, which are absorbed by the roots of plants and serve to nourish them. This change only takes place provided the temperature and moisture are suitable and the ground be properly tilled. Drought and frost arrest the change, and excess of moisture, by closing the pores of the soil, does the same thing.
Organic manures are economical in the long run, because if the weather is adverse they bide their time until the advent of 'fine growing weather.' If one season prove unfavourable, a large amount of the organic matter remains in the soil to nourish the next crop. This is not the case when soluble chemical manures are used.
That it is necessary to put dung upon the ground if we are to maintain the fertility of the soil has been the experience of all peoples in every age.
Fig. 35.
I will now allude to a diagram (fig. [35]) which represents by a curve the yearly produce of barley, in bushels per acre, grown continuously on the same plots of ground for forty years, but with this difference, that one plot (represented by the upper curve) received 14 tons per annum per acre of farmyard manure, while the other, represented by the lower curve, has been unmanured continuously. This diagram has been constructed from figures given by Sir John Lawes and Sir Henry Gilbert in the 'Transactions of the Highland and Agricultural Society of Scotland' for 1895. I have replaced fractions by the nearest whole figure. The fluctuations of both these curves are very great, and it will be noticed that they are exactly parallel to each other. This teaches us that weather is the most important factor in agricultural success, and shows the extreme danger to the farmer of 'placing all his eggs in one basket,' as has been done by the so-called farmers of the Far West, who have attempted to grow wheat only by the process of scratching the prairie, without returning any dung to the soil, and many of whom have been financially swamped by the first bad season.
Taking the average of the forty years, it will be found that the produce of the manured land averaged 49 bushels per acre per annum, while the unmanured land gave only 16½ bushels.