EXHAUSTION OF THE SOIL.

Patea Mail, Volume V, Issue 413, 2 April 1879, Page 2

EXHAUSTION OF THE SOIL.

Wn-will preseully give analysis of sample soils, to illustrate the subject, and to enable the reader to follow us ; but w,i will first consider what part of the plant is obtained from the soil-. If we take any vegetable produce,- as Wood, Wo know that it contains a large quantity of water which can be driven off by heat; further heat will drive off a vast amount of gas, which flames soid Iveoines invisible ; when' the gas is a! I driven off there remains charcoal, which is less than half of the original weight of the* wood. Fu : rthot burning drives this off also into the air, and presently only a little white ash is left ; all the rest has parsed into thin air, gone in fact where it came from. “ From air thou art, and into the air thou shalt return'’ may be said to the whole of a plant, or tree, ex cept only the while ash, left after thorough burning. The charcoal, which is all carbon, except tbe ash, looks so like a solid earthly substance, that we are inclined to look first to the soil for its origin. Indeed, until about forty years ago, it was never questioned that the carbon of the plant, which is nearly half of its wholo weight, is obtained from the soil. But it is now known that if soil contains carbon, its amount does not decrease through crop after crop beingproduced. Many experiments have shown that plants w : li thrive in soil that contains no carbon. The oil palms of the West Coast of Africa grow in moist sand, without a trace of vegetable matter. It is clear, then, that the presence of vegetable matter in the soil, is not one of the necessities of plant life ; by far the greater part of every plant is made up of material collected from the air. How the air becomes possessed of an unfailing supply, will be explained by a moment’s Ih night concerning the immense quantities of vegetable matter that are continual!} 7 vanishing into the air from fires and decaying vegetation. It is estimated that over every acre of land there are suspended in the air, 20,000' bs of carbon. It is true that soils that contain much organic

matter, are, as a rale, very fertile ; bat this is because the presence of decayed plants has ensured the presence of the ash, or the mineral parts of those plants, which

are necessary for the new crop. To return then to the soil : it supplies to the plant only the white ash left after thorough burning. The amount of this ash is very small in proportion to the total weight of the crop. Tims, a ton (2,2401b) of Italian rye-grass hay yields Usßlb of ash, a ton of red clover hay 12041b, a ton of white clover hay 141 b, a ton of lucerne hay 21141b. These sevetal small amounts are all that the soil supplies, per ton, to the crops mentioned. If these crops were removed from the land altogether, there would of course bo a dead loss to the land of the materials supplied to them ; if they j be eaten on the land, a large portion is i restored in the droppings of animals, but I there will still be some loss incurred by the removal of stock whose bones, flesh, hair, wool, and horns, all contain a portion of the ash supplied by the land. There is, therefore, continually a loss taking place, whether slowly by grazing, or more rapidly, by cropping. The ash bears a very small proportion to the whole mass of the soil from which it is drawn. A crop of ten tons per aero of white clover hay, would only take from each acre 141741b50f ash, while it has been recently stated in an American journal that an acre of soil, six inches deep, and thoroughly dried, weighs about 2,410,0tt0 lbs, or about 1,714 times the weight of ash in a ten ton crop of white clover. Unfortunately, however, these six inches of soil would not grow 1,714 ten ton crops of clover, because some of the ingredients of the clover ash may bo very scantily mixed in the soil, and because though they were all plentifully present, yet some sixtenths, or seven-tenths, of the soil would be of a material that enters but sparingly into plants (namely, silica, or pure sand, which is only utilised as a glaze to stiffen ■the stalk, and is found in large masses after

the burning of a stack of straw). We will,now give an analysis of three samples of soil, each of which, for convenience, is divided into 1,000 parts. The first column represents soil fertile without manure, the second soil fertile with manure, and the third barren sdil.

In order to facilitate comparison, we will also give a table like the foregoing, from excellent authority* show the analysis of a hundred,pounds of the ash of various crops ;—•

It will he observed that those substances that are absent from the barren soil, as phosphoric acid, enter largely into the composition of the ash of the various kinds of grain given in the second table. The reason of barrenness will be obvious. In the laud that is fertile with manure, it may be noted that potash and soda are wanting. The second table will show that these substances compose almost one-third of the ash of our chief kinds of grain. Manure, to be effected on this land, must, of course, include these absent substances. Of the grains given in the second table, the land in the second column could most easily be made to grow oats; for it requires but little chlorine, and loss soda and potash than the other grains.

To return again to the question of the exhaustion of soil, which as we have mentioned,- has taken place in a neighbouring colony ami in America. Let us suppose that the first column in the above table represent prime land. Every 1,000 lb of the' lirst six inches, say, of its surface contain ■Lj Hr of phosphoric acid,- or, according to the Am 'iican estimate already alluded to, about IO'.GOO lb per acre. Now, suppose wheat to bo grown cn this land every year, and removed, while the straw is left. In every 100 lb of wheat ash there arc 4G Ibof phosphoric acid, and consequently, if the crop represented about 220 lb of wheat ash per acre, there would be removed yearly 100 lb of this acid, and thj whole amount in the soil would bo exhausted in 108 years. Soil like that of South Australia must be very much less fertile than that with which we arc dealing; for even in the course of thirty years of very moderate crops some of the essentials of wheat life are about worked out But, however rich soil may be, a continual drain, however small, will exhaust it in time,-and either tin’s generation or some other will suffer for that kind of farming which keeps the drain going. To attempt to farm continually on the strength of the original wealth of the soil, is, as we have before said, only comparable to trying to live on a single

deposit in the bank. Land that has- lost one. or more of the elements of fertility may be laid fallow, but following would not create a fresh supply of what was lost. Some people have accustomed themselves to speak and think of land “ resting,” as if it were an animal, and only needed cessation from employment to renew all its vigour. No idea could be more erroneous. It were as wise to let the empty Hour bin rest, in the hope that it would refill Itself. E nihilo nihil fit, phosponc acid, or what not, will neither drop from the sky nor originate soontaneously in the soil. The only way to re-fertilise exhausted soil is to restore that which we have robbed it of. There is, after all, an analogy between the resting of land and that of an animal. If a beast be rested, and not fed, it will work again,

bat more feebly, and so on till it can work no more ; but if it be both rested and fed, its fitness for work may be said to continue the same from day to day. It is so with land; let it be rested and fed. We are aware that hundreds of practical farmers would say, “Fancy farming won’t pay.” If manuring be fancy farming, it is certain that one day it will have to pay, or men will starve, and if it is not begun in good time, there will be all the more up-hill work when it is begun. What then is the value of “ rotation,” if it does not restore the strength of the land ? Is it not. the object of “ rotation ” that one crop should restore what another has taken away ? Certainly not. It is only intended to enable the fanner to make the best of what is already in the land. Thus, if rye were grown year after year, it is evident, from the table above, that it would exhaust the phosporic acid sooner than any other crop would, and would at the same time leave the chlorine untouched; this substance would bo very useful to wheat and barley, hut could not bo used by them, owing to the absence of phosporic acid, which the rye had exhausted. Now, if instead of growing rye year after year, we suppose a “ rotation ” to be observed, the chlorine and ail other substances of the soil will be used in turn. Rotation defers exhaustion, but only to make it more complete when it arrives.

£ a o 45 si BS 3 • c % zz a r °-r- S * *-si i-H k— *=• c3 « Organic matter .... . 97 50 40 Silica (ill sand and clay) 648 833 778 Alumina . 57 51 91 Lime * 18 4 Magnesia . 8^ 8 1 Oxides of Iron . 61 30 81 Oxide of Manganese .. . 1 3 0* Potsh . 2 trace trace Soda ) as com- ( 4 Chlorine.:, f mon salt ) 2 Sulphuric Acid 9 0* Phosphoric Acid • H Carbonic Acid . 40 4-i Loss in analysing . 14 h 1000 1000 1000

c3 cc 0) g O a Js £ ci zn « o c .£ 'B .£* "S u o *2 H o "S "5 c5 'a r 5 ctf P o o w Ps K* w H pPotash and Soda.. 2(5 32 33 324 11 45 51 03 Lime .... 3. (x n 5 H 7 8j 1H 2 Magnesia ....12 10 H 104 id 2 (14 3 5 Oxide of Iron .... I 04 04 14 o i 1 04 33 04 04 Phosphoric Acid.. ....4a 44 2(1 484 45 5 Hi 18 Sulphuric Acid .. .... — R)J . 24 1 3 i 44 15 4 Chlorine 04 5 ? 0k 7 14 54 6 Silica .... i 24 23 04 H (10 04 2 14 — — — — — — —— — — 100 100 100 100 100 100 100 100 100