Agricultural Chemistry.

Poverty Bay Standard, Volume IX, Issue 954, 22 June 1881, Page 1 (Supplement)

Agricultural Chemistry.

Mb. R. W. E. Mclvor’s Lecture at Cambridge. ('Continued). THE CIIEMISTY OE SOILS. Well, now, it is a curious and remarkable fact in connection with the chemistry of soils that those substances that are most required by plants occur in least abundance, if we except potash. For example, the mineral constituent which crops require in the greatest proportion is phosphoric acid, and that is a mineral substance which occurs in the generality of soils —indeed, it is the exception when the case is as I say —is least abundant. Suppose you take a piece of average land and crop it for a succession of years, you notice that ultimately it begins to fall off in fertility. Now this falling » off in fertility may go so far as to enable you to say that the land is, for practical purposes, exhausted ; and it would be natural to suppose that the cropping had removed from the soil all the available plant food, but such is not the case. The crops have simply removed from the soil, more or less completely, that substance which exists in it in least abundance, or, in other words, the minimum constituent; and the restoration of what will restore the maximum fertility, your land will be able to yield fresh crops. So that the exhaustion of the soil is due to the removal of the constituent which occurred originally in the least abundance. Now, Messrs. Lawes and Gilbert and other eminent agriculturists in different parts of the world, have proved to us that the constituent which is most readily exhausted in the soil is nitrogen, which occurs in the soil in. the form of ammonia and nitrates. The next least is phosphoric acid. A system of artificial manuring must be based upon the belief that these two substances occur in land in smaller quantities than the others, and therefore the artificial manure you employ must contain those two in " larger proportion than the others, it must restore the nitrogen and phosphoric acid, the substances in the soil which are most easily removed by rea- * son of thir occurring in smaller proportion than the other substances. FALLOWING. Well, suppose you have exhausted a field, it naturally occurs to the Australian farmer to substitute the naked fallow’ for manure, he turns the land up and exposes it to climatic influences, puts in a crop, and finds the fertility of the soil has been restored. He exhausts the land again, after fallowing, and he ultimately comes to the conclusion that naked fallowing is a thoroughly efficacious substitute for manure, that, in fact, manures are unnecessary so long as he can fallow his land. Although this applies to rich soils, it does not apply to poor soils, because fallowing is nothing less than a system of working upon the capital of your land instead of the interest. By fallowing you liberate some of the lecked-up food, which in cropping you take away. You fallow again, and liberate more locked-up food, but in many soils you will reach a point at which you have exhausted, not only the available constituent, bit that which is in a locked-up state. When you reach that stage, in spite of yourselves, you must have recourse to the practice of manuring. The only true system of permanently maintaining the fertility of the land is to put back those substances of which the soil has become deficient. You will ask me,

“ HOW IS A FARMER TO KNOW WHAT CONSTITUENTS HAVE BECOME DEFICIENT ? ” The first thing he would very likely do is to send a sample of his soil to an agricultural chemist. I can assure you that in nine cases out of ten an analysis of soil is of no practical value to the farmer, no matter how careful the scientist. Soil analysis is not of that practical value which many men seem disposed to put upon it. There is a means, which, owing to the difference in the New Zealand manure market, you farmers cannot carry out, but which I may as well explain to you. Suppose you wish to know what your soil has become deficient in, or what manure would benefit it to the greatest extent, you have simply to take a portion of your land, may be a corner of one of your wheat fields, divide it into different plots, and to each plot apply a different fertiliser, ha.ing a statement from your manure dealer as to the precise composition of

those manures. You will leave a portion unmanured, and the fertiliser which gives you the best results will be the one which contains the substances in which your land is most deficient, and in this way the farmer may obtain a result which will be more valuable to him than any scientific examination by a chemist will be. In Victoria they have all these manures, but here the variety of your manures is limited. I have no "doubt that before long you will be able to apply these simple means to test your soils. THE WAIKATO SOIL. After a careful inspection of some varieties of your land I have come to the conclusion that it is essentially of granitic origin. It consists of decom-. posed granite principally. In driving along the road we occasionally stopped the trap, and I examined the rocks and pebbles to see the general character of the soil, and, so far as I could judge from the parts I saw, I think the district altogether may be described as one having essentially a granitic origin. You will naturally ask me, “ What is granite soil worth ?” Now, that depends a great deal upon the composition of the granite. There are 1 granite soils in different parts of the world, particularly in Aberdeenshire, the soils of which take the same form, that are very poor in some constituents of plant food, but rich in others. In some parts of Victoria there are granite soils as good almost as you can obtain from bluestone rocks, fertile, and yielding abundant crops. I will go so far as to say that the soils of this district are as good as some granite soils I have seen in Victoria, but I will say this —by careful cultivation they are capable of being developed into agricultural soils. They abound in a very peculiar mixture of silica and different kinds of pumice-stone silicates of various descriptions, and, unlike the ordinary sandy soils, they have a very considerable retentive power for water. Indeed, when I sat down on a heap of earth it seemed perfectly dry—this was before the recent rain set in—and Mr. Firth called iny attention to the fact that this heap, though it appeared perfectly dry, was quite moist, and in removing some half-inch or inch from the top, it appeared as wet as if it was clay. I would have been much better pleased to have found a larger proportion of clay in the soil, but fortunately there is deposited through the soil a large amount of—to use a term more easily understood —I will say white sand, which many of you have mistaken for pipe-clay. It is not clay, but white sand, or silica. It has many of the properties of clay, and retains water very well. I have no doubt but that, by careful cultivation, you will make this granite land into good soils and capable of bearing profitable crops. HOW TO WORK GRANITE SOIL. You will ask me, “ What general principles would you, as an agricultural chemist, lay down for working such land?” Now, that is a question which is somewhat difficult for me to answer, but my impression is—and, of course, I am presuming that the land has been taken out of its natural state, and has been sweetened by exposure—that a process of green manure would be very beneficial. You may put in clover or some other crop and feed it for a year or two, and that having been done you can put in a crop of wheat. To keep down the natural plants, you must maintain some system of rotation, so as to enable you to keep down the natural plants until they disappear. By fallowing, and such a plan as that I have no hesitation in saying you will ultimately clear your land and get an excellent sole of grass. Tn the process of cultivation you will find that the fertilising system that will perhaps best improve the condition of your land will be the use of green manure and gypsum, or plaster of Paris. By means of clover and gypsum the American farmers have worked hundreds of thousands of acres of sandy land, because it is valuable for plants, and valuable as an agent for rendering available for crops the food that is stored up in the soil in an unavailable state ; and it is more through this indirect action than any direct action that gypsum has produced the effect I have mentioned on the American soils.

(To be Continued.)