Agriculture.

Poverty Bay Herald, Volume X, Issue 2026, 15 September 1883, Page 2 (Supplement)

Agriculture.

The Chemistry of Agriculture. From the Chemistry of Agriculture ; by R. W. Emerson Maclvor, F.L.S., dc. SOILS : THEIH FORMATION, CHEMICAL COMPOSITION, AND PHYSICAL PROPERTIES. Geologists tell us that there waa a time when the surface of our planet wa3 devoid of the loose covering we terra soil, and consisted of hard crystalline rocks, upon whioh no form of vegetation anywhere existed. It may, therefore, be taken for granted that the non-combustible matter of soil has resulted from the slow decay or disintegration of the primitive rooks. Before we can proceed to learn how nature has effected, and continues to effect, the conversion of rock into soil, it is desirable that we should, first of all, know something about the rocks themBelveß. Well, when we come to carefully examine the structure of a piece of granite, basalt, or other crystalline rock, we find it to be composed of a mixture of crystals or distinct particles of different kinds. Those distinct substances are termed minerals. The minerals of the crystalline rocks are of two groups — First, quartz, or silica ; second, silicates of potash, soda, lime, magnesis, and iron, together with clay, which is a chemical compound of silica with a substance known as alumina, and hence is is termed silicate of alumina. Let us briefly study the composition of the more important silicates present in rocks. Felspar. — This mineral consists of silica combined with alumina and potash, soda or lime. It forms a large constituent of volcanio rocks and granite. In the latter it occurs associated with two other minerals, mica and quartz. It is an easy matter to detect the, three minerals by a careful examination of a piece of granite. The felspar may be flesh-colored, brown, or white, and is lees transparent or glass-like than the quartz : the mica occurs in the form of thin shining scales, which can be split up into thinner scales with the point of a knife. The following table will serve to show the composition of four kinds of felspar :— 3 s> B §L 2- a xr.---g. s. * r a. so * O ** o ■» •§§§ £SS£ || oI o . »??? £ 5 1 ii o « w » « m i m SO F * co to d £L O 2, O OS «O W H v g » cb • aiHAwcbcnit'gp O • © © -q 00 O Oi • V ri The orthoolase and albite were from granite ; the labradorite and oligoclase from volcanic rocks. Mica. — •This is also a widely distributed mineral, and hvo species are distinguished by mineralogists, one of which iB remarkable for the large amount of magnesia it contains. The following are analyses of the two kinds : — Mica. Potash. Magnesia. Silica .. .. 46-36 .. 42-65 Alumina .. 36-80 .. 12-96 Potash .. .. 9-22 .. 6-01 Soda . . . . none . . none Lime .... -30 .. 2-06 Magnesia .. none .. 25-75 Oxide of iron 4-53 . . 7-11 Water .. .. 206 .. 3-15 99-27 99-69 Common potash mica is a component of granite and also of its associate rocks, syenite, mica-slate, and gneiss. It is also found in basalt. Hornblende and Augitc.—Theße are the dark-green or black-looking minerals that are found in large proportion in trap rock and basalt. Syenite ia a rock resembling granite and differing from it by containing hornblende in place of mica. In this rock the felspar is generally red. It will be seen from the folowing analyses that hornblende and augite are silicates of alumina, along with lime, magnesia, and the lower oxide of iron : — « 1 1 1 r co * M D • (t> o, m I-" h-» >— ' h-» *- 5 PJ O*-«500 Mhj jjl o a o SO M (O M frU W 00 M O l« M H g 1 © O» Ul tO ST» <tt 3. l-> O Ol IO O5 CO K«O 'M to Ui W «O (OttOW H» o w ik o en w S i, w © M tO *. M O *■ » !O *• O X" tO 00 ©. *- C 5 2 (35 W « » M H i" These minerals contain neither potash nor soda. A dark green mineral know as olivine also occurs in volcanic rocks. It is present in considerable quantity in some Australian basalts. The following is an analyses of a specimen of the mineral as it occurs in Victorian Basalt : — Silica H-76 Magnesia . . . . . . 11-70 Iron oxide (lower) . . 10-85 Alumina 1-81 Lime ) Hft Magnesia] oo 100-00 These minerals constitute the bulk of our mountain masses, but they are associated with many others, which, however, we must pass over, as it would take us out of our depth were we to go much further into the mineralogical constitution of rocks. Before leaving the subject we must, however, say a few words regarding an important class of minerals termed zeolites, which occur disseminated through trappean and volcanic rocks in small quantity. These minerals are hydrated (or water-containing) silicates of alumina or lime with soda, or potash. The following represents the composition of a member of this clasß met with in basalt and allied rocks ; also in the seams of granite : — NATROLITK. Silica 48-68 Alumina . . . . . . 2636 Lime . • . . . • Jion.ft. Potash '23'" Soda 16-00 Water 9-55 100-82

cribed in containing their silica in an easily soluble state, and hence rocks containing them yield soils in whioh this substance exists in a condition in which it can be easily absorbed by the roots of plants. It will be seen from the analyses that each of these minerals would, on decomposition, yield a soil containing only two, three, or four of the substances which we know to be essential to a growing plant. Thus potash felspar would yield a soil in which potash would be abundant and lime and magnesia deficient; while lime and soda felspar would yield one containing sufficient lime, but whioh would be poor in potash and magnesia. We find, however, that rocks do not consist of one mineral only, but of a mixture of minerals ; and in some we find that the proportions are so regulated that the deficiencies of one mineral compensates those of another.

(To be Continued.)