Can the required proportion be definitely indicated?
From what source is the inorganic part of soils derived?
Do all soils decompose with equal facility?
How does frost affect rocks?
Does it affect soils in the same way?
The organic part of soils consists of the same materials that constitute the organic part of the plants, and it is in reality decayed vegetable and animal matter. It is not necessary that this organic part of the soil should form any particular proportion of the whole, and indeed we find it varying from one and a half to fifty, and sometimes, in peaty soils, to over seventy per cent. All fertile soils contain some organic matter, although it seems to make but little difference in fertility, whether it be ten or fifty per cent.
The inorganic part of soils is derived from the crumbling of rocks. Some rocks (such as the slates in Central New York) decompose, and crumble rapidly on being exposed to the weather; while granite, marble, and other rocks will last for a long time without perceptible change. The causes of this crumbling are various, and are not unimportant to the agriculturist; as by the same processes by which his soil was formed, he can increase its depth, or otherwise improve it. This being the case, we will in a few words explain some of the principal pulverizing agents.
1. The action of frost. When water lodges in the crevices of rocks, and freezes, it expands, and bursts the rock, on the same principle as causes it to break a pitcher in winter. This power is very great, and by its assistance, large cannon may be burst. Of course the action of frost is the same on a small scale as when applied to large masses of matter, and, therefore, we find that when water freezes in the pores[M] of rocks or stones, it separates their particles and causes them to crumble. The same rule holds true with regard to stiff clay soils. If they are ridged in autumn, and left with a rough surface exposed to the frosts of winter, they will become much lighter, and can afterwards be worked with less difficulty.
What is the effect of water on certain rocks?
How are some rocks affected by exposure to the atmosphere? Give an instance of this.