Analysing The Soil At Home

The determination of the relative proportions of sand, silt, clay, and humus in a soil is called a "mechanical analysis," as compared with a "chemical analysis," in which the kinds and the amounts of the different plant foods are determined. It is not always possible to have the soil analysed by a chemist, but it is always practicable for a farmer to determine himself, roughly, the relative amounts of the four ingredients that his soil contains. A mechanical analysis should point out the deficiencies of the soil much better than simply viewing it on the surface. A close examination of a handful of the soil will reveal much concerning its composition, especially if a microscope or even a pocket lens is used. Note the colour, whether dark or light; look closely for the tiny black particles of humus that are likely to be the cause of the dark colour, and are a sign of good texture and large water-holding capacity.

Rub the soil gently between the thumb and forefinger to determine the size of the particles. Are they mostly coarse or fine? If the soil feels distinctly gritty it probably contains a considerable amount of sand ; if it feels quite smooth and makes a very smooth, sticky paste when water is added to it, it contains a large percentage of clay or silt. Take a handful of moist — not wet — soil and squeeze it hard. If the ball of soil crumbles quickly and freely when the pressure is removed the soil contains sufficient humus or sand and is likely to prove of good texture and easy to work. If, however, the ball of moist soil retains its shape to a considerable extent, remaining hard and compact, it indicates that clay and silt predominate and that the soil will need to be handled carefully. A more accurate test for clay, silt, sand and humus may be made in the following manner. Take a small sample of moist soil, as it is found in the field, say a quart; screen out all except fine particles, and weigh it very carefully. Spread it thinly on a pan and set it in a very moderate oven or on the back of the stove, where it will dry slowly, but not burn. When it is perfectly dry weigh it again.

The difference shows the amount of water that the soil contains, all of which has been driven off as vapour of water. Place this dry soil upon a coal-shovel above hot coals, or on a pan placed in a very hot oven. The humus in it will begin to smoke. If the soil is kept very hot for two or three hours practically all of the humus will burn, leaving only the "ash" or mineral part of the soil. A fairly reliable measure of the amount of humus that the soil contains is secured by comparing the weights before and after burning. All soils that have a fair proportion of humus and are therefore most valuable for farming, should shrink considerably in bulk and in weight by burning. Separating the Sand, Silt and Clay. — After the humus is burned out of this soil the sand, silt and clay remain. These being pieces of rocks, or mineral matter, they will not burn like humus, which is vegetable matter. A simple way to separate the three ingredients is to put the soil into a tall, wide-mouthed bottle; one holding two quarts will answer, but a larger one is better. Fill this full of water and shake it violently until all the soil is mixed with water. Stand it on the table and watch the soil settle.

If the soil contains coarse sand this will settle almost immediately, being largest and heaviest. Medium sand and fine sand will settle more slowly. Part of the silt and clay will remain suspended in the water for many hours. After several days, or when the water is clear, all the soil will be deposited in the bottom of the jar; the sands on the bottom, then silt, and clay on top. These ingredients may not be deposited in well-defined layers, because sand, silt and clay are arbitrary terms, used to designate soil grains of certain arbitrary sizes, for the sake of convenience in describing them. In some cases the sand may grade into the silt and the silt into clay imperceptibly; in other cases ill-defined layers can be seen. In any case a close scrutiny of the way in which the soil settles and of its appearance after it settles will enable one to estimate roughly the proportions of sand, silt, and clay that it contains. It will pay a farmer to test the different types of soil on his farm in this way, and especially to test several different soils at the same time and compare them.

The results of these simple experiments will bear out and emphasise field observations on the agricultural value of these soils, or they may indicate a weakness where none is suspected. It is well to take a dozen or more samples of soil from different parts of a field in which the soil is all approximately similar, to mix them and to take from the combined lot the sample of soil that is tested. This makes it quite certain that the results obtained represent the field fairly. The Bureau of Soils of the United States Department of Agriculture is making a "soil survey." The types of soils in all the important agricultural sections of the country are being studied. About 100,000 square miles of land in different states have already been studied and reports issued. These reports should be very useful to the farmers in these sections. They may be obtained of the Division of Publications, Washington, D. C.