The Water-moving Ability Of Different Soils

Since soils are so variable in composition and in texture they naturally vary a great deal in their "capillarity," or their ability to move film water. This point is worth considering when selecting a farm; a soil through which water moves slowly is not apt to be very productive. The coarser a soil is, the less water it can draw up. Fill one lamp chimney with coarse sand and another with clay loam, both packed hard. Set both of them in a pan of water and note the difference in the amount of water that they draw up and the time it takes them to do it. For a while water will rise rapidly through sand, but it will not be drawn very high, because the spaces or tubes are so large. In the finer soils, especially those containing some clay, water rises more slowly, but it is drawn up very much farther. Humus increases the water drawing power of a soil. The importance of securing a soil with high capillary power lies in the relation this has to the water supply of crops.

Film water, on which plants feed, is drawn largely from the reservoir of free water below. It is important that a soil be able to draw water freely and rapidly in order to keep the roots constantly bathed in the life giving fluid. A large crop makes a tremendous drain upon the water in the upper part of the soil during a single day. If the sun is very hot, the amount of water lost by evaporation is large; even thorough tillage cannot entirely prevent the escape of water. This means that the supply of film water in the surface soil must be quickly replenished from below, else the plants will suffer. In some soils the water table is many feet below the soil in which the roots of plants feed; in such cases there is especial need that the water be able to move rapidly through the soil. Very sandy soils not only do not hold much water, but also have little power to transport it by capillary action. Very stiff clay soils, on the other hand, while they hold a large amount of water, regain water very slowly after having become dry, so that they frequently suffer much in a drought. When a clay soil dries it shrinks, and cracks appear not merely on the surface, but also to a depth of several feet. These cracks let in air which still further dries the soil ; the roots of plants may also be torn apart and exposed, All kinds of loams have excellent water-carrying power; this fact, together with their mellowness and fertility, makes them among the most valuable of farm soils.

How to Test the Water-holding Capacity of Farm Soils. — The points that have been brought out in the preceding paragraphs will be made more concrete to the reader if he will try a few simple experiments. Get a quart each of stiff clay, sand, and the black, spongy humus beneath forest trees. Put the three samples into a slow oven and dry them for two or three hours, or until they appear perfectly dry. Stand three lamp chimneys in pans and fill one with the dry humus, one with the dry sand and one with the dry clay. Pack each very tightly. Fill three quart jars with water and pour water slowly from one of them into the top of the chimney of humus; water the chimneys of sand and clay likewise from the other two jars. Pour in only a very little water at a time so as to allow it to settle slowly and wet all the soil thoroughly. Stop pouring water when the soil is wet to the bottom, and water begins to seep from the bottom of the chimney. Note first, how quickly water passes through sand and how little water it holds — it is leachy. Observe that the humus also absorbs the water quite readily, but holds much more of it. The clay takes up the water very slowly but holds a large quantity of it. The water left in each of the three jars shows the relative water-holding capacity of the soils.

The chimneys of soil represent actual conditions in the field ; the water held by the soil in the chimneys is film or capillary water, while the water that seeps out at the bottom of the chimney is free or standing water. The same results may be secured in another way by filling several flower pots with different soils and drying them in an oven. After weighing each pot of soil separately, add water to it very slowly until it seeps out at the bottom. Set the pots away to dram. When no more water seeps out, weigh them again. The difference in weight is the amount of film water that the soil can hold. The several types of soil on the farm may now be tested in the same way. Compare the water holding capacity of a sandy loam with a clay loam. If you have a stiff clay soil, fill one chimney with this, another with a sample of the same soil which has had some humus mixed with it, and a third chimney with another sample of the same soil which has had some sand mixed with it. A comparison of these three samples should point a profitable lesson. If you have a very sandy soil find the influence of adding humus to it in like manner. If accurate measurements are desired, weigh the dry soil and the same soil after it is thoroughly saturated. A good soil should be able to absorb at least one-half its own weight of water ; humus often holds almost twice its own weight of water, and sand from 15 to 25 per cent. Testing the Water-moving Ability of Soils. — The ability of a soil to move water by capillary action, and to draw upon the free water to supply the needs of plants, may be determined with a fair degree of accuracy in the following manner. Take chimneys of fire-dried and well-packed clay, sand and humus, as in the previous test, and stand them in a pan. Cover the bottom of the pan with half an inch of water. Note the water creep up into the chimney by capillary attraction. It rises very rapidly in the sand at first, but is carried only three or four inches high and then stops — the spaces between the grains are too large for it to be drawn higher. Humus takes the water up more slowly but eventually the soil at the top of the chimney is wet with the film water drawn up from below.

The clay absorbs water even more slowly than humus, but the surface soil of the chimney of clay is wet in a few hours. Now test in a similar manner the farm soils which you have to handle. See what effect mixing a little sand or humus with the clay has upon its water-drawing power, and what influence mixing a little humus with the sand has upon its capillary power. Compare the capillary power of sand when it is put into the chimney loosely, and when it is packed into the chimney very firmly; it may suggest the value of rolling. The chimney of soil is a field in miniature; all fertile soils draw up water from the water table by capillary action, as these samples of soil draw up water from the pan. One of the most important functions of the soil is here exemplified. Sometimes simple experiments like these show in tangible, concrete form, the results that may be expected from a farm practice that must be extended over a number of years in order to achieve these same results in the field.