In clay soil the particles are extremely small, as are also the spaces between the particles. Water is therefore taken up very slowly. It is, however, held tenaciously. When clays become wet, they are very sticky and cannot be worked. When they dry, they become very hard and crack. If cultivated at the wrong time they break into hard lumps and render further cultivation difficult. The adobe soil of the west is ofthis character. If the soil is nearly pure clay, it is useless for farming. If sufficient sand or humus can be added, it becomes valuable, since clays usually contain the elements needed by plants.
A soil having grains about midway in size between sand and clay is called a silt. This is usually a most fertile soil.
THE CRACKING OF ADOBE SOIL WHEN DRY.
It is the soil of the western prairies
and the great grain-producing states of our country. It holds water well, contains an abundance of plant food, and is easily cultivated. BetweenPRAIRIE SCE NE.
Showing modern methods of harvesting the crops from the fertile silt soil.
these three types-sand, silt and clay-there are all grades of soils presenting problems of various degrees. The problem of the farmer, however, is to maintain a soil which holds water but is well-drained, which contains the elements plants need, and which is mellow enough to be well aired and to let the plant roots grow.
46.Soil Water. -Although many soils contain every thing needfulfor the production of agricultural plants, yet the rainfall is insufficient or so unevenly distributed that these plants are unable to grow. This is true over a large area of the United States, and the same conditions of- ten prevail over the usually well-watered part of the country in times of drought. The question of increasing the water-holding capacity and of preventing the loss of water by evaporation, or in other ways is a very important one.
Experiment 52. -Weigh out equal amounts (about 100 g. each) of dried gravel, coarse sand and very fine sand. Put each of these into a four-inch funnel which has been fitted with a filter paper. Pour water upon each until all that can be absorbed has been absorbed. Allow each to stand until water ceases to drop from the funnel. Weigh again, balancing the weight of the wet filter paper retainer by a similar wet filter paper placed on the weight side of the scales. Which of these substances is capable of holding the most water? Since water does not penetrateinto the grains composing these different substances, the difference in water-holding capacity must be due to the different sizes of the grains.
If we dig deep enough into almost any soil we shall find water. Wells show this. Certain trees and plants have such long roots that they can reach the underlying water and flourishALFALFA ROOT.
A long root which has gone deep to seek water.
where other plants will die. When wetlands are so drained by tilling that the plants can send their long roots down to this constant water supply or water table, as it is called, they stand a drought much betterthan plants grown on undrained land where the water table has not so uniform a depth.
Experiment 53. -Place small glass tubes of several different bores in a dish of colored water. In which is the surface of the water higher, in the tubes or in the dish? In which tubes is it the higher, those of large or small bore?
Experiment 54. -Place two wide-mouth 4 oz. bottles side by side and fill one partly full of water. Put a coarse piece of cloth, or better, a lamp wick, into the water bottle and allow the other end to hang over into the empty bottle. Allow the bottles to stand thus for an hour. What happens? The force that causes the rising of water up small tubes, wicks and crevices is called capillarity.
Fig. 45.
Fig. 46.
Experiment 55. -Tie pieces of cloth over the ends of four lamp chimneys. Fill one of the chimneys with coarse sand, another with fine sand, another with clay, and the fourth with a deep black loam. Stand each chimney in a shallow pan of water. Allow them to remain for a week,keeping water in the pan all the time. Note how high the water has risen in the different chimneys at the end of an hour; two days; a week.
It was found in Experiment 49 that each little particle of soil was surrounded by a film of water, even though there was apparently no water in the soil. This film will be replaced if removed just as the water in the top of the wick (Experiment 54) was replaced by water flowing up the wick. Roots get a large part of their water by absorbing the water films of the soil particles.
If a region is well supplied with forests so that the rain as it falls is held by the moss, leaves and roots and protected from evaporation by the foliage, soil water will continue to be supplied to the surroundingA NATURAL SPRING.
Coming to the surface between rock layers.
open land long after it would have become dry had the forests been removed. Mountain soils have been found which hold back five times their own weight of water.
Gravity is continually pulling the soil water deeper and deeper into the ground. This deep soil water is frequently diverted tolower ground by impervious layers of soil or rock and comes to the surface as springs, or it may come gradually to the surface over a broad area a long distance away from where it fell and make a region, otherwise barren, fertile by subirrigating it.
It is often very essential to stop as far as possible this downward passage of water, or seepage, as it is called. The water in seeping through the soil dissolves plant food and if allowed to drain off would decrease the fertility of the soil. Whatever decreases the porosity of the soil will decrease the seepage and thus help to retain the plant food.
AN ARTESIAN SPRING.
A deep water-layer has been pierced and the water diverted to the surface.
This may be done by adding humus, and sometimes where the soil is very porous by rolling. At the time rain is likely to fall, however, the soil must be kept loose and mellow so that the water can sink into it.