“Plowsole.”—An artificial hardpan is very commonly formed under the practice of plowing to the same depth for many consecutive years. The consolidated layer thus created by the action of the plow (hence known as plowsole) acts precisely like a natural hardpan, and is sometimes the cause of the formation of a cemented subsoil crust simulating the natural product. This is most apt to occur in clayey lands, and greatly increases the difficulty of working them, while detracting materially from the higher productiveness commonly attributed to them as compared with sandy lands. Of course it is perfectly easy to prevent this trouble by plowing to different depths in consecutive years, and running a subsoil plow from time to time. In this case, also, lime will generally be very useful and be found to aid materially in the disintegration of the “plowsole.”
It is hardly necessary to insist farther upon the need of the examination of land to be occupied, for the existence of hardpan or other faulty subsoil, which may totally defeat for the time being the farmer’s efforts, or make him lose his investment in plantations after a few years. Probing by means of the steel rod described above ([p. 177]) or boring with a post-hole auger; or finally, if necessary, digging a pit to the proper depth (from four to six feet in the arid region), should precede every purchase of new or unexplored agricultural land.
Marly Substrata.—Among the causes of failure occasionally found in the case of the “going-back” of orchards, is the occurrence of strongly calcareous or marly substrata, at depths which in the humid region would not be reached by the roots, but in the course of a few years are inevitably penetrated by the roots of trees in the arid region. Then there appears a stunting of the growth, and sometimes a yellowing of leaves, or chlorosis, due to the influence of excessive calcareousness at the depth of four or five feet. For this of course there is no remedy except the planting of crops which, like the mulberry, Texas grapes, Chickasaw plum and others, are at home on such lands; which in the Eastern states are naturally occupied by the crab apple, honey locust and wild plums.
CHAPTER XI.
THE WATER OF SOILS.
HYGROSCOPIC AND CAPILLARY MOISTURE.
When it is remembered that from 65 to over 90% of the fresh substance of plants consists of water, the importance of an adequate and regular supply of the same to growing plants is readily understood. But it seems desirable, before discussing the relations of water to the soil and to plant life, to consider first the physical peculiarities which distinguish it from nearly all other substances known. That it is colorless, tasteless, inodorous, and also chemically neutral, alone constitutes a group of properties scarcely found in any other fluid. But its special adaptation to its functions in relation to vegetable and animal life are much more fundamental, as is shown in the table of its physical constants as compared with other well-known substances, given below.
PHYSICAL FACTORS OF WATER COMPARED
WITH OTHER SUBSTANCES
(PER UNIT WEIGHT).
| Capillary ascent in glass tubes of one mm. diameter. | |
| Water | 14 mm. |
| Alcohol | 6 mm. |
| Olive oil | 1 mm. |
| Heat Relations.. | |
| Density. | |
| Water at 0° C. (freezing pt.) | .99988 |
| Water at 4° (Maximum density) | 1.00000 |
| Water at 15° C. | |
| (ordinary temperature) | .990 |
| Ice at 0° (freezing pt.) | .92800 |
| Specific Heats. | |
| Water | 1.000 |
| Ice | .502 |
| Steam | .475 |
| Clay, Glass | .180-.200 |
| Charcoal | .241 |
| Wood | .032 |
| Gold, Lead | .032-.031 |
| Zinc | .096 |
| Steel | .119 |
| Heat of fusion. | |
| Water (Ice) | 80 Cal. |
| Metals | 5-28 “ |
| Salts, (incl. silicates) | 40-63 “ |
| Heat of Evaporation. | |
| Water at 20°C. | 613 Cal. |
| “ “ 100°C. | 637 “ |
| Alcohol | 209 “ |
| Spirits of Turpentine | 67 “ |
Summarizing the meaning of the data given in the above table with respect to organic life, we see, first, that water rises higher both in the soil and in the tissues of the plant than any other liquid. Second, that as its density decreases in cooling after a certain point is reached, it freezes at the surface instead of at the bottom, as other liquids do; and as solid water (ice) is lighter than fluid water, ice stays at the surface and is readily melted when spring comes. Third, since its temperature changes more slowly than that of any other liquid, it serves to prevent injuriously rapid changes of temperature in plants and animals as well as in soils. Its high “heat of fusion” also serves to prevent quick freezing of plant and animal tissues, so that the brief prevalence of a low temperature may be more readily borne. Finally, the large amount of heat absorbed in evaporation of water serves to keep both plants and animals cool under excessive external temperatures which would otherwise quickly destroy life.
Capillarity or Surface Tension.—In this table it will be noted, first, that water rises higher in fine (“capillary”) or hair tubes than the other fluids mentioned, which fairly represent all others. No other fluid approaches water in the height to which it will rise[66] in either soils or plant tissues. Were its capillary factor no higher than, e. g., that of oil or alcohol, trees could not grow as tall as we find them, and the water supply from the substrata, and all the movements of water in the soil, and hence plant growth, would be similarly retarded. It is easy to verify these differences by immersing a cylinder of clay soil (or a cotton wick) in water on the one hand, and in oil or alcohol on the other. Notwithstanding the greater fluidity of alcohol as compared with water, the latter will be found to fill the porous mass much more quickly.