The following information on the functions of water comes from Five Acres and Independence by M. G. Kains. Five Acres and Independence is also available to purchase in print.
*This chapter consists mainly of excerpts from the author’s Modern Guide to Successful Gardening.
Though many factors are essential to plant growth, perhaps the most important is water. No other plays so many roles. It dissolves plant food in the soil; carries these solutions to and through the plants; supplies hydrogen and oxygen which combine with other elements to form sugar, starch, oil, plant tissues, and many other compounds; keeps plant cells distended, thus enabling them to perform their functions; regulates the temperature of plants (and incidentally of the air) by transpiration from the foliage; and carries food constituents and soluble plant products from part to part inside the plants for storage, assimilation or growth.
The amount of unassimilated water living plants contain at any time is much greater than that of all other constituents combined—from 60% to 95% by weight. This indicates the importance of an abundant supply of water because these percentages must be maintained in order that plants may live.
Growth demands still more than the percentages mentioned. In addition to the amounts assimilated, a constant current of water must be maintained from the soil through the plant, then into the air as vapor. This amount is enormously greater than that assimilated or than that in the plant at any one time. In hot, dry, windy weather some plants transpire in 24 hours amounts of water equal to or even greater than their own weight! The amounts are also influenced by such factors as character of foliage (large or small, unprotected or protected by hairs, thick skin, etc.), by character of climate, size and development of plant and amount of available water supply in the soil.
Studies of plant growth have proved that each pound of “dry matter” in a mature crop has required from 200 to 900 pounds of water to develop, the usual range for common crops in the Northeastern states being from 300 to 500 pounds! During the growing season an average crop requires from 1 to 2 1/2 pounds for each square foot! If this were present on the surface all at one time it would mean a depth of 3 1/2″ to 9″! Hence the necessity of an adequate supply of water for plant development.
Water exists in the soil in three conditions; hygroscopic, free and capillary. Hygroscopic water is of no importance in crop growing because apparently plants cannot utilize it. It is neither moved by gravity nor by capillarity since it is not in liquid form.
Free water, that recognized by sight and touch, fills hollows during rains and sinks into the ground to the level of “standing water” (the “water-table”). In swamps this is at or above the ground surface; in shallow and undrained soils perhaps only a few inches below; in deep and porous soils often many feet beneath. This “free” water supplies springs and wells but is of use to plants only when near enough for capillarity to draw it up to the region of plant roots. When too near the surface it may injure cultivated crops by saturating the ground, for then it excludes air, makes the soil cold, prevents instead of favors nitrate formation, encourages formation of compounds poisonous to plants, retards decomposition of organic matter, and so on. It is corrected by drainage (Chapter 16).
Capillary water, the thin film of water that surrounds soil particles, is the main source of water in plant growth (bog and water plants excepted) and is the most effective form for dissolving and holding soluble plant nutrients. As its movement is from moist to less moist parts its general direction is upward from the water-table to the air. The distance it can move depends largely upon the character and arrangement of the soil particles and the air spaces.
Soils of close structure (clays, adobes and heavy loams) when cultivated properly usually have increased air spaces, greater capillary water movement and water-holding power than when mismanaged. In loose and coarse soils (sandy and peaty) the air spaces are already great, so stirring decreases capillary movement and water-holding power.
You may control the capillary water supply by decreasing water losses from the soil, by increasing water-holding power (draining off superfluous free water), and by direct addition of water (irrigation).
One of the most important ways to prevent loss of water is by tillage (Chapter 30). The other is mulching. As originally understood, a mulch (formerly spelled mulsh) is an application of manure or any other loose material such as leaves spread upon the soil surface to protect the roots of newly planted trees, shrubs, tender plants, etc. Today it is extended to include earth kept loose by surface tillage to check evaporation.
Though this last has become almost universal, some scientists doubt that a soil mulch alone lessens evaporation because, they declare, stirring the surface brings moist soil from below into contact with the air and wastes the water contained in it. Thus, they claim, evaporation is increased. Doubtless this is true when tillage is both deep and frequent but where only the surface inch or less is stirred by weekly raking, better crops are produced than in adjacent ones in which the practice is not followed.
When such loose materials as buckwheat hulls, shredded corn stover, chopped straw or granulated peat moss are available and cheap they are more effective than loose soil in checking moisture losses from tilled areas and when dug into the soil they add humus. For mulching trees and berry plants coarser materials may be used—marsh hay, straw, leaves, corn stalks, etc.
The most recent type of mulch is specially made, impervious black paper spread upon the ground and between or through which young plants are set. It is sold direct by the manufacturers and through garden supply stores.
It has been so successfully used by Hawaiian sugar and pineapple growers that experiments in the United States and Canada have been conducted, mainly with vegetables, because it is claimed that this material will shed water into the soil, conserve water in the soil by checking evaporation, increase germination, greatly reduce or even eliminate weeding and cultivation, increase soil temperature, hasten maturity, increase yields and produce larger, higher quality, cleaner crops. But results have varied so widely that the question is still open. Hence, to obtain additional comprehensive information, A. E. Hutchins conducted investigations during three years to determine effects of mulch paper in comparison with those of clean culture and to secure incidental information. His conclusions (slightly condensed) are quoted from Bulletin 298 of the Minnesota Experiment Station as follows:
1. A beneficial effect appears to be exerted by the paper. In the experiments the increases obtained do not, in most cases, appear to pay for the additional cost. 2. Mulch paper seems most beneficial with warm-season crops. 3. It seems to hasten maturity of certain vegetables and may be profitable with crops that have a relatively high market value.
4. It also appears to be most beneficial under conditions unfavorable to the optimum development of the crop such as poor soil, deficient precipitation and low temperature. As there is no precise way by which climatic conditions can be predicted in a given locality, its value from this standpoint can be determined only after the growing season is past. Under favorable growing conditions, often little beneficial and sometimes a detrimental effect is produced.
5. Its effect varies with local climatic conditions, with each crop grown and, to some extent, with different varieties of the same crop. Therefore each grower must determine the value of paper for his particular crops and for his local conditions. 6. Warm season crops of high acre value and yield grown intensively are most likely to give the best results. 7. Paper should not be used on low value crops. 8. It largely eliminates weeds in the covered area and thus conserves the moisture and fertility they would use. It also cuts down the cost of cultivation but this item is offset to a large extent by the added cost of laying and caring for the paper and by the additional labor involved in planting and transplanting when it is used.
You may increase the supply of capillary water in the soil by drainage, fertilizing, adding humus and by watering or irrigation.
Soluble salts in fertilizers in the surface foot of soil tend to draw water from lower levels to dissolve them and to dilute the solutions. As evaporation of the water in these solutions is constant at the soil surface water flows upward to maintain a balance. Hence a fertilized surface soil is normally moister than an unfertilized one.
Doubtless the most feasible way to increase water supply in the majority of soils is to maintain the crumb-like structure that favors the ascent of capillary water from below. The most important factor in attaining this end is not mechanical operations such as plowing, digging or cultivating but humus in adequate supply. Tillage operations permit particles of humus to wedge themselves between the mineral particles where they act as sponges to soak up and hold water raised by capillarity from standing water below. Thus humus becomes the most important reservoir of water for plants (Chapter 23).
In spite of all cultural precaution so far discussed to assure and conserve water supply artificial methods of watering are often necessary or at least advisable, especially in localities where long dry spells are common. Some of these methods (watering pot and hose) are capable of exceedingly limited serviceable application; others are infeasible under various specific conditions (furrow systems on sloping ground); and still others are applicable anywhere that water is needed (overhead irrigation).
In view of the figures already quoted, the limited use of watering pot and hose is evident. Moreover, each entails work that may be avoided by more adequate methods. As ordinarily used—sprinkling only the surface—these two garden accessories do more harm than good. Mere sprinkling encourages the roots to develop near the surface where they are almost certain to be injured by summer heat, especially if watering is neglected for a few days in dry weather.
If the same amount of water were applied once a week instead of a little every day the soil would be soaked (theoretically) many times as deeply, the roots would develop deeply and the plants benefit in proportion. Still better than hose is irrigation.