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III. Interactions of Water and Soil
M. B. RUSSEXL
Water, either in liquid or solid form, has been the major vehicle for
the movement, segregation, and deposition of soil parent materials. When
soil detachment and movement are accelerated by water movement resulting from man-induced changes in land use, a soil-management problem
is created. Major problems have resulted in many agricultural areas. No
discussion of the many important agricultural, social, and economic
aspects of erosion control will be attempted here, although it must be
recognized that such phenomena are an important facet of the over-all
hydrologic cycle and have many far-reaching effects on crop production.
Within the more restricted framework of current crop production
practices, water has important effects on the amount and kinds of nutrients found in the soil and on their availability to plants. In the permeable soils of humid regions where there is an annual flux of water
through the solum, planning a fertility program involves consideration
of the loss of soluble soil constitutents by leaching. The supply of soluble
ions such as nitrate must be matched to uptake by roots if excessive
losses are to be avoided. On soil having an appreciable base exchange
capacity, leaching of other nutrients is less critical; but, in very sandy
soils, these also must be applied at relatively frequent intervals in small
amounts matched to crop needs if efficient fertilizer use is to be obtained.
In subhumid and arid regions the soluble salts are not leached from
the soil but tend to be concentrated at the soil surface as the result of
movement of water to the surface and its loss by evaporation, The concentrations of total soluble salts or of sodium are sufficient in some soils
to prevent their use in agriculture. In subhumid and arid soils the possibility that toxic salt concentrations will develop in the surface soil must
always be considered when irrigation water is supplied. In all irrigation
systems in such soils, water applied as rainfall or irrigation must be
sufficient to provide a net downward flux of water through the soil profile.
Waterlogging, caused by excessive water and inadequate drainage,
State of Oxidation of Certain Soil Constituentso
in well-oxidized soils
so,- Fe+ + + (ferric)
Mn++ + (manganic)
in waterlogged soils
Complex aldehydes, etc.
NI and NHa
WATER AND ITS RELATION TO SOILS A N D CROPS
results in significant changes in the level of oxidation and solubility of
several important nutrient ions and soil constitutents (Robinson, 1930).
A summary of such changes is given in Table IV. Because of the altered
solubilities and ionic forms associated with anaerobic soil conditions,
plants subjected to waterlogged conditions may show either toxicity or
deficiency symptoms. A more detailed discussion of plant responses to
excessive water will be found in a later section of this article.
On the purely physical side, water modifies the thermal properties of
the soil largely through an increase in heat capacity accompanying the
increase in volume fraction of water in the soil. For this reason, both
the magnitude and rate of diurnal and seasonal changes in temperature
are smaller in wet soils than in dry soils. The movement of air into and
out of the soil also is highly dependent on the amount of water present
in the soil voids.
Finally the effects of water on mechanical behavior should be mentioned. Dry soils are able to support heavy loads, because of the rigidity
of the individual soil particles and the high internal friction of the soil
mass. But added water reduces internal friction, making the soil susceptible to plastic deformation and consolidation. As water is further
increased, the soil takes on the properties of a fluid and loses its loadbearing properties entirely. This extreme change in the rheologic properties is of agricultural importance particularly as it affects the susceptibility of soils to compaction when subjected to traffic, as in tillage
operations. The effects of moisture content and organic matter on the
compaction that will result from a given amount of impact are shown
in Fig. 11.
FIG.11. The effect of moisture content on soil compaction. Soils A and B contain
2.8 and 4.1 per cent organic matter, respectively (Free et al., 1947).
M. B. RUSSELL
OF WATERBY SOIL
Water arriving at the soil surface moves laterally over the surface,
infiltrates the soil, or is impounded on the surface until it returns to the
atmosphere by evaporation. The division of incident water into these
three portions is determined by the amount and intensity of the precipitation and by the slope and water-intake characteristics of the soil surface. Water impounded in small irregularities on the soil surface evaporates rather quickly, contributing little to crop production, and can be
considered as a loss of water from the agriculturally important part of
the hydrologic cycle. Standing surface water may contribute to the incidence and spread of crop diseases, and, if present for extended periods,
will cause serious crop damage by restricting the movement of oxygen
into, and carbon dioxide out of, the soil.
Water that moves over the soil surface concentrates in drainage ways
and reaches stream channels. Such water, accounting for about onefourth of total precipitation of the United States, is a major source of
water for irrigation. Erosion, one of the important consequences, of runoff, reduces the productive capacity of agricultural lands and the capacity
of water-storage structures. Although both of these phenomena are of
great significance as factors affecting crop production, they are outside
the scope of this article, and are not discussed herein.
The entry of water into soil and its retention in the rooting zone for
use by crop plants are processes of paramount agricultural importance
in the hydrologic cycle. The capacity of a soil to store water is described
above as a function, primarily, of its depth and porosity. The amount of
water that actually enters the soil also depends on porosity, as well as
on previous moisture content and the intensity and duration of precipitation. Infiltration tends to be higher in warm months than in cool months,
and is greatly affected by the type of vegetative cover on the soil. It is
correlated positively with organic matter content, the percentage of large
pores, and state of aggregation of the surface soil. The rate of water
intake decreases with time during a given infiltration period, partly because of the plugging of larger pores in the soil surface that accompanies destruction of the surface aggregates. A protective cover of living
or dead vegetation dissipates the kinetic energy of the falling raindrops,
greatly reducing such aggregate destruction and slowing the rate of decrease of infiltration. The effects of soil texture and type of crop on the
rate of water infiltration are shown in Fig. 12.
Water that enters the soil is held by capillary and surface forces. The
affinity with which it is held is a reciprocal function of moisture content.
Thus, at high moisture contents, water will move out of the soil by
WATER AND ITS RELATION TO SOILS AND CROPS
FIG.12. The effects of texture and vegetative cover on infiltration (Musgrave,
gravity, whereas at lower moisture it is retained even against extracting
forces several thousand times that of gravity. Moisture desorption curves
representing the functional relation between moisture content and the
security with which the water is held by the soil have proved to be a
very useful way of describing soil-water relations. Figure 13 shows typical desorption curves for three soils of widely differing textures. The
curves illustrate the fact that soils of similar gross moisture-storage ca-
FIG.13. Idealized moisture-retention curves for three soils.
M. B. RUSSELL
pacities may differ greatly in the amounts of water retained at a given
energy of retention.
The energy of retention of soil moisture is expressed in terms of an
equivalent negative pressure, called soil moisture tension. This important
moisture parameter can be measured in situ by soil moisture tensiometers.
Such measurements are useful in analyzing soil moisture movement and
water usage by plants. Although tensiometers are limited to tensions of
1 atmosphere or less, this covers the range in which most moisture-flow
occurs and in which 50 per cent or more of the water use by plants is
found, Soluble salts in the soil also affect water uptake by plants. The
effects of such salts on the energy status of soil water is described by
the osmotic pressure of the soil solution. Therefore, in soils containing
significant quantities of soluble salts, soil moisture tension and osmotic
pressure are summed to describe soil moisture-plant relations. This sum
is called the soil moisture stress.
When water enters the surface of a dry soil, it moves downward,
under the influence of attractive forces and gravity, in the form of a more
or less well-defined wetting front. If the amount of water added is insufficient to cause the wetting front to reach the lower end of the soil
mass or to contact a zone of moisture saturation, the downward movement will stop within a few hours after water entry at the surface is
discontinued. The moisture content of the wetted portion of the soil
column, in quasi-equilibrium with drier soil below, is known as field
capacity ( FC ) This parameter represents, for most practical purposes
in well-drained soils, the upper limit of soil moisture used by plants. For
many soils of medium texture it is approximately equal to the water retained by a soil at a soil moisture tension of % atmosphere.
The lower limit of plant-available soil moisture is known as the
permanent-wilting percentage ( PWP ) , This biologically determined parameter is measured by growing dwarf sunflowers in a standardized quantity of soil. When the plants have attained a given size, no further water
is added to the soil and the plants are allowed to wilt. When the wilting
reaches a predetermined severity, the plants are removed and the moisture content of the soil is determined. The moisture percentage so obtained is called the PWP and represents for most practical purposes the
lower limit of plant-available water in a soil. For many soils it is closely
approximated by the moisture content held at a soil moisture tension of
15 atmospheres (FAP ).
The movement of water into and through soil is the result of a potential gradient. In saturated soils the soil moisture tension is at or near
zero throughout the entire soil mass. Under such conditions the driving
force causing flow is the gradient of the gravitational potential, and
WATER AND ITS RELATION TO SOILS AND CROPS
water thus flowing is commonly known as gravitational water. This is the
water that flows downward through the soil into the drainage channels.
Water flow in saturated soils is described by Darcy’s Law, u = ki, which
states that the velocity of flow, u, is proportional to the hydraulic gradient, i. Hydraulic conductivity, k, a parameter dependent on the properties of the fluid and the porosity of the soil, is widely used to characterize drainage and water-transmitting behavior of soils.
The flow of water in unsaturated soils is more difficult to analyze.
In such situations, flow occurs as a consequence of gradients in soil
moisture tensions and/or the gravitational potential. The vector sum of
these two gradients is termed the net driving force, and may result in
flow in any direction. The proportionality between the velocity of flow
and the net driving force is a function of the properties of the liquid
and of the volume and configuration of the liquid-filled pores. This proportionality is expressed as hydraulic conductivity and is highly dependent on the volume fraction of water in the soil. The effects on moisturetransmitting ability of changes in degree of saturation of soils of different
texture are shown schematically in Fig. 14. It is seen that the relative
MOISTURE CONTENT .-c
FIG.14. Curves summarizing the effects of texture and moisture content on the
water-transmitting ability of soils.
water transmissibilities of coarse and fine-grained soils undergo a reversal
at relatively low degrees of unsaturation. In view of recent investigations
emphasizing the importance of moisture transmission as a factor affecting
water uptake by plant roots, the curves in Fig. 14 give added significance
to the effects of soil texture on soil-water-plant relationships.
Water also moves through unsaturated soils by vapor transfer. Such
movement is a consequence of gradients in the aqueous vapor pressure
in the soil atmosphere. Since the vapor pressure of water is strongly
M. B. RUSSELt
temperature dependent and insensitive to soil moisture changes in the
plant-available moisture range, the flow of water vapor in soil is largely
the result of temperature differences, with vapor moving toward areas
of lower temperature. Because vapor transport occurs through the airfilled voids, its importance as a moisture-moving process increases as the
degree of unsaturation of the soil increases. Vapor transport is probably
responsible for much of the seasonal changes in the profile distribution
of water in soils in which the moisture content is well below field capacity.
Although the gross water-storage capacity of a given soil is essentially constant, the amount of water present usually undergoes rather
wide seasonal variations. In humid regions of the United States, the soil
profile is normally recharged to capacity at the beginning of the growing
season. Depletion of subsoil water reserves occurs during the summer
months, when evapotranspiration exceeds precipitation. During the fall,
winter, and early spring, precipitation normally exceeds evapotranspiration, and the subsoil is recharged. In regions where total annual precipitation is less than potential evapotranspiration, the subsoil is seldom recharged to capacity, and efficient water management becomes the prime
consideration in land use.
The annual depletion and recharge cycle that characterizes soil moisture storage emphasizes the importance of the process of water entry
into the soil. For rainfall to be fully effective in using the soil storage
capacity, its rate of fall must not exceed the rate at which water can
infiltrate the soil.
Where topography is uneven and annual precipitation is less than
annual potential evaporation, or only slightly greater, major attention
should be placed on maintaining the highest possible infiltration rate
for efficient use of water. This can be accomplished by providing a protective canopy of living or dead vegetation over the soil surface as much
of the time as possible, and by developing a stable well-aggregated surface soil. On sloping land, water-retention practices that increase the
time available for water infiltration are also effective in increasing profile recharge.
In summary, it may be said that the soil acts as an important waterstorage device that smooths out month-to-month fluctuations in water
supply available for plant use. The capacity of the soil for storage of
available water is determined by its texture and porosity and the rooting
volume of the crop. The rates of movement of water into and through
the soil are important in determining the effectiveness of a soil as a
storage medium of water for plants.
WATER AND ITS RELATION TO SOILS AND CROPS
IV. The Soil Environment and Root Development
Purdue University, Lafayette, Indiana
The amount of soil water that is available to a plant is determined
by the moisture characteristics of the soil, the depth to which the plant
roots extend, and the proliferation or density of the roots.
The moisture characteristics, such as field capacity and wilting percentage (commonly used as limits of plant-available water) are peculiar
to a soil, and are a function of texture, structure, and organic matter.
Little can be done to alter these limits; greater possibilities lie in changing the characteristics of the plant, enabling it to extend its rooting system deeper into the soil, thereby enlarging its reservoir of water.
The density of root proliferation in the soil is also an important consideration. Water in unsaturated soil moves very slowly, and only a
distance of a few centimeters. It is necessary for the plant to have roots
that completely ramify in its rooting zone in order to utilize all water
available within the reservoir.
Plants vary genetically in their rooting characteristics. Some, such as
onions and potatoes, have a sparse rooting system and are unable to use
all the soil water within the root zone; others, such as the forage grasses
and sorghums, have very fibrous, dense roots. Lettuce has a single tap
root, whereas corn extends out a distance from its base. Alfalfa has
a deep root; that of Ladino clover is very shallow. Whether a plant is
an annual or perennial is another factor affecting its water relations. An
annual plant must extend its roots down into the soil to make available
all the water it can potentially use. A perennial rooting system is already
established as to depth, and needs only to extend its small roots and
root hairs to be able to utilize the entire amount of soil water. Rooting
characteristics of plants are difficult to observe. However, techniques
have been developed for studying root distribution and activity (Weaver,
1926; Bloodworth et al., 1958; Lipps et al., 1957).
Plants may be limited in their rooting by other factors than genetics.
Any factor that will affect the vigor or condition of a plant may be expected to influence its extraction of moisture from the soil. Physical conditions of the soil, such as moisture, aeration, compaction (bulk density),
and temperature can limit or enhance the growth of roots. Chemically,
the pH, fertility, and salinity of the soil have been shown to influence
the rooting of plants. Crop-management practices, such as cutting the
top growth at different physiological stages, and the cultivation and
cutting of surface roots alter rooting habits and the ultimate water
economy of the crop. The presence or absence of beneficial or harmful
soil organisms and diseases and insects, as brought about by any of these
physical or chemical soil conditions, may limit root growth and the ability
to absorb soil moisture.
A number of recent reviews (Hagan, 1952; Richards and Wadleigh,
1952; Lutz, 1952; Russell, 1952) consider over-all plant growth as affected
by soil conditions, Root development is discussed as part of the presentation. The intent here is not to repeat the reports of these reviewers, but
to use them as a basis for discussing additional, recently acquired knowledge of the effects of soil environment and root development.
Soil moisture below the wilting point or at saturation is detrimental
to root development. Roots of some plants have been observed to extend
into a dry soil layer if a portion of the root is in a moist area, but they
were unable to absorb radioactive phosphorus from the dry soil (Hunter
and Kelley, 1946b). The absorption of nutrients from a dry soil may be
of importance in humid regions where the major portion of the fertility
lies in the surface soil. If roots can obtain moisture from deeper, infertile
soil, but are not able to utilize the essential plant nutrients in the dry
surface layer, it may be necessary to keep the entire soil profile moist
in order to maintain a proper moisture-fertility balance. In the main,
this would not be as serious a problem in arid sections where soil is
younger and less differentiated. This may account for some of the differences in thinking on the importance of irrigation frequency to maximum
It is generally agreed that plants are able to extract very little, if
any, moisture from soil, the major portion of which is below the wilting
point. The importance of root extension into dry soil layers could be
threefold: (1)roots penetrating from a moist surface soil through a dry
layer down into a moist subsoil region would have an additional water
supply; (2) roots in a dry area would be available for water absorption
at a subsequent rain or irrigation; or (3) roots may be able to take up
plant nutrients from soil deficient in water.
Cell elongation in roots and hypocotyls have different demands on
the water supply to maintain growth (Ronnike, 1957). Studies of lupine
seedlings grown in sphagnum media of various water contents have
shown that hypocotyl elongation is totally inhibited at diffusion pressure
deficits (DPD) of 8 to 10 atmospheres, while root growth continued at
DPD values far beyond 15 atmospheres,
Winter wheat grown in Nebraska has been found to penetrate into
soil that is below the 15-atmosphere percentage (Kmock et al., 1957).