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III. Interactions of Water and Soil

III. Interactions of Water and Soil

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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









Russell (1952).

Normal form

in well-oxidized soils



so,- Fe+ + + (ferric)

Mn++ + (manganic)

Reduced form

in waterlogged soils


Complex aldehydes, etc.

NI and NHa


Fe+ (ferrous)

Mn+ (manganous)





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).






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
















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.



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 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



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



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.



IV. The Soil Environment and Root Development

D. Wiersma

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).

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III. Interactions of Water and Soil

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