Chapter 03
Chapter 03
Chapter 03
MODES OF CULTIVATION
175
176 CHAPTER 13 SOIL AND WATER MANAGEMENT
loose and require no primary tillage at all. As such, secondary tillage aims to
loosen the soil to a relatively shallow depth, generally less than 20 cm. The
implements suitable for secondary tillage are disk harrows, spike harrows,
seeps, rotary hoes, cultipackers, and various other tools that work the soil
to shallow depth and help to disrupt crusts where they occur. All too often,
however, such implements are efficacious in the short run (e.g., in preparing
a seedbed) but ultimately contribute to the degradation of soil structure by
grinding down the soil’s natural aggregates.
In recent decades, the advent of chemical herbicides has reduced the impor-
tance of tillage as the primary method for the eradication of weeds, though the
high cost of such chemical treatments and their ancillary environmental effects
limit their application, especially in developing countries. At the same time, the
formerly prevalent practice of inverting the topsoil in order to bury manures
and plant residues has become a less important function of tillage in modern
field management. Plant residues can, and in many cases should, be left over
the surface as a stubble mulch to protect against evaporation and erosion.
An essential task of agriculture is soil structure management, as it affects
water infiltration and runoff, wind erosion and evaporation, gas exchange pro-
cesses, as well as planting and germination of crops. Here we find that tillage
practices suitable in one location may become harmful in another. Arid-zone
soils with low organic matter contents and unstable aggregates are particularly
vulnerable to compaction, crusting, and erosion. The precise effects of various
modes of tillage must be defined in each case for tillage to be practiced effi-
ciently and sustainably.
Tillage operations are especially consumptive of energy. The amount of
earth-work involved in repeatedly loosening, pulverizing, inverting, and then
recompacting the topsoil is indeed very considerable. In a typical small field of
1 hectare, the topsoil to a depth of only 30 cm weighs no less than 4000 tons.
Fig. 13.1. Universal shape of a moldboard plow for deep primary tillage.
MODES OF CULTIVATION 177
Fig. 13.2. Horizontal and vertical rotating harrows for shallow secondary tillage.
SOIL COMPACTION
planting can be confined to the narrow strips where planting takes place rather
than be carried out over the entire surface as was the practice in former times.
An extremely important factor is the timing of field operations in relation
to the state of soil moisture. Operations that impose high pressures should, if
possible, be carried out on relatively dry soil, which is much less compactable
than is moist soil.
Irrigation can do more than merely raise the yields of specific crops; it can also
prolong the effective crop-growing period in areas with extended dry seasons,
thus permitting multiple crop growing per year where only a single crop could
be grown otherwise. With the security provided by irrigation, additional inputs
needed to intensify production further (e.g., pesticides, fertilizers, improved vari-
eties, physiological treatments, environmental controls, soil amendments, and
tillage) may become economically feasible. Irrigation reduces the risk of such
expensive inputs being wasted by crop failure resulting from lack of water.
The practice of irrigation consists of applying water to the part of the soil
profile that serves as the root zone, for the immediate and subsequent use of
the crop. Inevitably, however, the initiation and the continuation of irrigation
in a given area induce a series of processes that can profoundly affect the both
the on-site and the related off-site environments, and not necessarily for the
better. Over time, some of its potentially self-destructive effects may make the
very practice of irrigation unsustainable.
and flora. The alternative to discharging the drainage into the river is to convey
it to the sea (which may be quite distant), to lagoons or wetlands (whose fauna
and flora, however, may also be vulnerable to the pollutants), to environmen-
tally isolated evaporation basins in the desert, or to very deep aquifers where
the pollutants are diluted. All of these alternatives may be quite expensive to
carry out, and perhaps unsustainable in the long run. The Kesterson Reservoir
in California is an example where the discharge of drainage from irrigated
areas into wetlands has resulted in damage to wildlife due to the accumulation
of waterborne toxic elements.
DRAINAGE REQUIREMENTS
The term drainage can be used in a general sense to denote outflow of water
from soil. More specifically, it can serve to describe the artificial removal of
excess water, or the set of management practices designed to prevent the occur-
rence of excess water. The removal of free water tending to accumulate over
the soil surface by appropriately shaping the land is termed surface drainage
and is outside the scope of our present discussion. The removal of excess water
from within the soil, generally by lowering the water table or by preventing its
rise, is termed groundwater drainage, which is an integral aspect of sustainable
irrigation management.
The artificial drainage of groundwater is generally carried out by means of
drains, which may be ditches, pipes, or “mole channels”, into which ground-
water flows as a result of the hydraulic pressure gradients existing in the soil.
The drains themselves are made to direct the excess water, by gravity or by
pumping, to the drainage outlet, which may be a stream, a lake, an evapo-
ration pond, or the sea. In some places, drainage water may be recycled, or
reused, for agricultural, industrial, or even residential purposes.
Because drainage water may contain potentially harmful concentrations of
salts, fertilizer nutrients, pesticide residues, and various other potentially toxic
chemicals as well as biological pathogens, it is not enough to “get rid” of it. A
major concern is the eventual consequence of its disposal. Therefore, the first
requirement of drainage management is to provide a safe outlet for the effluent.
The recent emphasis on modes of agricultural management that minimize chemi-
cal inputs may help to lessen the problem posed by the persistence of some of
these chemicals in the environment.
Various theoretical and empirical methods have been proposed for designing the
optimal drainage system for different sets of conditions, considering the attributes
186 CHAPTER 13 SOIL AND WATER MANAGEMENT
Table 13.1 Prevalent Depths and Spacings of Drainage Tubes in Various Soil Types
of the soil, the climatic and hydrological regime, and the crops to be grown. The
ranges of depth and spacing generally used for the placement of drains in field
practice are listed in Table 13.1. In Holland, the country with the most experience
in drainage, common criteria for drainage are to provide for the removal of about 7
millimeters of water per day, and to prevent a water table rise above 0.5 meter from
the soil surface. In more arid regions, because of the greater evaporation rate and
groundwater salinity, the water table must generally be kept much deeper. In the
Imperial Valley of California, for instance, the drain depth ranges from about 1.5
to 3 meters, and the desired water table depth midway between drains should be at
least 1.2 meter. For fine-textured (less readily permeable) soils, the depth should be
greater still, especially where the salinity risk is high. Since there is a practical and
economic limit to how deep the drains can be placed, it is the density of drain spac-
ing that must be increased in such circumstances.
with the external environment, which includes both natural ecosystems and
other human enterprises. More specifically, irrigation projects should ensure
that water supplies of adequate quality are and will continue to be available,
the salt balance and hence the productivity of the land can be maintained, the
drainage effluent can be disposed of safely, public health can be safeguarded,
and the economic returns can justify the costs.
The sine qua non of ensuring the sustainability of irrigation is the timely
installation and continuous operation of a drainage system to prevent water-
logging and to dispose safely of excess salts. All too often drainage creates
off-site problems beyond the on-site costs of installation and maintenance,
since the discharge of briny effluent can degrade the quality of water along its
downstream course. Where access to the open sea is feasible, solving the prob-
lem is likely to be easier than in closed basins or in areas far from the sea. In
those cases, the disposal terminus eventually becomes unfit for human use, as
well as for wildlife. Hence the importance of reducing the volume and salinity
of effluents by such means as improving the efficiency of water use, a task that
in itself can bring economic and environmental rewards. Modern irrigation
technology offers the opportunity to conserve water through reduced trans-
port and application losses, coupled with increased yields per unit volume of
water.
methods can raise yields while minimizing waste (by runoff, evaporation, and
excessive seepage), reducing drainage requirements, and promoting the integra-
tion of irrigation with essential concurrent operations such as fertilization and
pest control. The use of brackish water has become more feasible, as has the
utilization of sandy, stony, and steep lands previously considered unirrigable.
Additional potential benefits include increased crop diversification and crop-
ping intensity; i.e., the number of crops that can be grown in succession each
year.
The traditional method of irrigation consisted of flooding the land to some
depth with a large volume of water so as to saturate the soil completely, then
waiting some days or weeks until the moisture stored in the soil was nearly
depleted before flooding the land once again. In this low-frequency, high-volume,
total-area pattern of irrigation, the typical cycles consist of periods of exces-
sive soil moisture alternating with periods of insufficiency. Optimal conditions
occur only briefly in transition from one extreme to the other.
In contrast, the newer irrigation methods are designed to apply a small,
measured volume of water at frequent intervals precisely to where the roots
are concentrated. The aim is to reduce fluctuations in the moisture content
of the root zone by maintaining optimal (moist but unsaturated) conditions
continuously, without subjecting the crop either to oxygen stress from excess
moisture or water stress from lack of moisture. Moreover, applying the water
at spatially discrete locations, or even below the surface, has the effect of keep-
ing much of the surface dry, thus helping not only to reduce evaporation but
also to suppress proliferation of weeds.
Since the high-frequency irrigation systems can be adjusted to supply water
at very nearly the exact rate required by the crop, the irrigator no longer needs
to depend on the soil’s ability to store water during long intervals between irri-
gations. Hence, water storage properties of the soil, once considered essential,
are no longer decisive in determining whether a soil is irrigable. New lands,
traditionally believed to be unsuited for irrigation, can now be brought into
production. Examples are coarse sands and gravels, where moisture storage
capacity is very low and where the conveyance and spreading of water by surface
flooding would cause too much seepage.
Of particular interest is the method of drip irrigation (also called trickle
irrigation) and its many variants, such as microsprayer irrigation or spitter
irrigation. Collectively called microirrigation, these methods have been gain-
ing acceptance in many areas. The idea of applying water slowly, literally drop
by drop, at a rate that is continuously absorbed by the soil’s root zone, is not
an entirely new notion. What has made it practical is the development of low-
cost weathering-resistant plastic tubing and variously designed emitter fittings.
System assemblies are now available that are capable of maintaining sufficient
pressure in thin lateral tubes to ensure uniform discharge throughout the field,
as well as ensuring a controlled rate of drip or spray discharge through the
narrow orifice emitters, with a minimum of clogging. A variant of the system
is the subsurface placement of a perforated or porous tube that can ooze water
continuously with practically no loss due to evaporation.
The application systems described have been supplemented by ancillary
equipment such as filters, timing or metering valves (enabling the irrigator
190 CHAPTER 13 SOIL AND WATER MANAGEMENT
Fig. 13.5. Partial-area wetting around orchard trees under drip irrigation.
WATER-USE EFFICIENCY AND WATER CONSERVATION 191
Fig. 13.6. Radiation and water balances on a plant under localized irrigation.
of irrigation at a high frequency; e.g., several times each day throughout the
growing season. The application rate can be adjusted according to the variable
(weather-determined) evaporative demand and to the stage of crop growth.
The circular pattern of irrigation can be seen from the air by airline passengers
flying over large sections of the US Great Plains.
WUEag = P/W
where P is crop production, either in terms of total dry matter or of the mar-
ketable product, and W is the volume of water applied.
Since only a fraction of the applied water is actually absorbed and utilized
by the crop, we need to consider the various components of the denominator
W, as follows:
W = R + D + Ed + Es + Tw + Tc
Here, R is the volume of water lost by runoff from the field, D is the volume
drained out of the root zone by deep percolation, Ed is the volume lost by
evaporation during delivery and application of water to the field, Es is the vol-
ume evaporated from the soil, Tw is the volume transpired by weeds, and Tc is
the volume transpired by the crop. All these volumes pertain to the same unit
area and time period.
ENGINEERING APPLICATIONS OF SOILS 193
In addition to its role in agriculture, the soil is used for a variety of pur-
poses in engineering practice. The soil serves as a foundation for many struc-
tures, including roads, dams, and houses. The soil also serves as a building
material—for example, in the manufacture of bricks and the construction
of earthen dykes and adobe houses. Dry soil is a good thermal insulator.
Ceramic materials are made of fired clay, in the making of which the par-
ticles are fused under heat, and the mass hardens and becomes relatively
impermeable.
The tendency of some clay types, especially smectite, to expand when it
imbibes water and to contract when it dries can cause buildings, roadways, and
pipelines to subside and even to shatter. To avoid failure, structures established
over expansive clay must be based on especially strong foundations resting on
deep piles or underlying solid bedrock. In some cases, entire cities that are built
over saturated clay (Bangkok, Houston, and Mexico City are prime examples)
may subside gradually due to the consolidation of the clay underneath.
194 CHAPTER 13 SOIL AND WATER MANAGEMENT
Fig. 13.7. Inefficient irrigation, downward spriral (a) vs. efficient irrigation, upward spiral (b).
Engineers who deal with the soil as a building material or as a foundation for
buildings must consider the mechanical properties of the soil in various states
of wetness and under various stresses. The concept of soil strength expresses
a soil body’s ability to bear loads and to withstand compressive or shearing
stresses without failing; i.e., without shattering or collapsing. Soil strength is
generally a function of the soil’s degree of compaction, or its bulk density; i.e.,
the mass of dry soil per unit bulk volume. To make the soil stronger, therefore,
engineers usually try to compact it by means of machines that can knead and
compress the soil to a high bulk density. An important consideration is the
moisture content of the soil at the time of compaction. Each soil has an optimal
moisture content at which a given compactive effort can produce the maximal
density and strength. A soil that is too dry will pulverize under stress, and a
soil that is too wet will deform when kneaded (as does dough), but in neither
state will the soil compress and become stronger. Hence, an intermediate con-
dition is desired at which the soil is moist but not saturated and therefore most
amenable to compression.
An important use of soil is in the disposal of wastes. Domestic sewage
(including garbage and human wastes), industrial wastes (which may be toxic),
surplus building materials, and even spent medical supplies are commonly
applied to so-called “landfills”. In many cases, the soil’s capacity to decom-
pose or immobilize such materials may indeed render them harmless. In too
many cases, however, the soil’s limited capacities are exceeded, so that harmful
agents may leak into the larger environment. The best facilities are designed
and controlled landfills that are isolated from the environment by means of
compacted clay layers, durable plastic linings, or concrete structures designed
to minimize and control the venting of gases to the atmosphere and the leach-
ing of liquids to groundwater and surface streams.
ENGINEERING APPLICATIONS OF SOILS 195