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CHAPTER 5. FERTILIZERS AND THE EFFICIENT USE OF WATER

CHAPTER 5. FERTILIZERS AND THE EFFICIENT USE OF WATER

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224



FRANK G . VIETS, JR.



inches of annual precipitation is evaporated or transpired from land and

water surfaces. Practically all this evapotranspiration is from land in

farm crops, grass, or timber. As urban and industrial demands for water

grow, agriculture will be pressed to make more efficient use of its

decreasing allotment. This pressure will, of course, be greatest in desert

and semiarid areas where the competition for water is keenest.

Man must have food and fiber. All food and natural fibers require

water for their production. Agriculture, faced with increasing competition

for water and an increasing demand for its products, must become more

efficient in its use of water. Using water more efficiently is a problem

not confined to agriculture in the United States. Growing populations all

over the world point up sharply the need for better use of limited water

resources, especially in semiarid and arid regions, and for more research

information on how this may be achieved. Fertilizers, when needed, are

among the best tools for increasing production of food and fiber, but

what happens to water demands when fertilizers are used?

B. SOMEPOPULAR

VIEWPOINTS

Many farmers and irrigation technicians have the impression that,

when fertilizers are used, water requirements are greatly increased

because the crop is larger. The following are some excerpts from a recent

report on irrigation requirements : “The estimated water requirements

of small grains are 12, 15, and 18 acre-inches per ton of yield on soils

of land classes 1, 2, and 3, respectively. . . . The water requirement of

potatoes has been estimated for the purpose of this report as 2.0 and

2.5 acre-inches per ton of yield . . . sugar beets will require 1.5 and

2.0 acre-inches of water per ton of yield on class 1 and 2 lands . . .” The

important part of these statements is not that water is required on poor

land of class 3 compared to the best land of class 1,which is undoubtedly

true, but rather that within a land class the water requirement is proportional to yield. Since proper use of fertilizers on a land class is one

way to increase yields, it follows from this statement that fertilizers will

increase water demands. Here is another statement, made by a county

agent at a dinner honoring winners in a corn-growing contest, connecting

higher yields with higher irrigation requirements: “The high plant

populations used today require six to eight irrigations instead of the two

that growers used before.”

The view that yields twice as large require twice as much water is

one extreme point of view. The other extreme may be that yield of

crop has no effect on water requirements. Empirical and aerodynamic

methods of estimating evapotranspiration do not allow for fertilization

and size of the crop. The Blaney-Criddle (1950) method, widely used in



FERTILIZERS AND THE EFFICIENT USE OF WATER



225



and and semiarid regions to estimate consumptive use of water, uses

only mean monthly temperature, monthly percentage of annual daytime

hours, and an empirical coefficient, characteristic of the crop, to estimate

consumptive use for the season. In the Penman (1948, 195613) formula

the variables are sunshine duration, temperature, wind velocity, and

relative humidity. Only air temperature is used in the Thornthwaite

(1948) formula.

This viewpoint is based on the fact that evapotranspiration is a

physical process and that when water is nonlimiting, evapotranspiration

is almost entirely a function of meteorological conditions and is dependent “scarcely at all on the nature of the vegetation as long as it is green

(i.e., in a stage of vegetative growth) and effectively covers the soil”

(Schofield, 1952). What constitutes complete coverage of the soil,

however, is now being questioned. Makkink (1957) noted that the

discrepancy between evapotranspiration of grass in lysimeters and

calculations of it by the Penman formula depended on grass length. Tanner et al. (1960) showed that “fully grown corn does not provide

sufficient interception of radiation to assure that the potential evapotranspiration condition of a fully covered surface is realized.” They worked

with corn in 40-inch rows, oriented either east-west or north-south, and

stand densities of 13 and 22 kiloplants per acre. Likewise, Denmead

and Shaw (1959) found that corn in Iowa approached the condition

of a “green crop” for only 2 to 3 weeks from shortly before silking to

about 16 days after silking.



C. EARLYSTUDIES

Research on water requirements and water-use efficiency can be

roughly divided into two periods. The early period began with the work

of Sir John Lawes at Rothamsted before 1850 and culminated in the

classic studies of Briggs and Shantz in the second decade of this century

when agriculture was pushing into drier and drier regions of the Great

Plains of North America. Parallel developments were taking place in

Europe, particularly in Russia (Maximov, 1929). The second period is

the present, in which new concepts of energy exchange and radiation

efficiency in CO2 assimilation are being introduced. Richards and Wadleigh (1952) reported little current activity in this field.

Briggs and Shantz (1913b) reviewed the work of 23 separate investigations on water requirements of plants (water transpired per

unit of dry weight) as affected by fertilization of soils in containers,

beginning with the studies of Lawes on the effects of manures on water

requirements of five different kinds of crops. Briggs and Shantz concluded: “Almost without exception the experiments herein cited show a



226



FR.4NK G. V E T S , JR.



reduction in the water requirement accompanying the use of fertilizers.

In highly productive soils this reduction amounts to only a small percentage. In poor soils the water requirement may be reduced one-half

or even two-thirds by the addition of fertilizers. Often the high water

requirement is due to a deficiency of a single plant-food element. As

the supply of such an element approaches exhaustion, the rate of growth

as measured by the assimilation of carbon dioxide is greatly reduced

but no corresponding change occurs in the transpiration. The result is

inevitably a high water requirement.”



D. PURPOSE

OF THISh v m w

The purpose of this review is to survey the current status of knowledge on the effects of fertilizers on the evapotranspiration of plants and

the efficiency with which that water is used in dry matter and salable

crop production. Attention must be given both to conditions in which

water is nonlimiting and to conditions in which it is limiting for evapotranspiration. Also, the effects of fertilizers on root extraction of water

and on infiltration and runoff cannot be laid aside in any discussion of

the efficient use of water. The effects of water on efficient use of fertilizer

and nutrient uptake of plants are not reviewed here. The science of

evapotranspiration in relation to meteorological factors is now under

such active investigation that there is high probability that anything

written at this time will soon be outdated.

The brief reviews of fertilizers and efficient water use given by

Kelley ( 1954), Haise and Viets (1957), and Haise et al. (1960) show

that fertilizers can increase the efficiency of water substantially. However,

Richards and WadIeigh (1952) stated: “Present data seem to indicate

that large decreases in growth and yield resulting from deficiencies of

nutrients and moisture may cause only nominal increases in water requirement.” And further, “Adequate data are not available for a quantitative evalution, but the foregoing evidence indicates that the effect of

nutrient deficiencies and soil moisture stress on the water requirements of

crops is small.”

II. Definition of the Problem



A . DEFIXITIOS

OF TEAhfS



In the broadest and popular sense, efficiency in the use of water

for crop production means growing as much crop as possible for the

“water used,” whether that water is actual evapotranspiration, irrigation

water applied, or rainfall. Each of these has sigdcance in specific

situations. However, much confusion can and does arise unless the



FERTILIZERS AND THE EFFICIENT USE OF WATER



227



“water u s e d is defined. For example, water applied may include variable losses by runoff and/or deep percolation. Amount of rainfall does

not give information on how much water is lost by runoff or leaching.

Measured evapotranspiration is the only basis having broad application

for making comparisons of the effects of fertilizers on water use and on

the efficiency of that water use.

The terminology used in this review is similar to that employed

by Haise and Viets (1957); it is as follows:

Water requirement: The ratio of weight of water absorbed by the

plant during the growing season to the weight of dry matter produced

by the plant during that time. This definition ignores the quantity of

water metabolically used by the plant, which is relatively small. Briggs

and Shantz used water requirement and transpiration ratio interchangeably. See Haise and Viets (1957) for a discussion of the value of this

term.

Evapotranspiration or consumptive use: The sum of the volumes of

water used by plant growth in transpiration and evaporation from soil

or intercepted precipitation on an area in any specsed time, divided

by that area. Usually expressed as acre-feet or acre-inches per acre, or

more simply, feet or inches.

Water-use eficiency: The weight of dry matter or marketable crop

produced per unit volume of water used in evapotranspiration ( E T ) .

As used here for comparative purposes, the acre is usually the unit of

area and the inch is the unit of depth.

Water-use efficiency =



dry weight/acre



ET in acre-inches/acre



- dry



weight



ET in acre-inches



Metric units are often preferable. If both the dry matter and ET are

expressed in units of mass, then water-use efficiency becomes a true

ratio. In mass units, water-use efficiency is similar to the term “efficiency

of transpiration,” which Maximov (1929) says was first used by L. A.

Ivanov in a series of lectures at St. Petersburg in 1913. Effciency of

transpiration was defined as the grams of dry matter produced per

kilogram of water transpired.

The advantage of the term water-use efficiency is that the emphasis

is put on the water, which we are interested in using most efficiently.

Comparison of fertilizer effects on production per unit volume of water

and even comparisons on alternative use of water are easily made, i.e.,

alfalfa vs. corn, or even tons of sugar from sugar beets vs. tons of steel

from a steel mill. The term has at least one disadvantage unless care

is used in making comparisons. Increasing water-use efficiency may



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