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II. Soil and Climatic Adaptation

II. Soil and Climatic Adaptation

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is warm, will emerge in 5 to 7 days (Smith et d.,1961; Nagata, 1960;

Hartwig, 1954 ). In addition to affecting rate of germination of soybeans,

temperature also affects rate of growth and the time required for the

plants to shade the ground between the rows, an important consideration

in weed control as well as yield. Smith et al. (1961) found that soybeans

planted on May 5 had shaded only 59 per cent of the ground in 2 months

but those planted on June 5 had shaded 86 per cent. Hartwig (1954)

found the rate of growth increased markedly as temperature at planting







M A Y 10

A P R I L 10

FIG. 3. Diagrammatic comparison of relative height and width at 6 weeks after

emergence for the average of four varieties of soybeans planted at Stoneville, Mississippi, April 10, May 10, and June 10, 1944-1951.

time increased. Figure 3 shows the relative growth 6 weeks after

emergence on the dates indicated.

Temperature affects blooming date, as pointed out by Garner and

Allard ( 1930). They stated that sustained summer temperatures below

75' to 77'F. will ordinarily delay blooming, a decrease of 1" causing a

delay of 2 or 3 days. Variation from year to year in date of flowering of a

given soybean variety planted on a particular date is due chiefly to

differences in temperature, whereas differences between varieties are due

chiefly to their response to length of day.

There is a minimum temperature for most growth processes, which

for all practical purposes appears to be about 50°F. Parker and Borthwick (1943) found that floral induction was greatly inhibited at 50°F.or



lower, and Brown (1960) found no growth of prebloom soybeans at

50°F. Using 50°F. as the base, Brown and Chapman (1961) calculated

heat units required to mature soybean varieties adapted to the upper

Great Lakes region. Good agreement was obtained between required

and available units for varieties grown in the northern part of the region,

but available units exceeded required units for varieties grown in the

southern part.








FIG.4. Mean maximum and minimum temperatures by 5-day periods at Urbana,

Illinois, and StoneviUe, Mississippi, during the growing season, 1948-1957. ( Source

of data: Climatological Data, Weather Bureau, U. S. Department of Commerce.)

High temperatures (over 100°F.) early in the season may have

adverse effects. For example, brief periods of high temperature greatly

reduce the rate of node formation and the rate of growth of internodes

(Howell, 1956). There is some indirect evidence that varieties differ in

their temperature requirements and that some are adapted to higher

temperature conditions than others. Varieties such as LEE grow well and

produce good quality seed at Stoneville, Mississippi, where seed of CLARK

is usually inferior. Green (1961) has shown that seed quality is adversely

affected by high temperatures during seed development. At Urbana,



Illinois, where am^ produces excellent seed, it matures when maximum

temperatures are approximately 70" and minimum temperatures around

50" (Fig. 4 ) . At Stoneville, it matures when maximum temperatures are

90" and minimum around 67". Although other factors are involved in

this, it appears that temperature may be a predominant one. The variety

LEE makes excellent growth and produces high yields of good quality

seed at Brawley, California, where daily maximum temperatures are

above 100°F. for June, July, August, and September.

2. Effect of Temperature on Composition of Seed

Temperature is one of the basic elements of the environment that

influences storage of oil in the seed. Howell and Cartter (1953, 1958)

showed that temperature during certain portions of the pod-filling period

was correlated with oil percentage in the mature seeds of soybeans

produced in the North Central and Gulf coast areas. The highest

correlation coefficients were obtained for the periods 20 to 30 and 30 to

40 days before maturity, results indicating that temperatures during these

periods exert a greater effect on oil level than those at other times. This

influence was subsequently studied under controlled conditions where

soybean seed produced in the greenhouse contained 22.3, 20.8, and

19.5 per cent oil when grown at temperatures of 85", 77", and 70"F.,

respectively, during the pod-filling stage,

To further determine the period of maximum sensitivity of oil

formation to temperature, the effect of a brief period of elevated

temperature was measured. One week of elevated day temperature

during the fourth to seventh week before maturity produced seed with an

oil content of about 22 per cent, compared to 19.6 per cent when

temperatures were elevated the second week before maturity. Thus, the

period of greatest temperature influence occurred prior to the period of

most rapid oil synthesis in the seed, a sequence indicating that the

influence of temperature was on the establishment of the metabolic

system for the conversion of sugars to oil rather than on a specific reaction


hlost of the studies by Howell and his co-workers have been with

daytime temperatures between 70" and 85°F. When temperatures under

these controlled conditions were raised to W",there was a seed yield and

oil content reduction indicating that the optimum temperature had been


3. Effect of Light on Plant Growth

Light is the source of energy for photosynthesis, as well as the control

of many plant growth processes. Light saturation of photosynthesis in



individual soybean leaves is at about 2200 foot-candles (Bohning and

Burnside, 1956), which is about one-fifth of the intensity of sunlight

at midday in the central part of the United States. By the time the plants

have reached an appreciable size, most of the leaves are receiving a

light intensity far below this value and by the time the foliage covers

the row, light intensity has been reduced to probably 2 or 3 per cent of

this value on the lower leaves.

During the flowering period, soybean plants produce three to four

times the number of flowers that finally develop into pods, the number

of pods that are finally set on the plant depending upon the vigor of the

plant during the time of blooming. If the plants are shaded during this

period, the proportion of pods that abort will be much higher, possibly

owing to lowering of the sugar level in the leaves or other imbalance in

the plant system. This effect of shading is of much significance in

considering the importance of the natural variations in light intensity

associated with periods of cloudiness, especially at critical periods in

plant development.

4. Effect of Photoperiod on Flowering and Maturity

In addition to furnishing energy, light also serves an important

function in regulating blooming and maturity. Our present extensive

knowledge of the effects of day length on flowering goes back more

than fifty years: Mooers (1908) concluded that the agreement in length

of season required by the same variety to reach maturity in the different

years when planted at a given date is striking, and that there is not only

a steady shortening of the season of growth as the date of planting is

made late, but also that this shortening is much more marked in some

varieties than in others. Garner and Allard (1920) recognized the significance of day length in the flowering behavior of soybeans and other

plants and termed it photoperiodism.

Later studies by Parker and Borthwick (1950), using different periods

of artificial light and darkness, determined that the length of the period

of darkness was the controlling factor. Since a soybean variety flowers in

the field only when the days are shortened below a critical value for the

variety, soybeans are called short-day plants. This photoperiodic response

is an important factor in soybean production. Research on photoperiodism

has been reviewed several times in recent years (Hamner, 1938, 1944;

Murneek and Whyte, 1948; Parker and Borthwick, 1950; Leopold, 1951;

Lang, 1952; Liverman, 1955; Doorenbos and Wellensiek, 1959; and

Howell, 1960).

One well-known example of photoperiodic effect in soybeans is the

delay in date of blooming and maturing of a soybean variety as it is




moved north. Figure 5 shows how the maximum day length on June 21

vanes with latitude, the difference amounting to some 80 minutes between

the latitude of Urbana, Illinois, and that of Winnipeg, Manitoba.

Assuming that a full-season variety at Urbana (CLARK, for example)

flowers around the first of July when the day length is about 15 hours,

this same shortness of day (long dark period) would not be reached in

the vicinity of Winnipeg until around August 10. A corresponding delay


















= 10






growing season. (Source of data: The American Ephemeris &d Nautical AlGanac

for the Year 1936. U. S. Government Printing Office, Washington, D. C., 1934.)

in maturing would mean that the variety would not ripen before frost

at the higher latitude. Actually, CLARK will be frosted most seasons when

grown at Madison, Wisconsin, only 3" north of Urbana.

The rather precise plant response to latitude is illustrated in Table 11,

which shows the average maturity date for the soybean variety LINCOLN

at several locations (Cartter, 1958). This delay in maturity illustrates

why soybean varieties are said to be adapted to rather narrow belts of

latitude. It becomes evident that the terms early-, medium-, or late-

37 1


maturing, when describing a soybean variety, are meaningless except

when related to a specific location and uniform planting date.

Johnson et aZ. (1960), using equipment in which the photoperiod

could be closely controlled, have studied rate of development throughout

the life cycle of the plant. They reported that under a d c i a 1 control

where similar day-length experiments differed by as much as a month in

planting date, the periods from emergence to flowering were essentially

identical, though the plants were subjected to quite different and

fluctuating environmental conditions. Only when the temperature drops

below an optimum value does it begin to play a major part compared to

photoperiod in determining rate of development following floral initiation.


Effect of Location on Maturity Date of



Madison, Wisconsin

DeKalb, Illinois

Dwight, Illinois

Urbana, Illinois

Eldorado, Illinois

Sikeston, Missouri

Stoneville, Mississippi

42" 34'

41" 50'



41" 8'

40" 8'

37" 52'

36" 23'

33" 25'



Oct. 2

Oct. 1

Sept. 27

Sept. 17

Sept. 8

Aug. 30

Aug. 12

In addition to control of blooming and maturing, the photoperiod

reaction also controls other functions of the plant. Soybeans produced

longer internodes when the plants entered the dark period with the

photosensitive pigment system predominantly in the red-absorbing formthat is, under incandescent supplemental light high in infrared than under

fluorescent light high in the red region of the spectrum. With either a

12-hour or 16-hour photoperiod, the stem length of AGATE, a very early

strain, was doubled under the rich infrared light and BILOXI, a late strain,

increased one-fourth when compared to plants exposed to fluorescent

light (Downs, 1959).

5. Efect of Soil Moisture on Growth

The period of germination is critical for soybeans; then excess

moisture or prolonged drought may be injurious. After the plant is

established, it withstands short periods of drought and is not seriously

retarded in growth nor reduced in yield by a wet season, provided weed

growth is controlled.

Runge and Ode11 (1960), in corn, corn, corn, soybean rotation at

Agronomy South Farm, Urbana, Illinois, 1909 through 1957, found that



above-normal precipitation during July (period of major vegetative

growth) and from mid-August to midSeptember ( grain-filling period)

increased soybean yields, but abundant rainfall during other periods

decreased yields. In early spring there is normally plenty of moisture

and above-average precipitation is detrimental. In the first half of August,

soybeans are in the early pod stage. An inch of rain above the average

during the last week of August or the first week in September increased

soybean yields nearly 2 bushels per acre.

Moisture deficit for 2 to 4 weeks immediately after flower bud

differentiation reduced vegetative growth and caused heavy flower and

pod dropping, according to Fukui and Ito (1951). They also observed

that a sudden increase to high moisture after a severe drought caused

another severe pod drop. A water table in the root zone is injurious,

especially if it is near the surface early in the season and during the late

fall (Fukui et al., 1954). Also, short periods of excessive moisture supplied

to the plants after the period of bud differentiation resulted in very poor

yields (Fukui and Ito, 1952).

It has been shown that the crop is more susceptible to drought injury

during the pod-filling period than during earlier stages of growth. An

early and a late variety may be affected very differently at a given

location during a period of moisture tension, in accordance with the

stage of pod development at the time (Howell, 1956). This varietal

interaction to a short period of drought in late summer is frequently

observed in the evaluation of soybean strains and must be taken into

consideration in a testing program.

111. Time of Planting and Varietal Adaptation


Probably no single cultural factor is more important to soybean

production than planting date. The effect of planting date was observed

by Mooers (1908), in Tennessee, who planted several soybean varieties

at several dates. When planting was delayed from May 15 to July 15, the

delay in maturity of MAMMOTH YELLOW, a late variety, was only 19 days

for the 60-day delay in planting, but for ITO SAX, an early variety, the

delay was 52 days. Twelve years later, the significance of day length in

the flowering behavior of soybeans was described by Garner and Allard

(1920), who explained these phenomena as changes in day length

accompanying changes in planting date. Time of planting was shown by

Johnson et aZ. (1960) to affect development of the soybean plant at

various stages through day-length response.



I. Maturity

Date-of-planting studies have been conducted in nearly every area

where soybeans are extensively grown. In the Midwest, the average

maturity date was retarded approximately 1day for each 3 days’ delay in

planting, although the varieties behaved differently ( Weiss et al., 1950;

Osler and Cartter, 1954). In Illinois, using four dates of planting from

May 1through June 12, the maturity date of the genetically later strains

was affected less by delay in planting than was that of the earlier strains.


the earliest variety, was delayed 16 days for a 43-day delay

in planting, but for WABASH, the latest variety, maturity was delayed only

8 days.

Workers in the southern part of the United States have reported

similar results for date-of-planting studies. In Virginia, LEE, a late

variety, was delayed only 8 days at Warsaw and 10 days at Petersburg

for a 61-day delay in planting (May 5 to July 5), while CLARK, an early

variety, was delayed 21 and 29 days, respectively (Smith et al., 1961) . In

Mississippi, WABASH, a very early strain for that area, matured August 20

when planted April 10, and September 22 when planted June 20, a delay

of 33 days for a 72-day delay in planting. For the same planting dates,

ROANOKE, a late variety, showed only a 5-day difference in maturity

(Hartwig, 1954). Since in Mississippi the maturity date of many strains

is essentially unaffected by planting dates from April 10 to May 10,

dividing the acreage between two adapted varieties of different maturity

is a surer method of spreading harvest dates than planting one variety

over a range of planting dates.

Leffel (1961), in Maryland, using several varieties of group IVYV,

and VI maturity (Morse et al., 1949) at five dates, May 22 to July 18,

concluded that the duration of the periods from planting to first flower,

of flowering period, and from termination of flowering to maturity

decreased with each delay in planting date. The reduction in the intervals

flowering to termination of flowering, and termination of flowering to

maturity, tended to be similar for each of the maturity groups studied

(Fig. 6 ) . The decrease in the interval from planting to flowering, as a

consequence of delayed planting, was greatest for the late varieties. These

conclusions are in substantial agreement with Mooers ( 1908), who

wrote that his field notes “show in extreme dates of planting of Mammoth

Yellow less than a week’s difference in the period between flowering and

maturity so that the variation in length of season takes place almost

entirely previous to flowering.” Somewhat similar results were shown by

Abel (1961) working under irrigated conditions in the Imperial Valley

of southern California.

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