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You Are Here Sunflower Magazine > Sunflower Root Growth


Sunflower Magazine

Sunflower Root Growth
April 2000

Grower experiences and scientific investigations have long testified

to sunflower’s capacity to root down to moisture in a dry summer. While

it may not merit the title of “Super Plant,” there’s no question that in

comparison to many other crops, sunflower’s rooting depth and ability to

extract moisture from lower in the soil profile at least justify the

adjective of “Superior.”

Several years of research by soil scientists Steve Merrill, Don

Tanaka and Al Black (now retired) of the USDA Agricultural Research

Service’s Northern Great Plains Research Laboratory near Mandan, N.D.,

have documented the sunflower plant’s impressive rooting abilities.

Their work specifically examined sunflower’s root growth behavior in

field experiments that compared (1) various tillage systems, including

fallow, and (2) sunflower versus other crops grown on the Northern

Plains.

The Mandan group used “minirhizotrons” to observe root growth.

Minirhizotrons are clear plastic tubes which are installed in the soil

(at a depth of up to six feet) to allow observations of root development

in a nondestructive manner. Miniature video camera equipment

(“microvideo”) was inserted inside the minirhizotrons every week to 10

days and systematically positioned every two inches down to the six-foot

depth. It magnified tiny (and short-lived) roots, thus providing the

researcher with a more meaningful measurement of root growth than can be

achieved by digging trenches for a one-time visual observation of roots.



The Mandan group conducted two series of sunflower root growth

experiments.

The first, conducted in 1992 and 1993, was in a long-term cropping

systems experiment using a spring wheat/winter wheat/sunflower

rotation. They looked at three tillage managements: conventional-till

(disk-based), minimum-till (undercutter-based) and no-till.

Minimum-till sunflower grown on fallow ground also was observed.

In the second series, conducted during 1995-97, they investigated

the root growth of sunflower as compared to that of spring wheat and six

other crops: soybean, dry bean, dry pea, canola, crambe and safflower.

Each crop was grown in a three-year minimum-till rotation after winter

wheat, which in turn had been preceded by spring wheat. The soil type

was a silt loam. Long-term annual precipitation at the south central

North Dakota test site averages just under 16 inches; seasonal is about

12.8.



That were the findings of the ARS investigations?

In 1992 they measured the relative distribution of sunflower root

growth during the period of maximum root length (flowering to

post-flowering plant stage). They found that at 6.1 feet, the sunflower

under no-till had a greater maximum rooting depth than did the ’flowers

under minimum-till (5.3 feet) or conventional-till (4.4 feet). Sunflower

on fallow ground had a maximum rooting depth of 5.5 feet.

Total root length growth was measured in both 1992 and 1993. Two

interesting — though not surprising — findings occurred. First, the

total length of roots peaks around bloom and seedset, and then declines

toward the latter part of the season as plants mature and dry down.

Second, signifi-cantly wetter soil conditions translate into less total

root length growth.

In the Mandan observations, total root length growth peaked at

approximately 10 miles of root per square yard under 1992’s “normal”

precipitation. In 1993, with seasonal rainfall more than double that of

1992, total root growth length peaked at about five miles of roots per

square yard.

Where, in the soil depth profile, was the median root length growth

— i.e., that point at which half of the root length growth was above and

half below? The greatest depths, both median and maximum, occurred with

the no-till and fallow treatments. The greatest median depth in 1992

(“normal” precipitation year) was 3.2 feet under no-till, while that of

1993 (“high precipitation” year) was 2.5 feet under fallow. As for

maximum depths, the deepest in 1992 was 5.3 feet under the no-till

treatment, while that of 1993 was 4.8 feet under fallow.

The 1995-97 root growth measurements included spring wheat,

sunflower, safflower, three annual legumes (soybean, dry bean and dry

pea), and two crops of the mustard family (canola and crambe).

“Out of the eight crops studied, our observations indicate that

sunflower is outranked only by safflower in terms of both maximum and

median depths of root growth,” the ARS group reports. The average

sunflower median root length depth was 2.4 feet across the three-year

period, while its maximum depth was 4.8 feet. Safflower’s median root

depth was 3.0 feet, while that of spring wheat was about 2.8 feet. The

median of the other crops ranged from 1.5 feet (dry beans) to 1.9

(crambe).



Of what value is this root growth information to producers of

sunflower and other Great Plains crops?

Procuring accurate, effective observations and measurements of the

dynamics of root growth of crops like sunflower gives researchers “an

immediate estimation of just where in the soil profile particular crop

plants are capable of extracting water and nitrogen resources, and just

when in the cropping season this capability of the crop species occurs,”

explains soil scientist Steve Merrill.

“We know in a general way from lts of past research that more

deeply rooted crops like sunflower can use more water and take mineral

nitrogen from deeper in the soil profile. But what is a really

‘practical’ application of this research information?”

In answering this question, Merrill points out that cropping

systems are very complex agro-ecological systems. The type of root

growth research he and his colleagues conducted, “is a key type of

‘linking’ information — linking together many different aspects and

‘submodels’ or ‘subroutines’ of practially oriented ‘user’ models and

decision tools on the one hand, and comprehensive research models for

use by scientists on the other.”

Another useful element of the root growth work, Merrill suggests,

“is the idea of our farmers getting compensated in some manner for

conservation tillage and no-till managements that cause cropping systems

to sequester more carbon in the soil.” How does carbon get into soils

in the first place? Predominantly through plant root growth. “So

accurate and effective modern research information about crop root

growth dynamics must be viewed as an important component of carbon

sequestration research,” the ARS scientist concludes.

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