Sunflower Root Growth
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
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
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
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
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
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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