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 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.  Back to Minimum Till/No-Till Stories Back to Archive Categories
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