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Give Sunflower Roots a Fighting Chance

Friday, April 15, 2005
filed under: Optimizing Plant Development/Yields

One of the most common perceptions is that sunflower will break up hard pan soil compaction. It’s true that sunflower roots have a stronger ability to break through compacted soils than other crops. And if sunflower has the proper condition to grow through the compaction layer, the plant leaves behind root residue channels that provide drainage and rooting paths for future cropping.

But there are times when even sunflower roots can’t break through a severe soil hard pan.

Last year showed considerable evidence of compaction impacts on a number of Colorado sunflower yields. Sunflower planted before June 10 went into very dry soil and had stands that no amount of rainfall could compensate for – some as low as 4,500 to 9,500 plants per acre. Sunflowers planted after the June 14 rain event germinated into 4 weeks of continued rainfall. Sunflowers – being as lazy as any plant – only developed deep roots in the absence of soil compaction.

Dryland yields in Colorado were variable – a number of dryland fields were 400 to 900 pounds per acre, with some no-till fields being half of neighboring tilled fields. At the same time, other producers saw their best yields ever. In order to make sense of those disparities, it is important to understand the physiology of the sunflower root system and its development.

As soon as the shoot of the sunflower seedling reaches the surface and starts photosynthesis, the taproot begins its journey downward. The growth point of the sunflower root is at its tip, across the full face. The growth of new cells on the tip exerts “hydraulic” downward pressure against the soil. As long as the soil particles yield under the available press, the root grows unimpeded.

If the root meets increased resistance, the root stops extending and reduces its tip diameter until it can push through the soil…or it finds a path of less resistance, often to the detriment of the plant. This means the sunflower plant has a more horizontal, shallow root system than what is desired, is less able to utilize available subsoil moisture and nutrients and more susceptible to other crop stresses such as insects and heat.

Generally, a good soil for crop production contains about 25% water and 25% air by volume, a 50% combination referred to as “pore space.” The remaining 50% consists of soil particles. And anything that reduces pore space can result in a dense soil with poor internal drainage and reduced aeration, James Frisby and Don Pfost point out in the University of Missouri extension ag engineering publication “Soil Compaction: The Silent Thief.”

This ratio of pore space and soil particles makes the issue of soil compaction easier to understand. Quite simply, compaction occurs when soil particles are pressed together and pore space is squeezed out of the soil. Without enough sizable pores, it can be difficult or nearly impossible for plant roots to grow and penetrate the soil. It can also be difficult for air, water, fertilizer and pesticide to infiltrate compacted soils, robbing plant roots of essential nutrients, water and oxygen.

Water acts as a lubricant for the soil particles to move out of the way of the advancing root while the air spaces make room for the soil particles to move into. The absence of either water or air space in the soil results in greater soil solidity, so even in the absence of compacted layers, it is important that sunflowers root down before moisture levels are depleted by soil drainage, evaporation, rising soil temperatures or plant uptake.

The time necessary for a root tip to resize and resume growing may be two to three days, and that time loss in root development can never be made up – resulting in lost yield potential. The evidence of resized roots can be seen in pulling plants and examining the root structure. The resized taproot will often be reduced 50% in diameter, which will give a similar reduction in water transport capacity. Unfortunately, by the time you can pull the plant and see the evidence, you have already lost out on the harvested rewards.

Soil compaction can be a problem even where it might not be obvious. The accompanying pictures were taken at the Akron, Colo. ARS station, and when the NuSun plot field was planted, no one thought that there was a problem. It certainly had not been apparent in other preceding crops, but at the field day it was obvious there were root problems.

Evaluating soil compaction

In the fall or while the planter is still in the shed are good times to look for compaction. There are several ways: pull a standing stalk from last fall and look. Dig out a shovel full of soil and see if it fractures on a line when it falls from the shovel. Make a cut down the side of a posthole with your pocketknife and feel for changes in resistance. Alternatively, take a tiling spade to the field and press down. When the shovel stops moving downward with pressure near the 4 to 6 inch mark below the soil surface, suspect a compacted layer.

Or, borrow a penetrometer from your extension agronomist, (penetrometer results are only numerically accurate when the soil is near water holding capacity). You can also make your own penetrometer (see page two of the following online publication: - be patient as it downloads. Also, see the article “Evaluating Soil Compaction” online at Click on the “Sunflower magazine” link, then “ view archives” and then “fertility.”)

If there is a problem, what can you do?

•Move a little dirt. I have often set the trash whippers deep to move the dry

dirt aside and let the opener cut a slice through the pan. Seeds germinated in the moist compacted soil will never have to size down later.

•Knock the rust off of the ripper. Subsoiling at an angle across the field is effective, but may not shatter all of the compaction. I found that after one ripping, about half of my plants might still show the symptoms of compaction.

•Create a root path. Try zone tillage, whether it is early pre-plant or at planting time, cutting a slot through the compaction beneath the seed row will promote the fastest root development, and it will drain rainfall deeper into the root zone where it is separated from high evaporation rates.

Keep in mind that when tilling to address compaction, rip only dry soil. Experience suggests that ripping wet soil will not fix the compaction issue.

– Dean Sonnenberg

Sonnenberg is a longtime sunflower grower from Fleming, Colo. He is president of the National Sunflower Association, and also runs Sonnrise Manufacturing, which builds sunflower harvest attachments and other custom equipment.

Online Resources to Learn More About Soil Compaction

Managing Soil Compaction, M Petersen, P Ayers, and D Westfall, Colorado State University

Soil Compaction: The Silent Thief, J Frisby, D Pfost, University of Missouri

Soil Compaction: Problems and Solutions, H Kok, R Taylor, R Lamond, S Kessen, Kansas State University

Several compaction related fact sheets can be found among the University

of Nebraska soils publications at this address. Choose “Search Publications” then enter “soil compaction.”

Soil Compaction – Causes and Consequences, a northern perspective from the University of Minnesota

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