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10/5/2013 7:00 AM

To Remediate Soil Compaction

Dry weather is pervasive across Pennsylvania this fall. This means that even many of those perpetually wet spots have dried up, opening an opportunity to alleviate soil compaction using subsoilers.

Extension soil management specialist Sjoerd Duiker points out that compaction leads to increased bulk density of soil, reduced porosity, increased penetration resistance, and reduced water infiltration and percolation.

Soil compaction also increases the potential for denitrification and ammonia volatilization due to reduced aeration and water infiltration. Uptake of potassium and phosphorus is reduced due to root growth inhibition and reduction in soil biological activity.

So it is important to minimize compaction. But common field activities using heavy equipment — especially when soil is wet — trampling by livestock and soil tillage may have caused soil compaction in the past.

To identify problematic soil compaction, use both soil and crop observations. The soil compaction tester, or penetrometer, is one means of compaction detection, but it is not useful now since soil conditions are too dry.

Instead, take a shovel and dig a hole with a vertical face perpendicular to the crop rows. Push a knife horizontally into the soil profile for about 1 inch starting from the top. You may be able to feel a distinct layer with higher resistance just below common tillage depth. If roots are clearly restricted at a certain depth this may call for action.

Also evaluate the soil structure. Compacted soil is massive and does not fall apart in small aggregates, restricting root growth to the cracks between clods.

Research has shown that our most common agricultural soils do not benefit from subsoiling unless they are severely compacted. However, if you decide that subsoiling is justified, it is time to decide which subsoiler to use and how to set it.

With the recognized benefits of surface residue preservation, modern subsoilers do not turn surface soil over. They therefore generally have narrow shanks, are not parabolic and may have attachments that actually help to keep residue in place.

Different types of subsoilers can be used without disturbing surface residue much. Some have large winged points that heave the soil and cause much fracturing of the soil, even between shanks. Others, such as the paratill unit, have bent-leg shanks that come down straight, then curve sideways on a 45-degree angle before the tip again turns downward.

Research at the Soil Dynamics Lab in Alabama has shown that paratill shanks do maximum subsurface fracturing, take less power per shank than straight shanks and do minimal surface residue disturbance.

Despite what some manufacturers may say, Duiker believes it is best to use subsoilers only under reasonably dry soil conditions. This means you should not be able to make a ball of soil by kneading it in your hand. The soil has to be that dry everywhere in the profile to the depth of tillage.

Contemporary subsoiling is meant to be a one-pass operation so that crops can be planted immediately after subsoiling without secondary tillage.

The choice of attachment is very important because it determines surface residue reduction to a large degree. Soil tends to blow out’ behind the shanks (especially when run at higher speeds) so attachments are available to push soil back to create a suitable seed bed.

To achieve soil conservation goals, more than 30 percent residue cover should be present after subsoiling and planting, so attachments should not cover residue but leave it on top.

Kickback mechanisms are another necessity on subsoilers. If they are not present, shear bolts will have to be replaced on a regular basis in our rocky soils, making subsoiling an arduous task.

Next is the depth to which the shanks should be set. The subsoiler should be set about an inch below a compacted layer. A tractor that can pull the subsoiler needs to be available. Depending on soil conditions, you should count on about 40-50 HP available per shank.

To Consider Taking <\n>Another Alfalfa Harvest

Not harvesting alfalfa 4-6 weeks (critical period) before the first killing frost has been a long-standing recommendation. Penn State forage specialist Marvin Hall tells us that is still very safe advice.

However, the dry weather some areas experienced in August may moderate that recommendation. Perhaps the risk of a cutting during that “critical period” is outweighed by potential increased profits.

Assessing the risks can be helpful in making the “best” decision. Older alfalfa stands are more prone to winter kill or to suffer winter injury following a fall harvest than younger alfalfa stands.

Alfalfa varieties with moderate resistance to several diseases and sufficient winter hardiness have greater tolerance to stress from fall harvesting than less disease-resistant or winter-hardy varieties.

Adequate soil pH and fertility minimizes the risk of fall harvesting by allowing alfalfa plants to develop properly and be healthier. Alfalfa on well-drained soils is less likely to suffer winter injury than alfalfa on poorly drained soils.

Alfalfa harvest schedules that do not allow the alfalfa plant to flower once during the season predispose the plant to winter injury. Dry weather, especially in August, causes alfalfa to store excess root energy reserves making it more winter-hardy. Leaving 6 to 8 inches of stubble when taking a fall harvest will reduce the risk of winter injury.

Quote of the Week

“The thankful receiver bears a plentiful harvest.”

— William Blake

Leon Ressler is district director of Penn State Cooperative Extension for Chester, Lancaster and Lebanon counties.

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