Subsoil: Digging Deep into Hardpan without Over-Tilling

Subsoil: The term implies tilling in an out-of-sight area called the “subsoil.” Building these zones may be the most important

Tractor plows farm land. Understanding why you till and how deep to till are important.

tillage practice, and it also may be the least understood. To understand why it is necessary to till the subsoil, we must first look under the ground into the real world of agriculture.

We will find mysterious things lying underground that can severely limit yields, regardless of how well we have managed the surface zone. Let us begin by taking a slice of the soil to see what is under there. The farmer should begin by looking for evidence of worm activity in the upper 6 to 12 inches of soil. Worms are a beautiful sight for those who understand their value. Their burrows enhance soil aeration and soil fertility. The burrow linings thrive with microbial activity. Once the soil passes through a worm’s digestive tract, the castings will be much higher in nutrients such as phosphorous and potassium than the nearby undigested soil.

We may also see a number of vertical holes that are formed by a different worm, commonly called the “night crawler.” These holes may be as large as 3/8 inch in diameter and extend from the surface to deep into the subsoil. In other areas we may not find night crawlers because while some soils may have them, others do not.

As we dig deeper, we often see the dark topsoil abruptly end. And as we dig more holes, we become even more aware of the various depths of the dark topsoil.

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Prairie grasses form a deep, dark topsoil because they have massive root systems that regenerate annually. The grass roots have decayed annually over thousands of years and have formed a black soil that is high in humus and extends to the depth the grass roots can grow.

Identifying the Hardpan

You will be able to tell where the natural pan starts by identifying where the black soil stops. The natural pan was in place before the prairie grasses became established. Prairie grass roots, just like the roots of our agricultural crops, could not penetrate the pan.

Unlike prairie soils, timber soils do not form a dark topsoil, for two major reasons.

First of all, grasses, with their extensive root systems, do not grow under the shade of a thick forest. And second, tree roots are woody structures that live for many years. As a result, there is little decay under the ground, and humus cannot accumulate. Trees drop their leaves onto the surface of the soil, and therefore do not form deep humus layers.

As we look into a 24- to 30-inch hole, we will see several different soil layers. The only exception to this would be if it is “muck” soil or has been windblown or water-deposited.

The Salatin Semester

Subsoil lies under the topsoil, and the soils finally grade into glacial till or glacial rubble that has not yet had time to transform into soil. As we look into the underground soil profile, we know that the soil has been there for 10,000 to 12,000 years, since the last glacier receded. During this time span, it has become what we call “middle- aged,” having a serious problem in the “waist” area.

Soil ecosystems are composed of combinations of various-size particles such as sand, silt and clay. Because of the different amounts of rain that fell over thousands of years, the water has moved the smallest particles downward to different depths.

This settling, plus chemical cementing, has formed a zone that is more dense than the soil above or below it. In most Midwestern soils, the bottom of this layer varies from 12 to a little over 20 inches below the surface. This belt-line bulging of finer particles is the reason these soils are considered “middle-aged.”

The primary reason for deep tillage is to make a slot through this dense, natural layer so that roots and water can enter freely into the subsoil. With this hardpan in place, we are farming with both roots and water up, on top of the hardpan, making our crops vulnerable to weather extremes of either flood or drought. As we carefully dig a cross-section of the soil, we may find roots making a right angle, running along the top of this hardpan.

This is a sight you will not forget, because it is convincing beyond doubt that a problem exists. This root- and water-limiting barrier may be our number-one yield-limiting factor, because it places a farmer at the mercy of weather extremes.

Testing for Tillage Depth

To deep-till correctly, a farmer must know the depth of the problem and make certain the zone-builder penetrates through it. Also keep in mind, when using either a penetrometer or knife, the soil must be moist. A pan cannot be found in dry soil because the profile will be too hard for either a knife or penetrometer to move through. Dry soil particles will not separate easily.

A penetrometer can be used to locate the depth of a hardpan. It can be slowly pushed into moist soil until a resistance point is reached. The resistance point will be the pan. The penetrometer can then be pushed until it abruptly breaks through. At this point, the bottom of the hardpan, or “breakthrough depth,” is reached and can be measured.

If it is difficult to find the pan with a penetrometer, we turn to a second instrument, a knife. A hole 2 feet or more in depth can be dug with a spade or posthole auger. Then a strong-bladed knife is inserted 2 to 3 inches into the sidewall near the bottom of the hole. It is pulled slowly upward, and as it reaches the pan, it will suddenly be nearly impossible to pull further. The knife test is the simplest and surest way to find the bottom of the pan.

Here are some rules to observe when building your zones:

  • Determine the depth of the natural pan to make certain the shanks of your zone-building equipment penetrate at least 1 inch below it.
  • Make a narrow slot through this zone without bringing subsoil to the surface.
  • There is no advantage in using a shank point more than 2 inches in width.
  • Build your zones when the soil is moist. Moist soil requires less horsepower and will cause less wear on the points.
  • With some exceptions, zone-building should be done in the fall.

Tilling in Clay Subsoils

Soils that crack open under dry conditions contain clay. Clay expands when it is wet and shrinks when it dries, to form drought or shrinkage cracks.

Drought cracks may be over 2 inches wide and extend down as far as 3 feet. While you may think this gives a subsoiling effect, it does not, because the clay expands when it becomes wet, and the crack will close again. Since clay is the smallest particle of soil, it is the one that settles downward first. In a clay soil, the pan will be so solid that it must be slotted through when the soil is moist and the clay expanded.

The shank on your zone-building equipment will remove an expanded clay core, leaving a void that can be filled with normal soil. When the soil next dries, the slot will become wider and will not close when it becomes wet. If you “shatter” this clay zone when it is dry, the shatter lines will swell shut when it rains, in the same manner as a drought crack.

With certain exceptions, we recommend zone-building in the fall on medium- to heavier-textured soils in order to prevent the severe wheel-track compaction that can occur if it is done in the spring.

Determining Slot Placement and Depth

Correct positioning of the slots is important. Let us assume you farm a corn-soybean rotation and both are planted in 30-inch rows. You also plan to pursue the entire zone-building system next season. Your first priority is to purchase the equipment necessary to begin zone-building. This first step should be done immediately after harvest, because the slots that are built will set the stage for a successful zone- management system for the crops that follow.

Make an all-out effort to cover your entire farm. This operation is so important that you should follow the combine, running the equipment around the clock if necessary.

Soybeans will normally be harvested ahead of corn. It is important to start zone-building on the soybean ground where corn will be planted the following year. Soybeans appear to stand drought a little better than corn, so by zone-building the soybean ground ahead of the corn crop, you will protect the corn from weather extremes.

This does not mean, however, that you should stop zone-building after completing the soybean stubble areas. You must also immediately work the cornstalks with the purpose of covering the entire farm.

Exactly where should the zone-built slots be placed? They should be placed directly into the center of the last year’s rows and directly under where the following year’s soybean rows will be planted. These narrow slots will be hard to see when you go to the fields next spring with your entire system.

Stay in the center of the rows while planting. Don’t think you can plant corn back into the old corn rows. This has often been tried, with a resulting loss of yield. Remember, our first Tillage Commandment states that we cannot allow our tillage system to place limits upon yields.

We want to leave the old corn or soybean root mass to decompose without being disturbed. These roots decompose to provide nutrient-laden passageways deep into the soil for the following season’s growing crops.

Let us assume you have zone-built slots on 30-inch spacings across your entire farm. Are you now through deep tilling? The answer is no, because you must start over the following year in the 1-year-old decayed rows. After this, you will have a slot every 15 inches.

Keep in mind that the top priority is to place a slot every 30 inches during the first year. The goal of 15 inches apart is not quite so urgent, but it should be completed even if it takes two or more years to finish.

Let us assume your soybeans are drilled. In this situation, you put markers on the zone-building equipment and begin slotting the drilled soybean stubble just as if you were operating the corn planter. Be certain to remember on which side of the field you started, and start planting on that same side.

You may think you do not drive straight enough to keep the slots under the planter rows. Don’t worry, because those slots will still do the job even if they wander 4 to 8 inches from the rows.

The 15-inch slot spacing is as important for drilled crops as for wide-row crops. The roots of every drilled row will enter these slots and plunge into the subsoil to provide the crop with true protection from weather extremes.

Another factor we must address is the depth of the tines. Remember that most farms have a natural pan with bottom levels that range between 12 and 20 inches from the surface. In most Midwest soils that were formed in place, we find the bottom of the pan at 16 to 18 inches. This can vary, depending upon erosion or soil deposition.

We explained earlier how to use the knife test to find the bottom of your soil’s pan. If the depth is 17 inches, you will need to make certain your zone-building equipment runs 18 inches deep. It is essential to penetrate through the pan so the roots and water can enter the subsoil.

Subsoil Benefits Under the Row

The third question inquires about the function of the slots. A complete understanding of these will hurry you into the fields to zone-till.

Let us begin by visualizing the zone-tilled area with slots directly under the rows and spaced 15 inches apart across your entire farm. In effect, there is a funnel under the rows that allows roots and water to be funneled directly into the subsoil below.

Imagine this funnel effect on the crop throughout the season. In the spring, the soils will warm much faster because the system manages soil aeration, soil water, soil organism activity and the adsorption of heat. It takes many more BTUs to heat water than to heat the same volume of soil.

By keeping the soil moisture at full-field capacity, soils will heat up very quickly in sunlight. Once the soil water exceeds the maximum capillary thickness that a soil particle can hold, the free water begins to accumulate in the soil’s air spaces. This causes the heat requirement for soil warming to increase dramatically and also prevents air exchange which further slows soil warming.

The “funnel effect” causes the water to soak into the subsoil to keep the lower water reservoir fully charged. This, along with deep roots, allows crops to produce abundantly during periods of drought, in comparison to those crops being farmed with their roots and water “up.”

On average, because of hardpans we farm only to a depth of 8 to 12 inches. Every time this top zone floods with water, all of the oxygen is driven from the soils. Roots cease functioning after two to four days of flooding, and nutrient and water uptake is drastically reduced. Worse yet, the root hairs on the outer root tips die.

Root hairs are the only part of the roots that adsorb nutrients and water. Certain mycorrhiza root fungi attach to the older roots and adsorb some nutrients in exchange for sugar from the root. These organisms also suffer when the air spaces flood with water. Anything that causes soils to become anaerobic can stop all nutrient uptake from the root hairs and symbiotic fungi attached to the roots.

Once the water drains and air reenters the soil, the root ends must begin to grow so they can develop new root hairs for nutrient and water adsorption. Soil life, which includes the root fungi, also takes time to recover.

This recovery time required for new root growth and soil life to return represents a loss of growing time. The plants have undergone stress and a degree of permanent damage that can reduce yields even though growth returns. Action must be taken to reduce the impact of this type of stress.

It is an interesting observation that depending upon the temperature, plants can thrive in flooded areas for approximately three days before they start to wilt while standing in water. It takes a few days for the oxygenated rainwater to run out of oxygen. Hydroponic plants can continue thriving while their roots are in nutrient-laden water as long as the water is oxygenated.

The point to be made is this: Every time soils flood or become waterlogged, we can easily lose seven to 10 days of growing time. If we can eliminate this loss of growing time by using a zone-tillage system, we can dramatically increase yields.

The final yield of a crop reflects the total amount of effective growing time it has during the season. We may be losing as much as 40 to 50 percent of a season’s growing time, because of the many yield-limiting factors we have failed to remove.

The slot that enters into the subsoil directly under the row removes many major yield-limiting factors. For example, corn growing over a such a slot will have its roots into the subsoil by the time it is 12 to 15 inches tall. These slots are the heart of the system and eliminate the most serious yield-limiting factors associated with air and water management.

By Donald R. Schriefer. This article was originally published in the October 2000 issue of Acres U.S.A.


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