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Soil compaction

by Dave Edwards 

It’s been said that “good farmers understand the value of well-tilled soil…and the dangers of over-tilled ground”. Soil compaction, resulting in what is commonly called a hardpan, has been recognized as a potential limiting factor in crop production since farmers transitioned from horse-drawn plows to heavier tractors and more aggressive tilling strategies in the early 1900’s. From a wildlife manager’s perspective, where planting food plots has become “standard practice” on most properties, it is important to learn and understand farming techniques and soil management practices. Doing so will not only help you grow successful food plots to enhance the wildlife value on your property, but will help you get the most out of your time, efforts, and money. Subsoiling is a soil management strategy commonly used by farmers to minimize soil compaction and maximize crop production, but also has application when managing quality food plots for wildlife. However, subsoiling is often overlooked and/or misunderstood by many food plot managers. In my travels as a wildlife consultant, I work on properties across the country and routinely see food plots where the manager has “planted by the book”, meaning they have properly amended the soil to ensure desired pH and adequate nutrients are available, prepared a smooth seed bed, and planted seed under ideal conditions, yet as the crop matures it becomes distressed or stunted. In many cases, the hunter/manager doesn’t recognize the ”underlying” problem because these plots produce respectable crops and generally do well until roots reach the compacted soil or the plot experiences a dry or wet period resulting in stressed or dead crops. More times than not, the culprit is an unseen layer of compacted soil lying slightly below the surface of the ground. Under these conditions, food plots are producing forage but do not experience optimal growth, or in some cases fail, essentially cheating the manager (and wildlife) of results for their efforts. By identifying and addressing soil compaction problems, managers can significantly enhance the growth and survival of a food plot allowing them, and the wildlife they are managing for, to gain the maximum benefit from their efforts.

Understanding Soil Compaction
Soil compaction results in a densely compacted layer of soil that lies between the topsoil and the subsoil. This compacted layer is often called “the hardpan”. Generally speaking, depth of hardpans vary with soil type and farming strategies but are often 4 -12” below the surface of the soil and are caused by the weight and pressure of tractors (and other equipment) on the soil and repeated disking/tillage over several years that loosens top soil. This causes the finest particles of soil (clay) to migrate downward, accumulate, and bind creating a very dense layer. In food plots, we commonly find hardpans 6 – 8” below the surface of the soil, which is coincidentally the depth most harrows plow/break the ground. It may help to imagine the hardpan as a layer of concrete below the surface of the soil.

Probing, water runoff due to compaction and compaction in different soil types

As you would expect, water and oxygen do not travel well through hardpans, thus during periods of excessive rainfall, water may lie in puddles on the surface of the hardpan then evaporate before it can seep down into the deeper subsoil. In some cases, standing water deteriorates and kills forages before it evaporates. Similarly, during periods of low rainfall, topsoil of food plots that have a hardpan dries out quickly because of its inability to draw moisture from subsoil resulting in stressed or dead crops. In my experience, the tell-tale sign of soil compaction issues is when food plot crops respond quickly to, or appear very sensitive to either a mild drought or significant rain event. That is, the food plot appears to get very dry or very wet easily, causing crops to stress. As such, these are usually the first food plots to start showing signs of drought during a dry period. Because soil compaction occurs below ground and out-of-sight, it is more difficult to detect than many other more obvious factors affecting forage growth and production. The old saying of “out-of-sight…out-of-mind” certainly holds true here as many managers don’t even think about it.

Detecting Hardpan
Hardpans can be easily detected in food plots using a soil probe, which is a 2 – 4’ metal rod – sharpened on one end to penetrate the soil, and a handle on the other end to assist in pushing the probe through the soil. You can make a probe out of rebar or purchase one from a forestry supply company for about $75. The best time to check for hardpans is when the soil is not extremely wet or dry. Insert the probe at various locations across the food plot. As the probe is inserted, the force required to move it through the soil should remain about the same unless a hardpan is reached. Upon hitting a hardpan, it will take much more effort to push the probe. It is important to understand that a “natural pan” normally exists between topsoil and subsoil layers of most soils, which will cause some resistance when probing the soil. This is the area where these two layers merge. However, a true “hardpan” will be very obvious as it is much more densely packed and harder to penetrate with a probe. From my experience, hardpans in food plots are often 6 – 8” below the surface, which is the depth at which most disks plow, and may be 2 – 10” thick, depending on soil type and age of the field. Also, hardpans can sometimes be detected while plowing a field. Because a hardpan is…well…hard, disks of a harrow will often bounce as they hit the hardpan. In some cases, disks may bite into the hardpan and bounce along the hardpan causing the tractor to jerk. These fields begin to get “wavy” as you ride across them due to the bouncing of the harrow while plowing. Although this is not a technical method of detecting a hardpan it is something good to know if you experience this in your food plots while plowing.

Breaking a Hardpan
Breaking the hardpan is often referred to as “subsoiling”, which breaks up the soil to depths of two feet and fragments layers of compacted soil allowing water and roots to penetrate into the subsoil. While there are a handful of different implements that can be used to subsoil, a subsoiling chisel plow – simply referred to as a subsoiler – is the most common. Subsoilers do not invert or turn the soil like a moldboard plow used to prepare the field to plant; they are simply very heavy duty steel shanks tipped with blades that drive deep into the soil and are pulled along to break the hardpan below the surface. While some soil is brought to the surface as the subsoiler is pulled along, the only evidence of disturbance on the food plot will be small trench-like furrows or lines left by the shank(s) of the subsoiler as it passes through the soil. As you can imagine, subsoiler plows cause significant soil drag and require a tractor or dozer with at least a 50 horsepower engine. A good rule of thumb is 10-15 additional horsepower is required per shank on a chisel plow. Most plows have 1-5 shanks that are set 9 – 12” apart. If you do not own a subsoiler, many companies rent them.

Due to the significant soil drag and horsepower needed to subsoil, which results in burning a good bit of tractor fuel, do not pull the subsoiler any deeper in the soil than what is needed. Deeper means more fuel which means more money. As previously mentioned, hardpans are normally 2 -10” thick. When assessing hardpans with a soil probe, you should be able to determine the thickness of the hardpan. As you push the probe into the soil, you will feel pressure as you reach the top of the hardpan. As the probe pushes through the hardpan and into the subsoil, the effort needed to push will be significantly less. Note the depth at which pushing gets easier as this will be the bottom of the hardpan and the depth you will need to set the subsoiler when plowing.

 

Keyline plow used during Keyline workshop in Alentejo

Keyline plow

Keyline plowing uses a subsoil plow method with a very flat plow shank (about 8%) to slice through the soil and create channels below the surface slightly off contour. These channels help break up soil compaction, create a place for new roots to grow with less effort and direct water more easily. Compost tea or EM can be inserted directly while plowing to optimise the process.

Keyline methods enable the rapid development of deep biologically fertile soil by converting subsoil into living topsoil. Keyline pattern cultivation enables the rapid flood irrigation of undulating land without terracing. Incidental results are the healing of soil erosion, bio-adsorption of salinity and the long term storage of atmospheric carbon in the soil as humus.

 

For more articles on wildlife management topics such as food plot, tree planting, predator control, fertilizing and liming, soil tests, supplemental feeding, fish stocking, weed and grass control in timber and ponds, timber thinning, and many others visit Wildlife Trends. They offer bi-monthly issues from wildlife managers and biologists that help you take the guess work out of your land management.

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