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What You Need To Know About Dirt

While sometimes ignored by hunters, soil composition is among the most key influencers of plant growth.

What You Need To Know About Dirt

Photo by kram9/ShutterStock

Back in the 1970s, when I developed a wildlife management program within Stephen F. Austin State University’s School of Forestry, no course saw more student resistance than the one on soils. “Why do I need to know about dirt?” I often heard. Here, nearly a half-century later, I’ll answer that question.

I was trained in ecology and wild animal physiology/behavior. Early in my career, I became very interested in what makes individuals of a species larger in some areas than others. I also sought to find out why some areas are simply more productive than others. In forestry, we have a concept called “site index,” a measure of the potential of a specific location to produce wood fiber. Simply put, if an area has a designated site index of 70, it means a tree planted this year as a seedling will be 70 feet tall in a specified period of time (originally 70 years). So a property with an average SI of 100 is far more productive than one with an SI of 70. As the primary factor for tree growth is available soil moisture, soil type has the greatest influence in delivering it.

It occurred to me that we might be able to develop a site index for some wildlife species. I was one of the early wildlife scientists to integrate Geographic Information Systems (GIS) into my research and thought it possible. GIS uses overlays of various types of maps to identify areas with specified shared characteristics. For example, we can overlay maps of topography, vegetation type, soils and characteristics/demographics of the animal species being studied. I was developing the Institute for White-tailed Deer Management & Research, so this species was the one I targeted for study.

Searching the scientific literature, I found nothing even remotely resembling what I was trying to study. As I come from a long ancestry of German gamekeepers, I then decided to search German sources. One name kept popping up: Erhard Ueckermann. So I got my hands on everything he had written about managing wildlife, including Der Rehwild-Abschuss (The Deer Shot). This was his treatise on roe deer (Capreolus capreolus). Ueckermann related the abundance and size of roe deer to several variables, with soils being one of the critical factors.

So what about soils makes them so important? Let’s start with the basics.

Many elements are found in all soils, but their abundance varies widely. For growing whitetails, some of these are far more crucial than others. Photo by Amenic181/Shutterstock

There are three textural components of soils: sand, silt and clay. These are listed in order of the size of particles. Sands are the largest, being degraded rocks of various types. The color of the particles depends on the type of rock producing the sand. Silt is derived from tiny particles deposited by water in ancient times. Silts are of finer texture and, when rubbed between your fingers, feel like talcum powder. Lastly, clays are the smallest particles that can be highly charged so they stick to each other. In fact, they’re so sticky they often form an impermeable layer.

Sands have almost no ability to hold water; on pure sands you can run a water hose for hours without forming a puddle! Silt does a better job of holding onto water, while clays are very “jealous” of the water they contain. In fact, a pure clay soil can have significant water but not give any up to the roots of plants. Hence, both sands and clays can be very droughty. “Perfect” soil is said to be made up of one-third each of sand, silt and clay.

Organic matter added to the mix not only increases water-holding capacity but also improves available soil moisture in both sands and clays. The No. 1 factor affecting site index is the soil’s moisture-holding capacity and its willingness to provide water to growing plants. Exposure to sunlight can, in the case of sands, rapidly oxidize the organic matter, reducing water-holding capacity. In mixtures, the three particle types tend to move downhill inversely to particle size. In general, the best soils occur in bottomlands that aren’t frequently inundated by flooding.

Mineral Content

Soils are made up of chemicals (nutrients), which are classified as either macro- or micro-nutrients. These aren’t categorized by particle size but rather by the amounts used by plants and animals. The most common macro-nutrients include nitrogen, phosphorus and potassium (potash). These are particularly important to plants, as nitrogen is needed for leaf and stem growth, phosphorus for metabolic and reproductive processes and potassium for root growth. The three numbers you see on a bag of fertilizer (e.g., 12-12-12) roughly approximate the percentage of each of these (N-P-K) in the bag.

Nitrogen is available only in water-soluble forms such as ammonia, nitrates and nitrites. Elemental nitrogen isn’t available to plants or animals. Phosphorus, when present, can only be utilized with a pH value of close to 7 (neutral); too acidic or too basic soils “refuse” to give up their phosphorus. For that reason, we often distribute limestone on acid (low-pH) soils and sulfates on basic (high-pH) soils to improve phosphorus availability. Potassium is highly water-soluble and can be lost quickly from heavy rainfall.

As their categorization indicates, micro-nutrients are utilized in much lower amounts. Plants use a number of these, including boron (B), zinc (Zn), manganese (Mn), iron (Fe), copper (Cu), molybdenum (Mo) and chlorine (Cl).

They constitute in total less than 1 percent of the dry weight of most plants but are extremely important. Likewise, animals eating these plants utilize the micro-nutrients, plus selenium (Se), calcium (Ca), sulfur (S) and iron (Fe). Most of these interact with each other to form complexes affecting nutrition in ruminants such as deer.


Exact needs differ by species of ruminant. For example, deer in particular are what I call “copper hogs”; they require high amounts of copper, much higher than sheep and goats. In fact, custom feeds for deer can be deadly to sheep. For several years now, I’ve maintained that copper deficiencies also could be tied to the occurrence of Chronic Wasting Disease (CWD).

Soils can be too dry or too wet to benefit plant growth. Hilltops and ridges most commonly have the poorer soils, with less moisture availability. If you could cut a cross-section through a ridge leading down to a bottom, the height of the trees of the same age and species would be taller as you move downhill. This is due to the increase in both nutrients and moisture.

For several years we’ve studied the relationship between traditional site index for trees and the amount of forage grown on various soils and topographic locations. As with tree height, per-acre poundage of forage increases from the hilltop toward the bottom. Nutritional quality also increases with decreasing elevation.

Over this same time period, we examined other limiting factors for deer productivity. We now rank the nutritional needs of deer as: (1) digestible energy; (2) available phosphorus; and (3) digestible protein. Most landowners and deer managers tend to fixate on protein, yet in reality, it isn’t the most important factor. Deer can get along fine with no more than 17 percent crude protein, and older bucks probably need no more than 12 percent. Digestible energy (calories) is the most critical factor, followed by available phosphorus in the soil.

Phosphorus is used for a host of bodily activities, including cell division, reproduction, pH buffering and energy production. That’s why it’s considered a macro-nutrient. We routinely run soil analyses for phosphorus when evaluating properties for productive capacity for deer, and we find the vast majority are deficient.

When looking at a spot’s potential to consistently produce numbers of big deer, always study a soil map. Photo courtesy of USDA

Working with the late Dr. Jerry Stuth at Texas A&M University, we developed a way to test fecal pellets for dietary phosphorus intake, allowing us to assess how deer are doing nutritionally. This has let us home in on specific areas of a property where phosphorus is low. We then can ameliorate the deficiencies with fertilization.

In the northern portion of the Lower Peninsula of Michigan, we’ve found areas significantly deficient in magnesium. This deficiency translates both in livestock and deer to a condition known as “grass tetany.” Nursing females in particular are grossly affected and often die.

At Turtle Lake Club, manager Wayne Sitton and I discovered deer have developed a unique solution to the problem by using a natural mineral lick adjacent to the Thunder Bay River. This spot, which is rich in magnesium, had come to be known as the “Buck Lick” as far back as 100 years ago. It still attracts dozens of deer throughout the growing season.

The Best Soils For Big Deer

My friend and colleague Gordon Whittington recently has been looking at factors leading to large numbers of record-book bucks in certain regions. While a number of factors are involved, we find that many such bucks come from a band created by two geologic events. The first was the last Ice Age, which ended around 11,700 years ago. There also was an earlier ice age, ending roughly 55,000 years ago.

What do these events have to do with growing big deer? They were characterized by huge ice sheets moving southward, eventually reaching what are now Missouri and the Ohio River Valley. The large, slow-moving sheets changed the soils above and even below their southernmost extents.

Michigan, for example, has a significant amount of what we call “glaciated soils,” which are sandy and rocky in nature. They tend to have serious soil nutritional deficiencies, being nutrient-poor and very droughty. Both factors affect site index.

Soils just ahead of the ice sheet advances tend to be nutrient-rich. This stems not only from materials pushed ahead of the ice but also subsequent melting of the ice sheet, carrying rich nutrients downstream. Tied to glaciation, another significant climatic/geologic event had a profound influence on soil quality in North America. Post-glaciation, finely ground rock material (silt) was picked up during a particularly windy period and deposited across an area that includes Nebraska, Kansas, Iowa, Missouri, Illinois, Indiana, Ohio, Kentucky, Tennessee and Mississippi.

This unique soil type is called “loess” (pronounced “lers”), derived from the German word for “loose.” These are the most productive soils in the world and the primary reason these states are the prime agricultural area of the U.S. Loess deposits can be as thick as 20 meters and tend to be thicker as you move east and southeast from the Great Plains. The thickest occur along the Missouri River in western Iowa and in western Mississippi, along the namesake river.

Loess soils, deposited by eons of prevailing winds and influenced by ancient glaciers, give some areas an advantage in growing great deer forage. Map courtesy of

Loess soils are among the most fertile in the world. Their fertility is caused by the high silt content, which affects moisture available for plants. Site indices for loess soils also are among the highest in the world, translating to tall, straight trees, high forage yields and outstanding forage quality. If we combine maps of the ice sheet advance, distribution of loess and the distribution of Boone & Crockett bucks, there’s a remarkable correlation. There’s no doubt big bucks traditionally have come from areas with the best soils.

Of course, there are other hotspots for trophy bucks, but they too have significant fertility attributes that affect body size and antler quality. A notable example is South Texas, which is famous for big bucks. The areas in Texas producing high-end bucks also relate to the distribution of silt soils, deposited during a time when the region was inundated by a shallow sea. So as a result of their soils, some broad areas are more capable of producing big bucks, due to glaciation and loess/silt deposits. They aren’t the only places you can grow big deer, of course. Elsewhere, smaller areas with high-quality soils also have enhanced trophy potential.

Other Applications For Soil Information

As noted, the biggest deer are often associated with the best soils. High-quality soils also tend to support more deer, due to the increase in quantity and quality of forage, as well as added mast production. The soils-analysis techniques used to evaluate potential deer properties also can be used to home in on the most likely places for good bucks in general. The search could be for public land to hunt or for private land you’re considering leasing.

Of course, along with soils, on public land you also have to factor hunting pressure into your analysis; high potential for producing large numbers of deer doesn’t necessarily translate to the presence of many mature bucks.

Yet if you do your homework, there’s no reason you can’t identify specific areas where your odds are greatly increased. In selecting an area to hunt, I recommend you use the following criteria. First, a high proportion of bottomlands and drainages that aren’t frequently flooded. Next, highly productive soils. Then, diverse habitats. Finally, lower hunting pressure.

By using these criteria, as a hunter you can eliminate as lot of places and focus on those with the best odds. And putting the odds in your favor is what smart deer hunting’s all about.

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