Soils of the Milton Ranch
By Cliff Montagne
Introduction
Soils of the Milton Ranch form during the process of landscape transition, influencing the ecosystem and human system processes at the heart of the ranch’s existence. The Natural Resources Conservation Service (NRCS) Soil Survey of Musselshell County, Montana supplies background information on how the soils formed, describes soil mapping units, and provides extensive interpretations for uses, ranging from agriculture to recreation, wildlife, roads and buildings.
Soil Forming Factors
Soils are living and dynamic entities resulting from the interaction of “soil forming factors” operating on the landscape. Here we describe the traditional five soil forming factors and how they influence soils of the Milton Ranch.
Geologic Parent Material
The USGS bedrock geology map shows the east-west trending sedimentary layers. The more resistant sandstones tend to form ridges, leaving the less resistant silt and clay-rich layers to form the swales and gentler slopes. During the most recent glacial period, a tumultuous climate may have led to greater exposure of fresh rock and soil materials. The extensive alluvial deposits on the Milton Ranch suggest that the climate of the time, and possible lack of vegetation, may have led to much more active erosion and deposition than seen today. Erosion and deposition processes modified the topography from the original bedrock outcrops. Today on the ranch we can find bedrock weathered in place (residuum), as well as soil materials transported by water (alluvium), wind (loess), and gravity (colluvium). These materials have physical and chemical properties (such as particle size or texture, pH, and mineralogy) inherited from their bedrock parents, although biological activity and chemical processes have modified them to some extent.
Topography
The topography of the Milton Ranch is a direct reflection of the bedrock layers, and their resistance to erosion. As mentioned above, surface processes driven by wind, water, and gravity have modified the bedrock-derived topography to include alluvial fans, flood plains and slopes mantled with colluvium (soil materials moved downslope by gravity).
On the Milton Ranch, where moisture and temperature limit plant growth and biological activity, colder and moister north facing slopes may have darker surface horizons and produce more vegetation than drier south-facing slopes. Soils on the flatter alluvial deposits are often most ideal for capturing and retaining water for the growing season.
Climate
The post-glacial semi-arid climate has supplied sufficient moisture to support the grasslands of the ranch, along with limited forested areas. There has been enough water to support the wetlands, now hotspots of biological activity. Calcium carbonate and other salts, which limit biological activity, have been leached out of the topsoil by the annual precipitation.
Biotic Activity
Over time since glacial periods, plant growth has provided raw organic matter to grazing animals and to the soil surface where it can decompose and form humus. The dark color of humus gives clues to soil productivity. The organic matter richness provides fuel for microorganisms and stickiness leading to formation of “granular” structure in the soil surface zone. Granular (or crumb) structure is highly desirable for its ability to provide pore space for air circulation and gas exchange and to supply water for plants and other biota. The dark colored “bank” of organic matter provides the biological capital needed to support each succeeding year’s plant growth. We think of this organic matter as having accumulated over the past 10,000 or so years of post-glacial grassland productivity.
Time
These soil forming processes act together on landscapes over time. Where surface processes have been recently active, such as active floodplains, the soils are very young, with little development. In more stable landscape positions, the soils reflect the organic matter and development of soil structure expected over some 10,000 years of grassland vegetation, and often have B horizons enriched by clay and/or calcium carbonate.
A Soils Transect
An informative way to learn about the soils is to describe a soils transect across major terrain features. On July 18, 2014, a group, including geologist Karen Porter and ranch owner Bill Milton visited the Big Wall to observe the landscape and its features. The following narrative combines observations and discussions in the field with information from the soil survey. Our transect starts just north of the Big Wall, ascends to the top of the Big Wall, and continues south into farmland soils (where we had parked our vehicles).
On the Soil Survey soils map for the Big Wall East Quadrangle, in Section 29, R 27E, T 10 N, we start our transect on the gentle slope north of the Big Wall with unit 34B, the Yamacall clay loam on 2-8% slopes. This loamy soil with few rock fragments is a cold and dry, poorly developed soil, which has few limitations for cropping (climate and slope steepness). The slope steepens as we cross into map unit 39C, the Dellpoint calcareous loam on 4-15% slopes.
Coming to the base of the Big Wall, we see the influence of sandstone as the texture changes from calcareous loam to the fine sandy loam of the Yamacall-Busby complex, unit 189E. These soils have relatively deep 6 inch A horizons. And finally as we stand on top of the Big Wall outcrop, we are on the Rentsac-rock outcrop complex, a soil with many rock fragments of high lime content which boils when you drop dilute hydrochloric acid on it.
During the field trip, Karen pointed out the sandy nature of the Eagle sandstone, and here we actually see and understand the concept of the “strike,” or compass direction of a bedrock ridge, along with the dip of the rocks, here some 10 degrees to the south. Paradigm thinking changes with advances of human understanding. Karen showed us small pore holes in the flat and smooth top of the Eagle escarpment we were standing on. She asked us what these holes reminded us of, and someone said, “Shrimp.” “Yes,” Karen said, “in the 1950’s and 1960’s the geology profession grew by accepting the paradigm that features now locked in rock formations million of years old may be similar to living, or active, features now present somewhere in our environment.” And so we are looking at shrimp bore holes, similar to what we see in today’s oceans. The shrimp poop clay to provide granular pieces to keep the hole open in the undulating environment of the sea floor.
Karen also pointed out the capped and resistant layer on the very top of the outcrop, the pattern of the caps, looking almost like slabs of slate stacked up in accordion style, repeated on and off into the hazy distance. She reminded us that this Eagle Sandstone cliff was once simply a flat expanse of beach sand, buried beneath the thick sequence of younger sediments. Ground water would have percolated through this layer cake, providing opportunity for silica or other minerals to settle out, or precipitate, as cement.
The conversation turned to the soils, the products of nature redistributing and weathering the mineral grains of the sandstones, shales, and clay stones of this layer cake of rocks deposited during the era of the North American inland seas. Living organisms, plants, animals, insects, and microbiota, evolved with the changing landscape as the seashores retreated and the land emerged to provide bedrock for erosion and redistribution. This is not only a bedrock story, but the story of the ice ages and climate changes of the Pleistocene when this landscape was apparently subject to more erosion and slope wash than today. A tremendous amount of erosion was required to strip thousands of vertical feet of bedrock from this site and transport it downstream, ultimately to the Gulf of Mexico. Now this landscape, from bare sandstone outcrops, to steep slopes mantled with sandy debris, to clay-rich swales of the least resistant shales, provides the base for the biota we see today.
In this climate, temperature and moisture impose limits on biological activity. Plants depend on soil properties and landscape position to access water when the temperature is warm enough for biological activity. When water falls on the land surface, it can evaporate or percolate into the soil to be available for plant growth or microorganism activity. The water molecule is like a micro magnet, with one side charged positively and the other side charged negatively. In soil pore spaces water molecules can be attracted to surfaces of organic matter and soil particles as the result of weak electrical activity between molecules. One water molecule attracted to a soil particle then attracts other water molecules, forming thin films of water which can be accessed by plant roots and soil microorganisms. Water can be either plant-available or plant-unavailable, depending on the how tightly it is attracted in the soil pore spaces. Soils with loamy textures often have greater available water-holding capacities than clay-rich or sand-rich soils.
Moving south, we walk on the Twilight-Blacksheep sandy loam 132B and then the Twilight-Blacksheep sandy loam with 2-8% slopes.
Our transect ended on Map Unit 52B, the Eapa loam. This soil occupies alluvial fans and terraces. It has a thin A horizon 0-5in, but a clayey B horizon above a zone of calcium carbonate and gypsum accumulation. This soil has no cropland hazards, and is able to produce 28-32 bushels of wheat or 1.4 tons of alfalfa per acre. As the Miltons know, these soils are rated as severe for roads because of their slippery gumbo properties when wet. They are also not great building sites because of their high shrink-swell properties. They often have a sufficient salt content to quickly corrode anything made of metal placed in the ground.
Human Dimensions
The Milton Ranch is known as a “working landscape.” We imagine that it may have been more productive before European settlement, and know of the subsequent erosion and degradation due to crop farming, overgrazing, and drought. Now it is inspirational and heartening to see that planned grazing, and perhaps tools like “cocktail” cover crops made up of a wide variety of plants, can stimulate biotic activity and reverse past soil degradation. We can look forward to more soil building management activities which can bring the landscape back to a more productive working landscape, with room for wild and domestic species. We also note the critical role active participation plays in encouraging human connection with the landscape, its soils, and the biota the soils support. People, be they owners, managers, agency officials, or neighbors are more supportive of unfamiliar range management methods if they have been out on the landscape and participated in activities to learn about and evaluate the health of the land. Intimate on-the-ground experience leads to understanding of soil health properties such as soil surface structure and vegetative cover. In Holistic Management Ecosystem Process terms, Energy Flow, Water Cycle, Mineral Cycle, and Community Dynamics provide windows into the health and productivity of the landscape and its soils.
Soils of the Milton Ranch form during the process of landscape transition, influencing the ecosystem and human system processes at the heart of the ranch’s existence. The Natural Resources Conservation Service (NRCS) Soil Survey of Musselshell County, Montana supplies background information on how the soils formed, describes soil mapping units, and provides extensive interpretations for uses, ranging from agriculture to recreation, wildlife, roads and buildings.
Soil Forming Factors
Soils are living and dynamic entities resulting from the interaction of “soil forming factors” operating on the landscape. Here we describe the traditional five soil forming factors and how they influence soils of the Milton Ranch.
Geologic Parent Material
The USGS bedrock geology map shows the east-west trending sedimentary layers. The more resistant sandstones tend to form ridges, leaving the less resistant silt and clay-rich layers to form the swales and gentler slopes. During the most recent glacial period, a tumultuous climate may have led to greater exposure of fresh rock and soil materials. The extensive alluvial deposits on the Milton Ranch suggest that the climate of the time, and possible lack of vegetation, may have led to much more active erosion and deposition than seen today. Erosion and deposition processes modified the topography from the original bedrock outcrops. Today on the ranch we can find bedrock weathered in place (residuum), as well as soil materials transported by water (alluvium), wind (loess), and gravity (colluvium). These materials have physical and chemical properties (such as particle size or texture, pH, and mineralogy) inherited from their bedrock parents, although biological activity and chemical processes have modified them to some extent.
Topography
The topography of the Milton Ranch is a direct reflection of the bedrock layers, and their resistance to erosion. As mentioned above, surface processes driven by wind, water, and gravity have modified the bedrock-derived topography to include alluvial fans, flood plains and slopes mantled with colluvium (soil materials moved downslope by gravity).
On the Milton Ranch, where moisture and temperature limit plant growth and biological activity, colder and moister north facing slopes may have darker surface horizons and produce more vegetation than drier south-facing slopes. Soils on the flatter alluvial deposits are often most ideal for capturing and retaining water for the growing season.
Climate
The post-glacial semi-arid climate has supplied sufficient moisture to support the grasslands of the ranch, along with limited forested areas. There has been enough water to support the wetlands, now hotspots of biological activity. Calcium carbonate and other salts, which limit biological activity, have been leached out of the topsoil by the annual precipitation.
Biotic Activity
Over time since glacial periods, plant growth has provided raw organic matter to grazing animals and to the soil surface where it can decompose and form humus. The dark color of humus gives clues to soil productivity. The organic matter richness provides fuel for microorganisms and stickiness leading to formation of “granular” structure in the soil surface zone. Granular (or crumb) structure is highly desirable for its ability to provide pore space for air circulation and gas exchange and to supply water for plants and other biota. The dark colored “bank” of organic matter provides the biological capital needed to support each succeeding year’s plant growth. We think of this organic matter as having accumulated over the past 10,000 or so years of post-glacial grassland productivity.
Time
These soil forming processes act together on landscapes over time. Where surface processes have been recently active, such as active floodplains, the soils are very young, with little development. In more stable landscape positions, the soils reflect the organic matter and development of soil structure expected over some 10,000 years of grassland vegetation, and often have B horizons enriched by clay and/or calcium carbonate.
A Soils Transect
An informative way to learn about the soils is to describe a soils transect across major terrain features. On July 18, 2014, a group, including geologist Karen Porter and ranch owner Bill Milton visited the Big Wall to observe the landscape and its features. The following narrative combines observations and discussions in the field with information from the soil survey. Our transect starts just north of the Big Wall, ascends to the top of the Big Wall, and continues south into farmland soils (where we had parked our vehicles).
On the Soil Survey soils map for the Big Wall East Quadrangle, in Section 29, R 27E, T 10 N, we start our transect on the gentle slope north of the Big Wall with unit 34B, the Yamacall clay loam on 2-8% slopes. This loamy soil with few rock fragments is a cold and dry, poorly developed soil, which has few limitations for cropping (climate and slope steepness). The slope steepens as we cross into map unit 39C, the Dellpoint calcareous loam on 4-15% slopes.
Coming to the base of the Big Wall, we see the influence of sandstone as the texture changes from calcareous loam to the fine sandy loam of the Yamacall-Busby complex, unit 189E. These soils have relatively deep 6 inch A horizons. And finally as we stand on top of the Big Wall outcrop, we are on the Rentsac-rock outcrop complex, a soil with many rock fragments of high lime content which boils when you drop dilute hydrochloric acid on it.
During the field trip, Karen pointed out the sandy nature of the Eagle sandstone, and here we actually see and understand the concept of the “strike,” or compass direction of a bedrock ridge, along with the dip of the rocks, here some 10 degrees to the south. Paradigm thinking changes with advances of human understanding. Karen showed us small pore holes in the flat and smooth top of the Eagle escarpment we were standing on. She asked us what these holes reminded us of, and someone said, “Shrimp.” “Yes,” Karen said, “in the 1950’s and 1960’s the geology profession grew by accepting the paradigm that features now locked in rock formations million of years old may be similar to living, or active, features now present somewhere in our environment.” And so we are looking at shrimp bore holes, similar to what we see in today’s oceans. The shrimp poop clay to provide granular pieces to keep the hole open in the undulating environment of the sea floor.
Karen also pointed out the capped and resistant layer on the very top of the outcrop, the pattern of the caps, looking almost like slabs of slate stacked up in accordion style, repeated on and off into the hazy distance. She reminded us that this Eagle Sandstone cliff was once simply a flat expanse of beach sand, buried beneath the thick sequence of younger sediments. Ground water would have percolated through this layer cake, providing opportunity for silica or other minerals to settle out, or precipitate, as cement.
The conversation turned to the soils, the products of nature redistributing and weathering the mineral grains of the sandstones, shales, and clay stones of this layer cake of rocks deposited during the era of the North American inland seas. Living organisms, plants, animals, insects, and microbiota, evolved with the changing landscape as the seashores retreated and the land emerged to provide bedrock for erosion and redistribution. This is not only a bedrock story, but the story of the ice ages and climate changes of the Pleistocene when this landscape was apparently subject to more erosion and slope wash than today. A tremendous amount of erosion was required to strip thousands of vertical feet of bedrock from this site and transport it downstream, ultimately to the Gulf of Mexico. Now this landscape, from bare sandstone outcrops, to steep slopes mantled with sandy debris, to clay-rich swales of the least resistant shales, provides the base for the biota we see today.
In this climate, temperature and moisture impose limits on biological activity. Plants depend on soil properties and landscape position to access water when the temperature is warm enough for biological activity. When water falls on the land surface, it can evaporate or percolate into the soil to be available for plant growth or microorganism activity. The water molecule is like a micro magnet, with one side charged positively and the other side charged negatively. In soil pore spaces water molecules can be attracted to surfaces of organic matter and soil particles as the result of weak electrical activity between molecules. One water molecule attracted to a soil particle then attracts other water molecules, forming thin films of water which can be accessed by plant roots and soil microorganisms. Water can be either plant-available or plant-unavailable, depending on the how tightly it is attracted in the soil pore spaces. Soils with loamy textures often have greater available water-holding capacities than clay-rich or sand-rich soils.
Moving south, we walk on the Twilight-Blacksheep sandy loam 132B and then the Twilight-Blacksheep sandy loam with 2-8% slopes.
Our transect ended on Map Unit 52B, the Eapa loam. This soil occupies alluvial fans and terraces. It has a thin A horizon 0-5in, but a clayey B horizon above a zone of calcium carbonate and gypsum accumulation. This soil has no cropland hazards, and is able to produce 28-32 bushels of wheat or 1.4 tons of alfalfa per acre. As the Miltons know, these soils are rated as severe for roads because of their slippery gumbo properties when wet. They are also not great building sites because of their high shrink-swell properties. They often have a sufficient salt content to quickly corrode anything made of metal placed in the ground.
Human Dimensions
The Milton Ranch is known as a “working landscape.” We imagine that it may have been more productive before European settlement, and know of the subsequent erosion and degradation due to crop farming, overgrazing, and drought. Now it is inspirational and heartening to see that planned grazing, and perhaps tools like “cocktail” cover crops made up of a wide variety of plants, can stimulate biotic activity and reverse past soil degradation. We can look forward to more soil building management activities which can bring the landscape back to a more productive working landscape, with room for wild and domestic species. We also note the critical role active participation plays in encouraging human connection with the landscape, its soils, and the biota the soils support. People, be they owners, managers, agency officials, or neighbors are more supportive of unfamiliar range management methods if they have been out on the landscape and participated in activities to learn about and evaluate the health of the land. Intimate on-the-ground experience leads to understanding of soil health properties such as soil surface structure and vegetative cover. In Holistic Management Ecosystem Process terms, Energy Flow, Water Cycle, Mineral Cycle, and Community Dynamics provide windows into the health and productivity of the landscape and its soils.