2.1: Soil Physical Properties - Texture, Structure, Tilth

Why do we care about the physical properties of soil? Because these properties help determine how well a soil will grow good crops. Understanding our farm or garden soil texture and structure will help us make good decisions about how (or if) we can amend the soil to make it better, and at what cost. Soil's physical properties affect plant growth in many ways including:

1. Root penetration
2. Root aeration
3. Retention of nutrients, and
4. Water availability.

Soil's physical properties include its structure, texture, consistency (weight, pore space, and air), and color. The properties we will examine most closely are soil structure and texture. We will also define soil tilth and what that means for farmers and gardeners.

What is Soil's Physical Structure?

Soil's physical structure is important because if soil life cannot breathe and grow, it will die. The physical structure should be like a playground, a jungle gym for microbes, bugs, and plant roots so that they can move and wiggle and branch out and play hide and seek from predators. There should be equal parts open space for air and open space for water and the mineral scaffold of the monkey bars. The scaffold is delicate, almost lacy, almost as fragile as grapes. The scaffold of soil structure, like the bunch of grapes, is called an aggregate. We will look at structure again in more detail in a short bit after we define 'organic' soil and review what texture is.

Figure \(\PageIndex{1}\): The optimal soil structure is like a jungle gym or a bunch of grapes with open spaces. Images created by Microsoft Copilot, March 2025.

The physical nature of soil is intricately related to how soil behaves as a habitat for microorganisms. The better the structure, the better the habitat. Organic farmers want a lot of habitat.

Organic Soils (A Technical Definition)

There is a bit of confusion about what “organic” means. The name of the course I teach at Northeast Wisconsin Technical College is “Organic Soils, Nutrients, and Composting”. To be more precise, the course should have been named “Organically Managed Soils”. That is because there is another older, technical, definition of “organic” soil. This definition has nothing to do with whether or not synthetic chemicals or fertilizers have been used on the soil. Rather, it depends on how much decomposing carbon material (organic material) from plants is present in the soil. Mucks and peats are classic organic soils. Peat still contains some plant tissue structure (that is why it can be burned). Muck’s plant material is nearly completely decomposed. In these two true organic soils, peat and muck, plant material (organic matter) makes up about 20% of the weight soil, depending on how water-saturated the soil is. These soils are very thick and very deep. They store tremendous amounts of carbon.

For purposes of this book, however, unless specifically noted, “Organic soil” should be understood to mean “organically managed” soil, not muck or peat soils.

Soil texture

The texture of the soil is what we can feel when we rub the soil between our fingers. This depends on the size of the soil particles. The size of the particles in soils influences how productive the soil will be for growing plants. In mineral soils where the parent material is rock, there are three textures as seen in the table below.

Clay soils, because they have so many more tiny particles, have much, much more surface area than silt or sand. This large amount of surface area creates a huge reservoir of available area for nutrients and water films around each particle. Most soils are not ideal in texture. Areas that are very good for farming have a good balance of sand, silt, and clay. These loamy soils have good water-holding and nutrient-holding capabilities.

Figure \(\PageIndex{2}\): Soil aggregates from a dark clayey soil. Photo courtesy of Pepe Álvarez (Technical University of Cartagena, Spain).

Table \(\PageIndex{1}\). Soil textures. The dots are depictions of relative particle size, not actual size.

Sand	Silt	Clay
Large particles	Medium	Small
2.00-0.05 Size (millimeters)	0.05-0.002	0.002 and lower
		----------- clay particle size They are flat rather than rounded like silt or sand. When wetted, clay soils get slippery/sticky.
Water drains right through the large pores between particles. Sand is not weathered enough to release many nutrients.	The silt feels like powder and does not stick together well. It does contain mineral nutrients that are readily available to plants.	Clay mineral particles are so small they carry a slight charge that attracts water and nutrients in a film around each particle

In northeast Wisconsin, for example, we have some soil that contains relatively large amounts of clay. These soils stay wet longer than loams, and when they do dry out, they “shrink-bake” into a very hard surface. In some areas, the soils are very sandy (perhaps old lake shores). These soils do not hold water well at all. Farmers can plant sandy fields earlier in the spring because they dry out sooner than other soils, but by mid-summer, if the rainfall is scarce, then the moisture is gone and the plants will go dormant or die. When particles are larger than sand, and there are significant amounts of larger rock fragments, soils are said to be gravelly. Gravelly soils are typically not suitable for farming.

In our region, there are some small farms that had the misfortune of having all three types of soil running across their land in 'benches' or waves. It was impossible to farm these fields successfully because when the sandy areas were ready for planting, the wet areas were not. The wet areas did fine in dry conditions but the crops in sandy parts died. Consistent soil textures across a field help farmers manage them effectively.

Besides water-holding capacity, soil texture is important in the role of the nutrient reservoir. Nutrients will be discussed elsewhere, but the reader should be aware that soil texture is an important factor in relation to soil nutrients. Sand is the least “nutritious” soil and silt is a more fertile soil. Clay is actually a mineral formed by the recombination of weathered ions. It has a great amount of surface area and loads of nutrient ions.

Soil texture can be determined by several methods including the use of a hydrometer (a bulb) which floats in a standardized chemical solution. This texture test is performed in a laboratory. This is not a practical test for the average person. However, an average person can with some practice wet a small quantity of soil and ribbon it out between their fingers. Obviously, sandy soils cannot form a ribbon. Clays and silts do form ribbons with clay soils sticking together better than silt soils. If a child can make a clay pot out of the soil then the texture is definitely clay. If a ribbon feels gritty, then it is either a sandy clay loam or a sandy loam.

Texture Types

Various combinations of sand, silt, and clay form texture classes or types like those found on the triangle diagram. The ideal soil texture is classified as loam. A loamy soil has a combination of textures somewhere between 30-50% sand, 30-50% silt, and about 10-30% clay. A soil’s textural class, such as clay, clay loam, loam, sandy loam, or sand, is perhaps its most fundamental inherent characteristic, as it affects many of the important physical, biological, and chemical processes in soil. Soil texture changes little over time, no matter how the soil is managed, although we can have success in small ways in gardens.

Soils are classified by how much sand, silt, or clay they contain. The texture triangles shown in the diagram are a commonly used way to depict soil classifications based on texture. In the first triangle, each colored line represents 33% of a texture, you can see how soil is classified as a clay loam if it has equal amounts of clay (green line), silt (red line), and sand (blue line).

Figure \(\PageIndex{3}\). The percentages of sand, silt, and clay in the soil textural classes.

The textural class of soil (Figure \(\PageIndex{3}\)) indicates the coarseness or fineness of a soil’s particles. It is defined by the relative amounts of sand (.05–2 millimeters particle size), silt (.002–.05 millimeters), and clay (less than 0.002 millimeters). Particles that are larger than 2 millimeters are rock fragments (pebbles, cobbles, stones, and boulders), which are not considered in the textural class because they are relatively inert.

Soil Structure

Soil mineral particles are the building blocks of the soil skeleton. But the spaces between the particles and between aggregates are just as important as the particles themselves because that’s where most physical and biological processes happen. The quantity of variously sized pores—large, medium, small, and very small—govern the important processes of water and air movement. Also, soil organisms live and function in pores, which moreover is where plant roots grow. Most pores in clay are small (generally less than 0.002 millimeters), whereas most pores in sandy soil are large (but generally still smaller than 2 millimeters).

Can a person improve soil texture to improve crop yields? Well, no.

Soil texture cannot be readily changed by farming practices. It is just a function of the parent rock material. There are some cases where sandy soils can be mixed with organic matter and improved to retain moisture and improve plant viability. Also, clay soils can be amended with a lot of work and expense (adding sand and organic matter). Both of these changes are impractical on a farm field scale of more than a couple of acres, but they do work on garden-scale fields. Increasing organic matter in all soil texture types can improve the soil structure, but it cannot overcome the basic original soil texture. We increase organic matter through additions of compost and perennial plant roots to improve a soil’s structure rather than its texture.

“Structure modifies the influence of texture with regard to water and air relationships and the ease of root penetration.”

– Henry Foth

Figure \(\PageIndex{4}\): A loam-textured soil has a crumb-like structure. Photo AI generated by Google. March 2025

The pore sizes are affected not only by the relative amounts of sand, silt, and clay in the soil but also by the amount of aggregation. On the one extreme, we see that beach sands have large particles (in relative terms, at least—they’re visible) and no aggregation due to a lack of organic matter or clay to help bind the sand grains. A good loam or clay soil, on the other hand, has smaller particles, but they tend to be aggregated into crumbs that have larger pores between them and small pores within. Although soil texture doesn’t change over time, the total amount of pore space and the relative amount of variously sized pores are strongly affected by management practices. Aggregation and structure may be destroyed or improved depending on, for example, how much tillage occurs, whether good rotations are followed, or if cover crops are used.

The clusters or aggregates or “peds” are described using various terms like; columnar, blocky, platy, crumb-like, or granular (Henry Foth). The terms are relatively descriptive. The preferred soil structure is crumb-like or granular mainly because the peds are roundish and don’t fit snuggly together–and that is a good thing. This creates more pores. Platy or plate-like soils, on the other hand, are considered “tight” because of the flat, overlapping nature of the peds. We want soils light and airy, not dense and flat.

Sandy soils are considered to have no structure at all because each particle functions as its own ped since it is unattached to any other particle. If a clay loam is tilled or trampled when it is wet, the clay particles may slip down and form a layer of very fine particles. This is how plow layers form. When clay soil pores are collapsed, that soil is said to be puddled. Puddled soil or soils with plow layers are difficult to farm because when they dry out, they form huge peds (aggregates) called clods. I can walk my organic plowed fields in spring, but when I walk my neighbor’s field that has been continuous corn or soybeans for 15 years, the clay sticks to my feet. His soil produces “cement feet”–the puddled clay sticks tightly to my shoes making it nearly impossible to continue walking.

The physical nature of soil is intricately related to how soil behaves as a habitat for microorganisms. The better the structure, the better the habitat. And good farmers want a lot of habitat.

As previously mentioned, soil structure and how it forms aggregates can mitigate the influence of soil texture. Clay soils, if managed well, can form aggregates that are more favorable to pore space and therefore air exchange, root penetration, water retention, and nutrient cycling. Let me repeat this point, agricultural practices can influence soil structure by altering aggregation, sometimes for worse, but hopefully for better, as we will discover in further chapters. In organic agriculture, this is mostly accomplished by increasing the soil’s biological activity. The gums and sticky substances produced by living organisms help particles aggregate and give soil better structure. Conversely, many conventional agricultural practices tend to cause soil structure to become less crumbly and more flat. Sandy soils amended with organic matter have improved structure.

Tilth

Good soil structure and till-ability are sometimes referred to as good soil Tilth.

To illustrate the idea that agricultural practices can affect soil structure, and thus crop yields, Foth cites studies showing that cropping systems that include a legume in rotation with corn create “marked increases in aggregation” over continuous annual cropping. Cover crops or continuous well-managed pastures can also be expected to increase aggregation because they reduce the impact of raindrops, which break soil aggregates apart. A University of Wisconsin study that has been measuring the impact of farming practices for the last 25 years on plots near Madison finds the same results.

Organic farming practices can improve crop yields because we improve soil structure and function.

This point is an overlooked benefit of organic and sustainable agriculture…..the point that the way we farm, the tillage and cropping practices we use, can alone alter the nature of the physical properties of soils! Changing the physical structure of the soil aggregation quality will change our success as farmers and gardeners mostly because we increase the habitat for beneficial soil life.

Forget about the effects of synthetic fertilizers and herbicides, if we just tilled and rotated crops or used cover crops we can have a huge, beneficial impact on improving rather than degrading the quality of our soil. I’ll also repeat again, the easiest way to change soil structure is to increase the biological activity of our soil fauna. Increasing soil biology is exactly the opposite of the effect that conventional agriculture has. Organic and sustainable methods can have the same yields as conventional agriculture because we work to improve soil biology, thereby improving soil physical properties, and then plant growth – this is our alternative to adding synthetic fertilizers.

"Moisture, warmth, and aeration; soil texture; soil fitness; soil organisms; its tillage, drainage, and irrigation; all these are quite as important factors in the makeup and maintenance of the fertility of the soil as are manures, fertilizers, and soil amendments."

—J.L. Hills, C.H. Jones and C. Cutler, 1908

The physical condition of soil has a lot to do with its ability to produce crops, mostly because it anchors their roots. A very fundamental aspect of soil is its ability to hold water between particles and act like a sponge in the landscape. This phenomenon, capillarity (or capillary action), helps store precipitation, thereby making it available to plants and other organisms or transmitting it slowly into groundwater or streams. Also, water in the soil allows for the very slow but steady dissolving of soil minerals, which are absorbed by plants and cycled back onto the soil as organic matter. Over the course of many years, these small amounts of minerals build up as a pool of stored organic nutrients available for agricultural production.

Soil usually contains about 50% solid particles on a volume basis, with the spaces in between, pores, accounting for the remaining volume. Most solid particles are minerals, and organic matter is a small, but very important, component of the soil. The soil’s mineral particles are a mixture of variously sized minerals that define its texture.

Figure \(\PageIndex{5}\): Distribution of solids and pores in soil.

Degraded soil usually has reduced water infiltration and percolation (drainage into the deeper soil), aeration, and root growth. These conditions lessen the ability of the soil to supply nutrients or to reduce hazardous compounds (such as pesticides), and maintain a wide diversity of soil organisms. Small changes in a soil’s physical conditions can have a large impact on these essential processes. Creating a good physical environment, which is a critical part of building and maintaining healthy soils, requires attention and care.