Introduction to Soil: Physical Properties (Part 1)

This is the first in a series of posts about fostering and maintaining healthy soil in your farm or garden.

It’s summertime and the garden is calling! You’re ready to start planting, but do you know if your soil is ready?

In this blog post, we’ll learn to assess basic information about the physical properties of soil, including texture, structure, and moisture levels. This information will help you begin to create an informed farm or garden plan so you can build and maintain healthy, productive soil for many seasons to come. In future posts, we’ll cover chemical and biological properties of soil, as well.

What is soil?

In Gaia’s Garden, the late permaculturist Toby Hemenway describes soil as “miraculous. It is where the dead are brought back to life. Here, in the thin earthy boundary between inanimate rock and the planet’s green carpet, lifeless minerals are weathered from stones or decomposed from organic debris. Plants and microscopic animals eat these dead particles and recast them as living matter” (71).

He goes on to say, “Soil is alive. One key to having a garden that’s bursting with healthy plants, well-balanced insects, and thriving wildlife is to stuff the soil with as much life as possible” (71). Healthy soil is an ecosystem of living and dying organisms in a continuous cycle of birth and decay.

This underground, living ecosystem creates the ideal conditions for plants to emerge from seeds, sprout, and grow into delicious food for humans and other creatures to enjoy. Then, the plants, people, and other creatures all return to the soil and re-join the cycle. The ubiquitous Biblical passage comes to mind: “for dust thou art, and unto dust shalt thou return.” We depend upon soil, and then we become soil.

Another, more basic definition is provided by the UC Santa Cruz Center for Agroecology & Sustainable Food Systems in their Teaching Organic Farming and Gardening (TOFG) manual: soil is “a mixture of mineral and organic material that is capable of supporting plant life” (Miles, Part 2-7). The first step to fostering a living, thriving soil that can best support plant life, is to understand the current health and characteristics of your soil today.

In addition to being a medium for plants to grow, soil serves other core functions that are important to be aware of when designing a farm, garden, or landscape system.

Core Functions of Soil

  • As mentioned, soil “Supports the growth of higher plants,” which in turn supports the existence of land animals, including humans, on planet earth.
  • It is the “Primary factor controlling fate of water in hydrologic systems.” The type of soil you have, its structure, and how much organic matter it contains influence whether water from rain or irrigation runs off the surface or infiltrates into the groundwater. It also determines how quickly it drains through the soil, if at all.
  • It is “Nature’s recycling system for nutrients.” In a healthy soil ecosystem, nutrients are effectively extracted from decomposing organisms and rock materials, cycled through living plants, and then back into the soil ecosystem.
  • It serves as “Habitat for organisms.” Soil contains microscopic bacteria, fungi, insects like earthworms, and myriad other organisms. Each play their own role in the recycling of nutrients, decomposition of organic matter, and web of life that characterizes a healthy soil ecosystem.
  • It is an “Engineering medium” that can serve as a base for structures as well as a building material when building natural structures like cob. For the purposes of this blog post, we won’t discuss this function in detail. (Miles, Part 2-7)

Knowing more about your own soil will allow you to identify how your soil serves these functions in your own farm or garden. This knowledge will help you foster and maintain productive and healthy soils.

Here’s a great quote by C.W. Burkett from 1907 that remains true–and is perhaps even more pertinent–today in 2019: “All over the country [some soils are] worn out, depleted, exhausted, almost dead. But here is comfort: These soils possess possibilities and may be restored to high productive power, provided you do a few simple things.” (Magdoff 3)

To know what simple things to do to improve and protect you soil, you must first get to know your soil.

Soil Texture

Depending upon where you live, the soil in your farm or garden has different sized particles, categorized into three types: sand, silt, and clay. These particles make up the mineral component of soil. In ideal soil conditions, minerals make up about 50% of any given amount of soil; the other components are organic matter (0-10%), air (~25%), and water (~25%) (Miles, Part 2-9).


Chart source: PhysicalGeography.net

Each different soil we encounter around the world has been formed in its own unique way over time by factors such as wind, rain, temperature, topography, geologic activity, chemical reactions, plants and animals, and the original rocks the minerals came from. Your soil has a story! Studying its properties can allow you to work most effectively with its strengths and weaknesses to achieve your gardening or farming goals.

As mentioned, soil contains three particle sizes:

Sand is, well, sandy. It has visibly gritty, larger pores than the other two types of particles. It doesn’t hold water well. Imagine the sand you find at the beach.

Silt is smooth and powdery when dry, and greasy when wet. It has medium-sized pores. It’s the kind of soil you would find in a river bed.

Clay is hard when dry and sticky when wet. Think of clay used to make pottery. It has small pores, and it’s formed through the weathering of silt and sand.

This figure represents the difference in size between sand, silt, and clay. Sand can be coarse, medium, or fine.

Gardens on a mountain top will have different soil profiles from gardens near the beach or gardens built in historic floodplains. Even within one neighborhood, farm, or garden, your soil texture can vary.

Soil texture plays an important role in selecting plants to grow, understanding how well your soil will drain water, and evaluating how well it will hold onto and release nutrients in the soil.

Texture is a characteristic of soil that cannot practically be altered. As with any permaculture design, identifying constraints and existing conditions is important for developing a plan that is suited to the unique place on earth where you are growing.


This soil texture triangle helps you identify what kind of soil you have once you know what percentage of sand, silt, and clay your soil contains. This chart can be found on PDF document page 23 of the Soils and Soil Physical Properties chapter of the TOFG.

Estimating soil texture by feel

You can estimate soil texture by picking up about 2 tablespoons of soil in your hand. “Add water dropwise and knead the soil to break down all its aggregates. Soil is at the proper consistency when plastic and moldable, like moist putty” (Miles, Part 2-24). Squeeze the soil into a ball.

Then, start at the box labeled “Does soil remain in a ball when squeezed?” in the chart on PDF document page 22 of the Soils and Soil Physical Properties chapter from the Teaching Organic Farming & Gardening (TOFG), produced by UC Santa Cruz’s Center for Agroecology & Sustainable Food Systems. Follow the directions until you’ve identified your soil texture.

There are additional demonstrations and more information about soil in the TOFG manual.

Soil Jar Test: Estimating Soil Texture

Another simple way to estimate your soil texture is with a jar test. You’ll need a jar, such as a quart mason jar; try to find one with entirely straight sides because any curves at the bottom of the jar will make it more difficult to measure. You’ll also need a small amount of dish soap, some water, a garden trowel, and ~24 hours or more to let the jar rest before you measure.

Bring your materials into your garden, and scoop enough soil into your jar to fill it about half-way. If you plan on gardening or farming on a large parcel of land, you may want to repeat the jar test in different areas because soil texture can vary even within the same farm or garden. Be sure to label each jar.

Once you’ve filled the jar half-way with soil, add a small drop of dish soap, and fill the jar the rest of the way with water, leaving a small amount of room at the top so you can shake the water and soil together. Place the lid securely on the jar.

Shake, flip, and vigorously mix the contents of the jar. The dish soap will help break up clumps in your soil so that each particle of soil is separated. Once the soil is sufficiently mixed into the water, place the jar on a flat surface. Leave it there for as long as it takes for all of the particles to settle and the water at the top to be clear.

As the jar rests, the heaviest particles–sand–will sink to the bottom, followed by silt particles, and then clay. Once the water at the top of the jar is clear, measure the total height of soil.

Then measure the height of each particle layer, and divide each layer’s height by the total. Then multiply each number by 100 to find the percentages.

Once you have the three percentages for sand, silt, and clay, look up your soil type on the soil texture triangle on PDF document page 23 of the Soils and Soil Physical Properties chapter of the TOFG.

The Natural History Museum of Los Angeles County has created this one-pager of instructions for the soil jar test that includes additional information.

USDA Web Soil Survey: Soil Texture

Without even touching your soil, you can actually get a big picture idea of what kind of soil is likely present at the site where you plan to farm or garden. There’s an online tool called the Web Soil Survey (WSS), which “provides soil data and information produced by the National Cooperative Soil Survey. It is operated by the USDA Natural Resources Conservation Service (NRCS) and provides access to the largest natural resource information system in the world. NRCS has soil maps and data available online for more than 95 percent of the nation’s counties and anticipates having 100 percent in the near future. The site is updated and maintained online as the single authoritative source of soil survey information.”

This information is helpful if you are searching for farmland or a new property and want to get a sense of whether the soil might be suitable for farming. It can also provide you with a holistic idea of the soil types on a large parcel of land so that you can narrow your options to where might be best to start testing soil at a more precise scale. If you don’t plan on doing more precise testing, it can also give you a general idea of what kind of soil you have.

To read instructions for using the WSS website to learn about your soil, please click here.

Lab Soil Testing with A&L Western Labs

For the most precise analysis of your soil, you can submit a sample to a lab. They will be able to tell you not only texture, but also chemical and biological properties that we will discuss in future blog posts.

We recommend using A&L Western Labs for your soil sampling if you live in the western part of the U.S. They have detailed instructions for taking your soil sample on their website here.

Essentially, you want to obtain a soil sample from 6’’ deep, and your sample should contain soil from 10-20 locations within your garden or farm area. Using a shovel, you can dig up some soil from each spot and place it into the same clean bucket (plastic buckets are ideal because there isn’t the risk of contamination from metal).

Once you’ve collected all the samples, mix the soil together and then measure about two cups into a ziplock bag. Print and fill out the submission form from the website. Include this with the soil sample and ship it to A&L Western Labs.

We recommend using the S3C test, with graphics and recommendations per 1000 ft2. You can see all of the options for soil testing here. If you are in an urban area and/or are concerned about heavy metals in your soil, they also do metals testing through a separate submission form, but note that this kind of testing is much more expensive.

When you receive your results, they will offer recommendations for how to address any deficiencies in your soil. You will likely need to do further research into how much to apply of different amendments, and in a future blog post, we will share our top ten organic amendments!

For more on lab soil testing, read the “Reading and Interpreting Soil Test Reports” chapter of the TOFG here.

Soil Structure

In addition to texture, the structure, or tilth, of your soil is important for the growth of plants. “A soil with good tilth is more spongy and less compact than one with poor tilth. A soil that has a favorable and stable soil structure also promotes rainfall infiltration and water storage for plants to use later” (Magdoff 4). To assess the tilth of your soil, you can dig into it and imagine plant roots trying to grow into it. Does it have air space and room for plant roots?

The process of tilling soil with tractor implements or hand tools is designed to improve the tilth–but as with many things in life and gardening, there can be too much of a good thing. (Note that some people don’t consider any amount of tilling good–more about that in a future blog post). Over-tillage leads to soils with poor structure that compact easily and blow away in the wind. The Dust Bowl of the 1930s was caused by over-tilling the soils of the Midwest. Good tilth can be achieved with minimal-to-no tillage with the help of lots of compost and cover cropping over time. Plant roots can also help break up soil compaction, and if your soil is particularly compacted, you can choose cover crop mixes that contain deep-rooted plants like daikon radishes.

Related to soil tilth are soil aggregates, another element of the soil that you can test at home. “When mineral and organic particles clump together, aggregates are formed. They create a soil that contains more spaces, or pores, for storing water and allowing gas exchange as oxygen enters for use by plant roots and soil organisms and the carbon dioxide (CO2) produced by organisms leaves the soil” (Magdoff 4). Healthy soils contain stable aggregates that maintain their structure when exposed to water, wind, and tillage.

“Aggregate stability is the ability to withstand wetting and drying, wind, and actions such as tillage. This is key for water infiltration, gas exchange, root growth, and long-term resistance to wind and water erosion, and is an indicator of soil health.” (Miles, Part 2-12)

This video goes into detail about soil aggregate stability and how to test your soil at home:

Soil Moisture

Each time you prepare to do any significant tillage (tractor or hand-scale) or digging in your garden, there’s a crucial step to make sure you aren’t undoing all of your hard work to maintain soil structure and good tilth. You should check the soil’s moisture and make sure it isn’t too wet or too dry before you begin digging and planting.

Understanding soil moisture is also important for knowing when to irrigate. To save water and to prevent your soil from becoming soggy (which can cause root rot and fungal issues for your plants), you only want to irrigate when necessary. You also want to make sure your soil doesn’t become too dry, or your plants will struggle and eventually die.

Depending upon your soil texture, water will move through your soil differently. As described in the “Soils and Soil Physical Properties” chapter of the TOFG, “Soils that are high in clay content tend to have a lower permeability, while soils that are high in sand content tend to have a higher permeability. Soil texture not only affects how fast water moves through the soil, it also affects the pattern of movement. Water will move almost straight down through a sandy soil whereas it will have more lateral movement in a soil with higher clay content” (17). See the figure below for a visual example of this.

Thus, your soil texture informs how often and how quickly you should be irrigating your soil. You may adjust the frequency and duration of your drip irrigation system, for instance, depending upon whether water moves slowly or quickly through your soil.

To know how much water remains in your soil after irrigation or rain, you’ll need to estimate soil moisture. Soil moisture feels different in different soils, such that a soil might feel wet to you compared with another soil even when the two soils are at the same moisture level. This is another reason why knowing your soil texture is important.

Estimating Soil Moisture by Feel

To learn how to estimate soil moisture by feel, see the charts on PDF document pages 48-49 of the Irrigation–Principles and Practice chapter of the TOFG manual. As mentioned, you’ll need to know your soil type in order to assess your soil moisture.

Take a handful of your soil and squeeze it in one hand. Release your hold on the soil, and then try to form a ribbon by placing some of the soil between your thumb and forefinger and pressing your thumb up and against your forefinger. In soils with higher clay content, you should be able to form a ribbon-like shape that elongates as you push your thumb toward your forefinger. Now look at the chart to assess your level of moisture.

Assessing Moisture for Irrigation

If you’re assessing moisture to determine if you need to irrigate, you’ll want to irrigate when your soil moisture is in the 50%–60% of field capacity range for soil sampled near the root zone of the crop you’re growing. Letting soil moisture fall below this risks your plants losing access to water. Below 25% of field capacity (the first row of the chart), your plants cannot actually access the moisture that is in the soil. If your soil is less than 60% of field capacity, it’s time to irrigate. (Learn more about Field Capacity on PDF document page 7 the Irrigation–Principles and Practice chapter of the TOFG manual.)

Guidelines for Irrigation

For some general guidelines for irrigating different crops, see pages 51-53 of the “Irrigation–Principles and Practices” chapter of the TOFG manual. It has information for general irrigation rules and irrigation approaches for different vegetable crops.

Another resource is this “cheat sheet” with basic guidelines to follow when irrigating crops at different stages.

Assessing Moisture Before Working Soil

If you’re assessing moisture because you’re ready to dig in the garden to prepare beds for planting, you’ll also want to look for a 50-60% of field capacity moisture level.

In general, if the soil falls apart and does not maintain any shape after squeezing some in your hand, it is too dry to work with. Lightly irrigate the soil and check on it again later that day (if it’s a hot day), or the next day (if it’s a cool day).

If the soil forms a ball and you can see fingermarks where you gripped it, it may be too wet. Check the descriptions for your soil type, and if it’s more than 75% of field capacity, you’ll need to wait until the soil has gotten drier before doing any major digging or tilling to protect your soil from compaction.

Learning to assess soil moisture takes practice and experience. Knowing the percentage field capacity isn’t as important as paying attention to the soil. If it seems too wet to be digging, stop digging. If your plants look thirsty and the soil seems dry, go ahead and irrigate.

The important part is to pay attention and become familiar with your unique soil. After you’ve irrigated, feel the soil to get a sense of what it feels like when it’s truly wet. Go back the next day and feel it again.

Now that you’ve learned how to assess your soil texture, structure, and moisture, it’s time to get out there and play in the dirt! Really, feeling your soil and utilizing the tests described above is a great way to prepare for a successful season of gardening or farming.

Learn More

Up Next: Chemical Properties

Now that you’ve learned about soil physical properties and why understanding your soil is important, we’ll cover chemical properties of soil in part two.

Want to go deeper? Our recommended resources and references from this blog post are below.

This is the first in a series of blog posts about soil, so stay tuned for more information! You can also register for our Regenerating Watersheds & Soils weekend during our Permaculture Design Course. Dates and registration information is available on our website: santacruzpermaculture.com/permaculture-design-course/

Resources

Gershuny, Grace, and Joseph Smillie. The Soul of Soil: a Guide to Ecological Soil Management. 4th ed., Chelsea Green Publishing, 1999.

Gershuny, Grace. Start with the Soil: the Organic Gardeners Guide to Improving the Soil for Higher Yields, More Beautiful Flowers, and a Healthy, Easy-Care Garden. Rodale, 1993.

Hemenway, Toby. Gaia’s Garden: A Guide to Home-Scale Permaculture. 2nd ed., Chelsea Green Publishing, 2009.

Magdoff, Fred, and Harold Van Es. Building Soils for Better Crops: Sustainable Soil Management. 3rd ed., Sustainable Agriculture Research and Education Program, 2009.

Martin, Orin. “Your Soil—A Primer, with Some Strategies for Sustainable Management.” News & Notes of the UCSC Farm & Garden. Center for Agroecology & Sustainable Food Systems, University of California, Santa Cruz, 2011.

Miles, Albie, and Martha Brown. Teaching Organic Farming and Gardening: Resources for Instuctors. Center for Agroecology & Sustainable Food Systems, University of California, Santa Cruz, 2005.

“Soil Cultivation: Fundamental Concepts & Goals.” For the Gardener. Center for Agroecology & Sustainable Food Systems, University of California, Santa Cruz.

%d bloggers like this: