Let’s talk about soil particle size.
As you can see, clay is an extremely small particle. It is orders of magnitude smaller than sand. The size of particles greatly affects how soil holds water and nutrients and how plants interact with it. If you understand this, you can work with different soil types with success.
The smaller the particle, the more water the soil holds and the slower it drains. If you pour water into a hole dug into each one of these soil types, you will note that gravel drains the fastest, and clay, the slowest.
You will notice that clay or silt soil may stay moist for days or weeks after a rain, but sand or gravel can dry out much faster.
Clay also holds nutrients better for a number of reasons, both because of its size and its chemistry. It’s much more interactive and capable of exchanging energy with nutrients, minerals, and plants, thus making nutrients more bioavailable (available in a form that plants can access).
Clay soils, when wet, will continue to gather on your boots as you walk through it until they are pulled off or you can’t lift your feet. It’s extremely sticky soil. When it’s compacted and then dries out, it can be like concrete - almost impenetrable and very difficult to dig into.
If you’ve ever worked in compacted clay, it’s very real to you why you don’t want to walk on or otherwise compact garden beds with lots of clay in them. It becomes very difficult to dig in. When clay is dry, it hardens. Lots of cracking in dry soil is an indication of clay.
Silty soils don’t stick as much, and they feel a little greasy and smooth. When dry, they tend to be powdery. They can still get rock hard and difficult to dig into or open up when compressed.
Sandy soil may stick together when wet, but crumbles apart easily. It also compacts but is easier to dig into, even when compressed. The particles more easily separate.
Here's a simple soil test: Roll wet soil up in a ball. Roll it into a tube in your hand, and then bend it. Clay will stick together. Silt will break in places but stick together generally. Sand will break up where you bend it and probably elsewhere. If you keep rolling and add pressure, clay will stick to your hands, silt will somewhat stick and some will drop off in chunks, and sand will disintegrate into individual particles.
Most soils are combinations of these three and will have a mix of characteristics, but if one type of soil dominates, it will respond in above way.
These are important characteristics because it has a lot to do with how you design your water, fertility, and growing patterns! If you understand your soil type, you can design your garden to do well in it, regardless of the challenges.
GPH means gallons per hour. The above illustration shows how water behaves in different types of soil, and indicates how big the emitters on irrigation systems should be. The more sand, the bigger the emitters, because sand doesn’t hold water.
So, where would you want to put your irrigation in these three cases? As one example, in sandy soils, if I’m growing citrus, I spread the irrigation out over a wide area because citrus is a surface feeder and needs water to spread out more than it will with just one mini-sprinkler (see our section on irrigation for more info).
Most soils are not pure sand, silt or clay, but a mixture. A mix of around 40% sand, 40% silt and 20% clay, that has some organic material in it too is called a loam. This is the "Holy Grail" of soils. Loam is ideal because you have enough clay to hold moisture and nutrition, but enough sand to allow for decent drainage. This is an ideal situation for most plant roots. They don’t get flooded out or dried out and can access nutrition.
Most soils are not a perfect loam, but end up with one type of soil being dominant - sandy loam, or clay loam, or silt loam. It’s really useful to understand how to determine the physical type of soil you may have, and there is a dirt simple (yes, I said that) way to do it! It’s called the “jar test.” All you need is an empty glass jar, some water and something to dig with. See the below "Hands On Activity" for how to do this test and others mentioned in this section.
We routinely do these soil tests on client sites. It's a fast, simple, low tech way to determine a lot of information about their site. We'll share how to use this information in future lessons.
Verify Your Understanding
Dig a hole about 1 foot deep and 1 foot wide. Fill it with water and time how long the water takes to sink in. If it is saturated clay soil, this could take hours. This shows what your infiltration rate is. There are a number of things that can cause infiltration to slow:
Soil type (clay or silt)
High water table
Other blockage under the ground, like hard pan
Saturated soils
Compacted soil
Optional: You may want to try this exercise in different places on your site especially if you have different types of soil or water features (like some swampy land). Try this test in a section where you drive a car or truck or machine a lot to see what compaction does. Do this test in different seasons, or after a drought or very wet period. It really gives you an idea of how your soil holds water.
Write the results up in your site assessment.
Hands On Activity
Soil Jar Test
This is a way to determine how much clay, sand or silt you have in your soil. Take an empty jar, and fill it about ⅓ with material from the subsoil horizon. We’re not measuring organic material, just the mineral soil (clay, sand and silt), so you want to get below the layer where there is a lot of organic material. Chances are you’ll still get some organics in the mix, and that’s ok.
Fill the jar about ¾ full with water, shake it well so the soil mixes with the water, and then set it down. Watch the soil settle. It settles by the weight of the particle. Heavy sand will settle first, finer sand second (often, you just have one size of sand).
This happens within a few seconds.
Next to settle will be silt. That can take a few minutes or longer. It is finer.
Last to settle will be clay. This can take up to a week to fully settle out. It is very fine and you will be able to see it suspended in the water.
Once all of those are settled, you will be able to see them pretty clearly. Often they are different colors. But you’ll be able to see the different layers because of how fine each layer is.
Measure each with a ruler. Now, you want to figure out percentages. For instance, if you have 1 inch of sand, ½ inch of silt, and ½ inch of clay, the percentages would be 50/25/25.
You now can find out what type of soil you have by looking at the above chart. Place a dot by the percentage of each type of soil on its appropriate line, and then join those three dots along the lines laid out for you. This will give you your soil texture.
At 50/25/25, we end up with sandy clay loam, which is a great soil. You will have to do little to remediate problems of soil that is very sandy or silty or heavy clay. We’ll get into how to address those issues later but the first step is to determine your soil texture.
Note: On most sites, you’ll have one type of soil generally. On some sites you might have several. We looked at a site that had sandy soil with clay bands in it, and some limestone gravelly soil as well. In that case, feel free to do more than one jar test. On sites of any size, I often do at least 4 jar tests, sometimes more.
Determine your soil texture using the above chart. This is part of your site assessment. You now know some important things about your soil!
Hands On Activity
Determine your soil texture using the above chart. This is part of your site assessment. You now know some important things about your soil!
Hands On Activity
Watch this video on how to determine your soil infiltration rate. This is important because it tells you something about how often to water, and how good your drainage is. If you have poor drainage, your soil may be compacted and you may have to take some extra soil building steps to remedy that. Compacted clay is one place that I would till manure into - once. Other strategies would be to plant fodder radishes or Daikon radishes or alfalfa or other plants that open up compacted soils; use a broadfork to open and aerate soil without turning it; or add earthworms.
https://www.youtube.com/watch?v=gEtTL6PpXjg
After watching the video, do an infiltration test in your yard. As you can see in the video, you don’t have to be exact, you want a ballpark idea of infiltration rate. You will likely find different rates in different locations. Try a location in a well traveled area or where cars have been, versus.