Sand vs Silt vs Clay in Nutrients and Drainage
Sand, silt, and clay are not the same things. They are the three basic types of soils that differ for their particle size and they are all present in the majority of outdoor soils. Sand has a different drainage capability, nutrient retention ability than clay. If you are aware of this, you can step up your gardening game. Let’s dive in.
Sand, silt, and clay differ for their particle size: 0.05 mm to 2 mm for sand, 0.002 mm to 0.05 mm for silt, and below 0.002 mm for clay. Clay and silt have higher retention ability than sand, but a way poorer ability to drain.
Particle size affects the physical and chemical properties of the soil, making each of them suitable for specific situations.
You know now that the three different types of soil differ for their particle size. However, how particle size affects the physical and chemical properties of the soil? Ignore them, and your plants will suffer! Keep reading to know more.
Particle Size: How Physical Properties Are Affected
Each soil is given by inorganic matter. Essentially, rocks that in the course of thousands of years have been reduced to very tiny pieces (fraction of millimeters) by the action of wind and water. The particle size affects the physical and chemical properties of the soil. A physical property refers to the soil texture. A chemical property is related to its interaction with chemicals and nutrients.
Let’s start talking about physical properties.
Physical Properties: The Ball Pit Comparison
The main physical properties of any soil are:
To better understand how each property works, I will use what I call the “ball pit comparison”. Think of the soil as the aggregation of millions of spherical particles (this is not exactly true but makes the concept very easy to understand) inside a plant container. Each soil particle can be seen as a ball in a ball pit while the plant container is the box of the ball pit.
If each sphere in a ball pit is very large, then you have lots of space between them. Lots of space implies that water and air can easily pass through the balls from top to bottom.
Easy? Well done, you just discovered two physical properties: drainage and aeration.
Drainage is the capability of the soil to let water pass through.
Aeration is the soil’s capability to let air pass through.
Now imagine that you are inside the ball pit and you want to stretch your legs. Now you are the plant, and your legs are the roots. If there is lots of space (and so the air), it will be easy for you to stretch your legs and move around. The same applies to plant roots. It will be easier for them to develop.
On the other hand, if the balls are very small (imagine replacing football balls with tennis balls), now the box will be way more compact, and for you, it will be quite a task to stretch your legs. In this case, the plant will struggle to extend its roots.
Easy again? Well done, you discovered another physical property: friability.
Friability is the capability of the soil to break into smaller pieces when pressed or moved. A friable soil will let the roots of a plant pass effortlessly.
Now imagine that the ball pit is very deep, and you cannot touch the bottom with your feet. Moreover, suppose that you are really light so you can float in the ball pit. What happened if the balls are really big and someone moves you. You will fall on the side. However, if the balls are tiny, you will be more “pressed” so any external influence will not affect your position.
Easy again? You just discovered the last physical property: anchorage.
Anchorage can be seen as a consequence of friability. This is the capability of the soil to provide a mechanical sustain for a plant. If the soil does not offer anchorage the plant may fall upside down with just a breeze. This is something you want to avoid, of course.
Effect On Plant Growth
Using the ball pit comparison, you know now the meaning of drainage, aeration, friability, and anchorage, and their intuitive relation with particle size. The next step, very easy now for you, is to see how these physical characteristics vary in the function of the soil type.
For simplicity, I will introduce you to the differences between the two extreme soil types as silt, with intermediate particle size, has average physical properties among the two.
One Extreme: Sand
Hence, sand might look ideal as the roots can receive all the water that arrives from the top (either from the sky or from you and me watering our green friends). Unfortunately, this is not true for drainage. A soil made entirely of sand is terrible news for many plants as the water pass through too quickly for the roots to have the opportunity to “drink” it. It is like someone opening and closing a tap so rapidly that does not leave you the time to drink.
Plants will not have enough water, and the soil will dry out quickly. The only countermeasure is for you to water it very often. However, this has also a nutritional drawback as too much water can “wash” away nutrients potentially present in the soil (if you add them through fertilizer, for instance). That’s why I do not recommend growing plants (with few exceptions like thyme that can thrive with a soil with a substantial sand component) in sandy soils. Sandy soil is very friable with not a great anchorage.
Do not get me wrong, though! An experienced gardener might tell you that you can grow everything in the sand. This might be true. However, you need lots of care and attention to compensate for the lack of anchorage and inadequate drainage of such soil (for instance, watering very often).
For more information on how to boost your plant growth, you can check the best potting soil article or the 2 aspects that make great any potting soil. It is also true that sand can be beneficial in a great potting mix.
Second Extreme: Clay
This is the other side of the spectra. With the smallest particles, this soil makes it difficult for gases to reach the plants’ roots and vice versa (inadequate aeration). This is especially true if clay soil is not moved frequently. It tends to be compact. The situation can get even worse with water. Indeed, the water cannot penetrate the soil or, once there, it cannot leave for a long time. This is something you want to avoid for your plants. Otherwise, they will have their roots wet for a long time (lousy drainage), ideal conditions for root rot.
Given how small the particles are, such soil tends to be very compact (not friable) and do provide good anchorage (probably too good) for your plants.
Effect on plants: clay soil is another no-to-go for plants. You can have some success with it, but only with lots of effort to compensate for the lack of drainage and aeration. You need to move the soil frequently and keep it just moist enough not to be wet (that causes waterlog) or dry (tends to become difficult to work with and for plants to grow through as dry clay is very hard).
Particle Size: How Chemical Properties Are Affected
The success of your plants strongly depends not only on the physical properties discussed earlier but also on the chemical properties. Here the topic can get really complex and here I am simplifying quite a bit to let you know the most important bits. These chemical properties are:
- pH level
- Nutrient retention capabilities.
The pH of your soil (for more precise details you can always use Wikipedia) tells you the concentration of “hydrogen ions” dispersed in it. If you are not familiar with “hydrogen ions” just remember that less of these “ions,” more acidic will be your soil. You should be familiar with “acidic” substances. Think about lemon juice, for instance. That’s acidic. It has a very low concentration of ions. On the opposite side, more ions make your soil alkaline (or basic).
The pH value is a number that varies from 0 to 14 (0 extremely acidic, 14 extremely alkaline, and 7 neutral, ideally like boiled water). For you as a reference, soil with a pH close or equal to 7 (neutral) is ideal. As always there are a few exceptions (like rosemary that thrives in slightly acidic soils), but at the start having a close to neutral soil (5.5 to 7.5) is adequate for many plants. Indeed, even if the ideal pH is 7 for many, this does not mean that a 6.5 or a 7.5 pH is totally bad, plants are not that sensitive.
Moreover, the soil pH changes over time depending on watering and the presence of organic material (that makes it more acidic). Hence, over time, you might want to perform pH test, as the one discussed in this article.
In general, sand is slightly acidic while clay is more on the alkaline side.
Effect on plants: the pH level is correlated with the capability of soil to hold and release nutrients to plants. A pH level far from the suggested one (5.5 to 7.5) will affect the ability of microbes of producing the nutrients that plants crave, and the concentration of plant-dangerous minerals (like aluminum) will also increase. A too low or high pH can also negatively change the structure of the soil, making it harder for a potted plant to thrive.
As the name suggests indicates the salt content in the soil. Plants, as well plant in general, will suffer in the presence of high salinity levels. Indeed, salt will “suck” water from their cell, damaging them. Salt is indeed used also as a natural way to kill unwanted plants in a field.
Generally, for herbs to thrive, the salinity should not be higher than 1 dS/m (dS/m is the unit of measure, called deciSiemens per meter). Just to give you an idea, seawater has a salinity of 45 dS/m and above. This also explains why adding saltwater (or even worse, table salt) to your soil is, in most cases, a terrible idea.
Sand, surprisingly, has a low salinity that varies from 0.3 to 1.1 dS/m. Indeed, when we talk about sand soil, you should not imagine a beach constantly wet with salty water. Sand, for soil, might have spent many years/decades, not in contact with seawater. Clay, on the other hand, generally has a higher salt content (0.6 dS\m). This is because its limited drainage capability causes water to stay for longer in contact with the soil so releasing more of its salt content that tends to build up over time.
Effect on plants: a salinity level above 1dS/m will challenge the ability of your plants to extract water from the soil at they need to compete with the water-sucking ability of salt. Symptoms to watch are: a wilted plant, leaves start getting yellow or brown and curl (also you can google “salt leaves burn”). You might even notice a white crust on the soil surface. That is salt! If the problem is not addressed (changing soil ideally) your plant very likely will die.
Measure Sand, Silt and Clay Amount: DIY Experiment
With 2 flat-side glass jars filled with ⅔ of water, I added 4 tablespoons of each soil sample to each jar. The soil samples were taken from the pathway and the other was from the backyard. Both jars were shaken vigorously to break down the lumps and separate each soil component.
I allowed the jars to sit for a couple of days until all the particles settle on the bottom. They were left undisturbed while the soil particles were suspended below the water surface. Some organic matters float on or below the surface.
During the soil suspension, I noticed the formation of different layers of particles. On the bottom are the largest and heaviest particles of sand, topped with finer granules of silt, and then a layer of clay. When they are all settled, I measured the height of the suspended sample and the height of each layer.
Here’s the measurement of each soil sample:
|BACKYARD SOIL||PATHWAY SOIL|
|Overall soil height||4 cm||3.1 cm|
|Clay Layer||0.5 cm||0.3 cm|
|Silt Layer||0.3 cm||0.1 cm|
|Sand Layer||3.2 cm||2.7 cm|
To obtain the percentage of each soil particle, I divided the height of each layer to the overall height of the soil sample. The settled backyard soil is about 4 cm in height. The sand layer is approximately 3.2 cm, while the silt and clay measure 0.3 cm and 0.5 cm, respectively.
Based on these available data, our backyard soil thus consists of 80% sand, 7.5% silt, and 12.5% clay.
For the pathway soil, its result yields 3.1 cm of suspended matter. Sand has the most settled particles up to 2.7 cm, while the silt and clay layer has approximately 0.1 cm and 0.3 cm of settled particles, respectively.
The layers made by the pathway soil are not so apparent unless you inspect them closely and see different particle sizes. Experiment shows that our pathway soil is made up of 87.1% sand, 3.2% silt, and 9.7% clay.
|COMPOSITION||BACKYARD SOIL||PATHWAY SOIL|
Percentage particle composition of soil samples
Using the USDA Textural Soil Classification Triangle, we can determine what type of soil sample we used. Locate the percentage of each layer and draw a line to represent each particle. The point where the lines intersect is the soil texture.
The backyard soil with 80% sand, 7.5% silt, and 12.5% clay, converge and lie on the area classification of sandy loam when plotted on the triangle. While the pathway soil with particle percentages of 87.1% sand, 9.7% clay, and 3.2% silt components, intersect on the loamy sand section.
It is paramount to determine the texture of your soil so you can assess if it can sustain optimum plant growth. This way you can decide what to add or amend to your soil to make it suitable for your plants.
Capability To Retain Nutrients: The Pit Ball Comparison
To understand the ability of soil to retain nutrients, you might need an entire university course. However, here, I will keep things simple, highlighting the central concept.
We know that every plant needs nutrients. We also know that such nutrients need to be present in the soil. How does the soil keep those nutrients? Easy, the surface of each soil particle can be seen as a nutrient magnet (most soils are negatively charged) that attracts plant nutrients.
If the soil particles are large (as in the case of sand) the available surface to attract nutrients is overall smaller than the situation in which you have smaller balls. Every single ball indeed has a lower surface, but, at the same time, you have way more balls that more than compensate for such aspects.
Hence, it is now easy to understand that clay is a way more nutrient-rich soil than sand. Indeed, due to its small particle size, it has more surface to attract nutrients.
Are Sand, Silt And Clay The Only Soil?
Hopefully not! You can create a large variety of soils by mixing those three (or two) types in different ratios. In this way, you can mitigate the defect of one soil type and bring the benefit of the other(s). That’s what every expert gardener (and gardening soil manufacturer) does: create their own soil with precise sand, clay, and silt ratio to reach the desired physical and chemical properties.
For instance, previously, I discussed the lousy drainage of clay. Sand, on the other hand, has the opposite behavior. Then, what will happen if we add clay to sand? Think again about the pit ball comparison. What’s going to happen if you add a few tennis balls in a box full of football balls? The tennis ball will cover part of the empty space among the larger balls.
Hence, water and air can still flow through the box, but this will happen at a slower rate compared to the original case. This is great news for your plants as water will flow slowly and so your plant roots will have time to drink it! You get rid of the problem of sand soil without getting the troubles of clay soil.
Of course, knowing the exact ratio to obtain the desired physical and chemical is a science (soil science), but I guess you got the concept: in real life, you will not have purely one soil type.
To have an idea of the soil type that exists just have a look at the soil triangle (very known among gardeners and soil specialists). Primarily, every side of the triangle is associated with a soil type concentration. This goes from one extreme (0%) to another (100%).
For instance, the red dot in the figure is referred to an almost balanced proportion among clay, sand, and silt. On this website you can play around to see how different soil proportion affects the final result.
Loamy soil types (there is a large variety as you can see in the graph) are very famous among gardeners (the most common is 40-40-20 of sand, silt, and clay). Indeed, they provide ideal conditions due to their balanced proportion of each type of soil, bringing the benefits of each soil type.
Potting Soil? What is Made Of?
As you can read in one of my previous articles potting soil, in reality, is not soil. Indeed, most of the time there is no sand, clay, or silt in it. One of the reasons is that potted plants provide a totally different challenge. It is crucial in this case for the soil to be as light as possible, with limited compaction issues.
A potting soil (more precisely potting mix) is a combination of peat moss, compost, perlite, fertilizer, and limestone. These are all medium that provides (if in the right proportion) the right balance of nutrients (compost), nutrients, water retention capability (compost), and drainage (perlite). Remember, none of these media is soil.
Is silt good for growing plants? Yes, for outdoor application silt is an excellent soil component due to its nutrient retention capability. However, should never be used alone but always in a mix with the other 2 soil types.
Can I buy sand, silt, and clay separately? Especially silt and clay are not very easy to find in the most common and larger retailers. It is possible to have access to them by querying construction companies.