Sand, Silt, And Clay: Differences You Need To Know


Soil is a crucial component for the success of your gardening experience. Hence, understanding the features and differences between the three primary types of soil (sand, silt, and clay) can make the real for a thriving indoor garden. This article has you covered. You will understand the main differences among these soils and how they types impact the development of your herbs. Let’s dive in.

What are the differences between sand, silt, and clay? Sand, silt, and clay, defined as “basic” soil types, differs for the size of their particles that vary from:

  1. 0.05 mm to 2 mm for sand
  2. 0.002 mm to 0.05 mm for silt 
  3. below 0.002 mm for clay

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 herbs 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 in very tiny pieces (fraction of millimeter) 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: 

  • Drainage
  • Aeration
  • Friability
  • Anchorage

Drainage

To better understand how each property works, I will use what I call the “ball pit comparison”. Think to 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 as 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.

Friability

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 the plant roots. It will be easier for them to develop. 

On the other hand, if the balls are very small (imagine to replace 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 to pass effortlessly.

Anchorage

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 much 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 my fall upside down with just a breeze. This is something you want to avoid, of course.

Effect On Herb 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 function of the soil type. 

For simplicity, I will introduce you to the differences between the two extreme soil type 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 any herb 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.

Sand is not able to hold water long enough to give a plant the chance to “drink it -Photo on Trend Hype

The herb 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 herbs (with few exceptions like tyme 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 boost your herb 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 benefical 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 herbs’ roots and vice versa (inadequate aeration). This is especially true if clay soil is not moved frequently. It tends to 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 herbs. Otherwise, they will have their roots wet for a long time (lousy drainage), ideal conditions for root rot.

Clay soil when dries out creates these typical cracks and get very compact

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 herbs. 

Effect on herbs: clay soil is another no-to-go for herbs. 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) neither dry (tends to become difficult to work with and for herbs to grow through as dry clay is very hard).

Particle Size: How Chemical Properties Are Affected

The success of your herbs 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:

  1. pH level
  2. Salinity
  3. Nutrient retention capabilities.

pH level

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 this “ions,” more acidic will be your soil. You should be familiar with “acidic” substances. Think about a 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). 

pH 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, a 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 herbs. 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 herbs: 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 herbs 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 herb to thrive.

Salinity

As the name suggests indicates the salt content in the soil. Herbs, as well plant in general, will suffer in the presence of high salinity level. 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.

High salt concentration in the soil can be harmful for your herbs

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 herbs: a salinity level above 1dS/m will challenge the ability of your herbs 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 herb very likely will die.

Capability To Retain Nutrients: The Pit Ball Comparison 

To understand the ability of a 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 herb needs nutrients. We also know that such nutrients need to present in the soil. How the soil keep those nutrients? Easy, the surface of each soil particle can be seen as a nutrient magnet (most soil 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 ball that more than compensate such aspect. 

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 bringing 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 to 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 herbs as water will flow slowly and so your plant roots will have time to drink it! You get rid off 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 a 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 specialist). 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. In 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.

The soil texture triangle used by many gardeners and soil expert

Soil For Your Potted Herbs? A Fake Soil

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 herbs 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, and water retention capability (compost) and drainage (perlite). Remember, none of these media is soil.

Do You Need Soil to Grow Herbs? The Water World Of Aquaponic

I discussed the property of soils given one aspect for assumed: herbs can only thrive in soil. However, believe it or not, this is not true! Many of the most common herbs you might have in a small pot on your windowsill (like basil) can thrive only with water, with no soil! This technique is known as aquaponics. Many common herbs like basil, mint, and parsley can fully develop with this technique. More on this in future articles.

Their plant roots will grow immersed continuously in water. Of course, water alone (tap or distilled) does not have the nutrients for any plant to survive.

Related Questions

Is silt good for growing herbs? Yes, for outdoor application silt is an excellent soil component due to its nutrient retention capability 

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.

Andrea

A young Italian guy with a passion for growing edible herbs. After moving to the UK 6 years ago in a tiny flat, it was impossible to grow herbs outside. So I start my journey in growing indoor and so I decided to share my knowledge.

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