Tony O’Neill, gardener and author of the popular “Composting Masterclass” and “Your First Vegetable Garden,” combines lifelong passion and expert knowledge to simplify the art of gardening. His mission? Helping you cultivate a thriving garden. More on Tony O’Neill
Clay soil is rich in organic material, but the very fine clay particles pack so tightly together that they limit the air that can reach roots.
Clay soil has several benefits, including high carbon content and improved cation exchange capacity (CEC), enabling it to hold nutrients better than other soils. Unfortunately, the same CEC also binds water, a root health challenge. However, heavy clay soil can be remedied, and here’s how.
Gardening soil structure is a proportional mix of clay, sand, or silt. Clay soil has little sand and silt content, whereas loam soil is a healthy blend of all three. Technically, the different proportional combinations of these three substances allow us to define soil in one of 14 categories.
Clay particles are super fine, and sand particles are the largest. Comparatively, if a sand particle is a baseball, silt is the size of a pea, and a clay particle is the size of a pinhead. Soil texture influences its response to water, air, compaction, and nutrition. Below are the characteristics of the different base textures:
|Field Capacity & Saturation Porosity
|Poor water retention
|Clod Forming Inclination
|Fair to Good
|CationtoExchange Capacity (CEC)
A word of caution! Mixing sand and clay doesn’t improve your clay’s drainage potential. Considering the illustration above, millions of pinheads can fit around the baseball. Therefore, adding sand has little effect on aeration or water drainage levels. The best way to aerate and enhance clay drainage is by adding compost.
Problems With Clay Soils for Plant Growth?
Clay soils have the advantage of holding lots of nutrients and micronutrients. However, heavy clay soils can be challenging and cause plants to underperform. The main challenges are:
- Clay soil compaction
- Clay particles are so minute they limit soil air movement
- Clay soils hold too much water, causing root rot
Clay Soil Compaction
Clay soil comprises many microscopic particles that combine and compact, limiting water penetration and drainage. After dry weather, clay is slow to rehydrate and, once wet, slow to drain.
Dry clay soils are difficult to work with and become so compacted that preparing a garden bed becomes challenging. On the other side of the spectrum, wet clay is so compact that it is much heavier than different soil, making it equally difficult to work with.
There are solutions, and we’ll explore them later in this article smaller the soil particles, the more compaction that occurs, although this soil type. Over time these particles get smaller and smaller, which makes the soil even worse. It can be seen during the summer when your soil goes as hard as concrete and cracks.
Clay Soil Air Movement
Leaf chlorosis (yellowing) is mainly caused by plant roots that are suffocating or drowning. When the soil pores are too small or filled with water, the soil cannot trap any air, and roots cannot breathe. Because clay soil particles are microscopically small, they leave no space for air.
Compare this with sandy soil where the pores are large. However, sandy soils can have poor structure and therefore needs aeration to keep channels open and air in the profile. Silt soils develop design but often weakly, being prone to compaction and hence require aeration and decompaction.
The best soils for structure can be clays, but their drainage rates can be meager, especially in wet and compact conditions. Sand also doesn’t hold air well, and water flows through because the organic matter is so low. Loamy soil is ideal for most plants, providing root aeration, water retention, and drainage to growing plants.
Soil microorganisms also need air to breathe, and their presence boosts soil’s porosity and aggregate formation. Whatever your soil profile, introducing microorganisms will help improve drainage, pore formation, and strengthen aggregate formation – all essential for healthy plants.
Clay Soil Water Retention
A soil’s ability to hold water is a product of cation exchange capacity (CEC), which is affected by organic material content. Sand, on the one side of the spectrum, drains 20 times faster than clay soil, mainly because the CEC is low. Clay has a CEC about ten times stronger than sand, so clay soil retains water well.
When large quantities of water are applied to clay soil, the soil becomes sticky, further preventing air penetration. The ground becomes hard when it releases this soil or dries out, like concrete and cracks.
The three reasons above are the leading causes of why clay soils are unsuitable for plant growth. However, there is a caveat to this; we will cover that as we proceed. Starting with amendments
Soil Amendments for Clay Soil
As we saw, loam soils blend clay, silt, and sand. Can we add silt and sand to clay to create our loamy soil? Unfortunately not. Loam is a natural blend shaped over millennia. The best way of improving clay soil is to add organic matter annually, preferably in thick layers.
The most effective way of improving clay soil is by adding compost, a blend of organic materials (materials that contain carbon) and microorganisms. The organic materials boost aeration, and the microorganisms improve the availability of plant nutrients.
First, start amending the soil to improve water, air, and compaction. It is important to note that this is not a quick process. Essentially we are terraforming our little section of earth, which takes time. Adding wood chips can help, but without oxygen, they may take years to decay.
Rather than adding wood chips and grass clippings to improve clay soils, use these to make compost and add it with its microorganisms to improve clay soil.
Adding organic matter to clay soils is a perfect practice as this will lighten up the structure and improve air circulation and water retention.
What can we add to improve clay soil for plant growth?
One can add many different organic matter sources to improve clay soils. The following list is not exhaustive but is the main one that is readily available to most gardeners.
- Farmyard manure
- Cover crops (Green Manure)
- Organic Matter
- Leaf Mould
Farmyard manure covers all vegetarian animals such as cows, horses, sheep, rabbits, poultry, alpaca, and even pig can all be used to improve soil quality. It is essential to note. You should not add meat-eating animals such as dog and cat feces; these contain harmful bacteria and pathogens.
It adds organic matter that helps to break down clay compaction, hold water, and create air pockets within your clay soils. It provides food for soil life. In turn, it feeds the soil, adding more organic matter as they leave feces behind and their little bodies as they die.
Cover crops are plants grown to protect the soil surface when it is bare or dormant. It provides food for garden soil life and adds nitrogen to the soil; some are even biofumigant. These plants also can add tons of organic matter to the garden soil.
It is usually done by chopping down and incorporating the plants into the soil. If you prefer no-dig or no-till gardening, chopping and leaving on the surface until the soil life, such as worms, take it down and incorporate it on your behalf.
Many different cover crops can be used for this. It deserves a post of its own. Lucky for you, I have already written that post, and if you wish to know more about this in detail, click here to be taken to that post.
This is one of the most popular ingredients to improve clay soil for plant growth. It has the same benefits as farmyard manure, but you can make this from home garden waste. Add this once a year and allow the worms to take it down or incorporate it into the soil.
Older gardeners will dig out a planting hole two spits deep (the depth of two spades), add copious amounts of compost and backfill the soil on top. It is ok if you prefer to dig, but if you are like me, who likes to disturb the soil as little as possible, it’s better to add a layer around 2-4 inches above the soil.
While making compost can seem daunting, mastering the skill is the shortest route to making rich soil and productive garden beds. I have a very informative blog post here that will show you exactly how to get reliable and the best quality compost for your garden. Alternatively, watch the video below.
Leaf mold is an excellent way to incorporate organic matter into the garden for soil improvement. Unfortunately, you cannot buy this in a garden center. You have to make it yourself. Why is it so good? It can hold vast amounts of water, is light, and allows air movement.
Have you ever walked in a forest or wooded area? The ground will feel soft, like a sponge under your feet. These are decomposed leaves that hold nutrients, water, and air. A bonus is that Leaf mold is fantastic at controlling weeds by blocking out the light.
Soil life loves leaf mold, but leaf mold will also bring fungal spores to your garden; these grow underground, creating tiny tunnels as the roots penetrate new areas. These tunnels carry air through the soil. The video below will show you how to make your leaf mold at home.
Using the four ingredients above, you will make the clay soil very good for plant growth, but it is important to note that this is a long-term project. It will not happen overnight and may take multiple years of applications to get the clay soil into a good enough shape for good plant growth.
FAQs on Is Clay Soil Good For Plant Growth? The Facts
Other products to add to clay soil to improve plant growth?
As discussed, clay soils are full of nutrients. It is the one good thing about clay soils, but one product called gypsum can be added to your clay soil to help plant growth.
Gypsum is calcium sulfate and a liming agent that can break up the minerals and particles of the clay soil, stopping them from sticking together. Gardeners use this product, but it doesn’t get spoken about in the text for some reason.
Is Clay Soil Acidic or Alkaline?
A lot of people think that clay soils are neutral. However, clay soils are usually more alkaline, although some plants, such as hostas, grow well in clay soils. Most will struggle to grow in unamended clay soils.
Even though plants will grow, their growth in unamended soil will never reach their full potential and could show signs of illness, such as yellowing leaves.
Although clay soils have issues with plant growth, It is not a lost cause. You can vastly improve the air, water, and compaction abilities by amending the soil, making clay the perfect soil for plant growth.
Clay soil is nutrient-rich and suitable for many plants. Once we have done the work and added the organic matter, we will have good soil for which many gardeners would die.
I hope you enjoyed this blog post about clay soil. I trust it answered your question thoroughly. If this interests you, why not consider subscribing to the blog so you don’t miss future content?