Cultivating Success: How Gardeners Triumph Over Clay Soil Challenges

Clay soil is distinctive in its goopy, sticky qualities when wet, and its brick-like texture when dry. It can wreak havoc on tools and backs alike. 

What causes clay to be so sticky and poor draining? Well, that has everything to do with clay’s unique molecular makeup.

Soil science

The author demonstrates the clay native soil in her beds

Clay particles are made up of minerals that stick together and are extremely small. This means the space in between particles is quite tight compared to the space between sand or silt particles which are much larger. This tight spacing is what causes clay’s notorious water holding capacity and hardpan.

This poor drainage can actually be used to your advantage though. One benefit of clay soil is that very little irrigation is required when the soil is properly mulched and protected from the sun.

I am fortunate with my farm that I only need to irrigate to get plants established in the beginning of the season, then I never need to water again. I will dig down into the beds periodically throughout the season to check on the moisture levels.

I find that if we haven’t received rain in a few weeks, the top two or so inches of mulch will be dry, but the actual native soil below will still be soft and damp with plenty of worm activity (worms can be used as an indicator to whether or not the soil is at the correct moisture level. Too wet and they can drown so they will not be present, too dry and they will also dry out so they won’t be there either). 

Uncovered clay soil will crack when dry

A double edged sword with clay is that anything you add will stay for quite a while, be it fertilizers or organic material (OM). This means you will spend less money on amendments than if you were working with sandy soil.

On the flip side, any chemicals or elements in your soil, like arsenic on old apple farms, will stay in your soil for much longer. This is why it is important to get your soil tested so you can know what is currently present in your soil before spending money on amendments, and so you can be aware of any potentially hazardous elements and chemicals.

There are two main methods for dealing with clay soil: working the soil, and building on top of the soil. There are pros and cons to both of these methods. 

Dirty work

Working the soil requires tillage or broadforking which can be tricky to get the timing right.

If the soil is too dry your tiller will either end up just bouncing uselessly against the hardpan or you’ll end up with a bunch of soil dust floating off into the air. If the soil is too wet, the tines will just slice right through and not actually move or invert the soil as intended. 

Additionally, clay is extremely sensitive to consistent mechanical work in the sense that if your tiller is always set to 6-inches deep, you will end up developing a subsoil hardpan right below that 6-inch mark. This means roots and water will not be able to easily penetrate deeper than 6 inches.

It will also take years of effort to improve drainage and porosity in the soil through adding organic material (OM), mulch (which will also break down into additional OM), and mechanical efforts. The benefits of working the soil directly is that you do not need to spend money on building beds or adding enough material on top of the clay to make a bed.

Build it up

With building on top of the clay, you create a space in which plants can immediately grow well. 

This is ideal if you are looking for a good crop starting in your first season. The OM and mulch you lay down on top of the native soil will not only leach into the native soil, but it will also invite biological activity which will ultimately help improve the native soil as well. 

Earthworm tunnels, for example, help aerate and break up the soil as well as create passages through which water can travel down into the subsoil, improving drainage. 

A shovel full of soil from one of the author's production beds with a distinct, thick layer of topsoil and the native clay soil below. Earthworm tunnels and soil aggregates are visible in the native soil.

Aside from the cost of building on top of the soil, the other drawback is that since the mulch and compost is simply sitting on top of the soil rather than having been integrated, early on it is at risk of being washed away in a heavy rain event. Over time this risk decreases as the bed integrates with the native layer.

Amendments and strategies that can help with clay soil include adding OM to add more air space and create soil aggregates that invite biological activity to help aerate and work the soil for you. 

Mulch and cover crops

Mulch is also incredibly important with clay soil because not only will it break down into OM over time, by keeping the soil covered you prevent it from drying out and turning into an impenetrable brick. If you can’t get a shovel through it, earthworms and other microbiological helpers can’t pass through it, and neither can plant roots. 

Cover crops are also helpful with adding OM and naturally tilling the soil for you. 

Tillage radishes and sunflowers have extremely thick and strong tap roots that can bust through clay (that is soft enough) and then rot in place adding OM to the native layer. 

It takes time and patience to see results with clay-busting cover crops.

A tillage radish from one of the author's production beds demonstrating straight growth, then a curve where it hit the native soil below the bed

My first season growing tillage radishes in my raised beds resulted in radishes that were beautifully straight and thick for the first 4 inches, where they grew through the mulch and OM filling the beds, but then the root turned where it hit the native soil since it was so hard. Each season it gets a little bit better though as more biological activity takes place and more roots make it down into the native soil and rot in place. 

Gypsum not for everyone

An interesting note: you may have heard of adding gypsum to your clay soil to help break up the clay. It turns out, this is only effective in certain parts of the country. 

In the Piedmont and Appalachian regions, the soil cation charge is slightly positive so it will not bond with gypsum particles, meaning the effects of gypsum for clay soil don’t actually work there. 

In other areas where the cation charge is slightly negative, gypsum will successfully bond with the clay and help make the gaps between the clay particles larger so the clay is not quite so dense. So, don’t waste your money if you live near me!

Final thoughts

Figure out what your priorities and budget are when working with clay soil. If you are on a timeline for production or have incredibly rocky soil, consider building on top of the soil. If you have patience, non-rocky soil, and easy access to a broadfork and strong back, consider working directly with your soil. 

A newly built production bed in the author's field that had not yet been seeded or mulched floated a few feet during a heavy rain event that involved roughly 12 hours of standing water

If drainage is a really big issue for your planting location, you probably want to create some sort of raised bed to hold your beds in place and keep your plants from drowning. I had beds literally float away because they sat in water for so long. With that much standing water for that length of time, any plants would have easily drowned. 

Within one year of mulching pathways and building gorgeous soil in the beds, any major rain events only leave an inch or two of standing water that drains within an hour versus 4 to 5 inches that would stay for a day or two. 

Soil science is so cool y’all!