How Do I Improve Heavy Clay Soil in the Garden?

If you are gardening in clay soil you know that clay can be challenging and challenging is probably putting it lightly. Clay soil can be very difficult to work with – it’s heavy, it’s dense, it’s difficult for delicate plant roots to penetrate. The saving grace of clay soil is that it typically is fairly high in nutrients and does hold moisture. The key is unlocking those nutrients so that they’re accessible to your plants and loosening up that soil texture.

I want to share with you methods for improving clay soil in the garden. You will know you have clay soil when you dig down and grab a clump and when you go to try and crumble it, it typically does not crumble but rather will form into a sticky ball.

Now one of the most common tidbits of advice given for dealing with clay soil is to add organic matter. It may not be entirely clear what exactly is meant by organic matter and there are so many options, how do you know where to start?

Organic matter basically refers to anything from nature, so plant or animal material and these materials are ideally used after they’ve  partially rotted down or in a composted state. And as far as what the best option for you, is going to depend on where you live and what your resources are. So my advice is to use what you have. Great options include biochar, earthworm castings, composted wood chips and good old compost.

How do you add organic matter?

Well the best and easiest way that I’ve found is just to add a layer of organic matter, at a minimum two to three inches thick you can go up to six to eight inches, to the top of your beds  and then dig or fork that in to incorporate it into the top soil. Now if you’re using compost or composted animal manure, I will typically add that to my beds in the spring before planting. With that you really don’t even have to work it into the soil. Just dump a layer on top and it will act as mulch and add nutrients and organic matter to the soil eventually working its way down  into the top soil.

Now adding organic matter to the soil helps improve the structure of clay soil in a couple different key ways. First, the addition of organic matter creates aggregates of the soil particles, physically separating them and allowing for better drainage and tilth. Which is kind of why you get this crumbly effect versus the sticking together effect of the clay soil.  Secondly, the addition of organic matter serves as food for earthworms and other beneficial soil microorganisms. Through this feeding process the worms and other beneficial organisms are converting that organic matter into nutrients essentially fertilizer for the plants in your garden. Worms in particular also improve soil structure and aeration by way of the holes that they dig while they’re tunneling through the soil.

Please ignore the all too common advice of adding sand to your clay soil to improve texture. Soil scientists warn that the ending result can actually be worse to deal with than the clay soil itself. Another tactic commonly used is to raise it up your beds or your rows or wherever you’re planting, even if it’s just a little bit. As you may have noticed clay tends to get waterlogged and compacted very easily. Raising up your beds helps fight these issues in two different ways.  Raised beds tend to drain more quickly.

As I mentioned clay soil is dense and sticky and tends to not get enough oxygen. Because of this texture, aerating is one of the best things that you can do for clay soil particularly when you’re starting out. Now, aeration is simply introducing pockets into the soil so that oxygen can penetrate. Now as I mentioned earthworms will do that job for you. Other techniques used for aeration are using tools like garden or digging fork or a broad fork and utilizing clay busting plants. A broad fork and a digging fork work in similar ways, in that they are introducing air pockets into the soil without disrupting it to the point where you’re causing damage.

And another great method of aerating the soil is using clay busting plants. Daikon or tiller radish is one of the ultimate clay busters. It has a nice long tap root that really busts through that clay soil and when those roots rot away they leave these nice big pockets and add organic matter to the soil as they rot. There are plenty of other crops that you can use to bust up clay soil. Anything with a long deep taproot and plants with extensive fibrous root systems.

And last but not least, cover that soil. Never ever leave soil exposed in the garden. This is important for all soils, but of particular importance for clay soil. If clay soil is left exposed, the heavy rains that we tend to get in the spring and fall will further compact and erode our valuable topsoil. By heavily applying natural mulches, you’re avoiding those two problems as well as adding valuable organic matter to the soil.

In adding organic matter, grass clippings are a favorite of mine, as well as leaf mulch, straw and chopped up cover crops. Focus on at least getting everything heavily mulched in the fall. If you come to plant in the spring and the mulch is still too heavy just pull back a little area so that you have room to plant and leave everything else in place. But if you live in an area where like slugs in heavy mulch are a problem, in the spring you can pull everything off your beds and just throw it in  your compost pile.

By taking these steps, whether it’s all of them or just incorporating a few,  you will begin improving your clay soil almost immediately. But don’t expect amazing loamy loose soil your very first season. Improving clay soil is a process and it does take some commitment. 

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