Does Topsoil Need to Be Compacted?

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We’ve published numerous articles about compacting dirt over the past few weeks. However, all of those were targeted toward compacting dirt for making a patio or laying pavers. What if you need to plant fruits and vegetables? What if you need to grow grass or make a garden? Does soil need to be compacted?

When discussing soil compaction in terms of plants, the first most important thing to keep in mind is that not every type of soil is suitable for them. Only topsoil is used, i.e., the soil removed off the top 12 inches during construction projects. It is enriched and then used for growing. But does topsoil need to be compacted?

The answer to this depends on what you’ve already done for preparation of your plants. Topsoil needs to be relatively loose for plants to grow and therefore doesn’t require rigorous compaction. Too compact and roots may not be able to grow. However, too loose and the soil won’t be able to retain moisture.

Does Topsoil Need to Be Compacted: The Specifics

Topsoil put into retaining wall acting as vegetable garden planter
Topsoil put into retaining wall acting as vegetable garden planter

At first glance, topsoil may look ready for your plants and seeds. However, the problem is often much deeper than the top layer. Even if you insert your finger in the soil and check for compaction, you might be able to do so for the top 3 to 5 inches, at most.

This would mean that your seedlings may thrive in the beginning and show signs of healthy growth but as their roots try to penetrate the 5-inch barrier, they may find some resistance because of the compacted soil. This would impede their ability to grow.

New construction homes or newly constructed houses invite heavy foot and machine traffic, which in turn leads to compacted soil.

Every plant needs a loose soil structure. You don’t have to compact topsoil for good growth. Just remember to water the soil a few times before you actually plant anything. The natural settling process is enough for plants.

Grass roots require at least 3 inches of loose topsoil to grow in. They will automatically take hold of the soil and bring the soil closer together – you don’t have to do a thing. In fact, grass roots can become so strong that they’ll be able to handle foot traffic or light machinery as well – even without compacting. Herbs require 3-5 inches of loose topsoil and compost (at least) to grow. They will thrive, however, if they have more room to grow. 10 inches is more than enough.

For vegetables that grow above the soil, for example egg plants, okra, chilies, cabbage, spinach, mustard, legumes, and more require at least 6 inches to grow and 10-12 inches to thrive. The soil must remain loose and lightly compacted. Tomato, a fruit, falls under the same category.

For vegetables that grow below the soil, you need at least 10 inches of loose soil to grow and around 16-20 inches (at least) to thrive.

Fruits are extremely fragile when seedlings whereas, vegetables are relatively resilient. If your fruit’s seed doesn’t have the right conditions, it won’t even try to grow! You need very loose and light soil around the seedling for it to grow. The top 4-6 inches must be loose in order to facilitate seedling growth.

However, fruit seeds become much more resilient as they grow. No matter how much space they have, they will grow into it – even compacted soil. As a general rule, we recommend you give them 8-10 inches of loose soil just to be sure. After that, even if there is heavily compacted dirt underneath, the plant will make do.

What Do I Need to Do to Prepare My Topsoil?

While topsoil doesn’t need to be compacted, you will have to loosen it every now and then with a rake or a hoe. This is because weeds are very resilient plants and can grow in even the harshest of conditions – all they need is water.

As soil gets watered, dries up, and gets watered again, it creates a crust at the top that covers the mulch and other organisms inside. This is good since it prevents the growth of weeds. However, if the soil doesn’t get watered as frequently, chances are that the soil will start to harden. This happens when there is too much clay content in the topsoil and not enough water.

A garden planter with loose top soil
A garden planter with loose top soil

You will have to manually hoe your lawn every now and then – especially before planting a new plant. When hoeing, remember that right after you are done, your soil will have roughly 70% air content between particles. You are going to have to compact it down to 50%. The best way to do that is by watering it once or twice.

To achieve that, you should ideally hoe the existing topsoil and add a 3-inch layer of compost, or a mix of topsoil and compost. Mix the soil using a hoe or a 6-inch auger bit on your drill. Go at least 6 inches deep to give all your plants a fighting chance. This will mimic the conditions plants are more likely to find in forests. Water the soil once and wait a day before planting anything.

Don’t just lay a 3-inch layer of compost, soil, or a blend of the two and expect your plants to flourish. Not only will your plants suffer, you will also end up creating drainage problems.

Depending on the preparations you’ve already made to your area, you may not have to compact your topsoil at all. If you have raked the topsoil or dug slightly to remove all the weeds thoroughly, create a level surface and then walk over the soil once. Keep your feet close and try to cover the whole area at least once. Then, leave it to settle or simply water it. If, while walking, you find that there are any soft spots, relevel the area by dragging some fill there and then walking over it. Make sure all areas are level and you’re done!

It isn’t a good idea to use a plate compactor or a garden roller here. You don’t want that level of compression, just have to make sure that there are no dips or hollows. If your plants’ roots find themselves in a hollow, there is a very good chance that the root structure will start to die off from there, especially if it is a vegetable. Okra, eggplants, spinach, chilies, and other similar plants die if they find themselves in hollows or large air pockets.

We recommend that you wait a few days after spreading and compacting the topsoil and actually planting something. Keep watering the soil so that it doesn’t harden up but wait a while before planting. This is to ensure that the dirt settles and consolidates. If there are any weed seeds present, you will be able to single them out and remove them easily.

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About Charlie D Paige

Charlie is a massive DIY fan, with dozens of DIY projects under his belt - ranging from tiling to electrics, and concrete pads to walls. Charlie loves tinkering, seeing how things works, the outdoors and playing with power tools... so is it any wonder that he's completed so many DIY jobs over the years?

Charlie loves spreading his hard-won DIY experience with the world via this blog.