Is Ground Frozen Solid: Determining If Soil Is Frozen

By: Susan Patterson, Master Gardener

No matter how anxious you are to plant your garden, it’s essential that you wait to dig until your soil is ready. Digging in your garden too soon or in the wrong conditions results in two things: frustration for you and poor soil structure. Determining if soil is frozen can make all the difference.

How do you know if the ground is frozen solid? Keep reading to find out how to tell if ground is frozen or not.

How to Avoid Digging in Frozen Soil

Although it may seem as if spring has arrived, it’s important to test the soil for readiness before working your soil or planting your garden. Several very warm days in a row may lead you to believe that the ground is ready to be worked. Be very leery of any early spring digging, especially if you live in a northern climate. Determining if the soil is frozen is paramount to your garden’s success.

How to Tell if Ground is Frozen

Just walking across your soil or patting it with your hand will give away whether it is still frozen or not. Frozen soil is dense and rigid. Frozen soil feels very solid and does not give way under foot. Test your soil first by walking on it or patting it in several locations. If there is no spring or give to the soil, it’s probably still frozen and too cold to work.

It’s best to wait for the ground frozen solid to break up naturally than to try to rush it out of winter dormancy. Soil that is ready for planting is easy to dig and yields to your shovel. If you begin to dig and your shovel seems to be hitting a brick wall, it is evidence that the soil is frozen. Digging frozen soil is hard work and the minute you realize you are working way too hard just to turn up the soil is the time to put the shovel down and exercise some patience.

There is never any sense in getting ahead of the natural sequence of events. Sit back and let the sun do its job; planting time will come soon enough.

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Read more about Soil, Fixes & Fertilizers

Overview: Soil and the Spring Thaw

You have all seen highway signs indicating the period of spring thaw and reduced truck loads throughout the northern US and all of Canada -- or walked across grass that is just beginning to thaw from the winter making deep footprints or tire marks that would never sink like that during a strong summer rainstorm. You may have noticed decks that moved or gates that stick, then un-stick. Some things sink, some rise and some do both. What is it about the spring thaw that makes landscaping so unpredictable and delicate?

Some cold climate science first, and then we will see how that affects our day to day structures in the north.

The first three to four feet of the crust of the earth receives heat from two sources: the sun and the core of the earth. In most of northern North America, the core of the earth sends up heat at around 7-12deg C (45-54degF) all year round -- well above freezing. It is the temperature above the soil that changes with the seasons. As freezing temperatures set in during the beginning of winter, the soil begins to freeze, from the top down. Since there is always heat from the bottom, it takes continuous cold from the top to drive the frost line lower and lower.

If there is no clay in the soil, the freezing moves down rather steadily, with little movement in the soil. When there is a lot of clay in the soil, things change. Clay soil freezes in layers, called ice lenses, drawing water up to the forming lens of ice and actually drying out the soil an inch or more lower down. Only when the soil under the lens is sucked dry will the cold temperatures move deeper into the soil and begin to form a new ice lens. Each lens expands, just like an ice cube in the freezer forms a bump on the top, and pushes the lenses above it even higher -- and hence we get frost heaving. For more details check out ICE LENSES.

Although ice lenses can grab onto the sides of posts and even foundations to lift them up, we avoid the more direct formation of ice under structural supports by putting our house foundations as well as fence (Fence post depth ) and deck (Supporting Outdoor Structures ) supports below the local frost depth.

Now let's go to the spring thaw. We still have a small constant heat coming up from below trying always to thaw the ground, but the real thaw comes more quickly from the top down. Imagine this block of ice that is sitting there just below the top of the ground. Depending on the depth of frost in your locality it might be as shallow as 6 inches in Vancouver to as deep as 4 feet in much of the rest of Canada. No rainwater or snow melt can percolate through this block of ice to flow into the water table. But the rain and the melt keep coming. That means that the little layer of thawed out soil on the top is quickly completely saturated. This is the marshy soupy landscaping that you should simply stay off of for a couple of weeks to avoid damaging it.

If there is a fair amount of clay in this soil -- it will expand and paving stones and other surface landscaping will probably heave temporarily upward, until this soil begins to loose its excess water. It is best to let this soil drain to a normal moisture content before undertaking any repairs, like that jammed gate, because clay soils will shrink as they loose water and the problem may just go away. If you walk on paving stones that are on a mushy foundation, you will force some of that clay out to the sides, and when it shrinks back to normal, the paving stone will drop lower than normal -- if you stayed off of it until the thaw was well advanced, this doesn't happen and the heaved stone will just go back to where it came from.

All saturated soils are too fluid and basically unstable. We actually have to wait until that hidden block of ice under this area finally thaws out, letting the water flow downwards and drain the surface before working on it or even trying to access winter damage. Even the highway trucks in our northern climate need to carry lighter loads until the frost is gone allowing the water to drain and the soil to return to its normal strength.

This frozen landscape that can't percolate off surface water is also the cause of most flooded basements. In this case, simple landscaping to run surface water away from the house will handle most of the spring basement flooding problem: Yard Drainage.

Why Soil Temperature Matters

Knowing the precise soil temperature is useful for getting your seeds to germinate, whether indoors or outside. It’s also a helpful guide for how well your seedlings or transplants will do if planted outside. The right soil temperature makes all the difference. If the soil is too cold, some seeds won’t germinate. Tomato and eggplant seeds, for instance, require warmer temperatures to sprout and might rot if left in cold, damp soil.

Sometimes checking the weather forecast doesn’t do it. It may look like spring has arrived, but the soil may still be frozen. Testing the soil temperature can be a more accurate guide than abiding by the last frost date info for your zone.

Did you know that just because the ambient air temperature is nice and warm that the soil temperature may not match up? In the winter, for instance, the soil helps to insulate the roots of overwintering plants. That’s another reason it’s vital to test the soil.

Which Fish are Suitable to use?

Now that you have a background of what fish fertilizer is, let’s take a look at what kind of fish you will be using. When it comes to fish, commercially, there are two options for companies to sell fish.

The first is the fish that people eat. These are usually all the fish that you know about like tuna, sardines, tilapia, and salmon. Now in many instances some of those fish are used in fish fertilizer in some way. This can be from a lack of demand for human consumption, or just the scraps being used like head, scales, and fins. Most of what is used in fish fertilizer is junk fish, scraps, or fish that cannot be consumed by humans because of high toxicity levels. For example menhaden spend a great portion of their lives in water that is contaminated with metals. Therefore they make perfect fertilizer, but not the perfect meal.

Fish fertilizer has many advantages for you and your garden. On top of that, a hidden advantage is the benefits it is doing for the mini ecosystem that is your backyard. Fish fertilizer is simply one of the best ways to grow quality crops and help renew the nutrients in your soil.

Fish fertilizer can also be 100% organic if you are self fertilizing or find the right brand to use. Fish fertilizers are have slower release rates than other types of fertilizer, therefore do not have to be applied nearly as often. When applying fertilizer like fish hydrolysates you are coming about as close as you can to burying a whole fish. Microbes love to feed on the organic matter and this makes for very healthy soil and very healthy plants. While fish fertilizer has a lot to offer there is also a downside.

How to Till a Garden

Follow these 11 steps to best till your garden:

    1. It’s best to prepare your spring garden in the fall and sheet mulch before rains set in.

To sheet mulch, first determine where your new garden will grow, remove any large weeds or shrubby plants, and wet soil using Gilmour’s Heavy Duty Front Control Watering Nozzle and Flexogen Super Duty Hose.

    1. Next, place a layer of cardboard over the soil surface.

The cardboard is a natural weed barrier and a source of carbon.

    1. Top the cardboard with one to two inches of weed-and-seed-free grass clippings and organic compost.

Then add a 1- to 2-inch layer of mulch over the compost, using leaves, bark, or another organic mulch.

    1. Again, wet the newly sheet mulched area using your watering nozzle and hose.
    2. Let nature do its work through fall and winter until spring.
    3. When weather warms and the soil is dry, double-dig your new garden area.

This is much easier to do after a season of sheet mulching. To double-dig, dig a 12-inch-wide trench with a spade that reaches to the depth of the spade (about 8 inches). Dig the trench from one end of your new garden to the other end, placing the soil from the trench on a nearby tarp or in a wheelbarrow.

    1. Use a garden fork to loosen the soil at the bottom of the trench.

Once loosened, but not turned, sprinkle a 2-inch layer of compost into the trench and gently work it into the soil.

    1. Next, dig another, separate trench directly adjacent to the first trench.

Again, dig this trench so it runs the full length of the new garden. As you dig, place the soil from this second trench into the opening of the first trench. It’s best to avoid mixing the soil when moving it from one trench to the other, so let it slide off your spade rather than turning it. This will help maintain beneficial soil structure.

    1. Work the base of the trench with a garden fork and spread a 2-inch layer of compost.
    2. Repeat steps 8 and 9 until you’ve covered the entire area of the new garden.
    3. Fill the final trench with the soil from the very first trench (the soil placed on a tarp or in a wheelbarrow).

Before planting, spread a final 2-inch layer of compost over the entire bed.

All About Frozen Ground

  • Introduction
  • Physical Science and Frozen Ground
    • How Does It Form?
    • What Affects It?
    • Where is It?
    • How Does It Affect Land?
  • Climate and Frozen Ground
  • Ecology and Frozen Ground
    • Plants
    • Animals
  • People and Frozen Ground
  • Studying Frozen Ground
  • Methane and Frozen Ground
  • About, How to Cite
  • Further Reading

Methods for Adjusting Soil Temperatures

Just because the soil is naturally a specific temperature, that doesn’t mean you can’t influence things a bit. Here are a few methods to warm or cool your earth.

Warming the Soil

What happens if it’s early in the season and the soil is still cold, but you want to start planting? There are many tools for giving your soil an extra boost of warmth, such as:

  • Plastic mulch
  • Row covers
  • Hoop houses
  • Greenhouse
  • Cold frames
  • Cloches

Each of these allows growers to get a head start on the season without waiting around for the sun and ambient temperature to work their magic on the soil. In some cases, these tools can significantly warm the ground by several degrees. Take caution to air out enclosed growing spaces, however, when days get super steamy. And don’t forget to water your seedlings and plants even if they’re under plastic row covers or cold frames.

The best way to ensure your soil is warm enough and ready in time for the gardening season? Choose an appropriate garden location that gets enough sun!

Cooling the Soil

What about altering soil temperatures to keep the soil cool? While positioning plants in the shade is a possibility (use containers that you can easily move around to do this) it’s not a practical solution for large raised beds or in-ground gardens.

If you want to grow plants that prefer cooler temperatures, like lettuce or spinach, it’s often tough in the high heat of the summer. It’s also more challenging to cool the soil than to warm it, but it is possible to offer up a bit of respite for heat-sensitive crops.

Water retains heat and moistened soil may be cooler than the air, which is why mulching is helping for plants that prefer cooler temperatures. Mulch has helped me prevent my lettuce from bolting too quickly in the summer.

Shade cloth is another superb tool for keeping the sun from harshly affecting your plants.

Indoor Soil Temperature

If you’re indoors and finding it tough to start seeds because your growing area is too chilly, heat mats are an excellent option. They’re a must-have accessory in my seed starting toolkit because my growing space happens to be in my basement.

It’s often much cooler downstairs than it is upstairs and while some seeds are happy to sprout in those temperatures, I’ve had trouble germinating nightshade plants in the past. I use my heat mats expressly for tomatoes, peppers, and eggplants.

Now that you have all the knowledge you need to test and manage your soil temperatures effectively, it’s time to get planting. Be sure to share any tips you have on getting your seeds sprouting in the comment section below.

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