Portland Oregon Landscape and Tree Care

Soil Care is Essential. About Compaction.


Copyright 2004 - 2016 by Mario D. Vaden


This image is an illustration representing soil particles of different sizes. The small red particles represent clay: the smallest particles. The angular gold particles represent silt, the next size up. And the single large piece represents a piece of sand, the largest of the 3 kinds. In reality, the size comparison is vastly different. Clay, comparative to sand is exceedingly smaller than I can convey here. Sand is enormously bigger than clay. The biggest individual sand particle (just one) can cover hundreds of clay particles.

soil care particles

The illustration may help convey that clay, although slow draining, does have pore spaces. And for that reason, even a single piece of sand can seal off hundreds or thousands of pores in soil.

Compare this advice with: French Drains

The inspiration for this topic stems from comments of people who believe that adding sand to clay soil will improve drainage. Many of them formed that idea from watching golf course workers put sand on greens. What they don’t realize is that many golf greens are comprised of mostly sand and the procedure is to drag sand into openings, not neccessarily coating the surface.

Soil information is applicable to many topics including drainage, fertilizing and beneficial microorganisms in soil. We will be learning or reviewing some essential information about soil. This knowledge has been “skimmed from the top” of the large soil science subject to teach the essential components of this guide.

This chapter could have been called “Essential Soil” but soil is not essential for plant growth. Plants and tree essentials include water, air and nutrients. Plants can grow without soil. Often, we see young trees and weeds growing in wet roof gutters or in moist gravel of decorative paths. Plants don’t need soil and this is important to understand.

Soil is not “the” medium that plants must grow in (consider mistletoe which can grow on trees). But soil is one of the most recognized mediums for plants growth in the Pacific Northwest.

What is soil? Soil, earth, ground – whatever you want to call it – varies from region to region. There are more variations and classifications than most people would care to hear about.

There is a difference between what soil is and what soil contains. For example, soil can contain air, moisture and bacteria, but none of those are soil. Soil is not air. Soil is not moisture. For the sake of simplicity, soil is comprised of varying amounts of sand, silt and clay. In addition to those, soils my also contain organic matter, humus, bacteria, water, nutrients, pebbles and more. But the main components of soil are sand, silt and clay.

In general, most soils contain varying amounts of these certain components; clay, silt and sand. The clay, silt and sand particles are called soil separates. The different proportions of these three kinds of particles in a soil determine its texture.

For example, if a soil is comprised of 70% clay, 20% sand and 10% silt; that soil is within the texture classification of clay soils. If another soil is 10% clay, 20% sand and 70% silt; that soil is within the texture classification of silt loam soils. If a soil is 100% clay, 0% sand and 0% clay, that is also a clay texture soil.

There are thousands of other possible combinations but all soils contain from 0% to 100% of each of the three soil separates which are clay, silt and sand.

Clay particles are the smallest and are all soil separates less than 0.002 mm in diameter. Silt particles are the next size up from clay. Silt particles range from 0.002 mm up to 0.05 mm in diameter. Sand particles or separates are larger yet – bigger than both clay and silt. Sand particles range from 0.05 mm to 2.0 mm.

A millimeter (mm) is approximately 4/100 of one inch. Whether or not we fully understand these measurements; these particles are tiny.

Soil particles may be tiny, but there are a lot of them – exceeding millions, billions and trillions. And if soil has not been compacted (crushed, squeezed, compressed), there can be more than millions or trillions of tiny air spaces between the sand, silt and clay particles. Those air spaces called pores are as important to plant health as the soil is.

Air space (pore) preservation goes hand-in-hand with soil preservation. A soil without pores is like the “kiss of death” to a plant; a tree. A soil without pore space may as well be a rock.

Imagine that a machine could shrink our bodies to a microscopic size; smaller than a flea. If we were smaller than fleas, soil particles would appear much larger to us; we could experience the difference. If we could be miniaturized, then soil particles would look as large to us as the objects in the photo above. To “microscopic people,” a sand particle would look like a refrigerator. A silt particle would be comparable to a watermelon. And a clay particle might seem as the size of a grape.

Since we are not microscopic, we can only understand this with the use of microscopes, learning, diagrams and our imaginations.

As you see from the measurements provided, soil particles vary in size. No matter what method we choose to understand the difference, silt particles remain larger than clay particles and sand particles are enormous compared to clay particles. The order of size from smallest to largest; is clay, silt then sand.

The pores or air space between soil particles is where water, air, humus, organic matter, microorganisms and roots will be found. Nutrients such as iron, nitrogen or phosphorus will be within the pores or on the surface of the soil particles. Since everything important to plant growth happens in the soil pores, it is vitally important to preserve the condition of soil.

With these basics, we can understand why topdressing a lawn with sand can ruin drainage. If a lawn has quite a bit of clay soil, it will be reasonably well draining or slow draining. Either way, it still has some drainage even if it’s as little as 1/100 of an inch of water per hour. The pores between the clay particles are tiny, but they are there. If sand is deposited on the surface, the sand grains will each cover hundreds or thousands of clay soil pores. Thousands of sand grains will cover millions of pores. So sanding is not an option for improved drainage. In light of that, topdressing with sand can work contrary to soil stablization by making soil more saturated.

If better drainage is desired (removal of surface water), try these:

1. Aerate the surface with an aeration machine or tool that punches cores of soil out. This will punch through the surface layer which often compacts and impedes downward movement of water.

2. Lime the ground exceedingly; well in excess of rates for pH correction. Lime can chemically shatter, “break open” or aerate soil by initiating capillaries or tiny fissures. Organic matter does a similar thing for soil in beds and gardens by leaching a complex sugar.

3. Install small French drains or catch basins in low areas. Sometimes in lawns, catch basins are undesirable. Consider using a French drain with a sand filled trench. Sand is a bad topdressing material on clay, but not in a trench where the entire content is sand. Just be sure to cover the drain tile with a fabric designed to prevent sand from clogging the perforated drain pipe.

Just to clarify about topdressing with sand; golf greens don’t compare to most residential lawns. On a golf green, the sand is not merely deposited. It is dropped in a thin layer, allowed to dry, they dragge or brushed into open aeration holes after machines have punched holes and removed the soil cores. The sand is dragged into the holes to keep the open holes from compressing shut. This allows water and air to move through the sod or thatch layer of the green more efficiently.

Sand can have it’s uses in soil such as gardens. If sand is added, be sure to add a lot of it. Even to where the sand is 50% of the soil content. When sand is that abundant, the edges of the sand grains can bump together and prevent greater compaction due to pressure caused by walking or rolling equipment.

But realize this: sand is little more than rock. Water cannot go through sand, but must go around the individual particles. Sand does not retain moisture and nutrients very well. So additional sand content means more material in the garden that does little for plants.

It can be better to add organic matter to clay. Many compacted clay soil areas have been converted to productive gardens by cultivating the clay soil and incorporating organic matter like composts. These are some of the finest soils.

Sometimes people choose the option for a sand lawn. That’s fine with light-rate increase-frequency fertilizer applications. And, the sand must be put down thick. Again, a thin topdressing on a clay-like soil will seal it. A thick application of sand will also slightly seal a clay soil, but not near as bad if the sand is spread thick in 4” to 6” layer. The top 1” may get a bit dry in summer, but the grass roots will easily be able to grow down several inches to the moist sand below. The nice thing about a sand lawn is that it really can’t compact. It’s like a boulder pile, but miniaturized. It just won’t compress much at all.

It would be a good idea to read our information about drainage too.

French Drains – Installing Effective Drainage - Functional Dry Creekbeds


There are many kinds of soil and each is different. There are some that won't compact. Like some soil in Ruch, near Jacksonville, Oregon, where an abundance of rock-against-rock prevents surface compaction. Or a few redwood groves where surface roots are very abundant and keep foot traffic off the soil.

Most soil that people grow landscape plants in will compact. Soil comprised almost entirely of sand is fairly resistance to compaction, but much of our landscape soils have silt and clay also.

Compaction occurs when soil particles are compressed closer to one another and air or pore spaces are reduced or eliminated. Reduction of soil pore space is bad for drainage, irrigation and movement of gases.

Water tends to lubricate soil particles and wet soil has greater potential for compaction that dry soil.

Here are ways to prevent or reduce soil compaction:

1. Don't drive heavy equipment or vehicles over soil, especially when the ground is wet. If this is unavoidable, schedule the traffic for when the soil is dry. Pressure can be reduced by laying multiple sheets of plywood over the soil surface.

2. Keep soil covered with mulch. Even rain drops can compact the soil surface and mulch prevents rain from impacting the soil particles directly. A thick layer of mulch can reduce pressure from foot traffic in areas where people walk through shrub beds. It's similar to how snowshoes work. Snowshoes disperse pressure from a foot over a larger area. Mulch disperses some pressure from the perimeter of a shoe sole outward – not near the degree that a snowshoe does, but it still helps.

3. Try to avoid mowing lawns when the soil is saturated, or the wettest, such as immediately after rain. Sometimes this is not possible but do this if you can.

4. Install adequate drainage systems whether it's one perforated drain line, a catch basin or light grading to prevent standing water. Standing water provides a heavily lubricated condition that is prime for compaction to increase. These are some of the worst areas in a yard, because soil in puddle areas compacts easily. Then the added compaction traps even more water which in turn escalates the compaction more.

5. Adjust each irrigation zone so your soil becomes moist from watering, but not saturated.

6. Instead of fertilizing once or twice a year, distribute the same yearly total of fertilizer into 6 or 10 smaller applications. All fertilizers EVEN ORGANIC FERTILIZER produce salts. Salts kill beneficial microorganisms in the soil. When those microorganisms remain alive, they occupy space within soil pores and provide resistance to compaction in soil (Pretend we have 100 men packed into a living room of a single story house with a flat roof. And pretend we lowered a 4000 pound car onto the roof with a crane. If all those men pushed on the ceiling with only 40 pounds of pressure per man, they could keep the ceiling from caving in. That is how simple this principle is.) If fertilizer is put on in smaller doses, there is less toxic salt leaching down after each application.

7. Try to reduce pesticide use. Pesticides also kill microorganisms that help soil and aid resistance to compaction. You may have heard that microorganisms will break-down or destroy pesticides like herbicides. And this is partly true. But the rest of the story is that those particular microorganisms are not always present in soil, and even if they are, millions and billions of good microorganisms still die from the chemical application. Herbicide reduction can be as simple as spot spraying individual lawn weeds instead of broad full coverage with spray or granules.

8. Have your lawn soil aerated with tines. Use a machine with hollow tines that lifts cores from the ground. This process does not eliminate compaction. It lifts good soil and partially compacted soil that breaks apart on the surface. This breaking apart is when the soil compaction process reverses as the core crumbles and disintegrates. Core aeration also helps irrigation and rain water infiltrate soil better. This works toward the reduction of standing water. DON'T USE SOLID SPIKE AERATION EQUIPMENT including the strap-on shoes with spikes on the soles. These don't pull cores but do compress and compact soil so the sides of the holes as the spikes enter the soil with force. Solid spike aeration equipment does the opposite of what it is sold to do. It is sold to aerate, but instead, it directly compacts soil particles.

9. Bridge traffic over soil. Often, small bridges or boardwalks are built over soil to prevent soil compaction. This is done for trees too. And it works - preventing thousands of footsteps from compressing soil. Compressed compact soil traps gasses in soil and that is harmful for trees. A bridge can be suspended only two inches off the ground and still be very effective.

10. Installation of paths or step stones can encourage or channel foot traffic into one small area. For example, people may be walking all over the turf, compacting soil, like at a university campus, or front yard where delivery or mail is carried. Many people have a tendency to stay on a path, reducing compaction randomly all over the area.

One last bit of advice - don't rototill soil until it becomes dust:

Many soils have a structure. When you break or squeeze a clod of soil, it can break into little fragments, crumbs or particles. As you applied pressure, it broke at small fractures, pores or cavities. That quality is good.

Some soil is so compact, that if you threw a moist clod at a brick wall, it would make a “thud” sound and drop to the ground without breaking into fragments. A soil like this is considered a “massive” soil and often has no structure.

Sometimes people rototill soil excessively so that it breaks down beyond just fragments, but even into dust. This is referred to as destroying soil structure.

Using too small of a rototiller in hard soil can result in over-cultivating, because many passes have to be made to get the tines deep into the ground. In a firm soil, use a large rototiller that has the power to rototill deep and break the soil into crumb size particles rather than dust.

A tiny rototiller like some of the 2 stroke engine types are okay in loose soil where one or two passes is sufficient.

Preserving soil structure is beneficial for drainage and the exchange of air and gases for soil and roots.

Damaged soil structure can be improved with gypsum, lime or organic compost. These help the soil develop fracture spaces and aggregated particles.