Improving Soil

by Rod Smith
Oregon Certified Nursery Professional

© 1999-2014 Rodney A. Smith
All rights reserved.

Soil is foundational to every garden. Every healthy, productive yard and garden starts with healthy, productive soil. Preparing the soil properly makes more difference than any other thing you can do. You cannot put on enough fertilizer and water to make up for poor soil.

Since soils are so different in different areas, it is necessary to know what soil is, what your soil is like and what to add to improve it.


Soil is made up of four main components: mineral, water, air and organic.


The mineral component is non-living material. It is divided by the size of the particles, into:
Sand: Rounded particles 1/12 to 1/500 inch (2.0 to 0.06 millimeters) in diameter.
Silt: Rounded particles 1/500 to 1/12,500 inch (0.06 to 0.002 millimeters) in diameter.
Clay: Flattened particles less than 1/12,500 inch (0.002 millimeters) in diameter.
Loam: Mixture of the above in roughly equal proportions.

Sand has large spaces between the particles, which allow air and water to move easily, so sand has good aeration and drainage. Clay packs down with only tiny spaces between particles so there is poor aeration and drainage. However, clay has about 1,000 times the surface area as the same volume of sand. More surface area means that clay will hold more water and more nutrients. Silt has some of the qualities of both sand and clay. Loam combines the best features of all three: aeration, drainage and storage capacity for water and nutrients. Often, soil particles are clumped together into crumbs, which create large spaces between the crumbs for aeration and drainage. Pebbles and rocks also increase drainage; sometimes so much that plants wilt from lack of water. Rocks also may interfere with root growth.


The amount of water in the soil is described in these ways:
Saturated: All of the spaces in the soil are filled with water.
Field capacity: Excess water has drained away leaving a film of water on each particle and air in the spaces.
Wilting Point: The film of water on each particle is so thin that plant roots can no longer pull enough water from the soil, so the leaves droop.

Plants grow best when the soil is at field capacity. Vegetable yields have been doubled by applying controlled amounts of water several times a day to well-drained soil to maintain field capacity. A simple way to maintain field capacity is to punch a small hole in the bottom of a coffee can and bury it near a tree or plant. Filling the can every day should maintain the soil near field capacity within two or three feet of the can. However, this approach does not work well on heavy clay soil since it has poor aeration and drainage. Plants grow better on clay soils if watered to near saturation, then allowed to dry before watering again.


Air in the soil is made of the same gases as the air in the atmosphere. Air doesnít move as freely in the soil so there may be too much or too little of certain gases in parts of the soil. Plant roots need oxygen to absorb water and nutrients. The lack of oxygen limits how deep roots can grow. Roots may grow thirty feet deep in well-drained sandy soil, but most roots are in the top foot of clay soil. Oxygen is replenished in the soil when water forces the air out of the soil, then fresh air is pulled back into the soil as the water drains away. Large spaces between soil particles and crumbs allow soil to breathe better. Deep watering helps the soil breathe much better than frequent, shallow watering.


The organic component of the soil is made up of living and dead plants and animals.
Living: Bacteria, Fungi, Insects, Worms, and Roots.
Dead: All of the above as they decay.
Humus: Dead organic matter that has decomposed until it is very fine, black and sticky.

Bacteria and fungi extract nutrients from the soil minerals and make them available to plants. Mycorrhizae are fungi that are partly in the soil and partly inside plant roots. They can transport a very large amount of nutrients into plant roots. Insects and worms create air passages deep into the soil. The carbon dioxide produced by roots becomes carbonic acid, which breaks down minerals to make nutrients available. Dead organic material provides rich nutrients for the living. It also holds the nutrients from applied fertilizers until the plants can use them. Humus sticks the soil particles into larger crumbs so there are bigger spaces for air and water.

Dead organic matter is decaying continually, so it needs to be replenished every year. Excessive nitrogen fertilizer will cause the dead organic matter to decay even faster; so more organic matter will have to be applied. Also, careless use of pesticides may harm or kill the living organisms and disrupt the soil ecosystem.


Soil testing is done to determine how much of each soil component is present in the soil. Soil testing laboratories can give a detailed and accurate measurement of the soils components. However, these simple tests can reveal much about the soil.


Dig a hole 18" deep and wide enough that the soil can be seen. Sand can be seen or felt. Black color indicates high organic matter; gray indicates medium organic matter. Red, tan, yellow or blue color indicates little organic matter and high clay. Yellow color indicates that there is little oxygen in the clay soil while blue color indicates that there is no oxygen. Therefore, no roots will grow in blue clay. Normally, the organic matter is mainly in the topsoil. Forest soils have a one to four inch thick layer of organic matter on top of mineral soil. Grassland soils have a much thicker layer of organic matter mixed with mineral soil. Wetland soil may have a layer of organic matter several feet thick because the lack of air in wet soil slows down the decay of organic matter. A dark layer under lighter colored soil indicates that the original soil was covered by fill dirt.


Fill a glass jar about half full with soil. Fill to the top with water. Shake the jar, but do not swirl. Let stand. Sand will settle in about ten seconds. Silt will settle in two minutes. Clay will take hours to settle. Larger pieces of organic matter will float for awhile. The thickness of the layers indicates the proportions of sand, silt, clay and organic matter.


Soil pH is a measure of hydrogen ion concentration. It is tested either with a chemical pH test, or by a pH meter. 7.0 is neutral. 4.0 is very acid. 10.0 is very alkaline. High rainfall and high organic matter produces acid soil. Low rainfall and high lime or sodium produces alkaline soils. Soils in the Willamette Valley naturally have a pH between 4.5 and 5.5 which is fine for acid loving plants such as rhododendrons, azaleas, camellias and conifers. Lawns, roses, lilacs, fruit trees, vegetables and many flowers prefer a soil pH near neutral, 6.5 to 7.0. The majority of plant nutrients are most available at slightly acid to neutral, though iron is more available as soil is more acid. Soil can be made less acid by adding lime (calcium carbonate), or more acid by adding sulfur or aluminum sulfate. Aluminum sulfate is especially helpful to produce good blue flowers on hydrangeas. Adding lime produces pink hydrangea flowers. Lime will raise the soil pH in a few weeks or several months depending on how finely ground the lime particles are. Sulfur takes about a year to lower soil pH since bacteria are required to complete the process.


There are thirteen essential nutrients which plants get from soil. The six that the plants need the most of are called Macronutrients. They are Nitrogen (N), Phosphorus (P) and Potassium (K) Calcium (C), Magnesium (Mg) and Sulfur (S). The other nutrients, which are needed only in trace amounts, are called Micronutrients. They are Iron (Fe), Manganese (Mn), Boron (B), Zinc(Zn), Copper (Cu), Molybdenum (Mo) and Chlorine(Cl).

Nitrogen encourages leaf growth. Phosphorus encourages roots and flowers. Potassium encourages general vigor. If one of the nutrients is not available, then plant growth will be slower or stunted, and leaves will be discolored. For example, lack of nitrogen causes the old leaves to turn yellow. Lack of iron causes the new leaves to be yellow. Nutrient deficiencies usually form patterns on the leaves that follow the vein patterns: sometimes along the veins, sometimes between the veins. Disease symptoms do not follow the veins.

Soil laboratories will test soils for nutrients as well as pH and organic matter. However, a general recommendation is: All soils are short on nitrogen; shallow rooted plants such as lawns and flowers need extra phosphorus and potassium; iron and sulfur are often deficient, especially around acid loving plants. Usually, the soil contains enough of the other nutrients, although some may be deficient in certain parts of the country. Boron is sometimes deficient in the Pacific Northwest.

Fertilizers are available to provide extra nutrients. The exact numbers arenít as important as the proportions. A 5-5-5 fertilizer works the same as a 15-15-15, except that it takes three times as much 5-5-5 to provide the same nutrients. The optimum fertilizer for lawns has an N-P-K ratio of 3-1-2, such as a 15-5-10. Fertilizers with similar ratios are interchangeable, except that some fertilizers, such as rose food, contain extra calcium to raise pH and some fertilizers, such as rhododendron-azalea food, contain extra sulfur to lower pH.

The nutrients are identical whether they come from organic or synthetic sources, but the source will affect how fast the nutrients are available to plants. Ammonia sulfate and water soluble fertilizers release most of their nitrogen in a few days and may burn plants if too much is applied at one time. Blood meal releases its nitrogen over a period of months. Organic fertilizers and specially treated synthetic fertilizers are slow release so they will not burn and the nitrogen will not wash away in the rain before plants can use it. Also, phosphorus is quickly tied up into insoluble compounds, so a slow release fertilizer is a much better source of phosphorus.

When fertilizing trees or shrubs, spread the fertilizer evenly from the trunk to twice the length of the branches. There are feeder roots throughout the area, not just at the drip line. Water soluble fertilizers can be applied to the leaves since they will absorb directly. This will green up the leaves, but it does not encourage growth.


The ideal soil would have sand, silt, clay and organic matter in about equal amounts. It would also be uniformly mixed to at least twelve inches deep. The subsoil would allow the excess water to drain away. No soil is ideal but soil can be improved with soil amendments and drainage.


Amendments that are commonly added to soil are:
Sand, pumice or Profile Soil Conditioner: to improve aeration and drainage.
Compost: to add organic matter, nutrients and to improve aeration and drainage.
Lime or Sulfur: to raise or lower pH.
Fertilizers: to add specific nutrients

For flower bulbs and root crops, the soil should be double dug to fifteen to eighteen inches deep. Double digging involves digging up the top shovel depths of soil and piling it to the side of the hole. Soil amendments are spread in the hole and worked into the lower shovel depth. Then amendments are added to the piles of soil as they are shoveled back into the hole. Bulbs can be planted as the hole is filled.

Soil should be worked when it is moist but not wet. Wet soil will be packed into hard clods, which are about like rocks. To check the soil for dampness, squeeze a handful of soil into a ball, then push your thumb into the ball. It should crumble, not dent like modeling clay. Dry soil is harder to work and the soil disintegrates into dust. Dust will turn into mud when watered, then dry like brick. Dry soil should be watered well then allowed to soak in for a day before tilling.

Soil should not be worked into dust. Leaving marble-sized to golf ball-sized clumps with give better aeration and drainage. Besides destroying the structure of soil, over working soil also kills off the mycorrhizae which are beneficial fungi which help move nutrients from soil into roots. Roto-tilling may be required to incorporate amendments into the soil but more mycorrhizae will survive if the soil is loosened with a spading fork. Simply stomp the tines into the soil and tilt back the handle to lift and fracture the soil.

Sharp sand or Masonís sand creates spaces much better than river sand, which has rounded edges and packs down tighter. Pumice or Profile Soil Conditioner are even better, since they are porous so they hold both water and nutrients. Compost is made of partially decomposed yard debris, bark dust, sawdust, manure or other organic matter. Fresh organic matter can also be used, but extra nitrogen fertilizer will be needed to help it decompose. Fresh manure may contain too much uric acid, which can burn roots. It is best if manure is aged for at least six months.

The best and easiest time to improve soil is to do an entire area at once, such as when planting a new lawn or landscape. The amendments should be spread evenly over the area and worked into the soil at least ten inches deep. A spade or spading fork works best for small areas. A roto-tiller handles large areas. For existing beds, the soil can be improved every time something is planted. A planting hole should be at least three times as wide as the root ball and about the same depth. Mix soil amendments evenly with the soil from the hole. I always shovel the soil into large plastic pots or a wheelbarrow to make it easier to mix the soil, and the soil isnít lost into the grass or mulch.


Drainage and aeration is improved by soil amendments, but there must be some place for the water to go. Yards should be graded so surface water drains off. Drainage sensitive plants such as Japanese maples, daphne and heather should be planted on a mound at least a foot tall or on a slope. Vegetables are best grown in raised beds, which provide better drainage and warmer soil in the spring for an earlier start.

In mostly flat yards, subsurface drainage might have to be provided. Perforated plastic drainpipe can be buried about a foot deep. It is important that drainpipes slope evenly so dirt doesnít clog up the low spots. Drain pipes can drain onto the street or into down spouts if allowed by local building codes.

Aeration can be improved on existing lawns and beds without tearing up the soil. Sod core aerators cut out plugs of soil and leave holes about six inches apart and four inches deep. Root feeders can be used around trees and shrubs to create holes eighteen inches deep. These holes allow air and water to quickly penetrate deep into the soil. The holes will stay open much longer if sand or Profile Soil Conditioner is raked into the holes to fill them.


Good soil is as important as any other element in a landscape. Preparing the soil properly will do more that anything else to make plants grow and thrive. First, determine what soil you have to start with and what can be added to make it better. Then, add the appropriate soil amendments to the soil whenever digging or planting. After that, continue to improve and replenish the soil with organic matter. The yard will be easier to work, and the plants will flourish with less water and fertilizer

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