Introduction

Video Welcome

Science & Math

History

Art & Literature

GIS & Mapping

Weather Project

Library

ChatRoom
Search
Trail Head
Base Camp

SOIL TYPES AND TESTING

INTRODUCTION

Soil can be described in many different ways, such as heavy, light, sandy, clay, loam, poor or good. Scientists typically describe soil according to its:

  • Color
  • Compaction
  • Moisture content
  • Organic content
  • pH
  • Profile
  • Structure
  • Temperature, and
  • Texture

Although each of these factors is important, three factors (texture, organic content and pH) are more important than the others. Regardless, we will provide a brief overview of all nine factors below.

Color

Soil color can provide information about organic matter in the soil, drainage, biotic activity, and fertility. The chart below can give you some insight into the condition of your soil just from its appearance. To identify the color of your soil, you should take a garden spade or shovel, and dig a shallow hole, at least 3" - 4" deep, and gauge the color (you should do this quickly before the sun can dry it out).


Condition
Color
Dark Moderately dark Light
organic matter

high

medium

low

erosion factor

low

medium

high

aeration

high

medium

low

available nitrogen

high

medium

low

fertility

high

medium

low

Compaction

To be healthy, a soil needs to be able to breath and water needs to be able to move through it reasonably easily. Compacted soils don't allow much air to circulate to the root zone and water (rainfall or irrigation) tends to just run-off.   This increases erosion and strips away vegetation and topsoil. A normal, loosely compacted soil helps to absorb and retain water, releasing it slowly, and allows the root zone of plants to "breath". These soils are generally more productive, since plants can grow much more readily. Dense, highly compacted soils typically have less plant growth, which increases runoff.

The rate of infiltration of water is an excellent indication of soil health. You can measure the water infiltration rate very easily:

  • First, get a large, empty coffee can and cut off the bottom.
  • Second, beginning about 3" up from the bottom, mark the inside of the can every " with a permanent marker, being careful not to cut your hand on the edges of the can.
  • Drive the can about 3" into the ground until the first mark is level with the ground (placing a board on the top of the can and pounding the board with a hammer will help drive the can into the ground. Be careful not to irrigate the area first, since this will prevent you from getting an accurate measurement of the infiltration rate.)
  • Fill the can with water clear to the top and begin timing the rate of infiltration. Measure the amount of water that has drained into the soil at the end of each minute for the first ten minutes. Determine the rate of infiltration in inches per minute by dividing the total number of inches of water that drained away in the can by 10 minutes. Knowing the actual water infiltration rate for your yard is critical if you want minimize the amount of water you use.
  • Repeat the experiment at several areas around your yard, being careful to record each location and its infiltration rate. If the infiltration rates at each location vary considerably, then draw a quick sketch of your yard, and plot the infiltration rate for that area. If you install an automated sprinkler system, you can adjust the emitters in each area to only deliver the amount of water that can infiltrate in a given amount of time. This will eliminate irrigation run-off from your yard or garden, while ensuring adequate soil moisture for plants.

Sand is the largest particle in the soil. When you rub it, it feels rough. This is because it has sharp edges. Sand doesn't hold many nutrients or water. Silt is a soil particle whose size is between sand and clay. Silt feels smooth and powdery. When wet it feels smooth but not sticky. Clay is the smallest soil particle. Clay is smooth when dry and sticky, or plastic when wet. Soils high in clay content are called heavy soils. Clay can hold a lot of nutrients, and some kinds can hold quite a bit of water, but the structure of clay doesn't let air and water move through it well. Most of the water in a clay soil is so tightly bound to the clay particles that plants can't get it loose.

Moisture

The amount if moisture found in soil varies greatly with the type of soil, climate and the amount of humus (organic material) in that soil. The types of organisms that can survive in your soil is largely determined by the amount of water available to them, since water acts as a means of nutrient transport and is necessary for cell survival. Soil moisture can be estimated visually, although this is quite imprecise. Soil moisture can also be determined by a soils laboratory. Soils labs typically dry a sample in an oven or on a hot plate (approximately 225 F for 24 hours) and compare the weight of the soil before drying to the weight after drying. The moisture content is reported as percent moisture on a weight basis. Several irrigation system manufacturers have developed soil moisture indicators that can be used to control irrigation more precisely, turning the system on only in areas where more water is needed and then only for the minimum time necessary to get the soil moisture back up to the desired level.

Organic Content

The organic content of soil greatly influences the plant, animal and microorganism populations in that soil. Decomposing organic material provides many necessary nutrients to soil inhabitants. Without fresh additions of organic matter from time to time, the soil becomes deficient in some nutrients and soil populations decrease. The amount of organic material can be determined by ignition. Organic material is made of carbon compounds, which when heated to high temperatures are converted to carbon dioxide and water. In the ignition process, a dry solid sample is heated to a high temperature. The organic matter in the soil is given off as gases. This results in a change in weight which allows for calculation of the organic content of the sample.

Oven-dry the sample to remove water (see soil moisture). Weigh a crucible and lid, evaporating dish and cover, or other covered container. Place approximately 10 grams of soil sample in the container, cover it and weigh the sample, container and cover. Place the container on a metal stand and heat it with a propane torch. Allow the fumes to escape, but not the soil particles. Heat the sample strongly after most of the gases have escaped; continue heating until there are no visible fumes. Cool the container, lid, and sample. Reweigh and calculate the percent of organic material.

Soil pH

Most people think that rainwater has a pH of 7, so it comes as something of a shock when they learn that rainwater (if its not polluted) has a normal pH of about 6 - 6.5, which is slightly acidic. This is due to dissolved carbon dioxide from the air, which reacts with water to form a dilute acid (carbonic acid), much like the carbon dioxide in soda. It should then come as no surprise that most plants grow their best at around the same pH*. You can determine the pH of your soil very easily using a universal indicator solution or pH paper, available at most hardware stores in the pool supplies section. To determine the pH, just put a small amount of the soil to be tested in a clear or white container, being careful not to touch the sample. Pour a small amount of universal indicator over the soil, then match the color of the indicator solution (not the soil) with the pH color chart. If you decide to use pH paper, pour a small amount of water on the soil sample. Touch the pH paper to the sample and match to color of the paper to the pH color chart.

(*Please note that some plants, such as rhododendron, camellias, azaleas, blueberries, ferns, spruce, pines, firs, and red cedar prefer soil that is more acidic, with a pH of 4.0 to 5.0. Other plants, such as beech, mock orange, asparagus and sagebrush tolerate soils with a pH 7.0 to 8.0. Above a pH 8.5, the soil is too alkaline for most plants, while if the soil pH is below 3.5 it will be too acid. You should also note that each layer of soil may have a different pH, which means that pH can vary within the soil, although the differences are usually not too great.)

Soil Profile

If you really want to know about your soil, the best way to start is to obtain a cross-section of the various layers. This can be done fairly easily if you use a soil core tool. A soil core tool is little more than a hollow tube 2 to 4 feet in length with a handle and cross piece like a shovel to help push it in. Once the tool has been inserted into the soil, it should be turned to loosen the soil and then pulled out. The resulting soil core can be easily examined to identify the various layers (each layer is also called a horizon) in the soil, the aggregate of which is called a soil profile. To determine a soil horizon, you simply mark where the soil changes color and/or general appearance.

Many soils have three major layers or horizons, top soil, subsoil and parent material. Depending on where you sample, the top zone may be comprised of actively growing plants and dead plant materials (for example, if you sample in your lawn.) The top soil is typically darker colored and usually has more organic matter, higher biotic activity, abundant roots, and commonly lower in nutrients than underlying layers. The first inch of top soil may be lighter in color because many of the nutrients may have been leached out by water, and organic material may have been partially oxidized by sunlight and heat. The soil  immediately below the first inch is usually somewhat darker, has many roots, moderate organic matter, and provides most of the nutrients for the plants. The next major layer is the subsoil. This layer is typically 1 to 2 feet below the surface and is characterized by a lighter color with much fewer, larger roots. The subsurface layer generally has less clay than the topsoil. The third layer, which may not be observable, is the parent material. This consists of unconsolidated, slightly weathered rocky materials from which soil develops. It is characterized by limited biotic activity and very few roots.

Soil Structure

Soil structure tells how the soil affects the movement of water, air and root penetration into the soil. The geometric shapes of the soil determine how it is put together. Words such as blocky (the blocks of soil are large, with the same number of cracks going horizontal as vertical), granular (the blocks of soil are small, with the same number of cracks going horizontal as vertical), columns (the blocks of soil and related cracks are generally longer in the vertical direction than in the horizontal), and plate-like (the blocks of soil and related cracks are generally longer in the horizontal direction than in the vertical), describe soil structures. To determine the structure of your soil, carefully break apart each layer and match its characteristics with the appropriate structural type shown below.

STRUCTURAL
TYPE
WATER PENETRATION DRAINAGE AERATION

columns

good

good

good

blocky

good

moderate

moderate

granular

good

best

best

plate-like

moderate

moderate

moderate

Soil Temperature

Soil temperature has a significant role in helping to determine the rate of plant growth, and whether a plant will even survive. The temperature in your soil changes greatly with depth. To measure soil temperature, find an area that is not in direct sunlight. Using a thermometer, measure the air temperature at shoulder height. Hold the thermometer still for about one minute (make sure your fingers are not on the thermometer bulb), read and record the air temperature. Next, measure the temperature at the surface of the ground. Put the thermometer flat on the ground and record the temperature after one minute. To determine the temperature below the ground surface, use a dowel that you have marked at 1 inch, 2 inches, 6 inches and 12 inches. Start by pushing the dowel into the ground till you reach the 1 inch mark. Remove the dowel and insert the thermometer for one minute, then remove the thermometer and quickly record the temperature. Repeat this procedure to obtain temperature readings at 2 inches, 6 inches and 12 inches. Take temperature readings at different times throughout the day at the same location. To compare with soil temperatures for areas in direct sun, just repeat using the same procedure but select an area that gets full sun. You will note that the soil temperatures in these areas are typically much higher than in the shaded areas.

Soil Temperature Conditions during growing season

Less than 40 F

no growth, bacteria and fungi are not very active

40 F to 65 F

some growth

65 F to 70 F

fastest growth

70 F to 85 F

some growth

above 85 F

no growth

Soil Texture

Sandy soil absorbs more than two inches of water per hour. It is very porous, with large spaces between soil particles. Little water is retained and the sandy soil dries out quickly. Loam soil absorbs from .25 inches to 2 inches per hour. The soil is loose and porous and holds water quite well. Clay soil absorbs less than .25 inches of water per hour. Clay soil is dense with few air spaces between particles and holds water so tightly that little water is available for plants.

Characteristics of Different Soil Types

It can be argued that no two soils are ever exactly alike. Although this is true, it is useful to group soils into categories. Three major categories of soil dominate our area.  These are:

  • Sandy soil
  • Loam soil, and
  • Clay soil

To figure out what type of soil you have, there are several easy methods.  The first, called the rope test, requires that you squeeze a moist, but not muddy, one inch ball of soil in your hand. Then rub the soil between your fingers. Sandy soil feels gritty and loose. It won't form a ball and falls apart when rubbed between your fingers. Loam soil is smooth, slick, partially gritty and sticky and forms a ball that crumbles easily. It is a combination of sand and clay particles. Clay soil is smooth, sticky and somewhat plastic feeling. It forms ribbons when pressed between fingers. Clay soil requires more pressure to form a ball than loam soil, but does not crumble apart as easily.

A second test is called a jar test and is very easy to do. Here's what you'll need:

  • 1 clean quart jar and tight fitting lid
  • clean water
  • soil sample
First, find an empty, clean quart jar (an old mayonnaise jar works very well for this test.) Fill the jar about 2/3 full with clean water.

quart jar with water.gif (8016 bytes)

quart jar with water and dirt.gif (7979 bytes) Next, take a sample of soil (break the large clods apart so it will fit through the jar opening) and fill the jar and water until the jar is nearly full, leaving about " of air space at the top. Screw on the lid and shake it vigorously for a minute or two, until all the soil particles are broken down into suspension in the water.

Now, allow the suspended soil to settle for about a minute, and place a mark on the side of the jar at the top of the layer that has settled out. This is the sand layer is comprised primarily of sand and larger particles.  Set the jar aside, being careful not to mix the sand layer that has already settled and wait approximately an hour. Now, place a mark on the side of the jar at the top of the next layer to settle out. This is the silt layer. Again, place the jar aside for a full day, being careful not to shake or mix the layers that have settled out. After 24 hours, or when the water is once again clear (more or less), place a mark on the side of the jar at the top of the final layer.  This is the clay layer. The percentage of each layer tells you what kind of soil you have.

Type of Soil Example of Test Jar
Sandy soils are found throughout Southern California, but are very common near the mountain foothills, along rivers and streams and certain coastal areas. Sandy soils are typically comprised of approximately 80 - 100% sand, 0 - 10% silt and 0 - 10% clay by volume. Sandy soils are light and typically very free draining, usually holding water very poorly due to very low organic content.

quart jar with sandy soil.gif (26180 bytes)

Loam soils are also common in Southern California, particularly in the valleys and flat areas (flood plains) surrounding rivers and streams. Loam soils are typically comprised of approximately 25 - 50% sand, 30 - 50% silt and 10 - 30% clay by volume.  Loam soils are somewhat heavier than sandy soils, but also tend to be fairly free draining, again, due to typically low organic content.

quart jar with loam soil.gif (25539 bytes)

Clay soils are very common in certain areas, particularly around urban areas where fill soils have been used to establish grade in subdivisions and developments.  Clay soils are typically comprised of approximately 0 - 45% sand, 0 - 45% silt and 50 - 100% clay by volume.  Clay soils are not typically free draining, and water tends to take a long time to infiltrate. When wet, such soils tend to allow virtually all water to run-off. Clay soils tend to be heavy and difficult to work when dry.

quart jar with clay soil.gif (25169 bytes)