Soil is more than weathered rock fragments. It is a complex, biologically-active environment composed of a mixture of inorganic material, nutrients, water, air, decomposing organic matter, plant roots, and microorganisms. Soil is the matrix within which our plants grow and thrive.
Permeability To Water and Air
Garden soil must be permeable to both water and air. Water moves into and through the soil by a network of pores. The larger pores, called macropores, allow water to readily move into the soil and excess moisture to drain through the action of gravity. The smaller pores, called micropores, provide spaces in which water may be stored for plant use.
What is desireable is a soil with a balance between macropores (for drainage) and micropores (for storage). If there is an excess of macropores, such as in very sandy soils, the soil retains less water, dries very quickly, and requires more frequent irrigation. Soils with an excess of micropores, such as clay soils, retain much more water, tend to become saturated, and take longer to dry out and warm.
After gravity has drained the free water out of a saturated soil, a substantial amount water remains stored in the soil. This is called the “field capacity” of the soil, and represents the maximum amount of water that can be stored within the soil micropores. Plants draw down this stored water as they grow, steadily reducing the water in the soil. As the soil moisture decreases, it becomes increasingly difficult for the plants to extract more water. At some point the plants start to become water stressed, hampering their growth.
Eventually the point is reached where the plants can no longer extract moisture from the soil, and the plant begins to wilt due to lack of water. This is known as the “wilting point”. Although there is still water in the soil, it is so closely bound to the soil particles that it is unavailable to plants. If additional water is not provided, the plants will die.
In addition to water, the soil must also be permeable to air, which is needed by plant roots. Air, and the critical oxygen it contains, diffuses into the soil through the soil pores. If pores are lacking, the air movement into the soil will be retarded, and plants will not thrive.
It is possible to physically damage the pore structure of your soil. Some of the most common reasons for the destruction of soil structure is by soil compaction, working the soil when it is too wet, or over-working the soil until all the pore structure is lost. All of these actions result in the collapse of the soil pores and the disruption of water and air movement and storage.
Productive garden soils are permeable to water and air, with a composition of approximately 50% soil, 25% water, and 25% air (by volume). Saturated or waterlogged soils contain about 50% soil, 45% water, and 5% air, while compacted soils contain about 70% soil, 25% water, and 5% air. Note that the real issue with compacted and saturated soils is not the water content, which remains at least 25%, but the lack of air, either through water displacement or by the loss of the soil pores themselves. This is why saturated soils are often easier to deal with (because they can be drained of excess water, opening up space for air), while compacted or over-worked soils are more difficult to renovate (the soil structure itself has been destroyed).
Soil texture is a description of how coarse or how fine a soil is. Soil texture is important for it impacts both permeability and the amount of surface area, which is where most soil activities takes place. Soil texture is primarily a function of the size of the particles it contains and their relative abundance. There are three general types of soil particles:
- Sand: Large, round, coarse particles, which give the soil a gritty feel. Sand particles have the lowest surface area/volume ratio of all particles.
- Silt: Smaller, round particles, but still visible to the eye, which give the soil a floury feel. Silts have a moderate surface area/volume ratio.
- Clay: Microscopic particles, flat in shape, which give soil a plastic, moldable feel when wet. Clays have a very high surface area/volume ratio.
Nearly all soils contain a mixture of these three soil particles. However, the percentages may vary widely, even within a single garden. Physical descriptions of soils are often based on the relative abundance of these three particles:
- Loams: Contain roughly equal parts of sand, silt, and clay. These are generally considered to be the best garden soils, for they have a good balance of macropores and micropores, and have good water holding capacity and permeability.
- Sandy loams: These have a higher percentage of sand than loam, and generally have a lower water holding capacity and higher permeability.
- Silty loams: These soils have a higher percentage of silt than loam. They have a higher water holding capacity, but drain less well.
- Clays and clay loams: These soils have a higher percentage of clay, are hard when dry and sticky when wet, and have a very high water retaining capacity and very poor permeability. These soils tend to be the most difficult for gardeners to work with, for they are dense, poorly drained, and poorly aerated.
In Grays Harbor and Pacific Counties, the soils in the valleys tend toward the silty loams. However, the impact of ancient glaciers can still be seen in localized areas where gravel beds have been deposited, the topsoil has been scraped away, ice-compressed soils have formed a sub-surface hardpan, and deposits of heavy clay are found. The beach and coastal areas tend to have very sandy soils, prone to drying and often deficient in organic materials and nutrients.
Soil organic material includes fully decomposed organic material (humus), partially decomposed plant material (similar to compost), and undecomposed organic material (dead roots, leaves, and other fresh plant material). This material is important for soil formation, for it serves as food for soil microorganisms, helps improve soil structure, and aids in the retention of water and the diffusion of air into the soil.
Adding composted organic material to your garden soil is the best way to improve the environment for your plants. However, avoid incorporating raw organic material, such as sawdust or freshly shredded leaves directly into your soil, for the microorganisms that break down uncomposted plant material will initially compete with your plants for the available soil nutrients.
Soil is the home of a wide variety of organisms. Some of these organisms can be readily seen, such as earthworms, moles, insects, and the roots of living plants. The larger organisms often feed on their smaller brethren, and play an important role in improving soil structure as they tunnel through the earth.
Other soil denizens can only be viewed with a microscope, such as bacteria, fungi, algae, protozoans, and the like. The primary role of these creatures is to break down organic material so that it may be utilized by plants. Some of these smaller residents are very beneficial, such as the Mycorrhizae fungi which aids plants in taking up nutrients or the Rhizobia bacteria, which fix atmospheric nitrogen so that it may be used by vegetation. Of course, some of these microorganisms are plant pathogens, which cause various diseases such as raspberry root rot and potato scab.
There are thirteen essential plant nutrients that soil provides. These nutrients are classified into three categories, primary, secondary, and micronutrients, based on the amount needed by plants:
- Primary Nutrients: Nitrogen, phosphorus, and potassium.
- Secondary Nutrients: Sulfur, calcium, and magnesium.
- Micronutrients: Zinc, iron, copper, manganese, boron, molybdenum, and chlorine.
If your soil is deficient in one or more of these nutrients, fertilizers may be applied. It is always best to use soil test results to determine the rate and type of fertilizer to apply.
Soil pH measures the acidity or alkalinity of the soil. The pH scale runs from zero (the most acidic reading possible) to 14 (the most alkaline reading possible). It is a logarithmic scale, so that each whole number change in pH equals a ten-fold change in acidity or alkalinity. For example, a pH of 5.5 is ten times more acidic than a pH of 6.6. If the acidity and alkalinity is balanced, the pH is 7.0.
Soil pH influences plant growth in several ways: pH affects the availability of plant nutrients, the availability of toxic metals, and the activity of soil microorganisms. Most garden plants prefer a soil with a pH range of 5.5 to 7.5. However, some plants, such as blueberries and rhododendrons, prefer a more acidic soil.
It is impossible to know your soil’s pH unless you have it tested. Luckily, soil pH is relatively easy to adjust. For example, the soil pH may be raised (made more alkaline) by applying lime or wood ashes, while the soil pH may be lowered (made more acidic) by applying elemental sulfur or ammonium sulfate. Your guide for any adjustments should be the soil test results.
Due to our high annual rainfall, which leaches out soluble alkaline salts and minerals, the soils in Grays Harbor and Pacific Counties tend to be acidic.
Soluble salts are deposited after fertilizers, compost, and manure areused. In drier regions, soluble salts may accumulate in the soil to the point where they may inhibit plant growth. This is more common in the drier eastern side of Washington State where there is not enough rainfall to remove the excessive salts.
In the Grays Harbor and Pacific County area the opposite problem may occur. Our area gets so much rainfall that soluble salts and soluble nutrients may be leached from the soil. The removal of the soluble salts generally does not inhibit plant growth, but the loss of soluble plant nutrients may require fertilizer replacement. Again, be guided by your soil test results.
The best way to find out if your soil is deficient in any essential nutrients, or is suffering from another imbalance, is to have it tested by a laboratory. The process is very straightforward: soil samples are taken from your garden, these are usually dried and sent in to a laboratory, which will then return a report describing the results, usually along with suggestions on how to correcting any deficiency or imbalance found. Gardeners are usually advised to have their soil tested every three to five years so that problems may be identified quickly.
WSU Cooperative Extension.
- Cover Crops. Describes the benefits of cover crops, different types of cover crops and their suitability for different situations, and how to grow and manage cover crops in your garden.
- Organic Gardening. A detailed guide to organic gardening in Washington State.
- Soil Management for Small Farms. WSU Extension Bulletin EB1895. Subjects include: soil and water; what affects the porosity of soil and its effects on irrigation; site and landscape factors; western Washington soils; soil organisms; nutrient management, fertilizers, and manures; adding organic matter; soil pH and liming.
- Backyard Composting. WSU Extension Bulletin EB1784. It explains the basic interactions of “fast” and “slow” composting, defines terms, and breaks raw composting matter into categories by moisture, porosity, and nitrogen content. Troubleshooting tips answer your questions and provide health and safety information. Learn how to build and turn a pile, what NOT to add, and how to use your finished compost.
Organic Farming Systems and Nutrient Management.
- Soils and Soil Testing. Links for gardeners and farmers to resources on soil sampling, soil testing, and soil test interpretation.
- Composts and Nutrient Management. Discusses the effects of compost applications on nitrogen availability and soil properties, current research, and useful links on composting.
- The Compost Connection. Washington State University’s Center for Sustaining Agriculture and Natural Resources. Links to sites about composting.
Oregon State University Extension.
- Soil Test Interpretation Guide. Oregon State University Extension Service EC1478. A brief introduction to soil testing and general guidelines for interpreting soil test results.
- Improving Garden Soils with Organic Matter. Oregon State University Extension Service EC1561. Discusses the importance of soil organic matter levels and provides suggestions for suitable soil amendments.
United States Department of Agriculture, Natural Resources Conservation Service.
- NCSS Web Soil Survey (WSS). Provides a digital soil mapping program for the United States.
- Soil Biology. Provides information on the creatures living in the soil that are critical to soil quality.