The Science in the Soil

by Clare Sullivan,
OSU Extension Agronomist, Small Farms & Specialty Crops

The Importance of Soil

A lot of people are surprised to learn that soil science is a profession, and a little confused about where “studying dirt” could get you in life. This is understandable, considering most of us were taught to not drag dirt through the house, rather than to pick up handfuls of soil and evaluate its potential. However, many farmers and gardeners, realize that soil is the foundation of everything and is a precious resource that requires attention. Soil supplies us with not only the food we eat, the textiles we use, the trees we like to sit under; but soil is also our earth’s water purifier, atmospheric gas exchange site, and source of many building materials. We literally would not survive without soil.

So what exactly is soil? The Soil Science Society of America defines soil as “the unconsolidated mineral or organic material on the immediate surface of the earth that serves as a natural medium for the growth of land plants.” Soil is the material beneath our feet that is made up of complex mixtures of minerals, organic matter, water and air, and is capable of supporting plant growth. We tend to use the word ‘soil’ when we are using this material (i.e. amending the soil in the garden to grow vegetables), while people tend to call it ‘dirt’ when it is smeared on their hands or shoes and not necessarily serving a purpose.

Why is it Important? What is the Function of Soil?

If you are interested in growing plants, soil is so important because it provides nearly EVERYTHING that your plant needs! Soil supplies plants with water, physical support, air for the roots, and 14 of the 17 essential plant nutrients (carbon, hydrogen and oxygen come from air and water). Soil also houses an infinite number of soil macro- and microorganisms, which are nature’s prime recyclers (and are the reason we aren’t being overrun by piles of detritus here on Earth!) Soil is made up of four main parts that all have important functions:

• mineral fraction (essentially different sizes of rocks)
• organic fraction (dead and dying things)
• water
• air

The mineral fraction makes up the largest percentage of your soil, and defines many characteristics of your native soil, including: soil pH, inherent nutrient content of your soil, water and nutrient holding capacity, and workability of your soil. The mineral fraction is made up of three different particle sizes — sand, silt and clay — and the mixture of these particles determines your soil texture. Sand is the largest and most coarse soil particle, silt is medium-sized and has a smooth feel, and clay is the smallest particle and gives your soil a sticky feel. Knowing your soil texture is important because texture has a large effect on the capability and limitations of your soil, and directly impacts the way you manage your land. For example, a sandy soil has large pores (the space between soil particles) that allow for good airflow but does not hold onto water and nutrients very well. This type of soil has good drainage, so you can work your ground earlier in the spring, but you will also need to irrigate more often. A clayey soil with small pores holds onto water very tightly, and you will need to wait longer after irrigation or precipitation before working your ground, but you can get away with watering less.

The organic matter fraction of your soil is also very important for water and nutrient supply. Soil organic matter includes fresh plant residues, soil microorganisms, and various stages of decaying organic matter — and organic matter helps ‘hold’ your soil in a structural pattern that facilitates good airflow, root growth, and water retention. Soils with increased organic matter have higher water holding and nutrient holding capacities, and organic matter also serves as a nutrient reservoir that provides crops with slow-release forms of nutrients throughout the growing season.

The water and air fractions of your soil are obviously important for ensuring your plant roots stay alive and healthy. Plants absorb the majority of the nutrients they need through their roots, and nutrients are transported to roots via soil water. We often say that nitrogen is the most limiting factor for plant growth, but in reality, water is the most limiting factor since it is needed to transport nitrogen to your plant. Roots, and also microorganisms, need air to breathe, therefore ensuring adequate air supply in your soil is essential. As described above, soil texture inherently influences pore space, but the way you manage your soil can be influential as well. Avoid working your soil when it is too wet because this destroys soil structure and decreases pore space. Pore spaces in soil are used for both water and air, and if all the pores are filled with water (i.e. you’ve over-irrigated), then there is no room for air in your soil.

What Makes Good Soil & What Role Can You Play?

Some qualities of your soil are inherent (such as soil texture), while other qualities can be influenced by you. You cannot change your soil texture, but soils with different textures are ‘good’ for different reasons. As described above, sandy soils provide ample drainage and air for plant roots, and clayey soils have a higher water and nutrient holding capacity. The most ideal soils for agriculture are loamy soils, which have a combination of sand, silt and clay particles (see texture triangle), giving you the best functional mix of the soil particles.

A typical soil is Central Oregon is described as a ‘Deschutes sandy loam’. This soil developed from pumice sand (think volcanic activity), so it is not surprising that the topsoil is quite sandy. Looking around, you see the natural vegetation is sagebrush, bunchgrass, rabbitbrush, bitterbrush — all plants that have low water and nutrient requirements. Native soils in this area tend to be sandy and shallow, low in nutrients, low water holding capacity, and low in soil organic matter. But don’t worry they have good drainage, and you can work with them.

You can influence how ‘good’ your soil is by the practices you use on your farm or in your garden. Two areas you can directly influence your soil are: soil structure, and soil organic matter. Soil structure refers to the way soil particles are arranged in soil aggregates, and affects your soil pore space. Typically, roto-tilling your ground (especially when wet!) degrades your soil structure and results in decreased pore space, which means lower rates of water infiltration and water retention. Leaving your soil bare and susceptible to wind and water erosion also degrades soil structure. Having living roots in the ground is one of the BEST ways of improving your soil structure — both microorganisms and roots exude glue-like substances that help bind soil particles together. Macro- and microorganisms feed off of living roots and the additions of above-ground materials, so keeping roots actively growing in your soil as long as you can (i.e. perennial grasses) is the ultimate way to improve soil structure.

However, if you are not growing pasture or a forage crop, it is not easy to keep plants growing year-round in Central Oregon. Another great approach to building your soil structure and soil organic matter is the addition of organic materials. This includes organic materials you apply (i.e. compost, manure), and materials that you grow on your land and return to the soil (i.e. cover crops, crop residues). To use organic materials effectively on your property, it helps to know as much about the materials as possible. Composts and manures all have different nutrient contents that will affect nutrient availability in your soil differently. Composts are generally thought of as ‘soil conditioners’ that help build your soil organic matter in the long-term, but are not a short-term source of nutrients. Some manures can provide a fairly rapid release of nutrients (i.e. poultry litter), but others can tie up nitrogen in the soil when they are applied. Cover crops that are legumes, or young and tender, will release nutrients to the soil within several weeks if incorporated; whereas cereal cover crops or those that are more mature will release nutrients more slowly. Knowing the nitrogen content of the material you are adding is ideal, or at least applying the material several months before planting your crop.

See Fertilizing with Manure and Other Organic Amendments for more information:
Plant Nutrient Requirements
As mentioned above, plants are supplied with 14 of their 17 essential nutrients from the soil; therefore, it is important to ensure plant nutrients are present in the soil. All crops have different plant nutrient requirements, but we can generalize a bit. Before talking about nutrients, however, you need to make sure your soil has the correct pH for growing plants. Soil pH is a relative measure of soil acidity or alkalinity, and soil pH influences the availability of nutrients, availability of toxic metals, and activity of soil microorganisms (see soil pH diagram). Most plants prefer a neutral to slightly acidic soil pH.

While there are 14 essential plant nutrients in the soil, start with making sure your soil has the right supply of macronutrients for your crop. Nitrogen (N), phosphorus (P) and potassium (K) are the building blocks for your plant, and are the three numbers you see on a fertilizer bag. For most crops, N is required in the greatest quantity; it is a major component of chlorophyll and amino acids (building blocks of proteins), and is responsible for leaf growth. Most vegetable crops require 100-200 lbs N/ac (3-5 lbs N/1000ft2). Phosphorus promotes healthy root growth, early shoot growth, fruit quality, and seed formation. Soil test requirement for most vegetable crops is ~25ppm P. Potassium is often required in quantities similar to N; it promotes plant vigor, disease and stress resistance, flavor and color enhancement, and regulates water pressure in plant cells. Soil test requirement for most vegetable crops is ~200ppm K. Sulfur (S), calcium (Ca) and magnesium (Mg) are also macronutrients. Sulfur can be low in sandy soil with low organic matter, and it is common to need to apply 20-40lbs S/ac here in Central Oregon. Soils in this region are typically sufficient in Ca and Mg.

For specific nutrient requirements of different crops, take a lot at fertilizer and nutrient management guides on the OSU website:

How do I test my soil?

If you’re interested in learning more about your own soil, the best way is to take a soil test. Taking a soil test involves using a soil probe or shovel to collect a representative sample from your garden or farm, and sending the soil to a lab for nutrient analysis. Here are some quick tips on soil sampling:

• Soil sampling should be done post-harvest (fall) or early spring
° Give yourself enough time to make any nutrient management decisions

• Take soil cores/samples from ~15 locations in field to tillage or rooting depth of (6 or 12”)
° Mix together to get a representative sample of field

• Make sure you are sampling soil only, not organic materials or fertilizer
° Scrape away any growing plants or freshly added organic amendments

• Conduct soil sampling at same time of year to compare across years
° Sample fields every 2-3 years, depending on your goals

• Use the same soil lab to compare your soil test results over time

The OSU Extension website has many great resources available to guide you through the soil sampling and soil test interpretation process:

• A Guide to Collecting Soil Samples for Farms and Gardens:
• Analytical Laboratories Serving Oregon:
• Soil Test Interpretation Guide:

photos courtesy of Clare Sullivan

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