Soil Composition

Written for GO Seed in Salem, OR

What is Soil?

Soil is the mineral and organic matter found on the surface of the Earth that serves as a medium for plant growth.

The top layer of soil, called topsoil can be classified as “shallow” when it is less than 10 inches deep and “deep” when it is more than 60 inches deep. Plants grow best in deep soil, where their roots can penetrate down to nutrients found further down in the soil profile.

These layers of soil are also classified as horizons. The “A” horizon, as pictured below, is the fertile topsoil, where most nutrients and microorganisms are found. Having a deep, healthy “A” horizon allows for better plant growth and higher yield.

Image: nrcs.udsa.gov

The top horizon, “O”, is where the newly deposited soil and organic matter is found. Tilling soil can help incorporate the organic matter deeper into the soil, adding more nutrients and air to the topsoil where it can be utilized by plants.

The “B” horizon is rich in minerals, but low in organic matter. This layer of soil is important for deep rooted plants such as trees and shrubs.

The “C” horizon is also known as the parent material and is the basis from which the current soil in the area began.

There are other horizons, such as the “E” horizon which is typically found in older, less disturbed soils and the “B” horizon which is the bedrock at the very bottom.

Soil includes more than just dirt and organic matter. Healthy soil includes a variety of voracious eaters, such as grubs and worms, munching on that decayed organic matter. These organisms can be quite beneficial in the soil, moving soil around, and allowing for increased water and air flow throughout.

Many microorganisms can be found in soil as well. According to retired Oregon State University nematologist, Kathy Merrifield, a single teaspoon of rich garden soil can hold up to one billion bateria, fungi, protozoa and nematodes.

Another vital part of the soil structure is the Rhizosphere. This is the part of the soil found around plant root systems where a large portion of biotic and abiotic processes occur. Much of the Rhizosphere is located in the fertile topsoil where root systems interact with the soil.

According to an article published in Frontiers in Environmental science, the roots found in the rhizosphere move minerals around and release root exudates such as mucilage, which has been shown to increase water content in the rhizosphere (Holz, et al., 2018). The Rhizosphere is also where a lot of microbial decomposition occurs in the soil.

General Soil Composition

Crop yield is directly related to soil composition, making it the most important aspect of soil management. A balance of air, water, organic matter, and minerals produces an ideal environment for plant productivity. If organic matter within the soil is not properly managed, crop yield is likely to suffer.

Cover crops are a tool used to enrich soils that lack nutrients or viability via plant-soil feedbacks. These crops can be tilled back into solis, thus increasing organic matter and adding extra nitrogen. The nutrients retained in the soil from a cover crop will benefit the next season’s crop.

In addition to increasing nutrients in the soil, cover crops help suppress unwanted weed growth and can also prevent wind and water erosion of deeper fallow soils.

Cover crops, especially clovers and legumes, make for excellent forage and produce food for native pollinator species. Whether used for grazing or wildlife, cover crops can be utilized effectively as both a food source and habitat, while improving soil quality.

Ultimately, cover crops are a great way to manipulate soil composition in favor of crop yield.

Selecting the Right Cover Crop

Sandy Soil

Sandy soil tends to be low in nutrients due to high acidity and large porosity. These soils are considered to be light because of their high proportion of sand and little clay. During the spring these soils are quicker to warm, but tend to dry out during the summer.

Cover crops for sandy solis:

• Hairy vetch

• Buckwheat

• FIXatioN Balansa Clover provides over 5,200/lb of dry organic matter per acre and does well in sandy soils.

Hairy Vetch is an excellent cover crop for sandy soils, but it requires a good potassium level in the soil. Hairy vetch thrives in sandy soils and rapidly releases nitrogen that will be available in the soil for the next crop. Buckwheat is a very easy-to-grow and easy-to-kill cover crop that can help suppress weed growth, especially in less-fertile soils.

Caption: Our FIXatioN Balansa Clover can have roots as long as 34 inches long

Clay Soil

Clay soil is high in nutrients, resulting in a heavy soil. This type of soil can hold in nutrients like potassium, calcium and magnesium. Clay soil holds a high amount of water and drains slowly. These soils dry out during the summer and remain wet during the winter months.

Cover crops for clay soils:

• Sweet clover

• Winter wheat

• Buckwheat

• Alfalfa

• FROSTY Berseem Clover features salt tolerance, high nitrogen fixation, and a synergistic relationship to alfalfa.

Cover crops with deeper roots, like alfalfa, break up compact clay while simultaneously pulling nutrients into the topsoil from the subsoil.

Silt Soil

Silt soil is known to be moisture retentive and to have a high fertility rating. By themselves, these soils drain well due to the medium-sized particles but are also more prone to erosion. By adding organic matter the silty soil can be bound creating more stable clumps.

Cover crops for silt soils:

• Winter Rye

• Winter Wheat

• Berseem Clover

• Annual Ryegrass

Tilling cover crops into the soil in the spring before planting returns all the nutrients that were collected by the crop back into the soil and adds organic matter.

Peat Soil

A healthy peat soil is high in organic matter and is great at moisture retention. Peat soil often is imported into gardens due to its optimum soil for base planting. Although peat soil often provides the ideal environment for growing, its scarcity results in limitations.

Cover crops for peat soils:

• Peas (winter pea, forage pea, and cowpea)

• Annual Ryegrass

• Sugar Beet

When planting cover crops for peat soils, the main goal is to prevent soil erosion.

Loam Soil

Loam soil is a combination of sand, silt and clay. This combination of organic matter and each soil type combats the negative effects of a single soil type, resulting in a fertile loam soil.

These soils’ predominant composition will determine it’s categorization as a sandy or clay loam. Loam soils are often easy to work with and when properly balanced, these soils tend to provide good drainage.

Cover crops for loam soils:

• White clover

• Berseem clover

• Buckwheat

• Crimson clover

• Winter rye

Selecting the right cover crop ensures that the crops being planted improve the soil health rather than deplete it. Using one or a mix of cover crops can prevent erosion, nutrient deficiency, and improve overall soil composition.

If soil type is unknown soil testing may be a good place to start. Soil testing will help determine pH and any nutrient deficiencies. Local Extension offices may provide soil testing or can provide information on where soil testing is available in the area.

Determining Soil Type

"Soil textural triangle" by NatureServe

Another technique for determining soil type is testing the texture of the soil by “ribboning” the soil. Each soil type will have a different texture and produce a different ribbon. The ribbon test is performed by first forming a bolus, or ball, of soil. To ribbon the soil, press the bolus between the thumb and index finger, pushing the soil away from the hand. Soils with high levels of sand will produce little to no ribbon, while soils with high clay content will produce much longer ribbons.

Photo: Agriculture Victoria: Assessing soil texture

Things to Remember

• Soil is a complex system of minerals, water, air, microorganisms and voracious eaters.

• The Rhizosphere is the layer of soil where primary root growth occurs. The rhizosphere makes up a large portion of the “A” horizon, or topsoil, where the bulk of nutrients are found.

• A healthy soil is the key to a successful crop and soil can be amended using cover crops to restore nutrients and organic matter.

• Understanding soil type and where soil is lacking in nutrients can aid in selecting the right cover crop.

There are plenty of resources out there to aid in determining soil type, nutrient and mineral levels, and even selecting a cover crop. The NRCS is a great resource for understanding soil characteristics and where to start on the journey to a healthier, more productive soil. You can also visit the GO Seed FAQ page for the latest information on cover crops!