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All About Soils

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All About Soils

The Cook County Extension office receives a lot of questions about soils and how to build up soils in an area where many of us have gravel or clay. Organics added to your soils are the key to changing the soil structure over time. You know you have pretty good soil when you can take a handful of the soil and squeeze it with your hand, when you open your hand it retains its form until you take your finger an gently poke it. If it breaks apart when you poke it you have pretty good soil tilth or structure.
If you have questions about how your soils work, the information below is based upon a great book, “Teaming with Microbes. The Organic Gardener’s Guide to the Soil Food Web.” It will help you understand much more how your soils work and what you need to grow great vegetables.

What are the characteristics of a healthy soil that will help your plants?

• Nutrient retention in your soil
• Improved soil structure (tilth)
• Improved disease defenses for your plants
• Influences soil pH for optimum plant growth
• Soil food webs are intact

How do we know if we have a healthy soil or not?
Most people start with a soil test.
When you do a soil test it usually tells you the percent of organic matter; pH (7 being neutral; acidic soils being below 7 and alkaline soils being above 7); amount of nitrogen, phosphorus and potassium (NPK).
What the soil test doesn’t give you is the soil texture; structure; whether you should garden organically or inorganically (what does that mean anyhow?), how to build up your soil to support a web system of local soil microbes, or how important the other macro and micro nutrients are to the success of your plants.

What does organic vs. inorganic mean when we are talking about soils/fertilizers?

• Inorganic or synthetic fertilizers: These are commercially prepared forms of nutrients. Nitrogen is obtained from the atmosphere and phosphorous and potassium are mined from the earth’s crust. They are generally concentrated. They don’t build the soil or support the microbial food web within the soil. A little bit of the nutrients are utilized at the root tip and the rest of the nutrients are often drained down through the soil until they hit the water table.

• Organic fertilizers: These fertilizers generally have a plant or animal origin. Animal manures or compost are some examples. Nutrients are contained generally in complex organic forms which will release more slowly over time. Nutrients are utilized by the microbes in the food web. They in turn release additional nutrients as waste materials and eventually die and decompose putting additional nutrients back into the soil.

In general, a garden should have about 8 percent organic matter in the soil or a little higher for vegetables.

Soil Texture:
This refers to the size of the mineral particles of the soil. Particles are sand, silt and clay. The relative proportions of those three make up your soil texture. “It takes 65 million clay particles to fill up the same amount of space as one grain of sand.” Ideal loam: 15% clay, 35% silt, and 40% sand, 5 – 10% organic matter.
These three particles are shaped differently and arrange themselves differently when grouped together:
~ loose & sandy
~ crumbly & light (loam)
~ compacted clay (particles are flat crystals that can pack together tightly leaving very little space for air)
You can’t really change the soil texture.

Soil Structure: (Tilth) This you can change. The soil structure refers to the way the mineral particles are arranged into aggregates (groups of particles loosely held together). The soil structure becomes a by-product of the decay of organic matter. You can change the structure of your soil with organic matter. The microbial life of the soil binds together the sand, silt & clay.

Organic matter – compost – humus – structure

Humus is the end product of organic decay. This decaying process usually takes place right under the ‘duff’ or layer of material you can recognize on the forest floor or the top of your garden. Humus is created by microbes. Humus is a fairly stable and complex carbon compound that serves to hold the pore structure of soil open. Little nooks and crannies in humus provide ‘condominiums for microbes’.

Pore space is a maze of minute continuous channels found throughout the upper layers of most soils. Very important for air and water flow. If there is inadequate pore space you may end up with air being utilized, no recharge of air occurs, and then you have anaerobic conditions that end up killing root systems.

Plant Roots: These act as places where ion exchanges take place. They are electrically charged with H+ cations. They give up these in exchange for anions like -NO3 (nitrate) or -SO4 (sulfate) or phosphate -(PO4). The cation exchange capacity of soils is based upon the amount of clay and organic matter it contains. Sand and silt have low exchange capacities.

pH: Every time a root tip exchanges a H+ (cation) it can be measured as pH or the concentration of H+ ions. The more H+ ions, the more acid the soil or solution. Root surfaces also take up negatively charged anions or hydroxyl (OH)- ions so that helps to balance out the exchange. pH influences the type of microorganisms that live in the soil. Certain fungi and bacteria are important to different plants to help them thrive.

Bacteria: They play a major role in plant nutrition. They lock up nutrients that might otherwise disappear due to leaching. They prefer cellulose over lignin. The bacteria remain on soil particles and that keeps the nutrients remaining in the soil. The bacteria in turn are eaten by protozoa and reduced to wastes. These nutrient wastes are picked up by the plant roots.
Bacterial slime raises soil pH; nitrogen-fixing bacteria generally require a pH above 7. Serve as barriers around roots to block entry of disease organisms.

Arachae: Discovered in the 1970s, these life forms are now considered to be the most abundant life form on earth. They are the most 
abundant ammonia oxidizers in soil – making nitrogen available to plants. They are decomposers of organic and inorganic materials. They produce methane – a key component of greenhouse gas. They are important members of the soil food web – much of what is still unknown.

Fungi: Fungi are unable to photosynthesize and have chitin in their cell walls instead of cellulose. They prefer lignin (woody materials). A teaspoon of good garden soil will have several yards of fungal hyphae. They can transfer nutrients from one end of the hyphae to the other. Phosphorus is especially made available to plants by fungi as they have the ability to release it from its chemical and physical bonds. Their hyphae leave microscopic tunnels in the soil that water and air can flow through. Fungi are the primary decay agents in the soil food web. The nitrogen released by fungi is the NH4 (ammonium form). Enzymes released by fungi are acidic and lower the pH.

Myccorrhizal fungi: There are two kinds: ectomycorrhizal fungi, grow close to the surface of roots and are associated with hardwoods and conifers. Endomycorrhizal fungi actually penetrate and grow inside roots and extend out into the soil. These are preferred by most vegetables, shrubs, perennials, annuals, etc. Some require specific plant hosts and become mutually dependent upon each other. Provide additional water and phosphorus to plants

Protozoa: They eat the bacteria, a few fungi and other protozoa. In turn, they provide food for nematodes and other worms.

Nematodes: (Non-segmented blind round worms) One teaspoon of good soil averages about 40 – 50 nematodes. Many of these nematodes made minerals or nutrients available to plants by releasing them from the protozoa or fungi they feed on. Nematodes need less nitrogen that protozoa so they release more nitrogen and make it available to the plants. Fungi and bacteria ‘hitch’ a ride on nematodes to different locations in the soil.

Arthropods: Arthopods have segmented limbs/bodies and an exoskeleton. Soil arthropods are important because they are shredders, predators and soil aerators. The presence or absence of some of these key arthropods can tell you about the health of your soils and the plants growing in them. Examples: springtails, mites, millipedes, ants, sow and pill bugs, etc.

Earthworms: Primary food source is bacteria although they will eat fungi, nematodes, protozoa and organic matter on or in which these microorganisms live. Worms can live for 15 years. They shred organic matter, aerate soil, aggregate soil particles and move organic matter and microorganisms in the soil. Vermicastings(worm poop) are 50% higher in organic matter than soil that has not moved through worms. Worms’ digestive enzymes unlock phosphate and other nutrients making them available for plants.
Bad news about earthworms: Forest floors that have been invaded by earthworms have completely altered the soil food web by drastically increasing the decaying of organic matter to point where it is not healthy for the trees and the rest of the soil food web. Earthworms are not native to the Great Lakes region. Their introduction has destroyed the ‘duff’ layer that dozens of understory plant species rely on to survive. Some of these native species cannot survive without that duff layer. Fungi, bacteria and other arthropods and even small animals are affected by disruption of the ecosystem that was developed without earthworms.

How do I know if I have a good soil food web present in my soil?

• If you have 5 – 30 earthworms in your garden soil in a square foot.
• Set a soil trap. Bury a quart-sized container in the soil so that the lip of the container is at the ground level. Put an umbrella over it or something to keep the rain out but keep it open at the soil level. Add a couple of moth balls. Leave alone for 3 – 7 days. Check to see what you have in your trap. If your soil is pretty good you will have centipedes, millipedes, other macroarthropods.

How can I use soil food webs to increase the health of my soil and the health of my plants?

Some plants prefer soils dominated by bacteria; other plants prefer soils dominated by fungi. They need nitrogen for building amino acids. Bacteria predominated soils have –NO3 (nitrate) predominately and are more alkaline. Fungi predominated soils have more –NH4 (ammonium) and are more acidic.

Fungal to Bacterial ratio (F:B)

1. Carrots, lettuce, broccoli, cole crops F:B ratio of .3-.8 to 1.0
2. Tomatoes, corn, wheat F:B ratio of .8 – 1 to 1.0
3. Lawns F:B ratio of .5 – 1 to 1.0
4. Conifer trees F:B ratio of 50 – 1000 to 1.0
5. Maples, oaks, poplars F:B ratio of 10 -100 to 1.0
6. Orchards F:B ratio of 10 – 50 to 1.0
7. Alder, beech, aspen, cottonwood F:B ratio of 5 – 100 to 1.0 (mature)
(Frankia bacteria fixes atmospheric nitrogen in alders)
8. Annuals: Prefer bacterially dominated soils
9. Perennials, shrubs: Prefer fungal dominated soils.

Add compost. Compost can inoculate an area with microbes to support soil food web organisms. Compost made with C:N ratio of 25 -30 to 1 is best. You can manipulate that ratio for vegetables to 25% alfalfa meal; 50% fresh grass clippings and 25% brown leaves or bark for more of a bacterial compost.

• Put down your compost first and then add mulch.
• Provide different kinds of mulch: leaves, grass clippings, wood chips, etc. for your trees, shrubs, perennials. These will provide different soil food webs.
• Rhizobia bacteria associated with legumes can be added to your beans, peas and will make nitrogen available in about two weeks after inoculation.
• A green mulch will provide bacteria to the soil; a brown mulch will add fungi to the soil. Peat moss used as mulch is sterile. Pine needles and cedar chips contain terpenes that can be toxic to many plants.
• Wetting your mulch grinding it up and working the mulch into the soil will support bacteria; coarser, drier mulch on top of the soil will support more fungi. Keeping your mulch on top of the soil larger (3/8” or larger) will prevent nitrogen being used up by bacteria. There will be enough nitrogen to feed the fungi.
• Keep your mulch layers 2- 3” to avoid blocking air and water resulting in possible anaerobic conditions.
• Active aerobic compost tea does work when brewed at room temperature. It takes about two days to make, use within 4 hours of completion. Do not use simple compost leachate or hanging a bag of manure in water. That results in anaerobic pathogens and possible alchohols.
• Rototilling and compaction of soils all have a negative effect on mycorrhizal fungi. Reducing this activity is key to keeping your soil food web healthy.
• Applications of synthetic fertilizers can kill microbes.
• If you add additional organic fertilizers, use low numbers.

The first step is to take a soil test. Call the extension office at 387-3015 to find out how to do this.


Cook County Extension office located in the CC Community Center building in Grand Marais does have soil testing available through the University. We do recommend a soil test before you start adding amendments to your soil. We also have an animal husbandry and gardening library available for folks who would like to check out more specific information on topics we discuss on Northern Gardening. Please call Diane at 218-387-3015 if you have questions or stop by.

Youth Spring Gardening Class

A youth spring gardening class will begin Tuesday, April 17 from 4 – 5 p.m. in the 4H log cabin building at the Cook County Community Center. The class will run every Tuesday through May 22. Youth can sign up for the After School Lounge in the I.S.D. #166 office and walk over with our youth coordinator, Melissa Wickwire, right after school, have a snack and be ready for gardening with Max Linehan and Penny Ortmann U of M Master Gardeners.

Container gardening workshop

Saturday, June 2, starting at 9 a.m. at the Cook County Community Center, our U of M
Master Gardeners will be hosting a container gardening workshop with topics that include: Growing Vegetables in Containers with Diane Booth; Spicing up Your Garden by Planting in Containers with Jane Horn; and hands-on container planting demonstrations with U of M Master Gardeners Nancy Carlson, Max Linehan, and Emma Bradley. Save the date!
Also, the Art Colony Potters will be making planters and garden art as a fundraiser for the ceramics studio. They will be sold during the workshop.
In conjunction with the Container Gardening workshop in June, the Cook County Master Gardeners will also be planning a container gardening contest this summer with prizes and a local garden tour on Saturday, August 18th. So if you are a gardener and would like to enter either your garden as an example of how people can grow in small spaces using containers or small raised beds, watch for more information about this in May and early June.

Cook County Co-op
If you’re planning to grow vegetables for the Cook County Co-op this summer, be sure to talk to Jeri Persons right away. The Co-op has new rules for local growers this year. Call Jeri at 387-2503.

Cook County Farm & Craft Market
The Cook County Farm & Craft Market will be making a table available for local growers who just have a small amount of produce to sell this year as well. Call Joan at 387-3101 to find out more.