Soils for Food Gardening

Soils for Food Gardening

South Whidbey Tilth Food Gardening Class, February 11,2008

Marc Wilson, Maha Farm and Forest, 360-579-2890

I. Why organic?

II. Soils with good Tilth

Minerals - The primary nutrients, needed in relatively large quantities, arenitrogen(N), phosphorus(P),and potassium(K). Secondary nutrients are calcium(Ca), magnesium(Mg) and sulfur(S) but are usually available in the soil. The micronutrients, needed only in small amounts, are boron (B), copper (Cu), iron (Fe), chloride (Cl), cobalt (Co), manganese (Mn), molybdenum (Mo) and zinc (Zn). N, P and K usually need to be managed carefully. Non-minerals, obtained from water and air and critical for plants, are Hydrogen(H), Oxygen(O) and Carbon(C).

Soil Temperature - plant specific. For example to germinate, Peas 45-75, Tomatoes 70-90. Warm soils are more biologically active

Soil pH - acidity vs. alkalinity. Most food plants prefer slightly acidic soil (5.5-7.0), minerals dissolve in acidic water making them available to roots.

Soil Texture - Weathered rock and organic matter form topsoil. Loams (clay and silt) are important for making nutrients available to plant. Glacial drift and outflow predominate on South Whidbey (mostly gravelly sandy loam or loamy sand).

Soil classification based on percent of clay, silt and sand is a general way of determining fertility in addition to % organic matter. Natural Resources Conservation Service of USDA surveys soils.

NOTES

Soil Structure - pores in soil for movement of water (with nutrients) and air, to hold up plants, to provide room for root growth, and to provide place for soil organisms. Soil should hold water but also drain well to allow aeration. 1/2 your soil should be pores. Loam plus sand, rocks, and organic matter are basis of structure. Treatments and earthworms can increase beneficial structure. Compaction destroys structure.

Ability to hold nutrients - cation exchange capacity, clay and humus

Soil food web - one handful of fertile soil can contain 6 billion microorganisms. Plants feed bacteria and fungi and nematodes, who then provide nutrients to roots. C:N ratio of 10 maximizes microbial growth, 30:1 is too low. Fungi and bacteria break down rocks and organic matter and provide nutrients in water soluble form to plants

NOTES

III. Soil treatments

• Plants need nutrients, air, water, and physical support. Ideally, soil should be 50% pores for water and air, 40% mineral, sandy loam or loam, and 10% organic. Soil should be moist but drain well to allow air into the soil. Good soil should have no rocks and little gravel. Most soils can be made productive primarily by increasing organic matter.
• When starting out, you need to understand what kind of soil you have and what needs to be done to create the conditions in which your plants will thrive, building the soil food web.What kind of parent material is your soil and what is the history of its treatment? Is it forest soil? Is it sod? Is it compacted? Is it well drained? Does it retain water? Most likely the pH is low(acidic) and you will need to add lime. You may want to build raised beds in order to increase the water drainage and aeration of the soil in our wet climate. You may want to test your soil to get a better idea of how to amend the soil, both for minerals and soil organisms.Most soils have a layer of organic matter, then a layer of mixed organic and mineral soil, then a layer with a little organic matter and finally the parent material (O,A,B, and C horizons).
• Double Digging - method of digging 2 shovels deep (16-18 inches), mixing top layer of soil and breaking up bottom layer. Often the top layers have been washed free of mineral salts which have accumulated in a hard, compacted layer deeper in the soil that restricts the flow of water. These salts, in high concentration, can be harmful to plants. Double digging breaks up this layer but leaves it deeper in the soil.
• Organic matter is the most important treatment for your soil. It contains all of the nutrients the plant needs as well as improves structure and water retention capacity. It encourages growth of organisms in soil.

Sheet mulch - place layer of newspaper or cardboard over ground and cover with manure or compost. Leave in place for a year.

Fall - Winter Treatment:

Mulching - Cover soil with weed free compost or aged manure or leaves or straw (cover with gunny sacks to prevent blowing away) or combination of these in late summer or early fall. Turn into soil with amendments before planting in spring.

Plant green manure (legumes, buckwheat or rye) in late summer and turn into soil in spring. Green manures can also be left in when leaving soil fallow (when not used to grow crops).

Spring Treatment:

Turn compost into soil with amendments. Manures may be added 90 days before plants.

Summer Season treatment - side dress with amendments and compost, mulch with compost. Avoid mulching with leaves or straw as it ties up nitrogen in soil making less available to plants.

NOTES

• Soil Amendments

Lime - increases pH (alkilinity) and adds calcium to soil. Slow acting. Whidbey generally has very acidic soils and so we need to add lots of lime at the beginning, eventually adding every 2-3 years (test).

Compost - slow release source of all nutrients needed, builds structure. Add at least 2-4 inches each year.

Aged manure - good source of N and all nutrients, check your source to make sure it is weed free and antibiotic free, salt caution. Amounts of nutrients can vary a lot depending on treatment, especially if washed by rain.

Fertilizers (rated N,P,K), different sources will vary somewhat on amounts

Commercial mixes are mixes of below to get "ideal" amounts.

Animal manures - chicken 3-2-2, cow 1-1-1, llama 2-1-1 slow release, bat guano 10-3-1, horse .5-.2-.6, pig .5-.4-.4 (disease caution). Compost is often made with manures and straw and other plants.

Others (typical NPK): Bloodmeal - 12-2-1, Fish emulsion - 6-1-1, Fish meal - 10-6-2, Cottonseed meal - 7-3-2, pesticide and GMO caution, slower relase, Soybean meal - 7-2-1, GMO caution, Feathermeal - 7-0-0, slow release, Bonemeal - 1-20-0 plus Ca, Alfalfa - 3-1-2, slow release, Kelp meal - 1-0-2, plus trace minerals, Greensand - 0-0-3 plus trace minerals, Gypsum - Ca and S loosen heavy clay soils, Sulfur - to lower pH, Rock Phosphate - colloidal form is slow release, Peat Moss - acidic, holds water

• Excessive nitrogen causes numerous problems, including pollution from runoff (algal blooms in streams, lakes and Puget Sound). Do not add N to legumes (they provide their own nitrogen through bacteria on their roots). Too much nitrogen will kill seedlings, burn plants and cause excessive leaf growth without much fruiting (esp. tomatoes). Nitrates available in soil at end of growing season will mostly wash out. Organic sources hold nitrogen in complex molecules that are broken down and made available to plants slowly.
• Working soil when saturated with water can cause compaction and loss of pores. Wait for dry days before working soils in spring.

III. References

How to Grow More Vegetables Than You Thought Possible in Less Space Than You Can Imagine. John Jeavons

Maritime Northwest Garden Guide, Seattle Tilth,

Washington State Extension Library,

Soil Biology Primer, Soil and Water Conservation Society,

Organic Nutrient Management and Water Quality,

Island County Soils Map Descriptions (1955),

Online soils maps,

NOTES