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What is Nature's Best Soil?
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Greetings!
In Nature plants have adapted to grow in nearly any situation possible where they can find a stable foothold and liquid water is present sometime during the year. The majority of land plants are adapted to live in soil that we call loam.
I'll have to apologize to anyone who has taken classes in soil science. I don't expect any of you to sit through a detailed study on soils, so I will over simplify this subject. I will be talking primarily about the top layer of soil where most plant root activity occurs.
Loam occurs when the soil contains relatively significant percentages of the three main types of soil particles SAND, SILT and Clay. All three types are primarily composed of oxygen and silicon with smaller amounts of aluminum, magnesium and iron.
SAND is a relatively rounded particle with a diameter ranging from 2 mm down to 0.05mm
SILT is smaller, but still relatively rounded particle with a diameter ranging from 0.05mm down to 0.002mm
CLAY is a flake-like particle less than 0.002mm.
Before going much farther I need to explain what plant's roots require from the soil.
1. MOISTURE Active plants require constantly moist conditions in the root area. The average tree can easily extract 50 gallons of water on a warm summer day. Without the soil's high humidity the roots can shrivel and die within hours.
2. OXYGEN Active plant roots need oxygen. Although plants produce oxygen in their above ground green tissues there is no direct oxygen transport system to the roots. The oxygen goes from the chloroplasts into the air through the soils pores and into the root tissue. Roots generally don't require as much oxygen as mammals and like fish, can exist indefinitely in well oxygenated water. Root and crown rot diseases in many plants are promoted by low oxygen levels in the soil. Roots can generally tolerate short periods (1-2 days) of low oxygen which often occurs after rain or irrigation.
3. MINERALS In order to grow plants require a source of minerals. Rich soils have the ability to store available minerals. Plants can exists for long periods without a source of minerals. When plants are lacking one or more minerals their foliage will often appear pale, but such chlorosis is not necessarily an indicator of plant's overall energy level.
4. SUPPORT Although the soil is essential for health of the roots it is a hostile environment for the foliage. The soil must be solid and stable enough to support the stem or trunk.
5. INSULATION Active roots prefer temperatures well above freezing but not much above 85 degrees F. Moist soil is a better insulator than dry soil.
Plants roots generally exist in a zone of soil from the surface to just below a foot deep with most root activity occurring within the top 6 inches. Tap roots don't exist unless the soil is highly permeable to an unusual depth. Plant roots can reach 25 feet deep in gravely soils in mountain valleys. Tree roots typically spread 3-20 times wider than the tree is tall. About 2/3 of a woody plant's total mass is above ground.
Molecules of water are attracted to the surface of soil particles and forms a very strongly held layer at least one molecule thick. The more surface area the soils particles have, the more water is attracted. Generally, smaller particles collectively have a larger surface area. The collective surface area is known as the pore space. Clay particles, being smaller, have a higher porosity than sand and a clay soil can hold nearly 3 times more water than sand. Clay soils have a higher porosity than sand.
Unfortunately this higher porosity does not insure that Clay soils breath. Most of the pore space in clay is occupied by water molecules. Pore size is more important when discussing the permeability (ability to breathe freely) of soil. Pores in a sandy or silty soil are the gaps created between the rounded particles of sand and silt. In physics the volume of empty space left when identical spheres are closely packed is 21-25% of the total. Therefore in sandy or silty soils you'd expect that about 20% of the soil is air space. It turns out that soils containing much more than 20% clay are called clay soils because all of the air gaps created by sand and silt particles are then filled with clay.
Any soil containing more than 30% clay is a clay-type soil (sandy clay, clay loam, silty clay, etc). Any soil above 50% clay has very poor permeability and is just called clay. On the other hand, sand must be at least 90% sand to be called sand. Just a little bit of clay ruins the permeability of sand.
The ideal soil for a farm is called sandy loam and is roughly 65% sand, 25% silt and 10% clay. Farms prefer a higher sand component to prevent compaction from vehicular traffic. The ideal soil for a home is called loam and is roughly 40% sand, 40% silt and 20% clay. The higher clay content holds moisture and nutrients better.
Nature has a way of creating higher permeability in any soil. Lignin, the glue that holds the strands of cellulose together in a plant, is released into the soil when dead plant tissues decompose. Lignin glues small soil particles into larger granules (granulation) with large air spaces in between. Plants roots along with Mychorrizal fungus strands will hold these granules of soils into a firm, structure riddled with pathways punched through by the activity of nematodes and amoebas. Ideal living soil resembles Swiss cheese and is about 1/2 mineral and 1/4 air and 1/4 water.
Up to now I haven't mentioned anything about organic matter. The organic component of soil is very small, but very important. It is usually in the form of humus, which are typically plant fibers that won't decompose further. Humus stores mineral nutrients and is usually associated with high activity of beneficial soil organisms. The USDA did a survey across the continent and found that the average organic content of the soil is 0.9%. The University of California did a soil survey of farms in our state in the year 2000 and were pleased that the organic content was a perfect 1%. They noted that a few farms had organic content of up to 40%, but these were only suitable for annual crops and irrigation had to be monitored carefully. There are many areas of the US that have 100% organic soils, but these are called peat bogs and only a few annual plants can flourish.
In a few lucky areas of the World there are extremely rich "black soils". These have a relatively high charcoal content of 2-3%. The charcoal is responsible for the black color. Unlike organic mulches or compost, charcoal is an inert form of organic matter. It, however, captures and stores minerals better than any material known (which explains its use in filters). Researchers in Central America could easily identify areas of the rainforest that were enriched by the charcoal created by human campfires 13,000 years ago.
There is another way to create black soil. When the tissue of dead organisms decompose anaerobically (without adequate oxygen) sewer gasses are produced which turn the surrounding soil black (the same happens in a sewer). If the soil is aerated, the black color dissipates. Sewer gasses are toxic to plant roots.
The USDA survey also showed that while the organic content of soil averages only 0.9%, the soil's surface is covered by an average of 5 inches of duff. Duff is a layer of dead leaves, flowers, twigs, etc. Mychorrizal fungi, bacteria, amoebas and other soil organisms consume the dead tissues and release mineral nutrients back to the nearby roots. It, rather than the soil, is the primary nutrient source for the existing plants. It is a bad idea to try to make your garden soil "rich" and "black" by amending it heavily with organic compost. If it turns black, you've just created a landfill unsuitable for most plants' roots. Even if it doesn't turn black you can cause trouble. Soil heavily amended with compost has low levels of oxygen leading to shallow or poor root development. Back in the 1980's (before I knew enough about soil) I had a retaining wall built so I could plant a rose garden against a short slope. I decided to make the soil as rich as possible so I filled it with at least 1/3 planting mix. I planted the roses and they all did fine. After about 3 years I pulled one of the roses out (to replace it with another) and noticed that all the roots below 5 inches deep were black and dead. At the time I figured that I had planted it too deep, but now realize that the planting mix killed the deeper roots. The rose plants compensated by growing additional roots closer to the surface.
If you want to improve your soil simply cover the surface with dead leaves and keep it covered and moist. Nature knows what to do. Farms do incorporate organic matter to increase their soil's nutrient levels, but the amount is fairly low at about 5%.
If you want to immediately improve the permeability of a clay-type soil, amend it with sand. This won't improve the drainage but does allow better oxygen penetration. If your garden is bedrock and requires a jackhammer to dig a hole, it is better to create raised planters filled with sandy loam. Do not purchase "top soil". Local soil companies create a totally unnatural "topsoil" by mixing sandy loam with compost in roughly equal amounts. This mix not only shrinks over time, it smells and it's great at killing sensitive plants. It turns out that some plants require soil with more air than others. These plants rot more easily when the oxygen level of the soil is low. Plants that survive in poorly aerated soils are easier to grow.
Survivors Include: Apple trees, Grasses, Lilies, Roses, Hydrangeas, Pines, Junipers, Cypress, Palms, Pear trees
Most Sensitive Plants Include: Avocados, Azaleas, Camellias, Ferns, Gardenias, Orchids, Persimmon, Rhubarb
Next week I'll discuss why potting soil is different from loam.
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Garden Notes
We will be at all 3 markets this Easter weekend.
So far, April's weather has been absolutely perfect. If April follows past trends we will get some hot weather just after mid-month (ideal for avocado flowers to set fruit) before the May-June gloom shows up. I actually expect gloom to be minimal this year, or at least less than last year (one of the gloomiest springs ever).
Official mildew season on roses begins in about 2 weeks and mildew on European grapes can start a few weeks after that. Many vineyards have a rose garden to alert them when to treat their grape vines. Hopefully you are growing hybrid grapes (like Venus, Lakemont, Himrod, Canadice, Einsett, etc.) that are unlikely to get mildew disease.
This month is when our suppliers have their most extensive selection of tomato varieties. We can't carry all (well over a hundred), but we will have our best selection for the next 3 weeks. The heirloom varieties Old Time Tasty and Persimmon are special requests from our customers.
TERRIBLE NEWS A citrus tree in Hacienda Heights (Los Angeles county bordering northern Orange County) has been found to be infected with HLB (Huanglonobing) also known as Citrus Greening Disease. This disease is responsible for the destruction of 200,000 acres of Citrus in Florida, 10 million trees in Brazil, and threatens the Citrus Industry around the World. Fortunately, California has been much more vigilant in monitoring and controlling the insect vector of this disease, the Asian Citrus Psyllid ACP, which is currently a very common, widespread pest in Florida. HLB is a bacterial disease which cuts off the tree's circulation. Unfortunately a newly infected tree is symptom free for up to 18 months. Also, at a glance, the ACP looks pretty much like an aphid infestation, otherwise common on citrus foliage. Currently there is no known cure for HLB and no known Citrus cultivars that are naturally immune.
GMO CITRUS TO THE RESCUE? Texas Agrilife Research (funded by the USDA and a Florida citrus producer) has created several GMO Citrus trees immune to HLB. They inserted two Spinach genes to create the immunity. The fruit is currently being tested tested to ensure its edibility and the flowers tested by bees. So far they have created GMO strains of Rio Red grapefruit, Ruby Red grapefruit, Hamlin orange, Marrs orange, and the rootstocks Flying Dragon, C-22 and Carizzo.
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AVAILABLE THIS WEEK
Edibles
Pepper quarts $3
SWEET Big Early, Cute Stuff Red, Orange Sun,
MILD Fushimi, Shishito, Anaheim, Ancho/Poblano, Padron
HOT Caribbean Red (Habanero type), Fresno, Jalapeno, Serrano
Squash quarts $3
Crookneck Yellow, Zucchini types
Tomato quarts $3
HYBRIDS Better Boy, Big Beef, Champion, Early Girl, Lemon Boy, Momotaro HEIRLOOMS Aunt Ruby's German Green, Black Krim, Cherokee Purple, Coustralee, Green Zebra, Kellogg's Breakfast, Old Time Tasty, Persimmon, San Marzano SMALL FRUIT Black Cherry, Grape, Yellow Pear, Sugary, Sun Gold, Sun Sugar, Sweet Million More Vegetables quarts $3
Shiso (green & red) Cucumber Japanese Suyo Eggplant Japanese Long Tomatillo Toma Verde More Vegetables 6-packs $3
Beets
Onions Red Creole, Sweet Spanish
Swiss Chard Rainbow
Herbs quarts $3
Arugala, Basil (many types), Caraway, Chamomile, Chervil, Catnip, Catmint, Dill, Lovage, Mint (many), Parsley, Rosemary (several), Sage, Stevia, Thyme (many types), and much more! Herb quarts $4
French Tarragon Fig Trees 1-gallon $22
Black Jack Black Mission Flanders Panache White Kadota Pomegranate 1-gallon $20
Desertnyi Ornamental Plants Centaurea Amethyst in Snow quarts $6
This perennial of the Aster family is native to the Middle East. The gray foliage forms a mound 1 foot tall and 2 feet wide. Each stem produces a sparkler-like bicolor flower. Blooms occur from spring through summer if spent blooms are removed. Established plants become tolerant of short periods of drought.  Gaura Belleza Dark Pink quarts $4
This is one of the best perennials you can grow. This cultivar of Gaura grows about 18" high and 12" wide. The foliage lays low during winter. In spring the deep burgundy new growth shoots upward and by mid-to-late spring is topped by spikes of rosy pink flowers. The bloom continues late into fall. Gaura is a North American native that becomes quite tolerant of short periods of drought after getting established and will perform for well over a decade. Carex oshimensis 'Evergold' GOLDEN SEDGE 1-gallon $9 Coleonema pulchellum PINK BREATH OF HEAVEN 1-gallon $9 Coleonema pulchellum 'Sunset Gold' BREATH OF HEAVEN 1-gallon $9 Convolvulus mauritanicus GROUND MORNING GLORY 1-gallon $9 Hymenosporum flavum SWEETSHADE TREE 5-gallon $35 Ophiopogon planiscapus 'Nigrescens' BLACK MONDO GRASS 1-gallon $11 Phormium 'GUARDSMAN' NEW ZEALAND FLAX 5-gallon $40 Romneya coulteri MATILIJA POPPY 1-gallon $12 Saxifraga 'Peter Pan' SAXIFRAGE 1-gallon SPECIAL $5 AND MUCH MORE! |
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Please visit us at our farmers market locations
EVERY FRIDAY 9am-1pm
Mission Viejo Farmers Market
Mission Viejo Library
200 Civic Center drive Mission Viejo
Exit 5 at La Paz going east, turn right on Marguerite, turn right on Civic Center
EVERY SATURDAY 9am-1pm
Old Towne Orange Farmers and Artisans Market
The Historic Villa Park Orchards Packing House at Chapman College
304 N. Cypress Street, Orange
3 blocks north and 4 blocks west of the Orange traffic circle (Glassell X Chapman)
EVERY SUNDAY 10am-2pm
The Great Park Farmers Market
The Orange County Great Park, Irvine
Exit 5 at Sand Canyon, turn south on Marine Way (parallels east side of 5), follow the signs.
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26285 Verona Place
Mission Viejo, California 92692
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