It's All About Soil

Understanding Soil Preparation

       Since about 1980 it has been horticultural dogma that incorporation of generous amounts of organic matter into the garden’s soil adds nutrients and creates better soil.   In 1998 I made a decision to commit heresy and deviate from this theory.  We no longer recommend incorporation of compost into any soil we intend to grow plants in.  We do recommend covering the surface of the soil with a 2-3 inch thick layer of organic matter.

       Before we go any further I will define some terms.  Organic matter is a substance that was once alive. 

Compost is organic material that is at least partially decomposed by natural bacterial and/or fungal activity.  (Commercially available planter mixes are composts.)

Humus is organic material that is fully decomposed, or won’t decompose further. 

Amendment is any material mixed into soil for the purpose of changing its physical or chemical properties. 

Pore space is the fraction of the soil volume (expressed as %) that is normally occupied by water or air.  The soil’s pores are the gaps that exist between the solid particles. 

Mulch is any material used to cover the surface of the soil and is usually coarser in texture. 

       Roots of plants require a minimum amount of moisture and oxygen in the soil to remain healthy.  Mineral nutrients like nitrogen, potassium, phosphorous, sulfur, calcium, iron, magnesium, boron, manganese, copper, zinc, molybdenum, chlorine and nickel are required for growth, however roots can survive for long periods without adequate amounts present.  Roots, specifically root hairs, cannot survive more than a few minutes in contact with totally dry soil.  Active roots cannot survive more that a few days without sufficient oxygen.  Note that water and oxygen are essentially, but not totally, mutually exclusive (water can contain some oxygen).  New plantings require a good balance between water and oxygen.  Most plants perform well in soil with a pore space of 10-25%.  Few plants (mostly grasses) thrive in soils with a pore space of less than 10%.  Soils that have pore space exceeding 30% usually do not store enough water for most plants. 

       Soils have four components.  Besides water and air (includes oxygen) soil particles include mineral and organic.  Mineral normally makes up about 80% of soil volume and the organic matter is typically less than 1%. 

       The mineral portion of natural soil is generally composed to three classes of particles: 

sand (particle size from 2 to 0.05mm)

silt (particle size from 0.05mm to 0.002mm) and

clay (particle size less than 0.002mm). 

Soils that contain significant amounts of all three particle sizes is called loam.  The ideal soil, as far as farmers and landscapers are concerned, is called sandy loam and is approximately 40% sand, 40% silt and 20% clay.  Soils with a high sand content has a greater pore space and is often referred to as light soil.  Soil exceeding 30% clay has very little pore space and is called heavy. 

       Think of clay soil like peanut butter in a jar.  You would have to add a lot of sand (over 70% by volume) to make the jar’s contents not feel gooey.  Conversely it only takes a bit of peanut butter (30% by volume) to make a jar of sand gooey. 

       Clay soil behaves similarly.  Soil acts like clay even if the clay content is only 35%.  At 35% it is possible to add a little sand and change the soil’s character.  If the soil is close to or greater than 50% clay, it would take a lot (impractical amount) of sand to change the soil’s character. 

       Most of the active roots of a plant are in a 4-inch zone of soil between 2 inches and 6 inches below the surface.   At the surface the soil is too hot or cold and often too dry.  Below 6 inches the oxygen levels are too low.  Although plants roots can reach over 100 feet deep in volcanic rock or fractured rock around fault zones it is not common for more than a fraction of the roots of plants to grow more than 1 foot deep.  With this in mind, we need only be concerned with the top foot of soil during soil preparation. 

       Nature improves soil, creates good friable soil, by the action of lignins released by decomposition of dead plant tissue.  Lignins, the glue that holds the cell walls of plants together, glues the loose soil particles (clay, silt and sand) into solid, building block-like clumps.  The hyphae (root-like structures) of Mycorrhizal fungi tie together and support the building blocks acting like the girders of a building.  Rooms, hallways, and doors are punched through the soil by the activity of nematodes, worms and protozoa.  A sterile amorphous soil is changed into a living, freely breathing structure that resembles a sponge or Swiss cheese.  This soil becomes highly resistant to compaction.  When Nature is left alone, the soil can support plants roots to a far greater depth than is normally observed in cities. 

       Mycorrhizal fungi (many species) are symbiotic partners to 95% of the species of land plants studied.  Truffles, on the roots of Oak trees in southern Europe, are an example.  These fungi are the dominant recyclers in Nature of dead plant tissue.  It is otherwise quite difficult for woody plants to obtain sufficient nutrients (particularly phosphorous) and water with their own roots.  Mycorrhizal fungi can completely cover the roots of a plant shielding it from diseases and pests.  In exchange, Mychorrizhae receive sugars from the plant. 

      Why do most plants need help?  Many researchers believe that the first land plants evolved at the edge of mineral rich ponds in coarse soil or volcanic rock.  The plants’ roots had ready access to water, oxygen and mineral nutrients.  When land plants migrated into heavier soils with less than ideal conditions, they needed help to recreate their “Eden”. 

       What can we do to improve the soil in our gardens?  There are several options:

 

1.        Allow Nature  Just maintain a mulch layer in your garden about 2-3 inches deep composed of a mixture of organic products.  Do not use significant amounts of chemical fertilizers or fungicides.  Your plants, along with natural soil organisms will utilize the nutrients and energy stored in the organic mulch to gradually create good soil.  If your soil is sandy loam, leave it alone!

 

 2. Recreate EdenIncorporate pumice, perlite, sand, gravel or decomposed granite to increase the pore space of the soil.We prefer pumice (a type of volcanic rock) because it is porous (70% air, 30% rock).  Amending the soil by about 25% by volume will allow air penetration into the worst clay soils. 

 

3.        Do Both  A combination of the above is most effective.  As each plant is installed create a zone several inches thick surrounding the root ball with soil amended with pumice.  This will provide oxygen to keep the roots healthy until Nature creates a breathable soil and allows the roots to venture out. 

Above all do not make the situation worse.  Avoid amending the soil with anything that can decompose.  Decaying material consumes oxygen.  For a while the organic matter’s size and shape may allow airflow.  Eventually the decomposing organic matter no longer has enough structure to create porosity and the oxygen level drops.  Decomposition under low oxygen levels creates acids and anaerobic decomposition creates sewer gasses.  These sewer gasses are deadly to roots and turn the soil black and smelly.  You’ve just created a “landfill” quite unsuitable to most plants.  At best, soil amended with organic mulches stunts plant growth and causes off-color foliage.  At worst it kills. 

       Unfortunately the black, high carbon soils, rich in nutrients, that occur naturally in many parts of the World have been misunderstood by many soil “experts”.  The black in these soils is from charcoal, not sewer gasses.  Charcoal is an inert form of carbon resulting from fires not decomposition.  Charcoal is not itself a nutrient source but dissolved minerals stick to it.  Bits of charcoal in the soil also provide an excellent habitat for soil organisms.  Charcoal can be added to the ground (if you can find a reasonable source) or you may add leonardite, a naturally occuring material related to coal. 

       I believe that amending with compost is the primary cause of crown rot in bedding plants, shallow root systems, stunted growth, premature dormancy and yellowing foliage in general. 

       There are some commonly available organic materials that cause less trouble.  Rice hulls, peat moss, redwood sawdust and coconut coir decompose relatively slowly and cause little damage in an otherwise porous soil.  Peat moss and coir are valuable for their water storage potential.  More intact pieces of wood or bark decompose slowly compared with finely ground wood or bark, but why incorporate these when the same materials work better, and cause little harm (presently or in the future), as a surface mulch. 

       LAWNS  Unless the soil needs to be raised, no amendments are recommended.  The U of Texas Agronomy Department has determined that the native soil (unless it is gravel) will always produce the best results.  If soil is needed, bring in sandy loam and make certain that some is tilled into the native soil to avoid a sudden change of texture.  The only amendment that could help is leonardite (humic acids) which can help the soil retain nutrients. 

       FLOWER BEDS  If your soil is decent, just leave it alone.  If the soil is clay, add 1 sack Laguna Hills Nursery Planter Mix (mostly pumice) to every 5 to 10 square feet and till it in 6 to 10 inches deep.  Kellogg’s AMEND (mostly rice hulls) can also provide good results and will cover twice the area.  Adding leonardite helps. 

       SHRUBS & TREES  Prepare just the soil immediately around the rootballs of container grown shrubs and trees by mixing Laguna Hills Nursery Planter Mix with the native soil at a rate of 25 to 50%.  Bare root plants perform best when the soil is not altered. 

       WHAT IF YOUR SOIL ALREADY CONTAINS COMPOST?  If your soil is more than 10% organic it may be impossible to repair quickly.  Nature will eventually fix it, however it may take a decade or more.  To get immediate results the amended soil needs to be replaced.  Soils amended with compost are usually a different color than the native soil and will have either an acrid odor or a sewage odor.  The bad soil can either be sent to a landfill or spread thinly over the rest of the garden. 

       If you are only planting seeds, bare root or field-grown (balled-&-burlapped) plants we can stop here.  There is, unfortunately, a problem with container-grown plants.  All container growers use organic materials in their growing medium (soil mix designed for containers).  Originally this was done to make the medium lighter because sandy loam weighs about 100 pounds per cubic foot.  50 years ago nursery plants were a lot heavier. 

       Soil researchers originally recommended that nurserymen lighten their soil mix by mixing peat moss and sand about 1:1.  Peat moss degrades after several years, but the sand will maintain porosity.  Peat moss has always been expensive and many nurserymen used redwood sawdust instead.  Although not perfect, redwood sawdust creates a fairly stable soil as it is highly rot resistant, lasting more than 5 years.  Unfortunately when the cost of redwood skyrocketed in the late 1970’s most nurserymen changed from redwood sawdust to fir shavings, finely ground fir bark and other composts.  Unfortunately as these organic materials decompose they create poor growing conditions due to declining porosity which not only causes root asphyxiation but also toxic chemicals resulting from further decomposition under anaerobic conditions.  This process happens when the plant is left in its container and also after the plant is installed in the ground.  The plant will show decent health if the soil surrounding it is relatively porous.  Results can be poor, or ultimately fatal, if the surrounding soil is heavy.   

       It is possible and often desirable to purge the soil ball of organic matter when installing plants sensitive to low oxygen levels.  Care must be taken to avoid excessive damage to the roots and to prevent desiccation for a week or so until the roots reestablish and regain full function.  A sharp stream of water is ideal for removing “soil”, however water can be conserved if a slender stick (sharpened chopstick) is utilized to remove the majority of the wood and bark pieces.   Without letting the roots ever get dry, carefully replace the soil with native soil or at least material that is mostly inert.  Water immediately and thoroughly.  If the plant is deciduous and dormant, success is likely.  If the plant is evergreen, it is essential to either modify the plant (pull off leaves) or modify the environment (put in shade) to minimize moisture loss during recovery.  This technique requires practice but is currently invaluable when working with plants that commonly fail from root rot diseases. 

       When the soil is not correct, a gardener can spend valuable time compensating for its deficiencies.  When the soil is properly prepared plants perform to your expectations. 

 

-Gary Matsuoka CCNP