Healthy soil and sustainable growing

["Tradingpost" <tradingpost@lobo.net>]

This must be one of the best soil articles

paul tradingpost@lobo.net
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Improving Clay Soils
Dump on the organic matter, work it in over time, then leave well enough alone
by Keith Baldwin
http://www.taunton.com/finegardening/how-to/articles/improving-clay-soils.aspx
the Fine Gardening magazine

All my jeans have red stains at the knee. I go through gallons of bleach to 
get the red out of my socks, and my blistered fingers are tipped on Mondays 
with red from attempts to remove chunks of clay from the working end of my 
12-pound mattock. But hey, when you live in the southern Piedmont, red clay 
is what you work with to "mold" a garden.

It was worse in my old garden in Cotati, California. I was much taller in 
those days—when it was wet, anyway—because it seemed I always had a least two 
inches of sticky clay soil on the bottom of my boots. I believe I could have 
taken a saw and built an adobe dwelling from large blocks of clay hewn from 
that ground.

Most of us are not blessed with a choice of soil. We're stuck with what we've 
got. And it seems like wherever I move, I get stuck with clay. By comparison 
to the clay in my old garden, the soil at my home now seems almost ideal. But 
it's clay nonetheless, and I've had to work diligently to upgrade the soil.

Not that the soil isn't perfect (to be ecologically correct) for its intended 
use. My part of the county isn't called the Big Woods without reason, and the 
clay soil in my back yard will grow some awesome oak, maple, poplar, and 
hickory, without special treatment of any kind. But try to kick a spade into 
the ground, and you're asking for a sprained knee.

If you face a similar problem, don't fret. Whether your soil has more or less 
clay than mine, so long as you understand what you're working with and keep 
at it over the years, adding organic matter and treating the soil structure 
right, you can get the better of clay soils and enjoy a productive kitchen 
garden.
Clay is more promising than it's cracked up to be

Before you try to manage the clay soil in your home garden, it helps to have 
a bit of background in soil mineralogy. The mineral fraction of a given soil 
consists of sand, silt, and clay particles. Clay particles are the smallest 
of the three, silt are intermediate in size, and sand, the largest. Clay 
particles, bound end to end and side to side in extensive planes, are stacked 
in a sandwichlike matrix and held together by electrochemical forces. This 
platelike stacking of horizontally arranged clay particles results in a large 
surface area.

Because individual clay particles are negatively charged, they have the 
capacity to attract and hold onto, or adsorb, positively charged elements 
(called cations) such as ammonium, potassium, calcium, magnesium, and other 
trace elements. Clay soils are relatively fertile because of this capacity to 
adsorb these important plant nutrients. Conversely, the single, uncharged 
sand particles in sandy soils lack the capacity to adsorb cations and thus 
they contribute very little to soil fertility.

Soil texture is an inherent property that you cannot change. Instead, direct 
your efforts toward improving soil structure.

Soil texture—The textural designation of a soil is determined by its relative 
portions of sand, silt, and clay particles, and indicates which of the three 
most influence the soil's properties. Sand, silt, and clay soil properties 
are obviously dominated by those respective fractions. For example, clay 
soils (generally more than 40 percent clay) are often poorly drained. On the 
other hand, well-drained loam soils are mixtures of sand, silt, and clay in 
roughly equal proportions, and are well drained. A sandy loam, however, has 
much more sand and much less clay than does a clay loam.

Soil structure—Soil texture is an inherent soil property that you as a 
gardener cannot change (except through extreme interventions). Instead, you 
should direct your efforts toward improving soil structure. Soil structure is 
defined by the manner in which soil particles are assembled as aggregates. In 
clay soils, clay particles are typically arranged along a horizontal plane in 
a platelike structure. When these horizontal aggregations are stacked high 
and consolidated over time, they can be quite tight and sticky. Your aim in 
improving soil structure is to achieve a looser, more crumbly or granular 
structural aggregation. A soil with the latter structure has a friable 
consistency and good tilth.

Tilth—Tilth is the physical condition of the soil as it relates to ease of 
tillage, seedbed quality, ease of seedling emergence, and deep root 
penetration. A soil that drains well (yet has water-holding capacity), does 
not crust, takes in water rapidly, facilitates aeration, and does not make 
clods is said to have good tilth. And with the right management strategy, 
good tilth is achievable in a clay soil.
Clay demands short-term and long-range strategies

My initial management strategy centered on adding as much organic matter as 
deeply as I could, because, generally, the application of organic matter to 
soil improves both structure and tilth, and contributes to improvements in 
overall soil health.

My long-range management strategy attempts to build available organic carbon 
and humus and to promote nutrient cycling through regular applications of 
compost, manure, and other organic matter, the incorporation of cover crops 
as green manures, rotations that include grasses and legumes, and reduced 
tillage.

Organic matter builds soil tilth in a couple of ways. First, the organic 
matter coats soil particles, physically separating clay particles and 
aggregates from each other. Second, and more important, microorganisms that 
degrade organic matter produce byproducts called glomalin that bind 
individual clay particles together into aggregates. Particle aggregation in 
the topsoil reduces crusting, increases the rate of water infiltration, and 
reduces erosion and runoff.

Though my ultimate goal continues to be to increase soil organic matter 
content in the garden, I was less concerned initially with the numbers than I 
was with achieving the benefits described above. I understood that in 
southeastern soils, it would be very difficult to increase the percentage of 
organic matter in the soil very quickly.

For one thing, even large amounts of organic matter initially incorporated in 
the soil will rapidly break down. The organic matter fraction remaining may 
resist further degradation for years or even decades, but stable increases in 
this fraction, called the humic fraction or humus, occur very slowly. Thus, 
any increase in stable organic matter is necessarily a long-term goal.
Compost plays a key role

Composts are integral to my clay soil management plan. Because of the 
humified nature of compost and its low concentrations of oxidizable carbon 
and available nitrogen, compost is relatively resistant to further 
decomposition, and additions of compost to the soil over time can increase 
the soil's organic carbon and humic matter content. I add compost not so much 
to provide nutrients as to provide stabilized organic matter that will 
improve the physical properties of the soil.
Read articles about composting...

I'm fortunate to have a commercial compost supplier close by my home, so I 
can take advantage of very reasonable bulk pricing, along with a guaranteed 
chemical analysis that allows me to apply the compost at appropriate rates. 
There is absolutely no way I could have built a garden in my clay soil 
without the 16-cubic-yard tandem truckloads of compost that rolled onto my 
property. Moving that compost one wheelbarrow load at a time to my vegetable 
and flower beds is a weekend ritual. The incorporation of the compost into 
the soil has been more problematic.

During the initial, pre-garden phase of my soil improvement plan, I managed 
to break up 10 inches of hard-packed clay with endless swings of my trusty 
mattock. I spread compost over the surface of the beds an inch at a time and 
rototilled it in. I was careful not to work the clay when it was too wet, 
because clay worked wet can result in some tenacious clods, very reluctant in 
their willingness to ever come apart again. I tried to work the compost deep 
into the soil, and along with it, I added lime and phosphorus, the clay soil 
in my garden needing both. When surface-applied, neither of these materials 
moves down through the soil, so incorporation to ample depth is very 
important to permit roots to grow into the subsoil. After 3 to 4 inches of 
compost, I had the "raised bed" I wanted. I called it quits (for the time 
being) and planted my garden.

In the years since, I have been back through most of the beds several times 
with the mattock and more compost, trying to get organic matter and lime 
incorporated deeply to break up the "seal" that forms between amended and 
unamended soil, and to get my beds raised back up. It's readily apparent that 
organic matter decomposes very quickly in the vegetable beds. In spite of all 
the amendments, the beds no longer seem "raised," and there's not much 
visible evidence of all the compost I've added. However, I can tell by the 
color, now a dark brown instead of red, and by the aggregation of the soil 
particles into a crumbly structure (of a kin with coffee grounds) that humus 
is accumulating.

Reworking a bed is always a fall practice when my rotation calls for 
fall-planted garlic or when potatoes or root crops will be planted early the 
following spring. I just can't count on the beds being dry enough in spring 
to work the clay deeply without messing up the structure I've worked so hard 
to build.
Cover crops keep on giving

Along with adding compost, a second strategy for adding organic matter to my 
clay soil is cover cropping. I plant cover crops any time the beds in my 
garden would otherwise be unplanted or fallow. Fallow periods provide little 
additional organic biomass while allowing the decomposition of organic matter 
in the soil to continue. Cover crops provide me with a variety of services. 
They contribute to the improvements in soil structure that I described 
earlier. They reduce erosion and increase infiltration. And they can smother 
weeds and even suppress weed seed germination. Many cover crops will also 
suppress pathogenic nematodes, for example root knot nematode.

I take a machete and scalp the cover crop at the surface, killing it but 
leaving the biomass in place.

Leguminous cover crops can fix significant amounts of nitrogen for use by 
subsequent crops. Through symbiotic associations with legumes, Rhizobia 
bacteria convert atmospheric nitrogen into an organic form that legumes use 
for growth. How much fixation depends on the length of the growing season, 
the local climate, and soil conditions. Nonleguminous cover crops, 
principally grasses or small grains, do not fix atmospheric nitrogen, but can 
be effective in recycling residual soil nitrogen remaining in the root zone 
at season's end, reducing leaching losses from the soil during the winter 
months. I often plant mixtures of grasses and legumes in the fall to reap the 
benefits of both. This past fall I planted hairy vetch and cereal rye, a 
proven mixture here. Crimson clover and rye or oats also do well in my area.

In mid-May, I become a terminator, literally putting these annual plants down 
to prepare the beds for summer vegetables. By then, the winter cover crops 
have provided my annual organic matter addition, as well as enough nitrogen 
fertilizer to grow my summer crops.
The less you disturb amended clay the better

Because of the many benefits that accrue to soil quality, it is clearly in 
your short-term interest to incorporate organic matter regularly and deeply, 
at least when beginning the process of whipping a clay soil into shape. Soil 
particle aggregation and aggregate stability, water holding capacity, 
drainage, nutrient retention, and plant root growth are all increased when 
organic matter is incorporated.

However, major losses in soil organic matter content can take place when the 
soil is inverted or mixed annually by tillage. Extensive tillage stimulates 
microbial activity (gives the little guys an appetite, so to speak), and the 
consumption of mass quantities of organic matter ensues. After your clay soil 
becomes more friable and you have provided a deep root zone for your garden 
plants, you should consider reducing tillage. Switching to a minimum-till 
system increases soil organic matter, soil organic carbon, total nitrogen, 
and soil microbial biomass carbon and nitrogen content, especially at the 
surface and in the top 2 to 4 inches. Because the soil remains undisturbed, 
fertilizers and other soil amendments do not become homogenized in the 
tillage layer. Plant feeder roots, therefore, tend to proliferate in the top 
2 inches of fertile topsoil.

For a number of years now, I have been experimenting with no-till vegetable 
planting in the spring. I take a big knife (i.e., a machete), make sure there 
are bandages available nearby, and scalp the cover crop at the surface, 
killing it but leaving the biomass in place. I pull the residue apart just 
enough to dig a small hole with a trowel, drop in my tomato, pepper, or melon 
transplant, and pull the mulch back around the stem. I plant large seeds the 
same way, except that I leave a mulch-free window for sunshine to strike the 
seedling when it emerges. For small-seeded vegetables, I generally stir the 
soil a bit more and leave a strip of mulch-free surface above the seed.

Fruiting is delayed a bit because the soil is cooler for a time, but overall 
production is not sacrificed. The mulch reduces weed growth and prevents soil 
that may carry overwintering disease pathogens from splashing up on foliage. 
Infiltration of irrigation water and rainfall improves, as does soil moisture 
retention.

The residue decomposes much more slowly than if I tilled it in. Slower 
decomposition can result in more favorable timing of the release of nitrogen 
from the legume cover crop.
A few sticky situations

There are a few circumstances involving clay soils that merit special 
consideration. In the western states, some clay soils contain excessive 
sodium (these are known as sodic soils). The adsorption of sodium by clay 
particles can cause aggregated clay particles to "disperse," or come apart, 
destroying friability and tilth. Typically, the remedy to this problem is the 
incorporation of gypsum (calcium sulfate) to provide soluble calcium that can 
replace the adsorbed sodium.

Unless gypsum is called for in a soil test report, however, it is rarely 
recommended as an amendment in clay soils. Most garden soils that have a 
proper pH (greater than 6.0) already contain adequate calcium.

Another special condition exists in those clay soils that shrink when dry and 
swell when wet. It is critically important that any tillage of these 
"expanding clay" soils (and other clay soils, too) occur only when soil 
moisture conditions are those that Baby Bear would like: "just right."

To determine "rightness," roll some soil between your palms into a ball about 
an inch in diameter. The moisture is right for tillage when a pinch between 
thumb and finger causes the ball of soil to fracture, break, or crumble into 
pieces.

Organic matter incorporated deeply by tillage is especially helpful in 
improving the soil structure of expanding clays.

A note of caution to gardeners who would attempt to improve clay soils by 
amending them with sand: This is a very risky business unless the sand (often 
incorporated as much as 50:50 by volume) is a very sharp sand. Otherwise, 
additions of sand to clay soil can create a cementlike creature that resists 
root growth and impedes the flow of air and water.