On Tuesday I went to a field day on no-till tobacco production. Tobacco is a warm season solanaceous crop -- like tomatoes or peppers -- that is usually transplanted into freshly-tilled soil in late spring. After fall harvest the remaining stubble is usually left to decompose in the bare soil until the next spring, when the plow comes around again.

In recent decades people have started to recognize that soil suffers when it's left bare, or routinely disturbed by cultivation. Bare soil is susceptible to wind and water erosion. Cultivation destroys the soil structure, further increasing its susceptibility to erosion. Cultivation also introduces a lot of oxygen to the soil very quickly, resulting in a brief boom in the microbial population, and a rapid depletion of the soil organic matter that the microbes eat. (Organic matter is a valuable component of soil because it holds on to the nutrients and water that plants need; soil microbes help release nutrients into the soil solution, making them accessible to plants, and exude sticky material that holds soil particles together, reducing soil's susceptibility to erosion.) In the long term, cultivation reduces soil organic matter content and soil microbial populations.

No-till grain production is now fairly common, but very few farmers grow transplanted crops, like tobacco, without cultivating. It turns out that one of them happens to be a sixth-generation Kentucky farmer who took the 'Introduction to Sustainable Agriculture' course that I co-taught last semester. The field day was at his farm.

We saw a nice demonstration of how soil that hasn't been tilled holds together better than soil that is routinely cultivated. Clods of soil collected from sections of the farm that hadn't been cultivated for 10 years were suspended in water next to a clod collected from a routinely cultivated section. You can see the clod on the left disintegrating while the clod in the middle holds firm:

After harvest the land is seeded to a winter cover crop that protects the soil from winter erosion, saves nutrients that might leach out of the soil in the absence of plants, and feeds soil microbes. The farm is experimenting with different winter cover crop mixes, most of which include a nitrogen-fixing legume species.

At the Kentucky State University Research and Demonstration Farm we often use a mixture of rye, which grows quickly and out-competes weeds; and hairy vetch, which fixes nitrogen and twines its way up the rye. Here are the two plants together, towering over a yardstick:

Nitrogen-fixing crops like hairy vetch harbor bacteria in their roots that are able to convert nitrogen gas from the air around us into nitrogen that is available to plants. A winter cover crop of hairy vetch can add more than 100 pounds of nitrogen to the soil per acre (Kansas State University pdf), enough to feed a nitrogen-demanding crop like corn.

Of course organic farmers have been using nitrogen-fixing cover crops for decades; the organic standards don't allow synthetic nitrogen fertilizer. Conventional farmers have known about the advantages of cover cropping, but using nitrogen fertilizer has long been cheaper than managing cover crops. Soaring fertilizer prices have changed that. Suddenly tactics like no-till production and cover cropping aren't just better for the soil; they're cheaper, too.

Michael Bomford provides research and extension services related to organic agriculture and small-scale renewable energy production through Kentucky State University's Land Grant Program.

The Sebastopol Demonstration Energy Garden is located within the Sebastopol Sandy Loam series, which is described as a moderately fertile, well drained soil. Permeability is moderately slow in the subsoil, runoff is medium, and the hazard of erosion is moderate.

Soil texture is determined by the relative proportions of sand,
silt and clay and organic components that soil has. Sand is the largest particle, silt particles are smaller than
fine sand but can still be seen by the human eye, and clay particles are
microscopic.

Sandy soil—tends to be very
light and dries out swiftly. Water drains very quickly and makes the
soil easy to dig. It is the first to warm up in the sun.

Silt soil—retains moisture and feels slippery when wet. Retains nutrients better than sand but does not dry out as quickly.

Clay soil—a
very heavy soil, it holds moisture for long periods of time when wet
and dries hard as a brick. Clay soil retains nutrients and is very
fertile but is heavy, sticky and very hard to dig. It is the last to
warm up in the sun.

Loam soil—the ideal
soil texture, it is composed of sand, silt and clay. The ideal loam
soil contains 40% silt, 20% clay and 40% sand and organic matter. Loam
is a separate category because none of its compontents account for more
than 50%.
Loam soils are ideal for most plants, although many plants grow well in non-loan soils.

8/3/07 8/11/07

So as to test soil texture, we performed a comparitive test of samples from different parts of the garden.

A) Soil sample from path; untreated

B) Soil sample from bed; ammended with mango mulch

C) Sample of purchased compost; mango mulch ammendment

D) Sample of the compost developed on site.

We filled recylcled mason jars a quarter full with a portion of each sample, added water and a couple drops of biodegradable dish washing soap, and allowed them to settle for a week. Based on the thickness of each settled layer, we determined the proportions of sand, silt, and clay in our soil.

Using a basic LaMotte soil test kit and a soil auger, we tested each sample's pH, and the quantities of available nitrogen, phosphorous, and potassium.

The pH of our soils ranged from 6-8 and it appeared that the more organic materials that the soil contained, the more alkaline it was. The phosphorous content of our samples ranged from medium to high (75lb/acre-100+lb/acre).

The nitrogen content of our compost is tremendously high compared with the other three samples which could be expected because our chickens are allowed access to the piles. Aside from sample D, merely trace amount of nitrogen were present. The potassium content of our samples correlated with the proportion of organic marterial in the soil. The purchased mango mulch expressed the highest content, followed by the compost we have developed.

Nitrogen- stimulates leaf and stem growth. Nitrogen deficiency
causes reduced growth and pale yellowish green leaves. The
older leaves turn yellowish first since the nitrogen is readily moved
from the old leaves to the new growth. If the soil is cold and wet,
nitrogen in the soil is not as available to the plants. Excess nitrogen
may cause potassium deficiency.

Phosphorus-is important in the germination and growth of seeds, the
production of flowers and fruit, and the growth of roots.
Phosphorus deficiency causes reduced growth and small leaves that drop
early, starting with the oldest leaves. Leaf color is a dull,
bluish green that becomes purplish or bronzy. Leaf edges often turn
scorched brown. Excess phosphorus may cause potassium
deficiency.

Potassium- promotes general vigor, disease resistance and sturdy growth. Potassium deficiency causes stunted growth with
leaves close together. Starting with the older leaves, the leaf tips and edges turn scorched brown and leaf edges roll. Excess
potassium may cause calcium and magnesium deficiencies.

Using a disecting microscope at a magnification of 30x we analyzed the
contents of each sample, here looking for the amount of sand, clay,
silt, and organic materials. At this magnification the mycelium of
fungi was visible.

A. B. C. D.

A. Mostly sand w/ small amounts of silt

B. Finer sand particles and more silt than A.

C. Mostly organic materials. Mycorrhizae present among other mycelial growth.

D. More silt than A and B. Mycellial growth present as well as organic remnants.

Using a microscope at a magnification of 600x we analyzed the biology of each soil sample.