The following essay reports on a 12-year long experiment (small scale) at
the CT Agricultural Experiment Station in New Haven. The quick summary is
that after 12 years the yields on a plot that recieved 10-10-10 and
limestone every year were about equal to those on the plot that had only
received one inch of leaf compost each year. The plot with only leaf
compost had a higher pH and other superior characteristics (lower bulk
density, higher organic matter, and greater water holding capacity).
Dr. Maynard is now doing further experiments using organic fertilizers, but
not, so far as I know, with the most valuable organic fertility
builders-cover crops and animals.
A mineralologist I know talks about the differing rates of leaching of
different elements from soil minerals, with many of the most agriculturally
important ones being leached first. This fit with the results that some
organic farmers here get from using fresh (unleached) basalt dust from
local quarries.
Living on the Earth, March 10, 1995; Organic Matter
The most important aspect of organic agriculture is the care of the soil.
Organic growers avoid using soluble, synthetic fertilizers such as
10-10-10, and instead use compost, cover crops, mulches, rotations, and
minerals to enrich the soil. Soluble fertilizers feed the plant and not
the soil. If they aren't absorbed immediately by the crop, these
fertilizers can leach into and pollute groundwater or streams. Fertilizers
also upset the soil ecosystem with their strength and acidity, and consume
a lot of non-renewable fossil fuel in their production.
A twelve year research project conducted by the Connecticut Agricultural
Experiment Station at their Hamden farm validates the practices of organic
growers. This research shows that it's organic matter which is really
important, and that just one inch of compost added to the soil each year
produces yields equivalent to and soil conditions better than annual
applications of chemical fertilizer.
In the experiment, station scientists set up five 300 square-foot plots,
fertilized them the same way each year for twelve years and compared the
cumulative yields of a variety of common vegetables.
On one plot they applied the standard amount of 10-10-10 synthetic
fertilizer and ground limestone each year. Ten-Ten-Ten contains 10
percent each of nitrogen, phosphorus and potassium, the major plant
nutrients. Ground limestone makes the soil less acid by providing calcium
and magnesium, two other important plant nutrients.
On the other four plots, they applied one inch of composted oak and maple
leaves every year, and a different percentage of the standard fertilizer
and lime -none on one plot, one-third on the second, two -thirds on the
third and a full dose of fertilizer and lime on the last plot.
After twelve years, soil scientist Dr. Abigail Maynard, compared the total
yields and soil conditions on each of the five plots. She found that the
plot with just fertilizer and lime had a total yield about equal to the
plot with just leaf compost. However by other measurements, the soil
amended with compost was much better. The compost enriched soil had more
than twice the percentage of organic matter. A high level of organic
matter is one of the best indicators of soil fertility and is the reason
the compost only plot was able to hold 50 percent more water and had 20
percent lower bulk density. Greater water holding capacity means that more
of each rainfall is held in the soil. As a result, plants need less
watering. Lower bulk density makes it easier for roots to penetrate the
soil in order to find the water and nutrients that they need.
The pH of the plots was one of the more surprising results. pH is a
measure of soil acidity. Most vegetables and important soil organisms do
best with a pH between 6.3 and 6.8, near neutral. The soil in the test
plots started with a pH of 5.6, typically acid for Connecticut soils.
After twelve years of lime and fertilizer applications, the pH was 6.5.
After twelve years of just leaf compost, the pH was 6.6; the compost only
soil was less acid than the limed soil.
Neither fertilizer nor leaf compost alone produced maximum yields. These
came from the plots with compost plus the fertilizer and lime. Adding one
third the amendments caused a 17% increase in yields. Adding either two
thirds, or the full amount caused a 25% increase.
Of course, a responsible grower, would test the soil each spring and add
only what was needed, whether it is organic or synthetic. A skilled
organic grower might add some limestone, perhaps ground rocks that contain
phosphorus and potassium, a few wood ashes, and maybe some alfalfa hay or
composted manure for extra nitrogen. This could easily boost the yield
near maximum and still meet organic standards.
Dr. Maynard did find that a few vegetables such as tomatoes, carrots and
chinese cabbage didn't do as well with just compost, and that the effects
of the compost took four or five years to show up, but once they did,
yields increased noticably.
Regardless of the percentage of amendments, however, all the plots with
compost had about the same pH, percentage of organic matter, bulk density
and water-holding capacity.
So, be sure to add organic matter to your soil building program this
spring.
This is Bill Duesing, Living on the Earth
(C) 1995, Bill Duesing, Solar Farm Education, Box 135, Stevenson, CT 06491
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