Compost’s Role in Landscapes

            We were
taught in school that productive agricultural soils generally contain from 2 to
5% organic matter; some required more than others depending on what was grown. In
soils with adequate amounts of organic matter, additions were not needed. But
that concept of “one size fits all” when it comes to organic amendments is
being challenged and the challengers are winning.

We have been educated that the ideal soil has 5% organic matter in it. But is that really enough for all plants?

            
We were
told by scientists that landscape plants would not benefit from the addition of
organic matter and that amending the soil at the time of planting was not
necessary. Those of us working in arid and desert soils of the West and
Southwest quietly challenged that generality and continued to add organic
matter like composts to our landscape soils. We could see the benefit.

What is a “healthy soil”?

            Research
during the past 20 years has challenged some of our previous thinking.  In many of our soils, regular additions of
organic matter and its decomposition to humus is a necessary step in maintaining
a vibrant and “healthy” soil and productive plants. Terms like soil health, soil food webs and soil
ecology have become mainstream now.

We have always been told that a healthy and vibrant soil contains lots of worms. But does their presence really indicate that’s all that’s needed?

            Composting
is the process used to convert organic matter into humus and employs many of
the same microorganisms that perform the same function in soil. The concepts
are very similar. The major difference is that composting provides a greater
degree of control over organic matter breakdown than if it were left to an
unregulated soil environment.

            Fungal
and bacterial colonies, earthworms and soil inhabiting insects feed on organic
matter. Microbial slimes and gums are produced when organic matter is converted
to humus. These byproducts of composting and organic matter breakdown help
cement soil particles together.

Most mushrooms are “saprophytic” which means they “feed” off of dead things, not living things. The presence of mushrooms in a soil is one sign that something in the soil is decomposing or breaking down. This decomposition is important for recycling, building organic matter and renewing life in the soil.

            This
altered soil structure is filled with voids that permit the entry, percolation
and exchange of water and gases. Improved soil structure or “tilth” is a major
benefit from the breakdown of organic matter or the addition of compost.

            Byproducts
from the decomposition of organic matter and the feeding by soil organisms
improve the soil further by altering the soil chemistry and providing organic
compounds that stabilize nutrients and assist in chemical reactions necessary
for plant survival.

Some companies capitalize on the idea that compost adds “life” to soils like this ad campaign by Viragrow, Inc.

            When
organic matter declines, humus levels decline as well. “Soil health” declines
and with it landscape plants suffer. The rate of decline depends on many things
including the type of soil, climate, management, nature of the organic matter
and other factors.

Renewing soil organic matter with compost

            If deteriorating
soil health is not caught soon enough, large volumes of quality soil organic amendments,
such as compost, are needed in a process called soil remediation to bring the soil “back from the dead”.

Compost additions to soil improve it by adding aeration, improving water retention while also improving drainage at the same time, and rebuilding life in the soil.

            With
these additions we see the improvement in soil health reflected in our
landscape plants; more vigorous and healthier growth, more tolerance to
environmental extremes, small amounts of fertilizer achieve greater results, less
water is needed and plants experience fewer pest problems.

Managing soil health

            As
managers of plants we must also manage our soils. The percentage of organic
matter in a soil, and thus its humus content, is terribly important. Most soil test
results provide the soil organic matter content.

            But, is
knowing the organic content of our soils enough? To know how much and when to
put it back, it is best to know how fast it disappears. Knowing the rate of
organic matter decomposition is a powerful management and budgeting tool. With
this type of knowledge we understand how often and how much compost to budget
for to maintain soil and plant health.

             Similar to the composting process, the most
powerful external factors controlling the conversion of soil organic matter into
humus and its eventual disappearance are moisture and temperature. If soils are
kept moist, accumulated soil temperature is the driving force in the loss of
soil organic matter. At higher temperatures, soils decompose organic matter faster
than in cooler soils. Hot, moist soils need additions of organic matter more
often than cool, moist soils.

Compost used as a fertilizer

            Organic
matter releases the nutrients it contains for plant uptake only when it
decomposes. By decomposing, plants benefit as well as the macro and microorganisms
that live in the soil. Decomposing organic matter also helps support beneficial
soil bacteria, fungi and earthworm populations. The decomposition process of
organic matter contributes to the breakdown of soil minerals which in turn further
release the native plant nutrients they contain.

There are enough nutrients in compost that it can act like a fertilizer. Composts very in their nutrient content so it is difficult to claim them as a fertilizer. Some contain more nutrients than others. It depends on the components used in making the compost.

            In ways not
well understood yet by scientists, the addition of composts to soils and plants
and their breakdown impact plant health for the better. These impacts can be
direct or indirect. Soils deficient in adequate levels of soil organic matter
contribute to plant health by improving plant vigor, nutrient availability and
uptake. This in turn improves plant tolerance to environmental stresses such as
heat, drought, disease, mechanical damage and insect pests.

Composts role in fighting plant disease

            There
are now dozens of reports on the suppression of some very common diseases in
many horticultural crops through applications of composts and compost teas. Numerous
studies in greenhouses have demonstrated the suppression of common plant
diseases found or transmitted in moist soils such as damping-off, root rots and
vascular wilts. Disease suppression has also been demonstrated on field crops.

Are available for making compost tea. They are very similar to making any kind of tea; the compost is put inside of a “teabag” and this compost is allowed to “steep” in the water. Of course good quality water is used. This compost tea can be sprayed on the leaves of plants where it can act like a fertilizer and suppresses some foliar diseases. This compost tea is perishable so it must be used relatively soon.

            Composts
and products formulated from humus have been used by landscape and turfgrass
professionals for 70 years.  However,
with the advent of inexpensive nutrient rich chemical and synthetic fertilizers
60 years ago the use of organics such as manures and composts as fertilizers has
nearly disappeared. It could be argued that with this disappearance we saw a
general increase in disease incidence and the pronounced use of pesticides to
control these diseases.

Compost used to “topdress” lawns

            More
recent research has demonstrated the potential for using sources of organic
matter such as composts and organic topdressings in the management of high
quality turfgrass.  Several turfgrass
diseases such as Fusarium patch, red thread, damping off, brown patch, dollar
spot and snow mold have been suppressed by topdressings of compost.

           It has
been further demonstrated that these suppressive effects were generally from
the biological activity of the organic amendments, not just because of
improvements in the chemical and physical properties of the soil. The disease
suppressive effects of organic amendments generally increased with increasing rates
of application and were not as pronounced when sterile compost was used.

All
composts are not the same 

Choice of feedstocks and how compost is managed
impacts compost quality. This in turn is reflected in landscape response. Producing
humus from organic matter takes valuable time and resources. Composts must be
monitored and submitted to test for nutrient content and biological activity.
Selecting the appropriate compost is critical in achieving soil improvement and
improving plant health.