How much organic matter is enough?
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.
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.
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.
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.