Submitted By: Mary Ensch
Until recently, a soil’s health was measured mainly in terms of its physical and chemical properties: its texture, structure, pH, mineral content, and so on. Today, attention has shifted to include something else as well: the ecology of the soil. Soil is a habitat for countless organisms, from microscopic fungi and bacteria to larger macrofauna, such as earthworms, centipedes, slugs, snails, etc. And we now know that these organisms—and the organic matter that sustains them—are key to the long-term fertility and viability of the soil. A healthy soil is also one that is not eroded, exhausted, or polluted. Poor agricultural and horticultural practices such as over-tillage have led to widespread soil degradation. Luckily, more and more people are embracing sustainability in every facet of their daily lives, from the clothes and household products they buy to the food they eat and the way they grow their gardens.
In the animal kingdom, you are what you eat; in the plant kingdom, however, you are what you absorb. About 90 percent of a plant’s living tissue is water, which is absorbed from the soil by the plant roots. Much of the rest of the plant (living and nonliving matter) consist of carbon, oxygen and hydrogen. The carbon derives from the atmospheric carbon dioxide that diffuses into the plant through leaf pores called stomata; the hydrogen and oxygen come from soil water that’s transported to the leaves and combined with carbon dioxide during photosynthesis to produce sugars.
Carbon, oxygen, and hydrogen are clearly vital to your vegetation; but plants must absorb 13 other elements—the essential nutrients—in order to complete their life cycles. These range from familiar elements such as nitrogen to lesser-known nutrients such as molybdenum. Each performs at least one important, non-substitute task. Nitrogen, for example, forms a major part of all proteins, including enzymes that control virtually every biological process in plants. Molybdenum is a component of just one particular enzyme, nitrate reductase, which enables plants to assimilate nitrogen into their cells.
Plants get their nutrients from the soil. They absorb them at the roots in the form of mineral ions—electrically charged atoms or molecules—dissolved in soil water. These are either cations (positively charged ions) such as ammonium (NH4+), potassium (K+) and calcium (Ca 2+) or anions (negatively charged ions) such as nitrate (NO3 -), phosphate (PO4 3-), and sulfate (SO4 2-). As roots absorb these nutrients, they actively pump out hydrogen ions (H+) and hydroxide ions (OH-) to maintain electrochemical balance in the plant tissue and stimulate further nutrient exchange with the soil.
The most accurate way to determine the nutrient content, pH, cation exchange capacity, and salinity of your soil is to have it analyzed by a soil-testing laboratory. The test results will tell you about the chemical properties of your garden soil, what nutrients (if any) are deficient, and how to address those deficiencies From the test results you can also determine which plants are best suited to the growing conditions in your garden.
A gram of fresh garden topsoil can contain billions of organisms, representing thousands of species. Protecting and nurturing their habitat is an important endeavor in and of itself, but it’s also the key to growing healthy, beautiful plants. Organic matter is the energy source that derives the soil ecosystem, or food web. As soil organisms break down organic matter, they recycle nutrients back into the soil, return carbon dioxide into the atmosphere, and contribute to the formation of humus, which--among its many benefits-- improves the soil’s capacity to retain nutrients and water. Nurture your soil organisms with annual additions of organic matter, such as good-quality compost, and they will, in turn, nurture your plants.
· Provide a well-aerated and evenly moist soil. Poor drainage restricts airflow and makes soil prone to anaerobic conditions, which encourage dentrifying bacteria and thus the loss of valuable nitrogen from the soil to the air. Wet, poorly aerated conditions also promote fungal root diseases.
· Provide a regular supplement of organic matter, such as compost or mulch. This provides food for your soil creatures but also enhances the environment in which they live. Moreover, good-quality, mature compost acts as an inoculant, adding diverse populations of beneficial organisms to your soil.
· Balance the carbon and nitrogen ratio of your compost ingredients. This will encourage a healthy mix of fungi and bacteria and associated feeders.
· Keep the soil pH close to neutral. Strongly acidic or alkaline conditions reduce microbial activity (and thus nitrogen cycling) in the soil. The most diverse populations of fungi and bacteria are present in soils with pH close to neutral.
· Avoid activities that cause compaction. For starters, don’t work the soil when it’s wet. Compaction collapses the pore spaces in which soil organisms live and breathe and prevents the healthy growth of plant roots.
· Avoid or minimize tillage of soil. It disrupts the food web and destroys soil structure. Fungal hyphae, in particular, are easily broken up by the soil disturbances.
· Respond to pest or disease outbreaks. Use the most effective and organic method possible so as to protect the beneficial organisms in your soil. Purchase beneficial insects and nematodes to control above-ground or below-ground pests, rotate vegetable crops and annuals ,and promptly remove diseased plant material from the garden.
- To learn more about healthy soil please read Healthy Soils for Sustainable Gardens, Niall Dunne, Copyright 2009 by Brooklyn Botanic Garden, Inc.