By Dr. Anne Sawyer, Extension Education and 2017 LAAS Alumna
Soil is so much more than "dirt" – soil is alive! Every teaspoon of soil is home to billions of microorganisms – bacteria, fungi, nematodes, and more – as well as larger organisms like insects and earthworms. All of these organisms have a role to play in the soil ecosystem. Bacteria and fungi, for example, break down dead plant and animal tissue, improving soil quality and providing essential nutrients for plants. Larger organisms, such as worms and insects, shred and chew organic material into smaller bits that bacteria and fungi can easily access, and their burrows create pathways for air and water to reach plant roots. Some organisms, such as mycorrhizal fungi, form special partnerships with plants and bring hard-to-reach nutrients directly to plant roots.
A healthy soil ecosystem provides plants with easy access to air, water, and nutrients. Here are six tips for achieving optimum soil health in your garden.
1. Test your soil!
Knowing the current state of your soil is the first step to creating an optimum soil ecosystem. Submit a sample of your garden soil to the University of Minnesota Soil Testing Lab, located on the St. Paul campus. Your soil test results will include information about soil texture, pH, nutrients and organic matter, and provide fertilizer recommendations for the plants you plan to grow. For more information, visit the Soil Testing Lab online or contact your county Extension office.
2. Add organic matter!
Organic matter is composed of living things (plant roots and organisms), dead things (recent plant and animal residue), and very dead things (highly decomposed material, or humus). As organisms decompose organic matter, nutrients become available to plants. Plants need a wide variety of nutrients for optimum health, just like humans. In fact, there are 17 essential nutrients that all plants need to grow. Organic matter such as compost can be a source of macronutrients like nitrogen as well as micronutrients such as manganese and zinc. Organic matter also improves soil physical properties such as aeration and water retention, allowing for healthy root growth.
3. Provide air and water!
Plant roots and microbes need access to varying amounts of air and water for optimum growth. Fortunately, soil is full of microenvironments – tiny habitats that differ in the amount of available air, water, and nutrients. Soil compaction and disturbance such as tillage can eliminate these important microenvironments, making it difficult for plant roots to penetrate the soil, absorb water and nutrients, and interact with beneficial microbes. To minimize compaction and provide an optimal growing environment, design clearly defined areas for planting and walking such as building raised beds separated by paths for access. Also consider using hand tools to minimize tillage whenever possible. Tillage can break apart soil aggregates, destroying microenvironments. Tillage also exposes weed seeds to light, making them germinate, and can leave soil vulnerable to erosion.
4. Keep it covered!
The top few inches of soil contain an abundance of microorganisms, organic matter, and soil nutrients. Mulch or cover crops can be used to protect this valuable topsoil from erosion and add rich organic matter as they decompose. Research has shown that raindrops landing on bare soil can splash soil particles as high as three feet and as wide as five feet. Mulching bare soil around plants prevents the splashing of soil particles and soil-borne pathogens onto leaves and stems, reducing the occurrence of disease. In addition, mulch and cover crops conserve soil moisture, minimize weeds, and reduce plant stress by moderating soil temperatures.
5. Rotate crops!
While most microbes are beneficial to plants, disease-causing microbes may overwinter in soil and plant litter. These pathogens prefer to infest and feed on certain plants. Planting the same crop in the same soil year after year can increase these pathogen populations. Rotating your crops reduces pathogens by eliminating their food source. Crop rotation also prevents nutrient depletion. Plants with long roots, like carrots, absorb nutrients deep in the soil; shallow rooted onions absorb nutrients in the top few inches of soil. Some plants add nutrients back into the soil. Legumes such as beans and peas add important nitrogen to the soil by forming a mutual relationship with rhizobia, root-inhabiting bacteria that take nitrogen from the air and convert it into plant-available form. When the legumes die, the nitrogen then becomes available to other plants in the rotation.
6. Cautiously consider chemicals!
Pesticides kill pests, but they also can kill beneficial soil microbes and insects. Instead of reaching for a chemical, always consider alternatives to pesticides first. Choose disease-resistant plant varieties and plants that will grow well in your site. Hand-pick larger bugs, such as Japanese beetles, and drop them in soapy water or knock them from plants with a blast from your garden hose. Use physical barriers like row covers, bags on apples, or fences to keep out larger critters. Plant damage from insects and diseases is often cosmetic and not necessarily life-threatening, and plants will over recover.