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CREATING AND MAINTAINING BIOLOGICALLY HEALTHY SOILS
By Tim Topham
A primary objective for successful organic gardening is to create soil conditions in which soil microorganisms will release the balance of nutrients that each plant requires. Plants do not get their food from the soil. Plants make their own food (protein and carbohydrate) using air, water, sunlight and ingredients from the soil, such as nitrogen, phosphorus, potassium and many other compounds and elements called plant nutrients.
You don’t need to be a soil scientist to have healthy, well balanced garden soil. A basic understanding of a healthy soil and how plants interact with a living soil is necessary.
Soil that has been maintained with chemicals (synthetic fertilizers, herbicides and pesticides) may have a very reduced population of soil organisims. The organic matter content may be very low or non-existant. Chemicals supply major nutrients --period-- in quick-release forms. Plants obtain fast growth, but long-term benefits are few. And living soil and living plants need far more than a few isolated chemical elements. Plants take then nourishment through infinitely complex biological processes that arc still not fully understood. To use chemical fertilizers to the neglect of compost is to disregard the soils need for life.
Organic Matter
Feed the soil not the plant. The most important goal of any improvement program is the promotion of microbial activity, it is the activity of microorganisms in the soil that makes nutrients available to plants. Adding organic matter to the soil in the form of compost or green manure is important to fertility.
Organic material includes all those things that, when left to nature, will decompose and become humus. Virtually all soil structure and nutrition problems can be solved with organic matter. The question is, how much should you add, and how much do you need?
Humus
Humus, the relatively stable end product of composting, is rich in nutrients and organic matter and highly beneficial to both the soil and crops grown in the soil.
The accepted standard for a healthy soil for food gardens is at least 5 percent humus in the first foot of soil, but a higher humus content will produce healthier plants. For comparison, consider that roses do best in a soil with 20 to 25 percent humus, and lawns are easiest to care for if their soil has a 25 percent humus content. Therefore, the 5 percent humus recommended for food gardens is actually a minimum standard. One inch of compost or fine organic material spread over the surface of the garden equals about 5 percent of the volume in the first foot of soil.
Unfortunately, there is a catch here. Soil organisms consume humus at varying rates, depending on the environmental conditions in and around the soil. There is a limit to how much humus can be efficiently processed by the microbiotic population in your garden soil, and that limit depends a lot on your gardening practices. Microoganisims will consume humus at a higher rate when soil is tilled excessively (several times a year in seed bed preparation and for weed control) or when it is left exposed to the elements A soil that is lightly cultivated, mulched during the growing season, and covered with a mulch or a cover crop during the winter will gradually increase its humus content.
Amounts to Use
A wheel barrow or garden cart dimensions can be measured to calculate how many cubic feet it will carry, or you can use a 30-gallon garbage can to measure out your organic amendments. Each can holds about 4 cubic feet of material.
Organic Material Needed to Cover 100 Square Feet
Depth% in firstAmount of Organic Matter
(in.)Foot of Soil(cu.yd.: cu. ft.)
650 2;54
4331.3;35
3251;27
2160.66; 18
180.33~ 9
The addition of compost to the soil improves overall fertility and cures some soil mineral deficiencies. Compost itself is not a direct fertilizer for plants. It is food for the billions of microoganisms in the soil that transform the compost into the soluble compounds that can be absorbed by the roots of plants. Compost, with its many oxygen-containing compounds, brings oxygen to these microorganisms, which is essential to their reproduction. As compost is created (during its decomposition process), certain acids are formed, which help to break down some of the rock particles in the soil, releasing nutrients to the plants that would otherwise not be available. When compost is freshly applied to the garden, it provides nitrogen, some potassium, and many of the trace minerals needed by the garden during the entire growing season. Finished compost is generally low in phorous. so add rock phosphate to the compost or directly to the soil.
Stay away from super phosphate fertilizers. They do make phosphate available immediately in water-soluable form. But the phosphate becomes bound up with iron or aluminum in acidic soils, or with calcium in alkaline soil, and thus is unavailable for plant uptake. Rock powder forms of phosphate beôome mineralized slowly and are available to plants over a longer time period. Add about 3 to 5 pounds of rock phosphate or some othei phosphorus source to a pile 4 by 4 by 4 feet in size.
About half of the nutrients that are present in compost are released for plant use during the first year. Half the remaining nutrients are released the second year, and so on after that. Most vegetable crops require about 1/2 pound of nitrogen for every 100 square feet. Since many gardeners have two and sometimes three successions of crops in their garden, at least 1 pound of nitrogen must be available for every 100 square feet, or 2 pounds for an intensively planted garden with two or three successions.
The amount of nitrogen in finished compost depends on the raw materials used to make the compost. Compost made from hay is higher in nitrogen than compost made from straw. Leaves have about half the nitrogen that manure has. Varieties of leaves differ, but studies have shown that about 75 pounds of leaves produce compost with as much nitrogen as a 5-pound bag of blood meal. For comparison, aged cow manure is about 1.4 percent nitrogen, and aged chicken manure is about 2.8 percent nitrogen. If your compost is made of leaves and grass it will probably have about 0.8 to 1 percent nitrogen. That percentage is sufficient to give you the amount of nitrogen most average gardens will need about 0.6 pound for 100 square feet. This means you can supply your garden’s nitrogen demands by adding a 1-inch layer of compost each spring.
Solving Problems with Compost
litre’swhat compost can do for the soil. builds soil structure
change pH improve drainage
improve fertility
neutralizes toxins reduce watering needs reduce insect problems reduce disease problems
reduce weed problems as a mulch and killer of weed seeds
stops erosion
welcomes worms
Mulching
The advantages of using organic mulches to blanket soil around trees, shrubs, perennial plants, and vegetables are many:
mulches moderate soil temperature
mulches make a more favorable environment for roots they reduce evaporation of moisture
they reduce splashing from rain or irrigation water reduce the spread of disease
block the germination of many weed seeds
and along with all the above mulches can give your garden a more finished look.
Whatever the type of plant, its rarely advised to mulch more deeply than 6 inches. Mulch layers thicker than that reduce air circulation in the soil, which will retard plant growth and slow down microbial activitiy in the soil. Also don’t pile mulch around the main stem of a plant where it enters the soil. The added moisture and insect haven that results could damage the plant you’re trying to help. Spread mulches so they cover the “dripline” of your plant.
Annuals and perennial. To avoid encouraging stem rot diseases, use a 2- to 3-inch layer of mulch around soft-stemmed animals and perennials.
Trees, shrubs, and vines. Even around older plants with thick bark, don’t crowd mulch around their bases.
Vegetables. Use mulches that decompose rapidly and add nutrients to the soil such as a thin layer of grass clippings or shredded leaves.
Pathways and slopes. Larger type mulches, such as shredded tree branches do the best job and will help control weeds and soil eroision while still allowing air to reach the soil.
Worms
Worms help make the soil perfect for plants. In passing soil and organic matter through their bodies they gradually make acid soil less acid and alkaline soil less alkaline slowly drawing any out-of-balance soil into the neutral range. Compost feeds earthworms as they eat their way through the soil in search of organic matter, they concentrate plant nutrients in their castings and build a soil structure that is perfect for roots. Worms are continually turning subsoil into topsoil. Reduce soil tillage of your soils. Soils that remain untilled have three to four times as many worms per square yard as soils that are tilled in spring and fall. The reason is not so much that tillage actually destroys worms as that it greatly accelerates the breakdown of the organic matter that worms need.
Applying mulches produces larger worm populations then bare soil even with compost worked into the soil. Soils that contain large numbers of carthwonns can absorb two inches of water (a very heavy downpour) in as little as 12 minutes. Soils without worms can require as long as 12 hours to absorb that much water.
Conducting an earthworm survey is a quick way to estimate soil fertility. If soil moisture content at the surface is 20 percent or more, try counting the earthworms in your garden soil. Using a shovel or spade, dig a cube of soil measuring roughly 1 foot on each side, and lay it next to the hole on the grounds surface. Break it apart and look for earthworms. You’ll find at least ten earthworms per cubic foot in a healthy soil. Repeat the procedure at several locations in your garden. If your soil’s average is fewer than ten worms per cubic foot, its time to add more organic matter to your soil.
Soil Testing
If you’re starting a new garden or if your plants in general are not doing well, havin{ your soil tested makes good sense. Test your soil before adding fertilizers or mineral amendments. Then use the results to help you decide which, if any amendments you need to add to your garden soil.
Testing Options
Basic do-it-yourself kits are available from garden centers or catalogs. Also soil testing laboratorys can be found in the telephone yellow pages. Choose a test appropriate to your needs and budget. If you are checking only for pH and major nutrients, a simple test may suffice. If you suspect your soil has a serious imbalance, a more expensive test may be justified.
Testing pH with Litmus Paper
Make a soil slurry. Mix a thick, muddy slurry of soil and distilled water in a clean container. Let the mixture stand for one hour. Place a litmus paper strip in the slurry.
Reading the results. Remove the strip from the slurry after one minute. Rinse it with distilled water and match it with the color chart. Each color represents a different pH value.
Many gardening books recommend that you have two different soil management strategies - one for your acid-loving plants like azaleas and gardenias and one for the majority of food plants which prefer a more neutral pH. There is increasing evidence that
von don’t need to worry nhont sneb thin~s once vcmr niants are estahlished assuming you add at least 1 inch of compost to your entire garden every year.
A study was done by the Department of Agriculture at the University of Connecticut, on a property that had soil with a nearly neutral pH. The gardener had planted some rhododendrons and azaleas many years previously without taking any special steps to prepare for these acid—loving plants. For years, this gardener had put 1 inch of compost around each plant annually (compost is almost always nearly neutral in pH). When a researcher went around the property and took soil samples, he found that the soil around the rhodendrons and azaleas was acidic, whereas soil on the rest of the property was still neutral. Scientists haven’t figured out how, but the plants seem to have created their own acidic environment.
It is still a good idea to add lots of peat moss or other acidic material to a neutral soil when planting an acid-loving plant such as azaleas. This helps the plant get off to a good start. But after that you needn’t worry about the the pH of the soil as long as you add compost every year.
Organic Fertilizers
All fertilizers are required by law to have the NIPK numbers on the label. These numbers are the fertilizers NPK ratio, and tell you percentages of nitrogen, phosphorus (in the form of phosphate) and potassium (as potash) contained in that fertilizer.
Labeling laws require that nitrogen numbers refer to the percentage of total nitroget in a product, while the numbers for phosphate and potash only tell you the amount of thosc nutrients that will be available the first year-- not the total amount available in the bag. And that’s an important distinction, because organic fertilizers generally are slow-release, so the P and K numbers on the organic fertilizer bags are lower than the ones on synthetic-fertilizers. But in reality, you get even more in the long run when you chose organic.
Types of Organic Fertilizers
Nitrogen Fertilizers:
Blood meal(12-2-0)
Fish meal(9-7-0)
Cottonseed meal(6-2-2)
Phosphorus Fertilizers:
Bone meal(1-1 1-0)
Rock phosphate(0-3-0)
Colloidal rock phosphate(0-2-0~)
Potassium Fertilizers:
Greensand(0-0-1)
Sol-po-mag(0-0-22
Calcium Fertilizers:
Limestone
Gypsum
Sulfur Fertilizers:
Elemental sulfur
Trace Minerals
Seaweed meal
Seaweed extract
For more information about soils and organics you can find numerous articles in most of Rodale Publishing Books and in their “Organic Gardening” magazine. Also “National Gardening” magazine, and “Sunset “ magazine have published articles about different aspects of soil management.
The cultivation of plants allowed our ancestors to settle and populations to grow. Human populations come and go, entire civilizations have risen and fallen with the health and depletion of their topsoils. I wonder what our decendents will say about our civilization 1000 years from flow or if there will be any humans left?
11/20/96
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