The Greenhouse as an Ecosystem
(Page 5 of 9)
November/December 1984
By Colleen Armstrong
2] USE THE BEST SOIL
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Never underestimate the importance of fertile soil in the greenhouse. We build our soils to provide high fertility, adequate organic matter, multiple nutrient-exchange pathways, and nutrient storage capacities. Diverse populations of bacteria, fungi, and actinomycetes digest organic material; their activity increases both soil fertility and carbon dioxide levels. Earthworms, springtails, ground beetles, and numerous soil animals excavate and aerate pockets and waterways.
More than a quarter of my indoor gardening time is spent on soil preparation or cultivation. That's understandable when you consider that greenhouse soil nutrients are tapped 12 months of the year. On the other hand, since these indoor soils are always warmer, organic matter decomposes at a faster rate.
Constructing a greenhouse garden bed requires three basic ingredients: topsoil, compost, and-depending on your region's soil typesand or clay. Clean topsoil should be the largest portion of your mixture. It's the basis for texture and is the best mineral source for your plants. It should contain 2% to 5% organic matter and have a slightly acid pH factor (between 6.5 and 6.8).
Compost provides many essential plant nutrients (nitrogen, phosphorus, and potassium), and it nourishes the soil's microbial population. Excess compost, however, will cause an imbalance and produce poor soil texture.
I suggest you build a compost pile while your greenhouse is under construction. When the structure is complete, the compost will be ready to work into the beds or containers. Whatever method you prefer for making the pile, let the material cook! In order to kill most pathogens and weed seeds, compost should bake at 180°F for two or three weeks. Before you mix compost into the soil, it should be screened through a 3/8"-mesh handmade cloth to eliminate any bulky material. If your topsoil has a clay base, sand will augment drainage and aeration around the plants' root zones. Use coarse, clean sand, or—if that isn't available—substitute perlite. If your topsoil is sandy or sandy loam, then work clay into the soil mixture.
Use the sources closest to you. Well-decomposed leaf mold, for example, can be substituted for compost or worked into the soil along with compost. The material should be black and crumbly and free from bulky debris. Be aware that undecomposed leaf litter in the soil is an invitation to scavenger pests such as thrips, leafhoppers, and pill bugs. While leaf mold is a well-balanced soil builder, it's often acidic and may need lime to sweeten it. If you are without compost or leaf mold, dehydrated cow or sheep manure is a risky third choice, but never use it while it's fresh.
Before adding any amendments to your batch, though, send a composite soil sample to your agricultural cooperative extension service. Every state has an office to serve you, and the price for soil analysis is a modest $2.00 to $5.00. Most of the reports are easy to interpret, but the agency's recommendations for fertilization may be couched in chemical terms and may be heavy on the dosage. A private testing facility specializing in organic agriculture, Woods End Laboratory (Orchard Hill Rd., RFD 128, Temple, ME 04984), will test your soil for nutrient levels and measure the percentage of organic matter. They'll send a humus chromatogram, which is a visual display of your soil's health.
If the soil is deficient in one or more nutrients, organic measures can remedy the situation. Nitrogen, phosphorus, potassium, calcium, and magnesium are taken up by the plant in larger quantities than are other elements and may be the first to become deficient. Keep in mind, however, that good soil management pays attention to physical as well as nutrient qualities in the soil, and that amendments will not improve soil texture or structure.
You should also practice crop rotation and keep rotation records. Heavy feeders—tomatoes, European cucumbers, peppers, and the like—should be replaced with legumes or root crops. Members of the cabbage family (brassicas), such as mustard, kale, broccoli, cauliflower, cabbage, and collards, should not occupy the same area more than once a year, because recropping with a brassica promotes clubroot, a disease that retards vigorous plant growth. A comprehensive textbook that can aid in understanding plant diseases is George Agrios's Plant Pathology (Academic Press, 1978, $23.25).
Compacted soil can be improved with the introduction of earthworms, peat moss, and leaf mold. Perlite, a volcanic material, can lighten up a nutritionally well-balanced soil.
Excess organic matter, which leads to poor soil structure, results from too much compost in the soil. In your soil-conditioning program, alternate leaf mold with compost . . . and occasionally revitalize the topsoil. Other natural fertilizers, such as seaweed extract, fish emulsion, and manure tea, should also be used.
SPECIAL SOIL MIXES: Our sidebar on special soil mixes is a guide for producing basic potting soil. Tiny, germinating seeds need a premium mix with light texture, good water retention, and adequate drainage.
If your greenhouse has poor air circulation, consider making of buying a sterile seedling mix. Damping-off (also known as rhizoctonia, pythium seed rot, or stem rot), a devastating disease, can wipe out your best batch of seedlings. It's caused by a soilborne pathogen that attacks sprouted seeds, roots, and succulent stems. If you've ever noticed a healthy seedling suddenly flop over with a brown, withered stem at soil level, you've seen damping-off.
If your greenhouse is properly ventilated, then you could try a seedling mixture of one part compost, one part peat, one part perlite, and one part vermiculite or clean sand. Both the compost and peat moss should be screened through a 3/8" wire mesh cloth, and the soil should be thoroughly turned.
Number 9 on the reading list is a fine, though dated, textbook on special soil mixes. In it you can find soil recipes for acid-loving plants, cacti and succulents, orchids, and so on.
WATER: When and how much to water depends on the time of the year, the weather conditions, the type of soil, and the depth of the container or bed. For large beds or boxes, poke your finger into the soil to see if the top inch is dry; if it is, add water. Small pots and shallow beds require frequent watering . . . usually every day.
Most beginners tend to overwater rather than underwater, though. You should, for example, avoid watering on cloudy, rainy, or snowy days. At these times, most solar greenhouses are cool and damp, and the air and soil may be completely saturated.
Watering in the morning will allow the leaves to dry out during the day, reducing the chances that harmful fungi will grow on wet leaves. Botrytis, stem rot, and downy mildew thrive when water coats the leaf surface, and water spots are unattractive on ornamentals—so restrict the water to the soil and keep it off the leaves.
The water should be warm (ideally, 55°F) and dechlorinated. You can preheat the following day's water in your greenhouse in garbage pails or barrels. If you own a large greenhouse, you might consider a subterranean irrigation system. It saves time and puts the water where roots can find it.
Soak the soil thoroughly once—rather than administering many light waterings—and use a nozzle, breaker, or rose to break up the gush of water into tiny droplets. (See "Greenhouse Tools and Supplies," or try your local garden center.) Seed flats, in particular, need a gentle rain of clean, warm water.
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