(number 8 on the reading list). This cooler is a backyard version of the commercial pad-and-fan system.

CARBON DIOXIDE: Normally, fresh air, exhaled breath, and a biologically active soil should provide enough carbon dioxide in an attached greenhouse. Keep in mind, though, that the hours of daylight, light intensity, temperature, and amount of carbon dioxide available affect the rate of photosynthesis and are interrelated. Within limits, higher levels of one environmental factor can compensate for a deficiency in another. For example, high concentrations of carbon dioxide (1,000 ppm) can increase lettuce yields when temperatures are lower than the optimum. Depending on the stage of plant development and all of the environmental factors mentioned, increased levels of carbon dioxide can improve yields and hasten plant maturity. Higher levels of carbon dioxide are particularly beneficial to many fruiting vegetables and to flowers.

Natural Soil Amendments

Lime:Most of our eastern soils are naturally acidic (and are becoming even more so because of acid rain). Applying pulverized lime will raise the pH into the range where nutrients are available to most plants. There are two kinds of lime: calcitic and dolomitic. The latter contains both calcium and magnesium, both of which are essential for plant growth. If the soil's pH is properly adjusted and of these nutrients is deficient, gypsum (CaSO 4 ) will correct the calcium problem, and epsom salts (MgSO 4 ) will add magnesium without altering the pH.

Blood meal:Dried blood is a natural soil amendment composed of 6% to 12% nitrogen. It may contain small amounts of phosphorus. Blood meal can be applied directly to soils or be used in composting to boost the nitrogen content. Dry fish meal is another alternative and contains about 10% nitrogen.

Bonemeal:Steamed, ground bone contains phosphorus in proportions varying from 15% to 30%. It can be worked into the greenhouse soil or sprinkled on compost.

Rock phosphate:Rock phosphate contains calcium phosphate as its major ingredient, with levels varying from 28% to 32%. This material, which is ground into a powder, is insoluble and is released slowly . . . even more slowly than bonemeal.

Wood ash:Burnt wood ash is an inexpensive source of potassium (3% to 7%). Mixed into the soil, it will raise the pH and provide small amounts of calcium, phosphorus, magnesium, and sulfur. I prefer to add wood ash to compost piles when the greenhouse soil's pH is ideal.

Greensand:This nutrient supplement is dredged from the ocean bottom and is rich in minerals (7% potash). Potassium in the soil will regulate the plant's water absorption and the movement of the liquid through the vascular system. Granite dust and kelp are other sources of potassium.