Edaphiles are as common as dirt. They may be seen scurrying about the garden in early spring, digging up little cupfuls of soil. They bring back the samples to their kitchen labs and mix them with various reagents, anxiously waiting for an array of test tubes to turn pretty colors. What they are testing for are pH and N-P-K values.

Soil is a surprisingly complex biological system made up of inorganic minerals, organic matter, soil organisms, air, and water. The study of all these ingredients in soil and how it relates to plants is the science of edaphology. pH is the single most important factor to test for. It means "potential (not percent!) for hydrogen" and it's units are distributed on a logarithmic scale from 1 to 14, seven being neutral. Free hydrogen ions (H+) measure acidity below 7, so a pH of 6 is ten times more acid than pH 7. A pH of 5 is one hundred times more acid than pH7. Alkalinity (prevalence of basic OH- ions) occurs on the higher side of the scale. The ideal soil pH range for most plants is 5.8 to 6.5. At this level of acidity, all the essential elements will be soluble and available in optimum amounts. Soils which are too alkaline will tie up many of the micro-nutrients such as zinc and manganese, which are needed for good plant growth. If you wonder why you can grow good broccoli but terrible carrots and beets, a high pH may be tying up phosphorous which stimulates root formation. Test the pH - it's cheap and easy.

Nitrogen (N) has a special relation to pH. It must be broken down by soil bacteria (which thrive at 5.8 to 6.6), before being absorbed by plants. Plants can starve even if liberal quantities of fertilizer have been applied if the pH is too high or too low. Nitrogen makes dark green foliage (chlorophyll and photosynthesis) and rapid vegetative growth. Too much can cause soft tissue, prone to disease and infection, and decrease calcium content (watch out for tomato blossom-end rot.) It increases the cation capacity of the root system and aids the uptake of other nutrients. Vegetables that benefit from extra nitrogen are lettuce, asparagus, cabbage, leek, chard and brussels sprouts.

Phosphorous (P) is essential for cell division, respiration and energy storage. It stimulates root formation, speeds up maturity, helps flower and seed development, and increases winter hardiness. It promotes good germination of seeds, healthy seedlings and general plant vigor. At pH 8 and higher, it is unavailable. Use extra phosphorous for plentiful, colorful flowers and for plump seeds and grains. Potassium (K) -(for Kalium) or potash, activates enzymes that control carbohydrate and nitrogen metabolism, is an osmotic regulator, and strengthens stomata. It is essential for fruit development, encourages elongation of roots, hastens maturity and improves shelf life. Apply extra potash for plump potatoes, crispy carrots, beautiful beets, tremendous tulips and dandy dahlias.

So there's the A-B-C's of N-P-K. Soil testing is fun and will reward you with results that will take you to a new level of gardening. Many types of kits are available now at all garden centres.

by A. Martinson