Marin Master Gardeners
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Garden soil needs a healthy balance

December 5, 2009
Martha Proctor

Last week we looked at two important properties of soil: texture and structure. This week, we learn the basics on the essential nutrients plants depend upon to thrive and the importance of soil pH. Soil is the source of a plant's nutrient needs. Any imbalance in the soil affects the health, growth and development of plants living there.

Essential nutrients

An ideal soil contains sufficient quantities of approximately 20 essential plant nutrients in a chemical form that roots can absorb. Each of these nutrients is deemed essential because it performs at least one important, non-substitutable task. Carbon, hydrogen and oxygen are drawn from air and soil water; the other nutrients are dissolved in the soil water and absorbed as ions by plant roots. Nitrogen (N), phosphorus (P) and potassium (K) are the elements plants need in the largest quantities. Elements needed in lesser amounts include calcium, magnesium and sulfur. Boron, chloride, copper, iron, manganese, molybdenum and zinc are deemed micronutrients as, although essential, they are needed in minute amounts. Research suggests sodium, silicon, cobalt, vanadium, selenium, aluminum, iodine and nickel are also critical to plant nutrition. Nutrient elements differ in the form they are absorbed by the plant, by their functions and mobility in the plant, and by the plant's characteristic deficiency or toxicity symptoms.

Soil water is the solvent in which plant mineral nutrients are dissolved.

Plant roots draw in the nutrients they need from the soil water.

Sandy soils have good aeration and drainage, but limited capacity to bind nutrients or water. Clay soils are fertile because their smaller particle size enables them to bind water and nutrients, but reduces drainage and aeration capabilities. If nearby, ions of nutrients can diffuse toward roots or be swept along with the water being taken up. When a plant takes up nutrient ions from the water surrounding its roots, that solution becomes deficient in those nutrients unless more water moves from other portions of soil toward the plant root. Active plant roots grow into new soil to help keep plants supplied with nutrients. A nutrient deficiency occurs when a nutrient is not in sufficient quantity to meet the needs of the plant. Nutrient toxicity occurs when a plant nutrient is in excess and decreases plant growth and quality.

An excess or deficiency of any nutrient in soil leads to recognizable symptoms in affected plants. The first signs of an inadequate source of N, P, K, magnesium or molybdenum in the soil are seen in older leaves because these elements can be remobilized within the plant and sent from older leaves into younger ones. Early deficiencies of calcium, sulfur, iron, copper, zinc or manganese show up in young leaves first because these elements are immobile. The most common nutritional problems in California are related to deficiencies of nitrogen, phosphorus, potassium, zinc and iron and toxicity symptoms caused by excesses in boron, chlorine, and sodium.

Soil pH

Soil pH is a measure of the soil's acidity or alkalinity. Soil pH affects the solubility of soil minerals, the availability of nutrients to plants, and the activity of microorganisms. The midpoint of the (1 to 14 point) logarithmic pH scale (7.0) is neutral. Numbers below 7 are increasingly acidic as the number decreases, numbers above 7, increasingly alkaline. A pH of 5.0 is 10 times more acidic that a pH of 6.0 and 100 times more acidic than a pH of 7.0. Most plants prefer a pH ranging from 5.5 to 7.5 or slightly acidic to neutral, the range in which all plant nutrients are most readily available. Soil microbes are most active in this range also.

Soils in California typically range from pH 5.0 to 8.3. Excessive acidity or alkalinity restricts certain nutrients from entering the soil solution and can release other nutrients and minerals in toxic amounts. In strongly acidic soils (pH 4 to 5.5), phosphorus, potassium, calcium and magnesium may be present but not in a form available to plants. In more alkaline soils (pH 8 to 10), the availability of phosphorus, iron, copper and zinc is significantly reduced. Rhododendrons, blueberries and azaleas prefer acidic soil and will show evidence of an iron deficiency, even if iron is present in the soil, if the pH is above 7. To ensure a balanced nutrient supply, it is best if soil is pH neutral (6.5).

The first step to improving your soil is to have your soil tested for pH and nutrient levels. The most accurate way to determine the nutrient content, pH, structure and texture of soil is to send a sample of your soil for analysis to a soil-testing laboratory. The soil test will tell you whether your soil needs lime or nutrients or both. Follow the lab's recommendations to avoid or correct nutrient imbalances in your soil. Do-it-yourself soil testing kits are also available at garden centers. Contact the Marin Master Gardener office to learn more.

Next week, learn how to improve your soil to have a healthier, more productive garden.

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