Marin Master Gardeners
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How to determine what makes your soil healthy

July 30, 2016
Martha Proctor

Healthy soil is the basis for keeping healthy plants as it is the source of the necessary nutrients plants need to grow.

To assess the health of your soil and to promote optimal root growth, it’s advisable to send a soil sample to a professional lab to be tested for the presence of nutrients and pH (the measure of the acidity or basicity of the soil in solution). The lab will provide an analysis of which nutrients are in adequate supply and which would benefit from being supplemented or modified. Soil pH greatly effects the availability of some plant nutrients to your plants. Less-than-optimal soil can be improved once you are armed with information on your soil’s pH.

A fertile soil contains the 14 essential nutrients critical for plant maturation and development in amounts favorable for optimal growth. Most nutrients including nitrogen (N), phosphorus (P) and potassium (K), considered to be the primary essential plant nutrients, are passively absorbed by the plant’s roots. N is important for green leafy growth and photosynthesis, P is vital for strong growth, root development and seed production and K promotes overall plant health and water absorption/retention. Three other essential elements (carbon, hydrogen, oxygen) are supplied by water and air.

The most common problems in California soils are related to the deficiency of N, P, K, zinc and iron as well as toxicity symptoms caused by excesses in boron, chloride and sodium. As California soils contain most of the elements known to be essential to plants, it is necessary to add only the ones that are deficient in a particular soil. An excess, deficiency or imbalance of an essential element leads to symptoms in affected plants.

Soils can be amended or fertilizer added to correct deficiencies in the soil. Fertilizers (packaged commercially available inorganic mixtures which contain N-P-K or natural organic materials) affect plant growth directly by improving the supply of available nutrients in the soil. An all-purpose fertilizer is labeled 10-10-10 and reflects 10 percent N, 10 percent P and 10 percent K. Thus the percentage by weight of a 50-lb bag is 5 pounds (or 10 percent) N, 5 pounds P and five 5 pounds K. The remaining 70 percent is composed of inert ingredients. Amendments (compost, mulch, straw, peat moss) influence plant growth indirectly by improving the soil’s physical properties (texture, structure, density) or chemical properties (pH, salinity).

Although present in the soil in favorable amounts, some essential nutrients may not be available to the plant roots if the soil pH is not in a suitable range. The “ideal” soil pH falls within a range from a slightly acidic pH of 6.5 to a slightly alkaline pH of 7.5. Soils are considered acidic below a pH of 5 and alkaline above a pH of 7.5. N, K, and sulfur (S) are less affected directly by soil pH but phosphorus forms less soluble compounds when the soil pH is not between 6.5 to 7.5. Most of the other nutrients are less available when soil pH is above 7.5, and are optimally available at a slightly acidic pH, e.g. 6.5 to 6.8. Lime can be added to raise soil pH; elemental sulfur can be used modify alkaline soils.

Test your soil — the necessary nutrients may be present but not available because of less-than-ideal pH. Outward signs of nitrogen deficiency include leaves turning light green or yellow. Similarly, purple foliage (on an otherwise green plant) is a telltale sign of phosphorus deficiency.

If your soil is healthy earthworms, beneficial microbes and other critters have better access to the oxygen they need to thrive. As they live and digest organic matter, they help create soil that is healthy and fertile. Amend your soil with organic matter like compost to provide nitrogen, phosphorus, potassium, and an abundance of other trace elements; add mulch to protect the soil.

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