Category Archives: Soil Testing

May Meeting Summary: Soil and Soil Amendments

Soil Amendments

Get ready to dig in some amendments to your soil!

by Eleanor Baron

Gardeners in attendance at the Capital City Organic Gardeners May 19 meeting enjoyed an overview of soil structure and biology, and received basic recommendations for creating healthier soil through use of soil amendments. Extension Educator Amy Ouellette tailored her presentation to organic soil-building techniques and provided valuable insight into understanding soil test results.

A basic overview of soil demonstrated that soil’s main purposes are to anchor plants, provide a reservoir or 16-18 nutrients and allow for water and air exchange. The ideal soil will be comprised of 25 percent water, 25 percent air space, 2 to 5 percent organic matter and 44 to 48 percent minerals (weathered bedrock). Soils in New Hampshire are very variable in composition because of the random nature of glacial deposits throughout the state.

Soil separates into clay, silt and sand particles. Because sand particles are many, many times larger than clay, the resulting surface area is many times greater. Therefore, sandy soil requires smaller quantities of soil amendments than clay soil to achieve a desired change. For this reason, Amy advised against gardeners limiting their soil testing to use of a pH test from the garden center. Without knowing the exact soil composition, it’s impossible to know the right quantities of amendments to use; a full soil test will recommend specific amounts.

Soil pH, a measure of hydrogen ions, is an important indicator of the soil’s ability to absorb and pass along nutrients to plants. If pH is either too high or too low, nutrients will simply leach away, unused by the plants that need them to thrive. Most of the elements that are needed for plants to grow are chemically most available between a soil pH of 6.25 and 6.75. New Hampshire soils tend to be acidic (low pH), because of granite content, and should be treated with dolomitic limestone (which contains calcium carbonate and magnesium carbonate) or wood ash to raise the pH. To lower the pH (for example, for blueberries), add elemental sulfur.

Soil organic matter is a critical component of overall soil health. Microorganisms, dying plant roots and small animals living in the soil all contribute to nutrient cycling, create air spaces and add materials that hold soil together. Leaving roots when harvesting plants contributes to building organic matter. Adding compost and composted manure is an excellent way to build up soil organic matter. Amy advised caution when using horse manure, due to the high carbon content of the wood chips or straw that are usually in the mixture. The high carbon to nitrogen ratio leaves the nitrogen unavailable to the plants. It’s best to compost horse manure separately and add it when fully composted.

Use composted animal manure carefully to avoid increasing the soil’s phosphorus levels too much. Amy pointed out that many organic gardens have soil with too much phosphorus, because of over-reliance on animal manures. (Compost made from all vegetable matter is not high in phosphorus.)

In summary, organic matter adds nutrients, allows the soil to hold water and contributes to the nutrient cycle. Maintaining good organic matter content in soil helps to prevent erosion, as does practicing gardening methods that leave soil undisturbed.

Although testing soil in the spring (or at any time) is a good idea, Amy advised adding soil amendments to correct pH in the fall When ideal soil pH has been achieved, legume cover crops should be used to help to maintain soil health.

New Hampshire soil is generally not ideal for growing vegetables and requires use of soil amendments to make it ideal. Achieving optimum pH will allow soil to hold and transfer nutrients to plants. Building up the soil’s organic matter will create a living, breathing soil structure with a life-giving nutrient cycle that’s sure to grow beautiful and nutritious garden vegetables.

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