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Horticulture Questions and Answers

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What does my soil test mean?

The basic soil test should give you several important pieces of information; a measure of pH (acidity/alkalinity), percentage of organic matter, and levels of available phosphorus and potassium. Looking at the results you'll notice there are no measures for nitrogen levels in your soil. Nitrogen, which promotes foliage or leaf growth, does not remain in the soil long. As a result, most soil tests do not measure nitrogen. Instead nitrogen recommendations are based on expected plant needs. Remember that excess nitrogen may injure plants, hinder fruiting, and retard fruit development. Excess nitrogen leaching from the soil may lead to ground water problems.

Soil pH is a measure of soil acidity or alkalinity and can affect availability of essential soil nutrients to your plants. Your soil is acidic if the pH is lower than 7, a neutral soil has a pH of 7. If your soil has a pH above 7 it is basic or alkaline. Follow the test recommendations concerning adjustment of pH. If your soil is alkaline you can add sulfur to lower the pH. You should add lime if your soil is too acidic. Add lime only as recommended by your soil test results.

Organic matter in the soil changes continually. How much organic matter you have in your soil depends on rainfall, temperature, aeration, and many other factors. It takes a lot of compost, manure, peat moss or mulch to make small changes in percentage of organic matter in the soil. Typically good soil contains 2 to 10% organic matter. If your soil contains less than 2%, add more organic matter in the form of manure, compost, peat moss or chopped leaves and mix with the top 6 to 8 inches of soil.

Phosphorous stimulates primary root growth and flowering in plants. Depending on the plants and vegetables you wish to grow, your soil test should show between 5 to 6 lbs. of phosphorous per 1000 square feet.

Potassium supports many functions in plants including strengthening cell walls and water regulation. If your soil is fertile, it should contain about 10 lbs. of available potassium per 1000 square feet. Your soil test will tell you how much phosphorous and potassium you need to add if any. Urban soils typically contain high levels of phosphorous and potassium. If your soil is high in either nutrient, do not apply complete fertilizers. Complete fertilizers, those containing nitrogen, phosphorous and potassium simply add more phosphorous and potassium. If your soil needs only nitrogen, add only that nutrient.