Soil and Bed Preparation for Growing Dahlias


Page 3

Determining the Kind and Condition of Our Soil Continued...

Next we want to learn things about our soil that can only be determined by a soil test. There are soil test kits that can be used by almost anyone, but these are not reliable. Having soil tested by a local University Extension Office or soil laboratory is the best course of action. One must specify what information is to be provided when submitting soil for testing. The questions to be answered should include:

What is the pH of our soil?

Acid soil is a result of the decomposition of organic matter. A variety of acids are produced as organic matter decays. Alkali is a property of many of the inorganic elements in the soil. Therefore soil that does not contain organic matter is not acid--its pH is 7 or above.

Acidity and alkalinity are measured on the pH scale. This scale runs from 0 (pure acid) to 14 (pure lye). The neutral point is 7--neither acid or alkaline. The scale is geometric not linear. That is from pH 7 the amount of acidity or alkalinity increases in geometric progression. For example, pH 5 is 10 times more acid than pH 6 and pH 4 is 10 times more acid than pH 5 and so on. Similarly, pH 9 is 10 times more alkaline than pH 8 and pH 10 is 10 times more alkaline than pH 9. Since few plants will survive in soil more acid than pH 4 or more alkaline than pH 8, pH 4 to pH 8 is the range that concerns most of us as gardeners. As mentioned earlier, dahlias prefer a pH around 6.5 as do most plants.

What amounts of plant nutrients are in the soil?

Dahlias, as do other plants, need a number of nutrients for their growth and development --especially nitrogen, phosphorous, and potassium. Additionally, they need iron, calcium, magnesium, and a variety of nutrients referred to as trace elements. The latter are required in such small quantities that it is unlikely that they will depleted from the soil. The soil test should give the amounts of each of the major nutrients available in the soil.

Is the soil contaminated?

Salt contamination is not uncommon. Sodium chloride or salts of calcium, magnesium, or potassium can burn foliage and stunt growth. Tissue at the edge of the leaves turns yellow, then dies. These salts can come from natural soil minerals, polluted water, or over fertilization. Commercial or industrial pollution may occur in some cases.

The non-professional gardener is not likely to need the assistance of a horticulturist to interpret the results of the pH test. However, a horticulturist will be needed to interpret the results of the all the other tests. The person who does the interpretation will need to know that dahlias will be grown in the soil prior to completing the interpretation.

Having decided the type and condition of our soil we are ready to modify it to suit dahlias.


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