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Friday, January 23, 2004
Gardening: Soil Tests
By Dr. Bob Black
Soil tests serve as a valuable guide for fertilization practices. Such tests generally indicate the amounts of available phosphorus, potassium, calcium and magnesium in a soil. In addition, the pH (acidity or alkalinity) of a soil is also determined.
Trained soil technologists interpret results of soil tests and recommend what fertilization practices are necessary to produce a desired effect.
At times, a soil may be too alkaline, and it is necessary that an acidifying agent be added to the soil. Such acidifying agents are sulfur, iron sulfate, aluminum sulfate and others. In many instances only a slight reduction in reaction is needed. In such cases, the use of an acid peat in the soil and adding acid forming fertilizers usually will do the trick.
Agricultural lime or dolomite is used to increase the pH, or "sweeten" the soil. The only way to know what should be added to a soil to change the pH is to have a soil test conducted. It is a mistake to try to adjust the pH without knowing what is needed and in what quantity.
We often hear of gardeners who add lime to their soil almost every year. In most cases, the lime is not needed and an undesirably high degree of alkalinity is reached. Thinking that they have not added enough lime, they heap more onto the soil making a bad situation even worse. This practice is like adding gasoline to a fire to put it out.
All we are really trying to point out is this--don't guess--soil test. Your guess may be wrong or completely out of proportion as to the amount of material you really need.
Many gardeners are desirous of soil tests when suddenly a plant dies or seems to be on the brink of doing so. After growing well for a period of years a plant will not suddenly wilt and die due to malnutrition. More often the reverse is true, that is, too much fertilizer has been placed about the plant resulting in death of the root system. It is rare, indeed, to find a garden soil so low in nutrients as to cause the death of a plant.
Some things soil tests do not tell you are the presence of insect or disease organisms and lack of water or too much water. In many instances, the sudden death of a plant can be attributed to one of these factors.
Poor drainage, particularly during wet weather, is one of the major causes of plant decline or death. We often forget that oxygen is needed in greater amounts than other elements such as nitrogen, phosphorus, potassium, etc. Wet soils contain little oxygen which is available to the roots. When this condition exists, the plant's root system ceases to perform its vital function of absorbing water and nutrients from the soil. If this water logged condition persists the root system will die, resulting in death of the entire plant.
Another thing not ordinarily revealed is the presence of herbicides or weed killers. The indiscriminate use of these chemicals often results in sudden death of plants.
The presence of nematodes in the soil will not be revealed by ordinary soil testing. These microscopic worm-like animals damage plants by feeding on their root systems. Affected roots appear shriveled, blackened and stubby and there generally will be an absence of white feeder roots.
The next time you have a soil problem you think might be related to lack of fertilizer, examine your growing conditions (water, drainage, insects, disease and nematodes) to see if these are not causing the problem. If none of these problems exists, then have your soil tested.
(January 23, 2004 issue)
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