By Jeff Rugg- Q: I was reading some notes in a garden catalog that said I should be testing my garden soil. I have had a vegetable garden for over 10 years, and I plant annuals around some of my shrubs and perennials. I have never tested the soil before, and I usually have a good crop. Would the soil tester do me any good?
A: A soil test could do you some good because it could tell you what the soil pH is. It might tell you that there is already so much phosphorus in your soil that adding more when you fertilize would be a waste of money. It might also tell you that your soil is deficient in one of the micronutrients.
There are about 20 chemicals that are necessary for most plants to grow, and a few more chemicals are required by some specialized plants. Of the 20, carbon, oxygen and hydrogen are supplied by air and water. Everything else is pulled into the plant from the soil.
There are six soil chemicals that are required in large amounts, and they are called macronutrients. They are nitrogen, phosphorus, potassium, calcium, magnesium and sulfur. The remaining 11 chemicals are called micronutrients, and even though they are only necessary in very small amounts, a soil that is deficient in any one of them can lead to poor plant growth or lack of fruit.
There are home test kits and soil-testing laboratories. Both will have instructions to follow on how to do the soil test properly. Some labs will only test for pH and macronutrients, which are usually not in short supply in soils that have been producing well in the past. The pH test is important because it measures the acidity of the soil, and the acidity greatly affects the availability of both macro- and micronutrients to the plant roots.
If you send the soil sample to a lab, be sure to say that it is for a vegetable garden or lawn or flowerbed. Labs are used to testing farm soil samples, and without knowing the sample came from a small plot of land, they would send the results back to you in the form of tons per acre instead of pounds per thousand square feet. The results would still be valid, but you would have to convert them from tons to pounds and acres to square feet.
After you get the results from the soil test, look at the pH result. If it is around 6.5 to 7.5 and everything has been growing fine, then let it go. If it is much higher or lower and you want to bring it close to a neutral 7.0, remember that this will be a continuing battle, and that the soil will return to its present pH in just a few years after you stop trying to change it.
It can take lots of sulfur to lower the pH or a lot of lime to raise it just a little bit. For best results, they need to be tilled into the soil three to four months in advance of the planting date, and at least six inches deep into the soil. Changing soil pH is easier in a vegetable garden that gets tilled every fall or spring than it is in a flowerbed full of rhododendrons or a lawn. Another soil test after the amendments have been added may show that the new pH has freed up some of the existing soil chemicals and additional micronutrients aren’t needed. But if they are needed, follow the label directions on a fertilizer that includes them.
Nitrogen is very mobile in soil. It is easily dissolved in many forms, and it washes away quickly. It is one of the primary pollutants of local streams, national rivers and even the Gulf of Mexico. Phosphorus, on the other hand, is not very mobile in soil and is often plentiful. Adding more of it may not help your plants grow any better. Potassium is between the other two in mobility, and some garden plants are helped when it is added.
Small applications of fertilizer are often helpful during the growing season when plants are actively growing and removing nutrients from the soil.