So, how did your garden do this year? Did everything get big and bushy? Put out tons of blossoms? Produce lots of fruit?
If, in spite of your best attention and efforts, your garden under-performed, you might think that the solution is to toss around a lot of fertilizer. But before you do that, take a beat and think this through. The problem may not be a lack of nutrients. The problem may be something else going on in your soil.
But to diagnose the problem, you have to know the current condition of your soil — whether it’s acidic or alkaline, if it’s deficient in any macro- or micro-nutrients, and whether it has unacceptably high levels of lead. To get that information, you need to test your soil.
Testing your soil
There are soil tests that you can buy at the garden center that will give you some of that information. These kits are quick and easy to use and will give you general readings on the pH level and nitrogen, phosphorus, and potassium levels of your soil. It’s then up to you to determine what to add to your soil to adjust it appropriately.
For just a few dollars more, however, you can do a mail-in test at a laboratory that will give you specific readings as well as recommendations for correcting your soil for specific crops. The University of Massachusetts at Amherst Soil and Plant Nutrient Testing Laboratory will run an inexpensive soil analysis that gives you good, detailed information that will make it easier for you to correctly amend your soil. See the lab’s website for information on the routine soil analysis as well as additional tests available.
For additional testing sites, the University of California, Davis, has put together a list of labs in California that will run soil tests for home gardeners.
Whichever way you test your soil, it’s important that you collect good soil samples. Different testing labs may have more specific instructions, but basically you collect a trowel-full of soil from 6 to 8 inches below the soil surface (or 4 to 6 inches in lawns) in at least 12 places in the area you want to test. Mix all the samples together in a clean bucket, breaking up soil clods and removing stones, roots or any other kind of debris, then spread it out on clean paper to air-dry. When it’s completely dry, collect 1 cup of soil to test, and send it to the testing service.
Pay attention to the pH
One of the things that soil tests can measure is the pH level. The pH scale is measured from 0 to 14 with 7 being neutral, below 7 being acidic, and above 7 being alkaline (also called basic).
The reason that pH level matters is that plants growing in soil that has either too high or too low a pH level will not be able to fully access the nutrients present in the soil. In that case, adding fertilizers won’t help the plant at all.
Most plant nutrients are available at a pH of 6.0 to 6.5, so that range is generally considered ideal. Some ornamental plants, such as ceanothus, daphnes, lilacs, hellebores, and clematis, do best in more alkaline soil.
On the other hand, most edible plants thrive in acidic soil. Some prefer it to be very acidic. Tomatoes, for example, need a pH level between 5.5 and 6.5 to perform best, while blueberries want the pH even lower — as low as 4.5.
Soils in different regions may have very different pH levels. If your soil test comes back showing that your soil is either more alkaline or more acidic than what your plants need, you can add amendments to raise or lower the pH level. It’s a gradual and ongoing process, but it’s not difficult to do.
Making the soil more acidic
To lower the pH level, making your soil more acidic, the most common remedy is to add organic matter such as sphagnum peat to the soil and work it in to a depth of 4 to 6 inches. Acidic mulches like pine needles can help to lower the pH level as well. Even adding used coffee grounds to the soil can help to acidify it.
Making the soil more alkaline
To raise the pH level, making the soil more alkaline, add pulverized limestone. (Be careful not to add dolomitic limestone, which also contains magnesium, unless you know your soil is deficient in magnesium because too much magnesium could be detrimental.)
It will take a year or more of regularly adding amendments to significantly raise or lower the pH level and keep it where you want it. But you should see your plants grow healthier and more vigorous when the soil is meeting their chemical and nutritional needs.