What Information Does a Soil Test Provide
Soil tests contain valuable information but have their limitations, unless you are getting specialized, expensive tests. Most basic soil tests provide the pH level of your soil, and measure the amount of potassium, calcium, phosphorous, and magnesium in the soil.
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.
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Why Should I Know My pH
PH is a measure of the soil’s acidity or alkalinity. The scale runs from 1 to 14. 1 is extreme acidity, while a soil with a pH of 14 would be extremely alkaline. The majority of garden plants prefer a slightly acidic soil, about 6.5 on the scale.
Knowing your soil pH is valuable information since many nutrients become unavailable to plants, despite their being in the soil, if the pH is either too acidic or too alkaline. In addition, soil microbes such as beneficial bacteria and fungi are more abundant and active when the soil pH is near neutral.
A soil test will determine the pH of your soil and recommendations for amending the soil to adjust pH will be given if necessary. Generally, adding lime if your soil is too acidic, or sulfur if your soil is too alkaline are recommended. This is another area where adding organic matter such as compost is beneficial since organic matter helps to maintain a neutral pH.
Should I Fertilize?
When you have your soil tested, recommendations will also be given to add specific fertilizer to your soil. My focus is on building soil, not on feeding roots so, with a few exceptions, I do not generally recommend using fertilizers. As a young (and poor) gardener, I never had my soil tested. After years of adding only compost, I decided one year to go ahead and have my soil tested. The report came back that my soil needed no amendments!
Fertilizer tends to give the plants a quick boost rather than a consistent level of nutrients. Chemical fertilizers are salts which build up over time and harm soil microbes and thus contribute to a degraded soil in the long run. They also have the potential to burn plant roots, and seep into ground water, contaminating our water supply. In addition, the manufacturing of chemical fertilizers require a large amount of fossil fuel to produce.
I highly recommend using organic matter to build your soil and to provide the nutrients that your plants need. Manure, compost, decomposing yard waste, grass clippings and wood chips will build soil that will provide nutrient-dense fruits and vegetables in your garden.
Recommended Soil Amendments
This, of course, is not a perfect world, and sometimes we do need to amend our soil. In my eBook, The Art of Gardening: Building Your Soil, I give recommendations for using natural products to amend your soil. I’m including a portion of that information for you below.
The following organic amendments may be used, following package instructions for use, if soil testing shows a specific nutrient deficiency of some of the most commonly needed nutrients.
First, though, a word about the numbers in the parenthesis – these indicate the amount of Nitrogen, Phosphorous and Potassium (N-P-K) in each amendment. So, if the N-P-K is 4-12-0, for example, this means that the amendment contains 4% available Nitrogen, 12% available Phosphorous, and 0% available Potassium. That, of course, only adds up to 16%. The other 84% would be inert or inactive ingredients, or other micro-nutrients.
For items which don’t come “packaged”, I’ve noted amount to use. Click on each amendment for information on where to purchase (affiliate links).
Blood meal (13-0-0)
Fish meal (10-7-1)
Alfalfa meal (2-1-2)
Composted manure (NPK varies) – add approximately 1 cubic ft per 100 square ft of garden.
Wood ash (0-1-3) – add approximately 1.5 lbs per 100 square ft of garden.
Crushed Granite (0-0-5)
Wood ashes – add approximately 1.5 lbs per 100 square ft of garden.
Crushed eggshells – add approximately 1.5 lbs per 100 square ft of garden.
Clam and/or oyster shells – add approximately 1.5 lbs per 100 square ft of garden.
Please note – Let’s say that your soil test indicates that your soil needs both potassium and calcium and you want to use wood ash. Do not add both the 1.5 lbs needed to raise the potassium level and the 1.5 lbs needed to raise the calcium level. 1.5 lbs total is all that is needed.