Why Soil Testing is Critical for Maximum Yields
Are you doing everything you can to take good care of your plants and soil, but your garden is still not growing as well as you'd like? Here's one simple tip that might turn your garden around…
Get your soil tested at a lab. Seriously! Would you go to your doctor, and say: “Hey, doc, I'm not feeling so good. But I don't want you to take any tests. Just look at me and tell me what's wrong!”
Your soil could look beautiful – soft, dark brown, loamy, and full of organic matter – and it still may not be able to grow decent crops. Plants thrive best in a well-balanced soil – one that is not too rich and not too poor in the various nutrients, with a pH level in the fairly neutral range (6.0 – 7.5).
It can be just as bad for your plants if any of the minerals are too high as when they are too low. An excess of one nutrient can cause deficiencies in other nutrients – even though there may be plenty available in the soil.
Casually applying fertilizer to your garden, without knowing what it really needs or how much it needs, can cause serious long-term problems. If you accidentally create an excess of one or more nutrients in your soil, it can take many years to correct.
The Benefits of Getting Your Soil Tested
Getting your soil tested can make the difference between having a mediocre or poor garden, or having one that is thriving! A lab test can tell you the levels of pH, macro-nutrients, micro-nutrients, CEC, and organic matter.
In the U.S., the easiest and cheapest place to get your soil tested is through your local Cooperative Extension office. You can find your local office by doing an internet search for “cooperative extension offices” in your state.
Here in Virginia, a soil test only costs $10-15. These lab reports are much more accurate and more detailed than home test kits (see this comparison of home test kits)
Depending on what you request, a soil test report from a lab can let you know:
1) if your soil has any nutrient deficiencies or excesses
2) how much fertilizer to use (though you may need to convert the chemical fertilizer recommendation to organic fertilizers)
3) if you need to modify the pH level by using lime or sulfur
4) how much organic matter is in your soil
5) how to best fertilize your particular type of soil, based on your CEC
Why Organic Matter Is So Important to Your Garden
You can usually request for the soil test to include the level of organic matter (decomposed compost) in your soil. If you want your garden to thrive, it's very important for your soil to include adequate levels of organic matter.
A soil that is rich in organic matter will extend the range of pH that your plants can tolerate, and improve their ability to absorb the nutrients in the soil. Organic matter can help make clay soil softer and easier to work in, and it helps sandy soil retain more moisture. It helps to retain soil nutrients, and also encourages abundant levels of beneficial soil life.
However, there is no one “perfect” level of organic matter. The optimum amount will vary, depending on your climate and soil type. A healthy sandy soil in a hot climate might only have 4% organic matter, while a healthy clay soil in a cool climate may have 9%.
No matter what your climate is, your garden soil doesn't need more than 10% organic matter to be healthy. More organic matter isn't always better. Some organic gardeners have developed serious nutrient imbalances in their soil by constantly adding large amounts of compost, manure, or mulch over many years.
Organic matter basically decomposes into carbon dioxide, water, and trace minerals. Some of these minerals can build up over time, while others will leach away, and this can create major nutrient imbalances if you continue to import large amounts of organic matter over a long time.
Once your garden soil has developed a healthy level of organic matter, you normally don't need to add more than 1/4” of compost each year (about 40 lbs per 50 sq ft), though very hot climates might require more. In my garden and climate, the thin layer of mulch that I use year-round supplies most of the organic matter that my soil needs each year.
Using Mel's Mix
That's one of the concerns I have with using Mel's Mix or other potting soil in garden beds. It decomposes rapidly. In my climate, I've seen about 25% of the mix disappear each year, and you have to keep adding large amounts of compost or potting mix to keep your beds full. Over the long term, that can create nutrient imbalances.
Some people believe that simply adding lots of compost to your garden will supply all the nutrients that your garden needs, but that's not always the case. Many of the composts available at local nurseries or big box stores are of very poor quality, or may be contaminated with a long-lasting herbicide (see: Don't Kill Your Garden With Compost!)
The Mel's Mix recipe blends several different composts in an effort to end up with balanced nutrients. However, if you mix together several poor composts, you will not necessarily end up with a great compost! So, if your plants are not growing well in your Mel's Mix, it might be due to your soil mix pH being off, or the mix having nutrient deficiencies or excesses.
If you are having growing issues involving large amounts of potting soil or Mel's Mix, it may be cheaper to test and amend that mix than to throw it out and start fresh. The university lab that handles Virginia's Extension soil tests will only test mineral soil (dirt) – not a potting soil like Mel's Mix. If you would like to get your potting soil tested, there are private labs you can use, such as A&L Eastern Laboratories, A&L Western Laboratories, or Logan Labs.
How Knowing Your CEC Can Help Your Garden Thrive
Soil test reports also usually include what's called the “CEC” – cation exchange capacity. In simple terms, it can tell you how many nutrients your soil is capable of holding and making available for your plants. The type and amount of your soil's clay content and organic matter determines your CEC.
By knowing your CEC, you will know if you can fertilize your garden just once a year in the spring, or if you need to side dress your vegetables with additional fertilizer during the growing season.
If you've noticed that your garden starts out looking great, but your plants start looking poorly or don't produce well by mid-summer, you might be dealing with a low CEC.
Sandy soil is usually low in CEC – less than 10, sometimes as low as 2. Some young clays common in the northern U.S. can be very high, over 30, though old weathered clays like we have in the southeast U.S. can be very low – below 10.
Soils with a high CEC (20-30) can easily hold onto the large amounts of nutrients needed to feed all of your garden plants throughout the entire growing season. Gardens with this type of soil can generally be fertilized just once in the spring.
Soils that have a very low CEC (below 10) cannot hold onto very many nutrients. If you try to apply a full season's worth of fertilizer in the spring, rainfall can leach out many of those nutrients before your plants can make use of them later in the summer.
With low CEC, your garden will usually benefit by splitting up the application of the fertilizer into 2 or more doses. The lower the CEC, the more frequently you should dose your garden, with smaller amounts. With a CEC of 7-9, divide the amount of your fertilizer into two, and apply half in the spring and half in mid-summer. With an extremely low CEC of 2-3, you may need to divide your fertilizer into 6 or more doses, and apply it monthly.
If your soil's CEC is above 10, but below 20, you'll need to pay attention to how well your plants are doing. Low-demand crops like carrots and beans may do just fine throughout the growing season with no additional fertilizer (beyond your spring application), but high-demand crops like the cabbage family and corn may benefit with with the application of additional fertilizer in the middle of the summer.
Where You Can Find Organic Fertilizers
If your garden is very low in just one nutrient, use a fertilizer that supplies just that one. If most of your garden nutrients are low, then you can use a general fertilizer blend.
There are a wide variety of organic fertilizers on the market. Some provide a single nutrient, and others provide 2 or more. You should be able to find just the right product for your own garden's fertility needs.
Here are two mail-order companies that offer a great selection (I am not an affiliate of either one):
You can find more sources of organic gardening products at Mother Earth News Organic Pest Control and Garden Products Finder.
Don't Waste Your Valuable Time and Money!
Don't spend all your hard work and money on your garden without investing in a soil test. It can often make the difference between having a garden that is doing poorly and having one that is thriving!