Almost everyone recommends getting a soil test, including most gardening web sites, USA extension offices and gardening experts. I disagree!
There are good reasons for getting a soil test, but the general advice of getting regular soil tests for homeowners does not make sense. Here’s why.
What is a Soil Test?
There are many different soil tests, but the one discussed here is the routine soil nutrient test for fertility. It usually tests for potassium, phosphorus, maybe sulfur, calcium, magnesium and pH.
If a soil test is such a good idea, why is it that most home gardeners don’t do them? I am a Master Gardener and we routinely recommend a soil test to our clients. At a recent meeting I asked my fellow Master Gardeners if they have ever had one done. Only 1 person had it done and that was years ago. If it is such good advice, but don’t we follow it?
Soil Tests Don’t Measure Micro-nutrients
A standard soil test does not measure micro-nutrients. You can upgrade for a more expensive test and get these analyzed, but what do you do with the information? Very few fertilizers actually tell you their concentration of micro-nutrients, so you really can’t figure out how much fertilizer to use based on your test results.
The reality is that most soils are not deficient of micro-nutrients. If you suspect such a deficiency – get a soil test done.
Soil Test for Nitrogen
Notice that the above nutrient list does not include nitrogen. Nitrogen is the most likely nutrient that is deficient in your soil but routine soil tests don’t measure for it. Why? The reality is that by the time you get the soil sample to the lab and they do the test, the amount of nitrogen in your soil has changed too much. So there is no point in measuring it.
You can get nitrogen measured, but the sample needs to be frozen and kept frozen until testing. The test costs extra and is not cheap. It is used in agriculture but is of limited value in gardens. Remember the value in your soil has already changed by the time you get the results, so you can’t use them to apply the right fertilizer.
Some home kits do measure nitrogen, but they only measure free nitrate, not total nitrogen and they are not very accurate. For more information on these see Soil Testing for NPK.
Why get an NPK soil test? It is the only way to know if you have a nutrient deficiency, and if you don’t know that, you don’t know which fertilizer to add to your garden. Therefore it seems to make sense to get soil tested. In fact, many people suggest that you should not fertilize until you get the soil tested so you know which fertilizer to add. But …… there is a fundamental flaw in this logic.
Farmers get their soil tested for the same reason. They use the results to tell them which fertilizer to use to maximize yield. This works because labs have many years of data to know the right amount of nutrients that will maximize a particular crop. If you are growing 100 acres of corn, this makes perfect sense.
It is more typical for home gardeners to be growing 15 different kinds of vegetables, 40 different types of perennials and half dozen different kinds of trees and shrubs. Except for a few of the common vegetables, labs don’t know the right amount of nutrients needed for these plants. That is why the lab results given to home gardeners are for some generic “garden plant” category.
Each plant would need a different NPK formulation to maximize their growth – labs can’t provide that kind of detail, and quite honestly gardeners don’t want to fertilize each plant differently.
Do Gardeners Want to Maximize Yield?
In the above description I used the term “maximize yield”. That is important for farmers, but is it important for gardeners? Not really. It is not critical for the ornamental garden. If a tree grows 2″ less than maximum potential – gardeners don’t care. If the garden phlox is 3″ shorter and produces 18 flower clusters instead of 20, gardeners don’t really care. Even in the vegetable garden, it is easier to add another tomato plant than spend a lot of effort maximizing the yield on the other 4 plants.
Remember that in order to maximize yield you have to also do everything else in the best possible way. Fertilizer is only part of the solution.
I don’t think home gardeners need or want to put a lot of effort into maximizing yield. They want a “good” yield but you don’t need a soil test for that.
Gardeners Over Fertilize
A common problem in home gardens is over fertilizing. People add too much fertilizer, both synthetic and organic. They tend to have high levels of nutrients in soil, except for nitrogen which is almost always low. A soil test will show you if the nutrients are too high as well as too low, so one benefit of a soil test is that it might get you to fertilize less.
Finding The Right Fertilizer
So you don’t believe me and got a soil test done. Let’s assume it shows recommends adding a fertilizer with an NPK of 12-1-4. Note that nitrogen is included in the recommendation even though it is normally not tested. Most labs will include it in the recommendation, purely as a guess because it is almost certain that you need more nitrogen.
Time to go shopping. You want to go organic so you look for organic sources that have a 12-1-4 NPK, but can’t find any. You decide synthetic might work better and visit every store in town and can’t find a 12-1-4 fertilizer. That is not unusual. There are relatively few available formulations on the market and many combinations that could be needed to match your soil test.
Now what? Simple, you buy several options and blend them together to form a 12-1-4. This does require quite bit of math and remember that this is all done with weights, not volumes, so you need a way to weigh everything. You also need to know the size of the garden so you can spread it at the correct rate.
Have I lost you? Is this all too much work? For most gardeners, including me, it is too much effort.
What do you do instead? You go out and buy a 10-10-10 and spread it around. That is so much easier. If that is what you end up doing, there is no point in getting a soil test.
By the way, you should never use a 10-10-10 fertilizer!
When Should You Get a Soil Test?
Get a soil test when stuff is not growing right. If plants are stunted or yellow, and the problem is not a pest problem, get the soil tested.
If you suspect heavy metal toxins in your soil because of past land use, or old lead-based paint flaking from the house, get a soil test for heavy metals.
Get a soil test if you are willing to follow directions and apply the right fertilizer mixture.
You can NOT determine a soil deficiency by looking at leaves, as so commonly suggested.
Better Than A Soil Test
Have I ever gotten a soil test? Yes, twice. I got them to get the right values so I could make some gardening videos including a video testing home soil test kits. I have never gotten a soil test to determine fertilizer needs.
Here is what I have done in all my gardens and what I suggest you do.
- Find some old time gardeners near you. Ask them about the soil. They might have some insight on nutrient levels and pH.
- Grow stuff. If most stuff grows well, you do not have a nutrient deficiency and don’t need a soil test.
- Apply 1-2″ of compost every year. To learn how to make compost read my book Compost Science for Gardeners.
- Keep organic mulch on the soil 100% of the time.
- Add a small amount of nitrogen to your vegetable garden (synthetic, manure, blood meal – it all works the same way).
The compost and mulch will add all the micronutrients and some of the macronutrients your garden needs. KISS the garden – Keep It Simple Stupid!