Lots of information in books and on websites recommend a soil test to help you select the right fertilizer numbers. This is very good advice but it has limitations.
Soil Test; Is it a Good Idea?
In the previous post I suggested that buying fertilizer without knowing what nutrients you have in your soil is a poor idea. You need to buy the nutrients that are missing from your soil–not some fertilizer numbers an expert told you to buy.
The only way for you to know what nutrients are missing from your soil is to have it tested. You can’t tell by looking at the soil, or plants, except in some special cases.
If a soil test is such a good idea, why is it that almost no home gardener does 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. It is good advice, but we don’t follow it.
There are two big reasons why people don’t have soil tests done; cost and effort. A good soil test is not cheap and it requires effort to collect samples, deliver them to the lab etc. It is spring and time to fertilizer–we want to get on with things–not wait for test results. These may not be great reasons, but they are realistic excuses. However, there are some more logical reasons for not doing a soil test.
Reasons for NOT Doing a Soil Test
Who Needs Fertilizer?
My garden is doing just fine and I don’t use commercial fertilizers. My 2,500 different perennials are growing and flowering quite well. Most of my 80 different kinds of daffodils bloom like crazy and have never seen bonemeal. If your plants are growing well, there is no reason to fertilize and therefore there is no reason to get a soil test. Don’t try to fix a problem you don’t have!
I don’t use commercial fertilizer, but I do add dead plant material back into the garden. I use a mulching mower so that grass clippings stay put and feed the grass. I mulch with wood chips in perennial and shrub beds; which slowly degrade and fertilize the garden. So I do fertilize–I just don’t buy fertilizer.
Soil Tests don’t Measure Nitrogen
It will probably come as a surprise to most of you, but regular soil tests do not measure nitrogen. But nitrogen is the nutrient that is most likely deficient in your soil. It is certainly the most important nutrient for lawns. You can order a nitrogen test but it is complicated and expensive, so home owners rarely get this done.
If the soil test does not tell you how much nitrogen you have in the soil, is it really that valuable?
Most labs will estimate the amount of nitrogen you need based on your location, and your crop. Another expert telling you which fertilizer to buy without knowing what is in your soil!
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. For some micro-nutrients, you can get specialized fertilizer that only contains one or two nutrients, for example calcium sulfate for calcium, and you can then add that according to your test result.
The reality is that most soils are not deficient of micro-nutrients. If you suspect that such a deficiency exists–get a soil test done before buying fertilizer.
The Need to Fertilize
Our minds have been conditioned so that they have a ‘need to fertilize’. We just don’t feel like good gardeners if we don’t fertilize. It is time to change that point of view.
You probably don’t need to fertilize if some of the following apply:
- You mulch with an organic material. It breaks down over time and is fertilizing for you.
- You add compost on a yearly basis.
- You return spent plant material back to the garden instead of sending it to landfill. Again you are fertilizing.
- Your plants are mostly growing well. Not all plants do well in all gardens–accept this fact. Adding fertilizer is rarely the answer.
- You have not done a soil test.
If I have not yet convinced you, please consider joining FA (Fertilizers Anonymous). You have a problem and you need help with your need to fertilize.
Next time you are at the garden center and you see someone loading up their cart with bags of fertilizer, shake your head and think to yourself (ie not out loud); poor fellow, he’s wasting his time, his money and his back, to do something that does not need to be done.
Do I need to worry if after fertilizing the plant, a big rain comes? Will all the fertilizer be washed away?
Yes and no. nitrogen is washed away quickly, potassium more slowly and phosphate very slowly – actual speed depends on the soil type and organic matter level.
Good Afternoon Robert,
I have gone through and read your blog posts on fertilizer
and my goodness they are so helpful! Thank you for clearing so much up.
A team and I are currently trying go create a device that will go through and collect samples of the soil in different areas on its own. This would save gardeners lots if time and effort in sampling their soil, and reduce over fertilization of gardens.
The machine will measure NPK and then decide based on that result how much fertilizer to dispense over a small area, then do that for the whole field. It holds 3 types of fertilizer: 46-0-0, 0-46-0, and 0-0-60. If it measures that there is too much potassium and not enough nitrogen, it will figure out the amount of nitrogen fertilizer to dispense over the soil. My team is trying to figure out the ratio of NPK for the machine to want the soil to reach. As in, it measures the NPK and now it needs to decide how much of each nutrient there is too little of. How can we decide how much of each nutrient there should be in the soil?
In other words, is there a specific measurement of nitrogen we want to be in the soil, and what is that measurement?
Thanks so much
The recommended amount will vary depending on soil type, but a soil lab can give you some approximations.
When you get it ready – I’d love to try it and do a review YouTube video.
So much has been explained and understood. I thank you
The soil pH ( acidity/ alkalinity) test could be extremely helpful:
you won’t get blueberries as well as rhododendrons in a sweet- alkaline soils.
One won’t get prize winning lawn unless soil pH is above 6.2
Think of pH value as a key, which unlocks the refrigerator ( soil) for plants to have proper nutrients..
Don’ t you agree?
PH can be very useful, but that is not the reason most people get a soil test done. An approximate pH can be ascertained by talking to neighboring gardeners. Do they have success with blueberries and rhododendrons? If not, it is not acidic.
I’m not a soil scientist, but I’ve never gotten how pH testing and nutrient testing could be useful. Just as there are microclimates in any garden, how could one soil sample really represent the garden as a whole? I live on small acreage, and in my front yard it is dense clay. The backyard is made up of a ton of basalt rock – some boulders,
and some the size of pea gravel (actually I think it was a volcano in the Boring Lava fields near Portland Oregon) and down in my lower garden it is wetland, creek, pond, and pretty sure it was all covered in river at one time. I realize that this is pretty big area compared to other gardens, but how could the nutrient profiles or the ph be similar in all areas? Not to mention that all areas have received different types of mulches, fertilizers, etc., and I’m not the only one who’s lived here and made gardens… I realize that not everyone has such large space as me, but can easily see how this would apply to smaller spaces. Would appreciate your input.
In a larger area, you would do several soil samples so you get data for each area.
The difference in wants and needs is a little shady to me. Do we actually need a flower garden, anymore than we need a green lawn without weeds? If we “want” a very green lawn without weeds don’t we “need” to fertilize it?
I agree whole heartedly with 95% of what you say and in most garden situations fertilisers are not needed. Where nutrients are deficient they can be recycled back or added in bulky organic materials such as compost.
However there are some situations where fertiliser helps or even is needed. For example in tubs and pots where long term plantings may become leached – I sometimes top dress my tubs and planters and of course those gardeners who liquid feed are fertilising. My lawn and veg garden are examples of other situations where I sometimes use fertiliser sparingly.
In the cases where I do use fertiliser I do use a professional balanced fertiliser – the same balanced analysis for virtually all those uses.
Great blog Robert!
You are correct that you do need fertilizer for potted plants, assuming that they are in a soiless mix, which is most common. In this blog I tend not to focus on potted plants, but it is a valid point.
The idea of a “balanced fertilizer” is a myth which I must write about one of these days. Balanced fertilizers do exist. It is any fertilizer where all three numbers (the NPK numbers) are the same. The problem is that there is no logical reason for making a balanced fertilizer. Many of the reasons for this are given in this post https://www.gardenmyths.com/fertilizer-nonsense-3-all-tomatoes-need-the-same-fertilizer/
that is, the person who is loading up on fertilizer is doing something that “may not” need to be done, depending on their particular circumstance…
My initial reaction to this comment was; he’s right–there are times when fertilizer is required.
But I have been thinking about this for a couple of days. Is there a time when someone does need to fertilize?
We are talking about gardeners and not farmers. It is certainly true that fertilizer may be needed to improve yields in a farming situation. But this blog is targeted to gardeners, not farmers.
Even if a soil test shows that there is a deficiency of one nutrient or another, does this mean we need to fertilize? I would argue that the answer is NO. We may decide to fertilize for esthetic reasons, ie we want more flowers in the garden or greener grass. But this does not translate into a ‘need’ to use commercial fertilizer.
Except for grass, compost or wood chips is a better alternative to commercial fertilizer. It takes too much compost to keep grass green and Keeping grass growing well will reduce weeds. But then we don’t need a weed free lawn–that is a desire for esthetic reasons, not a need.
Very sandy soil does need fertilizer, but again it can be added using organic sources.
Is there a situation where the gardener does need commercial fertilizer??