Soil Testing for NPK

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Robert Pavlis

Which fertilizer should you be adding to soil? How much fertilizer should you add? Why is my plant not growing well? These are all very common questions and a very common answer to them is; do some soil testing!

That may be the right answer, but there is much more to the story. Let’s have a look.

Soil testing for NPK
Soil testing for NPK

Soil Testing – Why do it?

The most common reason for soil testing is to measure the availability of plant nutrients in the soil. Soil tests can also measure things like pH, pollutants both organic and metallic, and humus levels, but the main reason for most soil tests for gardeners is to provide information about the level of nutrients.

Unless you have a special problem with your soil, you are mostly interested in the NPK ratio; nitrogen, phosphorus and potassium. We all know that plants need a certain level of these soil nutrients, and the only sure way to know what your soil has is to get a soil test done by a proper lab.

Along with your results, most labs will also tell you how much fertilizer to add, and what NPK ratio to use. So if your soil is low in potassium, for example, your soil testing results will indicate that you need to add a fertilizer which is high in potassium.

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Which Plants are You Growing?

In order for the lab to provide you with NPK fertilizer recommendations, they will ask you to specify the type of plant you are growing. Each type of plant has an optimum level of soil nutrients that will produce maximum growth. This works well for farmers. They might have 100 acres, of say corn, and they can fertilize specifically for corn. As a home gardener you probably grow 100 different types of perennials, shrubs and trees. It would be impossible to fertilize specifically for each type of plant.

To overcome this problem labs will provide fertilizer recommendations for “general” plant requirements. I guess that is better than nothing?

Nitrogen – The Key Nutrient

All of the soil nutrients are important – if any one is missing, the plant will not grow properly. So why do I call nitrogen the most important?

If you are taking care of your soil you will be adding organic material to it. You will be mulching with organic material, and adding compost to the soil. Dead plant parts will be returned to the soil either directly or as compost, to feed future plants. You will be building up the humus level in your soil. Doing this will add major and minor plant nutrients to the soil. They will rarely be deficient, unless you are gardening on pure sand.

The one exception to this is nitrogen. You might be adding significant amounts of nitrogen with your organic material, but there is a special problem with nitrogen. Firstly, nitrogen moves quickly through the soil. Common forms of nitrogen like nitrate, nitrite, and ammonia dissolve easily in water, and are ‘washed away’ with each rain. Nitrogen is also converted to N2 which is a gas that escapes into the air. You can be adding lots of compost to your garden and still have low levels of nitrogen, see Compost Fertilizer Numbers for more details.

One of the main crops home owners grow is lawn grass and we all know that grass requires nitrogen to grow well. Lawn fertilizer is probably the most common fertilizer home owners use.

For these reasons nitrogen is the most important nutrient for your plants, and the nutrient for which you need a soil test.

Does Soil Testing Measure Nitrogen?

Here is the surprise. Most standard soil tests do not measure the level of nitrogen!

The reason is quite simple. By the time you collect the soil sample, take it to the lab, and they analyze it, the level of nitrogen in the soil has changed so much that any reported value is meaningless. Remember, nitrogen is always being converted to other forms, especially a gas, and these changes take place quite quickly.

Standard soil testing does not measure nitrogen.

Farmers do require a nitrogen test. What they do is take a soil sample and freeze it right away. They take it to the lab frozen, and the lab keeps it frozen until it is tested. This will provide a correct nitrogen value. Home owners rarely have this test performed–it is complicated and more costly.

Should You Bother with Soil Testing?

The main reason to get a soil test is to get a value for nitrogen. Since the standard test does not give you this value it may not make sense to have a soil test.

Consider this. If you get a soil test without measuring nitrogen, how can you select the right amount of fertilizer? You can use a fertilizer with no nitrogen and the correct amounts of other nutrients, or you can guess at the amount of nitrogen you need – pick a number out of the air. Some labs, the University of Wisconsin for example provides a nitrogen recommendation even though they don’t measure it. These are best guess estimates based on historical results and general rules for a given geographic area.

Before getting a soil test done look at your garden. If things are growing reasonable well, then you probably don’t have a significant nutrient problem which means you don’t need a test.

Having said that, a soil test is a good idea if you are having problems growing things. If plants don’t grow properly, a plant nutrient in your soil may be too low or too high. The only way to know is through soil testing.

You might be interested in this post Fertilizer Nonsense #4 – Soil Testing and Soil pH Testers – Are They Accurate.

Should You Fertilize Without a Soil Test?

There are two ways to fertilize. If we are talking about buying a bag of commercial fertilizer then the answer is – NO. If you have not had a soil test done, you don’t know if a certain plant nutrient is low or high. You might have too much phosphorus already and adding more can be detrimental to your plants. Save your money and don’t add fertilizer unless you are fixing a specific known problem indicated by a soil test.

The same advice goes for home concoctions, and common products like bone meal and Epsom salts. Don’t use them without a soil test.

Lawn fertilizer is a bit of an exception. Grass does need higher levels of nitrogen to grow well, and unless it is growing well, it tends to be weedy. If you are going to add nitrogen, try to find a lawn fertilizer that only contains nitrogen (ex Urea), or one with low levels of P and K. Plant fertilizers with no phosphorus are now common and for most soil they are a better choice since most soil has plenty of phosphorus.

A second type of fertilizer is the organic type; compost, wood chips, straw etc. This type of fertilizing can always we done without a test provided you don’t go overboard. It adds low levels of plant nutrients, over a longer period of time. It is much less likely to add too much of any one nutrient.

Keep in mind that even adding organic matter can be a problem. Let’s say you have high levels of potassium already. Adding compost will increase the amount of potassium and make the problem worse. This is another example of organic not being good! Only a soil test can tell you if this is a problem.

Most gardeners do not need to be concerned. Talk to other gardeners in your area. If they add compost, and have good plant growth, you will have similar results.

References:

1) Photo Source: r. nial bradshaw

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

9 thoughts on “Soil Testing for NPK”

  1. Well that explains why my home soil test was giving suspicious results. I found it hard to believe that there was no more nitrogen anywhere on my property.

    Reply
  2. U said out of NPK we should assume that PK is usually present in a home garden. Does this mean that we should add urea to the soil? Also is urea a chemical and if so can u give natural and organic subsitutes for the same please.

    My garden soil with the home vinegar ph tests says its alkaline. This is for home vegetable garden patch. Now, how do I go ahead in preparing the soil? Should I try to add nitrogen in some form?

    Reply
    • If you only need nitrogen – you add nitrogen. Urea is a chemical, just like everything that gets added with compost or any other organic material.

      Reply
  3. I did a home soil test & it indicated that my raised bed needs all 3 NPK. I bought topsoil & compost mixture from a local soil farm so I’m so surprised! My plants aren’t thriving. They aren’t dying but are smaller than they should be & are Showing signs of these 3 deficiencies. My question is how do I add all 3 into an existing garden? Is it too late?

    Thank you!

    Reply
  4. Informative, thanks. If nitrogen is the most important nutrient, and the only really valid way to measure your concentrations accurately is to freeze the sample and hand carry it to a lab in the frozen state, that could be quite a deterrence. Can you recommend any alternatives to get decent N results or a direct-read, in the field kit(s) for measuring N K P with decent accuracy? Do you accept soil samples for analyses and if so can you direct me to the details?
    Thank you.

    Reply
    • You can use a home test kit, but that measures only nitrate, not available nitrogen. And cheap gardening kits don’t even give you a numeric number.

      Reply
      • I used the home soil test kit to measure nitrogen. It was helpful to even have a rough idea that some soil was deficient. I tested it on pure compost as well to give myself a reference point. The pure compost was very high in nitrogen so I felt like the deficiency in my soil made sense.

        Reply
  5. the information is very good. I learned a lot for this .
    But I have some doubts regarding measuring the NPK level can anyone suggest any kind of information

    Reply

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