Soil NPK Test for Gardeners: What You Need to Know

woman with raised beds garden

Most gardeners know that the quality of their garden soil is important, but aren’t sure how to evaluate the condition of what’s in their yard. Often I’m asked, “Should I get a soil npk test?

The health of your soil is is one of the main factors that affects the success of your garden each season. 

When your soil contains all of the nutrients the plants need they’ll grow larger and be more productive, the fruit and vegetables they produce will be a higher quality, and the plants can better resist disease and pest pressure.

Also, when we eat vegetables our bodies absorb the nutrients we need from the food. If your soil is lacking nutrients then your food will as well.

So, basically, if you don’t have healthy soil, all of your other efforts will be wasted. You simply cannot have a well-performing garden with unhealthy soil.

In this article you’ll learn all about soil, what plants need to grow, options for NPK tests, when and how to send away for a soil test, and what to do with the results.

building vegetable garden soil

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What is soil? 

Soil is made up of five different building blocks – minerals, organic matter, living organisms, water, and air.

Minerals are tiny particles of rock that have broken down over time. The size of the rock in your soil determines what type of texture you have – sand, silt, or clay. Most soils consists of a combination of all three of these sizes. 

Organic matter is partially broken down leaves, grass, mulches, and plants. It helps air and water move through the soil, retains moisture, and is food for soil life.

A teaspoon of rich soil can contain one billion bacteria! Living organisms like bacteria, fungi, protozoa, and earthworms help break down the minerals and organic matter into food for the plants.

Healthy soil is made up of almost 50% water and air.

nitrogen deficient leaves

On the left, nitrogen deficient bean leaf. Right, healthy and nutrient rich leaf.

What Nutrients Plants Need to Grow

There are three primary macronutrients that all plants need to grow healthy and strong – nitrogen (N)phosphorus (P), and potassium (K). They are often called NPK for short. Plants need these nutrients in the biggest quantities.

Nitrogen helps plants put on vegetative growth. If you don’t have enough nitrogen you may notice your plants turning yellow and/or stunted growth. If you have too much, your plants may grow big and green, but not produce any flowers or fruit.

Phosphorus stimulates flower bloom and root growth. If you’re lacking phosphorus you might notice leaves turning a bit red or purple.

Potassium supports the plants’ immunity to disease and also affects the quality of fruit. If your soil is lacking in potassium you may notice that some of your fruits are thin-skinned or small and your plants are more susceptible to disease.

If you go to the garden store and look at bags of fertilizer you’ll notice there are three numbers listed on the front. They stand for the percentage of the three macronutrients (NPK) it contains, like 4-5-7, or 12-0-0. (We’ll come back to fertilizer later.)

There are also three secondary macronutrients – calcium, magnesium, sulfur.

And several micronutrients, which are needed in a lesser quantity by plants – chlorine, boron, copper, iron, manganese, zinc, molybdenum, and nickel.

Vegetables need a lot of nutrients to grow well in your garden. How do you know whether your soil has all of these nutrients? 

woman in garden with harvest

NPK Test & More Advanced Option

One of the best ways to dive into the health and fertility of your garden soil is to collect some soil from your garden and send it away for a NPK test or other advanced soil test.

Getting a soil test done is particularly important if you’ve been having issues in your garden like yellowing plants, stunted growth, small harvests, and disease issues.

In this article about soil problems I shared that when I built my new raised garden beds the soil I purchased was very poor. This resulted in a very underperforming garden the first year.

Adding a commercial organic garden fertilizer did help address the issue in the short term, but I also wanted to do a deeper dive into what nutrients my soil was missing so I could work on building up the soil fertility over the long term.

If you’re also having soil fertility issues, or you’re just curious to learn about the soil testing process, there are two options I recommend for soil testing, depending on your level of interest and energy for this process.

Soil Testing Option A: Local Cooperative Extension NPK Test

If your state has a local university extension office they likely offer an NPK test. This is a simple and inexpensive test. The only drawback of these kinds of tests is that they only offer information about how much nitrogen (and sometimes don’t offer that), phosphorus, potassium, and organic matter your garden soil contains.

These NPK tests don’t usually test for any of the secondary macronutrients or micronutrients. Although my local soil testing lab will add on a test for calcium and magnesium for an additional fee.

If you live in a house that was built before 1979 there is a possibility you could have elevated lead in your soil. You should add on a lead test if possible. 

I would recommend this kind of test if in general you think your soil nutrient level is pretty good and you’re just curious if there are some simple things you can do to improve it further.

If you’re having major soil fertility issues, and especially if you recently ordered soil and built new garden beds, you might want to use option B instead.

How do you know if your soil has plenty of nutrients? The plants’ leaves are deep green during the season with no yellowing, the plants grow to full size, fruiting vegetables produce lots to harvest, and leafy vegetables continually re-grow enough for you to harvest at regular intervals.

If your garden is the opposite of this – yellowing leaves, underdeveloped fruit, small harvests – then the lack of health of your soil could be the culprit.

If you live in Wisconsin you can order an NPK test and send your soil sample to the University of Wisconsin Soil Labs.

soil test in vegetable garden

Stunted celery plant in my garden due to poor soil fertility.

Soil Testing Option B: Logan Labs

The first few times I sent my soil away to be tested I used option A above. But, when I built the garden at my current house I knew I had some major soil problems a month after planting when my seedlings turned yellow and stopped growing.

I just happened to be reading The Intelligent Gardener, which is all about soil health and testing. It’s a dense read, so I don’t particularly recommend it, but the author suggests using Logan Labs in Ohio for a more complete soil test.

Logan Labs will test your soil for pH, Organic Matter, Total Exchange Capacity, Sulfur, Phosphorous, Calcium, Magnesium, Potassium, Sodium, Boron, Iron, Manganese, Copper, Zinc, and Aluminum. You can also add on a lead test.

If you’ve been having problems with your garden that you think are linked to soil fertility issues I highly recommend this option. It would also be great for gardeners who are interested in learning more about soil testing and interpreting the results. A.k.a: garden nerds like me 😉

I’ve been using this lab for several years and the results have helped me repair my damaged soil and continue to cultivate a healthy soil ecosystem. I’m not affiliated with them in any way.

You can purchase the Standard Soil Test and see a sample report on this page.

When to Sample

You can obtain soil samples for the NPK test and Logan Labs test from your garden at any time of the year except when the ground is frozen. Late fall after your garden cleanup or very early spring after the ground has thawed are both ideal times.

Make sure you start the process with plenty of time to send the sample away, receive your results, and gather the amendments and fertilizers you’ll need to add to your garden.

abundant vegetable garden

How to Collect a Soil Sample

Step 1: Collect Soil Sample

If you have several different garden areas you need to decide if you want to take multiple tests or lump them all into one.

For example, I have a front and a side yard garden. It’s likely that the soil is a little bit different in each one. I could just take my samples from both the front and side yard and mix them together, or I could send away one sample for the front and one for the side so they’re tested separately. That would mean I have to pay for two soil tests.

To gather your soil sample scrape away surface litter from the garden bed. Use a spade to dig a V-shaped hole six inches deep; then cut a thin slice of soil from one side of the hole. Depending on how big your garden is, you’ll need to do this somewhere between 6-15 times and then mix them all together in a bucket.

You want to end up with at least two cups of soil representing various areas of your garden.

Step 2: Label Soil Sample

Measure out two cups of soil in a plastic ziploc bag. Mark the bag with your name, your address, and the sample ID on the outside of the soil bag. Keep a record for yourself of the area represented by each sample.

Step 3: Fill Out Submission Form

Print and fill out the Submission form on the soil lab’s website. Don’t forget to include your payment by check or credit card number.

Step 4: Mail Soil Sample

Package up the sample, submission sheet, and payment according to the lab’s directions on their website. Take it to the post office.

The lab should email you the results in about a week. Next, we’ll talk about what to do once you get your results back. 

Each lab’s website has more detailed directions for collecting and submitting your soil samples if you want more guidance.

mixing up npk test soil amendments

Interpreting Your Soil Test

Getting an NPK test or more in depth test will allow you to have a clearer picture of what exactly is going on in your garden soil. If you go with Soil Test Option A, NPK test through your local cooperative extension lab, they’ll send you a list of recommendations for your soil with the test results.

This document from the University of Wisconsin explains how to interpret the analysis.

If you choose Option B, the Logan Labs test, the report you get back will be more comprehensive than the NPK test results from a university extension office. See a Logan Labs sample report here.

As you can see, this report can be confusing. What does it all mean?

If you really want to dive into everything in the soil test, you could grab a copy of The Intelligent Gardener and the author walks you through all of the specifics. 

In all honesty, I like to cut to the chase. When I received my results I really just wanted to know what I needed to be adding to my soil to create a more healthy environment for my plants. 

So, I skipped to the next step as recommended in The Intelligent Gardener, which is to input my results into an online calculator that would spit out the exact nutrients I needed to add to my garden.

The co-author of the book, Erica Reinheimer, a soil analyst, has created her own program, OrganiCalc, to do exactly this.

poorly growing broccoli plant

Stunted broccoli plant in my garden due to poor soil.

How to Submit Your Results

There are several different options for testing and analysis on Erica’s Grow Abundant website. 

The Basic Package, which is recommended for home gardeners, includes a Logan Labs soil test, amendment recommendations, and written comments. You’ll be able to communicate directly with Erica if you have questions.

The Soil Amendment Recommendations Package is for when you already have a Logan Labs soil test and just want them to give you some recommendations and written comments.

Or you can choose their DIY Package and subscribe to OrganiCalc, which is a program that allows you to input your results from the soil test and automatically outputs the soil amendment recommendations. You can use it for a year as many times as you’d like.

You can also book Erica for a consultation if you want her help in explaining your soil test results and coming up with a plan. I booked a consultation with her one spring when to assist me with improving the soil in a particularly poor performing corporate garden I was hired to plant and maintain. She was a big help and very knowledgeable.

adding fertilizer to garden after npk soil test

Amendments For Your Garden

Basically, all of these steps are leading to the recipe of amendments you need to add to your garden beds to build the nutrient density of your soil and cultivate a more successful garden.

Here’s an example of the report that’s generated by OrganiCalc. 

The most important section is at the bottom, excerpted below. 

It’s a custom recipe created just for your garden based on your soil test results. This is exactly what you need to add to your garden.

garden soil test


For my soil, it’s recommending I add specific amounts of:

Azomite for trace minerals and micronutrients

Gypsum for Calcium

Feather Meal for Nitrogen

Elemental Sulfur for Sulfur



Borax for Boron

As you can see at the bottom of the table, the total is about 4 lbs. for every 32 sq. feet, which is a 4′ x 8′ raised garden bed. 

amendments for npk test

Sourcing & Mixing Amendments

If you’re up to this challenge of mixing your own recipe, you’ll need to source everything you need. Since some of these amendments are pretty heavy, I suggest trying to find as many of them locally as possible. The best places to purchase them are farm supply stores, grower/hydroponic suppliers, and garden centers.

(If you live in the Madison, WI area, I’ve found much of what I need at Jung’s Garden Center, The Bruce Company, Paradigm Gardens, and Midwest Bio-Ag.)

The more obscure micro-nutrients can be ordered online in smaller quantities. I usually order from Black Lake Organics. The OrganiCalc website also has a list of places to source ingredients.

I store all of my amendments in a plastic tub in the garage. I also have a digital scale to measure out the weights.

When I’m getting ready to plant in my garden, I mix up a few batches in a 5-gallon bucket.

adding fertilizer to garden bed after npk test

Applying Amendments to the Garden

The best time to add amendments to your garden is about two weeks before planting. Personally, I find it easier to do this right before planting since my beds are always mulched and I don’t usually prepare them until I’m out getting ready to plant.

Ideally, you would mix the amendments with compost and spread the mixture on your garden bed and lightly dig it in to the top six inches. I don’t always have compost available, so sometimes I just use the plain fertilizer mix.

If I’m preparing an entire bed for planting I spread the mix over the top of the bed in the recommended amount. Then I use a hard rake to lightly mix it in. Once the bed is prepped I simply go about planting seeds and plants.

When I plant seedlings I usually don’t prepare the entire garden bed, I simply dig holes with a trowel at my preferred spacing and put a handful of the mix into the hole.

You can see how I prep spring garden beds in this article and accompanying video.

Each time I replant in a garden bed, sometimes several times during a season, I add more organic fertilizer mix before planting.

woman in garden with carrots

Summary: What I’ve Done

This likely all sounds like a lot of work, but believe me when I tell you it’s made a HUGE difference in the health and quality of my garden. Most people who visit my garden remark on how big, lush, and productive my plants are. 

This is because I’ve taken the time to build up my soil fertility.

My experience with pretty much every garden I’ve ever had is that the soil has been lacking in nutrients. So, it’s been worth it to me to put in the time and energy to get to know my soil better and create custom mixes based on the results of my soil test.

I’ve been at my current house for eight garden seasons. I started with a Logan Labs soil test that first year and began to create my own fertilizer mix. Every few years I re-test my soil and adjust the mix based on the results and the recipe that OrganiCalc provides me. 

I continue to add my custom amendment mix all season long, every time I plant, because of the amazing results I’ve seen over time.

This Sounds Too Complicated! Other Options?

I know this is a big topic, so thanks for sticking with me!

If the NPK test or Logan Labs test seem like too much work, but you think your garden is lacking in nutrients, my suggestion is to start with simply adding a commercial organic fertilizer. This is a good place to start.

If adding organic fertilizer to your garden results in healthier and more robust plants, then you can simply continue down this route. Everything you need to know about how to use it and what kind of organic garden fertilizer to purchase. 

If you add an organic fertilizer and still think your soil is lacking in nutrients then you can pursue a soil test.

Conclusion and Warning

As I wrap up this article, I want to leave you with a caveat.

The way that all of these nutrients interact is very complex, so you should not go and start adding things to your garden willy nilly. That’s what I did the first time I discovered I had poor soil at my last house, which wasn’t a good idea.

You can do real damage to your plants by mis-using these amendments, so your two choices are to buy a premixed organic garden fertilizer or submit a soil test and follow the recommendations. Anything else is playing with fire!

At the very least you should get an NPK test. And if you have the interest, a more extensive test is preferable. 

There’s a bit of a learning curve when understanding soil nutrients, but once you go through the process of testing and amending your soil it’s an easily repeatable process from year to year. And the results you’ll see in your garden will speak for themselves!





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