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Soil testing in fall gives a head start on next year’s garden

St. Paul campus scene featuring the sign outside the soil testing lab. The sign reads, "Soil testing. Sample drop off."

If you’re new to gardening and lawn care, you might have questions about how to care for your soil and make sure it provides the nutrients your plants need.

How do I find out if my soil needs nutrients?

Did I add too much compost or manure this past year?

If fertilizer is needed, how do I choose the right product?

The University of Minnesota Soil Testing Laboratory has answers to these questions and more. Not only is the Soil Testing Lab an unbiased source of information, but they also provide recommendations and calculators to help make growing productive lawns and gardens easy. 

Soil sampling is the key to proper nutrient management, and fall is a great time to take a soil sample. This will save you time in the spring and you can get a head start on next year’s lawn and garden.

“The nutrient status isn’t going to change,” says Keith Piotrowski, a soil scientist and manager of the Soil Testing Lab. “You can go out in the middle of October or whenever fall actually hits, collect a sample and send it to us. We will get your report back to you in approximately two weeks.” 

Why take a soil test?

Soil tests provide a snapshot of your soil’s current nutrient levels, helping you to make smart decisions about whether to apply compost, manure or fertilizer and how much to apply.

The three main nutrients that plants require for healthy growth are nitrogen (N), phosphorus (P), and potassium (K). Although they are already present in your soil to some degree, your N-P-K levels may not be optimal for plant growth. Your soil might lack potassium, for example, or you might have more than enough phosphorus already. The goal is to make sure your plants are the most productive they can be without applying too many nutrients, which can lead to environmental issues.

“If you're growing tomatoes, you want the most tomatoes per plant; the healthiest, the best tasting. If you're growing flowers, you want the prettiest flowers. If you're growing turf, you want the healthiest turf,” explains Piotrowski. “By having a soil test done, you can get a snapshot of the nutrient status of your garden or lawn. It’s a good way to check and make sure that there are sufficient nutrients there for optimal growth.”

How to take a soil test

The soil sampling process itself is quite straightforward. The Soil Testing Lab’s Lawn and Garden webpage is the best place to begin. There, you can download and print the Lawn & Garden Soil Analysis Request Sheet. This form walks you through how to take a soil sample and how to submit samples to the lab. You can also pick up this form, as well as soil sample collection bags, from your local U of M Extension office. This short video shows you exactly how to take a soil test from your garden: 

Once you have submitted your sample to the lab, it typically takes approximately two weeks to receive your results. The Soil Lab tests about 6,000 samples every year from the public. Many are home growers and gardeners, but the lab also tests soils for researchers from all over the country – Massachusetts, Texas, and even Hawaii. They even verify other labs’ results. 

The regular soil test is sufficient for most lawns and gardens. But you can also test for soluble salts, lead, and trace elements (sulfur, micronutrients, etc.). High soluble salt levels can be bad for plant health and yield while exposure to lead in soil is a health concern for humans, especially young children and pregnant adults. Trace elements are important for some crops. These tests require additional fees (see the request sheet for more information). 

Understanding your soil test report

The image below is what your soil test report will look like. The regular test provides your soil’s N-P-K levels, as well as organic matter content, pH level, and estimated texture. The Soil Testing Lab’s fertilizer recommendations are highlighted in this sample report below:

Example of a soil test report with a red box drawn around a section at the bottom of the repot that says in handwriting "Your soil's results."

For most home growers, the lab’s N-P-K results provide enough direction to begin to amend your soil. For more information on interpreting your soil test results, visit the Lawn, Garden and Landscape Plants page on the Soil Testing Lab’s website or dive into this Extension publication: Soil Test Interpretations and Fertilizer Management (PDF).

Buying and applying fertilizer, manure or compost

Two large bags labeled "Fertilizer". The one on the left has a label that reads "0-0-60." The other one reads, "18-46-0" indicating their amounts of nitrogen, phosphorus and potassium.

The next step is to decide whether you’re going to add fertilizer, manure or compost to your garden. 

Unlike bags of fertilizer, it can be difficult to know how much nitrogen, phosphorus and potassium are in your compost and manure, and how much you are applying. If you're using compost or manure, you should test your soil regularly to make sure you aren’t adding too much phosphorus, which can pollute water. In addition, because compost and manure often lack nitrogen, a soil test may recommend adding products that supply N without adding P and K that your soil doesn’t need. This recent Extension Yard and Garden post provides tips and resources on how to use compost and manure more efficiently, and how to address over-application problems.

Bags of fertilizer often have N-P-K levels printed right on the front of the bag, so it’s just a matter of picking the closest nutrient ratio to your soil test results.

If you can’t find a product with the exact N-P-K ratio from your soil test, don’t worry, this is very common, says Piotrowski. “We always say – and the terminology on the website and on the report is – get as close as you can with the nitrogen level. That's what's necessary for growth and energy within the plant. From there, you can compromise a little bit for P and K.” 

Choosing the right fertilizer product for your soil is also important for environmental reasons. If your soil test suggests a 15-20-30 ratio of N-P-K but you use a bag of 18-24-12 fertilizer, for example, it will result in too much nitrogen and phosphorus but not enough potassium. Repeated lawn and garden over-application of nitrogen and phosphorus can lead to environmental and water quality issues. “It is a nightmare for the environment,” says Piotrowski. “You don’t want to overdo it.”  

Soil cannot only be over-fertilized, but it can also be under-fertilized. Soil that requires a 15-20-30 recipe but receives 10-10-10 (a common starter fertilizer) could result in your plants not having enough nutrients to reach their full potential. Even straightforward usage of your soil season after season can lead to depleted nutrients, according to Piotrowski. “Long-term growth, long-term farming or gardening, you run the risk of depleting certain resources.”

Extension’s website also has resources on managing soil and nutrients that will help you fertilize and manage your soil fertility based on the specific crop you want to grow. 

One issue you may run into when it comes time to apply fertilizer to your lawn or garden is deciding how much to apply. The instructions on the bag of fertilizer may not provide information on your garden size. If that’s the case, the Soil Testing Lab has a very helpful fertilizer calculator that converts pounds of fertilizer per acre to ounces of fertilizer per square foot – perfect for urban lawns and gardens. 

Still have questions? We can help

If we haven’t answered your question, don’t hesitate to reach out to your local Extension educator for help. You also can Ask a Master Gardener to get research-based answers to your specific questions. And Extension's Yard and Garden pages can help you identify and solve problems with your plants with our growing and watering guides.

Authors: Jack Wilcox and Paul McDivitt, UMN Extension communications

University of Minnesota Extension’s urban nutrient management communications is supported in part by Minnesota’s fertilizer tonnage fee through AFREC, the Agricultural Fertilizer Research and Education Council. Learn more at MNsoilfertility.com.

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