Talking with U of M

Talking soil health with U of M

Credit: Tejvan Pettinger

Spring is here and many Minnesotans will start tending to their gardens. Anne Sawyer, an Assistant Extension Professor at the University of Minnesota, answers questions about soil health and how it translates to healthy plants this growing season. 

Q: What should a home gardener know about soil health? 
Dr. Sawyer:
One of the most empowering things to know is that no matter how large or small your garden is, you are able to do good things for your soil! Your soil is alive, full of amazing microbes like bacteria and fungi, bugs, worms, spiders, and so much more. All of these organisms interact with each other and with plants to create a soil ecosystem. The more diversity we have in the soil ecosystem, the more it’s able to fend off “bad” bugs and plant diseases. In this way, gardeners are almost like farmers, feeding and tending the soil “livestock."

And the best part is that caring for your soil not only helps you to grow healthy plants, but can save you money and also protect the environment. A healthy soil can retain, recycle, and provide nutrients, allowing you to reduce your fertilizer use and, in turn, help prevent excess fertilizers and nutrients from getting into our water. The trick is to test your soil on a regular basis so you know which nutrients your soil can supply to plants, and which you may need to supplement with fertilizers.

Q: What is soil testing? Who should get their soil tested?
Dr. Sawyer:
Soil testing, in a general sense, is the process of collecting a representative sample of your soil and sending it to a laboratory that will analyze the characteristics most important for plant growth. The laboratory will then make recommendations for fertilizers and/or lime (to adjust pH) that are uniquely suited to the soil you have and the plants you want to grow. Soil tests can also look for contaminants, such as lead, that may impact human health.

Every gardener should get a soil test! If you haven’t had one yet, now is a great time to get one. Repeat your soil test every three years or so; you may want to test more or less often depending upon how intense your production is and/or how quickly conditions change in your soil. The University of Minnesota Soil Testing Lab is a great option for a soil test, but there are several other labs in Minnesota that offer soil testing, too. 

One of the most important aspects of a soil test is collecting a good sample. First, define the area you want to represent with a sample, ideally one that’s relatively uniform in soil type and growing conditions. Then, dig a hole to a depth of about six inches and use a spade or trowel to collect a vertical “slice” of soil from the side of the hole. Do your best to remove surface debris, like leaves or grass, from the “slice”, and put the “slice” in a bucket. Repeat this in five-10 randomly selected places around your garden or lawn. Thoroughly mix all of the “slices” together and remove about two cups of soil from the bucket. That’s the “sample” you’ll send to the lab.

For more in-depth information about soil testing, including how to collect a sample, check out “Soil Testing on Fruit and Vegetable Farms”. Also, here’s a neat U of M Extension video about why you should do a soil test.

Q: How can we improve the health of our soil? 
Dr. Sawyer:
There are several things we can do to improve soil health. The first is to feed your soil organisms by adding organic materials for them to “munch” on. Examples would be incorporating compost, using cover crops, or using organic (carbon-based) fertilizers. These organic materials, when decomposed by soil organisms, replenish soil nutrients that are removed by harvesting crops. 

Another thing to do is to protect the soil from erosion. Rain and wind can transport soil particles away from our fields and gardens, taking with them soil nutrients and organic matter. Not only does this impact our soil fertility, but it can also add to nutrient pollution in our waterways. Consider using some kind of mulch (straw or grass clippings) or living cover (such as cover crops between rows of peppers) to keep your soil in place.

It’s also a good idea to minimize the amount of disturbance to your soil. Try to avoid excessive tillage in the garden. For example, try tilling only where you’re planting, such as tilling in beds but not tilling walking areas between beds. Or consider trying hand tools, such as a spade or broadfork, to turn the soil without pulverizing it. You could even experiment with no-till gardening. Tillage breaks up the soil’s natural structure, exposes organic matter to decomposition, disturbs the soil organisms, and can make your soil more prone to erosion. 

And finally, test your soil to be sure you’re not adding too much fertilizer or organic materials (yes, you can overdo the compost - read more here!).

Q: What should we know about fertilizer? 
Dr. Sawyer:
I like to focus on helping people understand that while soil supplies lots of nutrients for plants, not all plants are able to get all of the nutrients they need from the soil alone. Some nutrients, particularly those that plants need the most of (nitrogen, phosphorus, and potassium), may need to be supplemented with fertilizers. That’s why a soil test is so important; you want to give your plants what they need to be their “best selves”, but you don’t want to apply too much because it can harm your plants and the environment. Here’s a quick primer I wrote about fertilizing in the home vegetable garden that talks about the nutrients that plants need, the nutrients that soil has, differences between organic and inorganic fertilizers, and much more.

Another thing to know about fertilizer is that finding the right one can feel overwhelming. Even when you get a soil test, the recommendations on the test may not match the fertilizers that you find at the garden store. A general rule of thumb is to match the nitrogen recommendation as closely as possible, but do your best to not add any more phosphorus than is needed in order to protect water quality. Potassium is more forgiving; it’s OK to be a little over or under with potassium. Note that the fertilizer bag or box will list the nitrogen (N), phosphorus (P), and potassium (K) content in that order (N-P-K).

Q: What does your work in soil health show? 
Dr. Sawyer:
My position is mostly focused on outreach and education, so I don’t really do research - and that’s fine by me! What excites me most about my work is helping people care about and take action to preserve and improve soil health and, by extension, water quality. I love working with home gardeners and gardening groups, in particular, because they are always fascinated by how interesting soil actually is. I can’t tell you how many times I’ve heard people say that they expected my soil talk for their gardening group to be really boring, but were pleasantly surprised by how interesting soil actually is. I never tire of hearing that!

Dr. Anne Sawyer is an Assistant Extension Professor at the University of Minnesota. Anne earned her PhD in Soil Science from the U of M in 2017, where she studied nutrient management and microbiology in perennial grasses grown for bioenergy. Her previous work experience includes Extension (horticulture), the National Weather Service (snow hydrology) and the National Park Service (resource interpretation).

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