How to amend your garden soil for next season

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Sara's vegetable garden complete with wilted squash plants on the righthand side. Sara Welch photo.

It’s not quite the end of the growing season, but it’s time to start evaluating how your vegetable garden performed and that starts with the soil.

For the past couple of years, one area of my vegetable garden has consistently underperformed. The tomatoes and peppers planted there grew spindly and weak two years ago. The beans planted there last year barely produced any pods. The squash planted this year withered and shriveled.

I thought, surely, the squash would survive — it can grow anywhere. Unfortunately, that was not the case. Few squash and zucchini were harvested from three rows of plants.

It has to be the soil. It’s the only feasible explanation for why that corner of my garden refuses to allow anything to grow.

Test the soil

Every year fertilizer is tilled into the garden and sprinkled when the seeds and starter plants are put in the ground. This year fertilizer was periodically added throughout the growing season and it didn’t change the results.

The only logical conclusion is that what’s missing from the soil was not being added — this is a common issue for gardeners trying to correct their soil without a soil test.

A soil test is the best method to determine what soil amendments your garden needs. A soil test measures phosphorus, potassium, calcium, magnesium, sodium, sulfur, manganese, copper and zinc for their plant-available content within the sample. Along with determining the amount and type of fertilizer needed to correct any nutrient deficiencies, soil tests also measure the soil pH, humic matter and exchangeable acidity. A soil test can give you a comprehensive view of your garden soil’s shortcomings and ensure you add the correct amendments to address them.

When collecting soil for a soil test, you should collect a representative sample, meaning the soil’s appearance, texture, color, slope, drainage and pest management practices should be similar throughout the area. In cases like mine, where one area of the garden has been particularly struggling or if the soil quality or characteristics are obviously different, a separate sample should be collected.

You should always label your samples and collect soil when the ground is not very wet.

Your local soil and water conservation district or extension office should be able to answer questions about having your soil tested and may even have soil test kits available.

Amendments to correct soil pH

An ideal soil pH for most plants is between 6.0 and 7.0, but many garden vegetables prefer more acid soil with a pH between 5.8 to 6.5. The results of your soil test and the growing preferences of the vegetables you plan to grow next season should help you determine how to adjust the pH of your garden soil. 

Amending garden soil in the fall is ideal because it allows enough time for the chemical reactions to occur that are necessary to change the pH of the soil.

Raising soil pH

If your soil is too acidic for the vegetables you want to plant next spring, you’ll need to raise the soil pH using agricultural limestone. 

Agricultural limestone comes in two combinations. Calcitic limestone is composed of calcium carbonate and dolomitic limestone is composed of calcium magnesium carbonate. Dolomitic limestone should be used in soils low in magnesium.

The soil testing lab will recommend your lime application rate based on your target pH and the soil test sample’s buffering capacity or tendency to resist pH change.

Lowering soil pH

Growing acid-loving plants or an abundance of calcium and magnesium in your soil may warrant lowering the pH. Elemental sulfur is commonly used to lower soil pH.

In this instance, the lab will determine the amount of elemental sulfur needed to lower your soil’s pH based on its texture, current pH and your target pH.

Elemental sulfur reacts slowly with soil, which is why it’s best to apply it in the fall. It should be worked into the soil to a depth of 6 inches. Soils with a sandy composition will require lower amounts of elemental sulfur than those that are more clay-heavy. No more than 15 pounds of elemental sulfur per 1000 square feet should be applied at once.

Organic amendments

Adding organic matter improves soil structure, increases nutrient holding capacity, improves drainage and water holding capacity, provides plant nutrients and increases biological activity in the soil. In sandy soils, organic amendments help bind the particles together and increase the soil’s ability to hold moisture and nutrients. In heavy clay soils, they bind to the small clay particles and form larger particles that have larger air spaces between them, allowing for better drainage and air circulation.

Fall is an ideal time to add manure or compost to garden soil because there’s enough time for them to break down and provide plant nutrients and other soil benefits by spring. In the case of manure, fresh applications to vegetable gardens are not recommended in the spring because food can be contaminated by microorganisms that are harmful to humans, such as Salmonella and E. coli. Applying it in the fall gives these harmful microorganisms time to die before vegetables are planted.

Manure and compost should be applied to the soil surface at a rate of ½ to 3 inches and then rototilled into the top 6 inches of the soil. These organic amendments should be worked into the soil when it’s dry to prevent layering.

Additionally, manures and composts contain high levels of phosphorus, so regular soil tests should be conducted to prevent phosphorus buildup when they are continuously applied.

Organic fertilizers like greensand, rock phosphate, kelp meal, bonemeal or bloodmeal are other organic amendments that break down slowly over several months when they are not overapplied. They can be used in place of manure or compost.

Cover the soil

Covering your amended garden soil with straw or leaves is a great way to insulate the soil and encourage worms and microbial activity during the colder months. This will help any organic matter you added to breakdown and improve soil quality come spring.

Cover crops

Cover crops are another way to improve soil health during the offseason. Much like covering your soil with straw or leaves, cover crops create a protective barrier during the colder months to encourage activity below the soil’s surface. They also pull nutrients up from the subsoil, remove excess water and return nitrogen and organic matter to the soil when you till them into the soil in the spring.

For more on planting and choosing cover crops, read How to plant a cover crop in your vegetable garden.

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Sara is Farm and Dairy’s digital editor. Raised in Portage County, Ohio, she earned a magazine journalism degree from Kent State University. She enjoys spending time with her daughter, traveling, writing, reading and outdoor recreation.

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