When and How to Amend Soil

A gardener amends their soil.

Depending on where you live, you might have rocky, heavy clay, sandy or loamy soil. However, the fruits, vegetables and flowers in your garden require lots of nutrients to perform their best. These nutrients are depleted year after year as you harvest crops and may no longer be present in your natural soil.

Horticulturists often speak of amending the soil, but what is a soil amendment? Here's a rundown on how to amend soil in your garden and why you should be doing it every year.

What Is Soil Amendment?

One could argue that there's really no such thing as "bad" soil since plants, fungi and bacteria have evolved for millions of years to fit various niche environments. Even the most inhospitable soils are teeming with life! But, simply put, a soil amendment is anything added to the soil to improve its physical properties.

Amending the soil is often confused with mulching. Mulching is adding layers of mulch, rock or sand to the surface of the soil to inhibit weed growth, keep soil in place, retain moisture or create a certain aesthetic. Mulching your soil can, but doesn't necessarily, add nutrients to the soil.

Soil amendments can be organic or inorganic, often improving the nutrient content in the soil as well.

Organic Amendments

Organic amendments can include compost, leaf litter, manure or other organic matter. When adding organic matter to the soil, make sure what you're putting into your soil has been properly treated. For example, compost — especially compost prepared at municipal facilities — is prepared by mixing together yard waste from many sources. This is a great recycling of resources, but you'll want to be sure the compost has been properly heated to kill off pathogens and seeds. Likewise, when using manure, use only aged manure. Fresh manure often contains large amounts of salts, which can be detrimental to garden plants and difficult to leach from the soil once it's been added.

Inorganic Amendments

Inorganic amendments include perlite, vermiculite, sand, biochar and granite. The type of soil you have will determine the right mix of both organic and inorganic additives. If you have too much sand, the soil may drain too quickly and leach nutrients. Soil with too much clay can trap water and the mineral salts hard water contains. Ideally, garden soil should be rich, loamy and free of foul odors. When you take the soil into your hand and squeeze it into a ball, it should hold together initially but crumble when touched.

A third additive worthy of mention is garden probiotics. Just as your digestive system requires "good" bacteria to function properly and to support your immune system, bacteria and fungi do the same in soil. You can easily add probiotics to the soil by purchasing packs of organic fertilizers that contain them.

How to Amend Soil

To amend soil and attain that rich, loamy property best suited for your favored plants, you first need to know what kind of soil you have. Here are a couple of insights into amending more difficult soil types. Remember, amending the soil takes time and patience. It could take a couple of years to get unhealthy soil back into shape.

Sandy/Rocky Soil

Take a handful of slightly damp soil from your yard and squeeze it into a ball. Now release and watch what happens. Does the soil easily fall apart? If you pour water onto the soil, does it quickly drain through? If so, you probably have soil with a high sand or gravel content. To better lock in moisture and nutrients, mix in organic materials such as compost, leaves or leaf mold, composted straw or aged manure. Add a 2- to 3-inch layer to the top of the soil, then use a shovel to mix the layer into the soil about 8 inches deep. Sandy soil also benefits from a top dressing of mulch or film to retain moisture. Rich, sandy soil is great for growing root crops such as carrots, potatoes and radishes.

Clay Soil

Does water pool up in your yard? And when you grab a handful of soil and squeeze it into a ball, does it hold its shape even if dropped to the ground? Large amounts of clay in the soil can be especially challenging, but it does have the benefit of retaining moisture and nutrients, and it can be amended over time. The key is to get the fine particles to begin to form larger clumps in a process known as flocculation. You can achieve this by slowly tilling large amounts of organic matter into the soil. This process can take a few years, but you'll be rewarded with nutrient-rich, moisture-retaining soil. Soil with high clay content is good for growing shallow-rooted crops that need higher moisture such as lettuce, Swiss chard, cabbage and broccoli.

Consider adding a cover crop such as winter rye, oats or clover during the off-season. These crops are planted into the soil, allowed to grow, then plowed under in spring. They provide organic matter and encourage the growth of beneficial bacteria and fungi in the soil.

When to Amend Soil

In general, add any soil additives and amendments at the end of your growing season. This ensures that anything you add that's high in nutrient content, such as manure, has time to age and be further decomposed by soil microbes. Fall is also a good time because it's often when you might be digging up old plants and have bare soil for ease of preparation.

If you'd like to learn more about your soil, read Burpee's guide to understanding the importance of soil.

Written by Derek Carwood, Greenwood Horticulture

Derek Carwood, a native of Northern California, currently resides in the Upper Midwest and has been involved in horticulture for over 30 years.  Derek holds a Bachelor's Degree in Environmental Sciences and a Master's Degree in Sustainability Education & Policy.  He has been heavily involved in education throughout his professional career and has volunteered and worked across the Americas, Europe, and Asia.  Most recently, Derek started Greenwood Horticulture focusing on both indoor and outdoor horticultural consultation, education, and design.

October 7, 2021
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