Although garden soil seems ordinary, it's actually a complex mixture of organic materials, minerals, and other nutrients. It keeps plants upright, channels water and air to their roots, and feeds their growth. The qualities that make for good garden soil fall into two categories: fertility and texture. Fertility is a combination of essential nutrients and a soil pH level that makes these nutrients available to plants. Texture refers to the size of soil particles, soil cohesiveness, and the soil's ability to transfer water and air. If you have poor soil, there are certain steps you can take to improve its fertility and texture.
When to Improve Garden Soil With Amendments
The best time to amend garden soil is when you're first establishing a garden bed. In an existing garden, soil amendments typically are an ongoing task, even if it's as simple as digging in some compost prior to each year's plantings. If your plants are growing well, there's usually no need to touch the soil. But if there's room for improvement, it's probably time to learn more about your soil and take action.
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Equipment / Tools
- Garden shovel or tiller
- Gardening gloves
- Lime or sulfur (as needed)
- Organic material (such as compost, manure, or peat moss)
Evaluate Your Soil
The only definitive way to know your soil quality is to have it tested. Your local Cooperative Extension Service likely provides this service for a nominal fee. Many nurseries also test soil. The soil report you receive will give you a wealth of information on your soil's texture, pH, and nutritional composition. It will even offer recommendations on what amendments to use, and in what quantities, to correct any deficiencies.
You can make a guesstimate of your soil quality by looking at your plants' health. If they are thriving, don’t fix what isn’t broken. But if your plants are yellowing or otherwise looking sickly, it could be worth testing your soil. It's best not to guess when it comes to amending your soil because it's difficult to identify exactly what it needs. For example, what appears to be a nutritional deficiency calling for fertilizer might turn out to be a pH issue.
Adjust the Soil pH
Soil pH is critical because plants can't properly take up nutrients unless the acid/alkaline levels are in their ideal range. If a soil test shows your pH is off, you will get a recommendation to add either lime to raise the pH or sulfur to lower it. In an existing garden, this should be done in stages, so you don't shock the plants.
It's generally recommended that you don't add more than 5 pounds of lime or sulfur per 100 square feet of the existing garden. If you're creating a new garden, go ahead and add the whole recommended amount. Rake the lime or sulfur over the garden, and then dig it in thoroughly with a shovel or garden tiller. Check the soil pH every couple of years, as the sulfur or lime will get consumed and need to be replenished.
Adjust the Soil Texture With Organic Material
Proper soil texture is essential to allow roots to take up moisture and air. Dense, clay-type soils can remain too moist, causing roots to drown, while sandy soils can drain too quickly for roots to absorb moisture.
The best way to improve soil texture is by adding organic material, such as compost or peat moss. Decaying organic matter helps sandy soil by retaining water that would otherwise drain away. And it corrects clay soil by making it looser, so air, water, and roots all can penetrate. Plus, in all soils it encourages beneficial microbial activity and provides nutritional benefits.
Common forms of organic material to amend garden soil include:
- Compost: Compost makes an excellent amendment, and it's free if you're composting your garden waste and kitchen scraps.
- Manure: You often can obtain manure from local farms and stables. It should be composted until it turns dark, crumbly, and odorless. Besides the smell, fresh manure has too much ammonia and can burn your plants.
- Peat moss: Peat moss is cheap and works well to loosen soil. It's also very dusty, so wet it first to make it easier to work with.
- Grass clippings: You can work grass clippings and other plant debris directly into a garden bed to decompose slowly. Be sure whatever you put down is free of seeds and hasn't been treated with pesticides or herbicides.
- Cover crops: Also known as green manure, cover crops are grown on unused soil with the intent of tilling them in and letting them decompose. The roots keep the soil loose, and the plants suppress weeds. Cover crops from the legume family, such as clover and vetch, also add nitrogen to the soil. Cover crops are most often used for vegetable garden sites.
Many garden soils offer perfectly good nutrition, especially if they contain lots of organic material. But if a soil test or poor plant health indicates the need for more nutrients, you’ll have the choice between natural organic products or synthetic fertilizers.
Synthetic fertilizer has some pluses. It's usually cheaper than organic fertilizer, and it acts more quickly. However, it doesn't actually amend the soil; it just feeds the plants. And in some cases it might damage the soil with its high salt content. Meanwhile, organic fertilizers release their nutrients over a period of time. You won't get an instant fix as you do with synthetic fertilizers. But you will get a longer, sustained feeding.
Fertilizers vary in nutrients. A complete fertilizer contains all three primary nutrients: nitrogen, phosphorus, and potassium. Plants have different nutritional needs, but in most cases a complete fertilizer will be the type to use. A soil test can make more specific recommendations on the fertilizer type and quantity. Follow label directions for the fertilizer application. Some granular forms are mixed into the soil, while water-soluble fertilizers are applied with a sprayer or watering can.
Tips for Improving Garden Soil With Amendments
Adding compost or another organic material is often the easiest way to amend soil. In some cases, a yearly application of compost might eliminate the need for all other forms of amendments. Plant-based composts are lower in salts than those containing decomposed animal manure. These composts are better for improving soil texture, though both do a good job of providing nutrients.
A surface mulch is not a true soil amendment, but it can serve that function if you dig it into the soil once it has broken down. An annual routine of digging in old organic mulch before applying a fresh layer can make for excellent garden soil.
Choosing a Soil Amendment. Colorado State University Extension