How to Improve Garden Soil With Amendments

Gardener Preparing Raised Beds with Rake in Vegetable Garden
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Dirt by any other name will still stain your clothes, but for gardeners, that dirt becomes soil as soon as there are plants growing in it. Although garden-variety soil seems ordinary, it is actually a complex mixture of mineral particles, elemental nutrients, and organic material that combine to form a medium that keeps plants upright, channels water and air to their roots, and offers them the nutrients they need to grow. No gardener who has struggled with poor soil will ever again take it for granted. Taking the time to improve or preserve your garden soil can spell the difference between a vibrant, green garden and one that languishes.

The qualities that make for good garden soil fall into two categories: good fertility and good texture. Fertility is a combination of essential nutrients and a soil pH level that makes these nutrients readily available to the plants. Texture refers to the size of the soil particles and their cohesiveness, and the soil's ability to transfer water and air.

How Plants Use Nutrients

Soil nutrients come from many sources, including decaying plant material, soil organisms, and fertilizer. The three primary nutrients used by plants are nitrogen, phosphorus, and potassium.

  • Nitrogen is largely responsible for healthy leaf and stem growth. In the soil, nitrogen is made available to plants by nitrogen-fixing bacteria that convert nitrogen into nitrates, a form that plants can use. Nitrogen does not remain in the soil for long. It gets used up by your plants and by decaying matter in the soil. It is also water soluble and can wash out of the soil rather quickly. Although some nitrogen is absolutely essential, an excess will cause a lot of foliage growth at the expense of flowers and fruit.
  • Phosphorus is very important for root growth. Flowering bulbs and root crops can always use some phosphorous. That's why bone meal is often recommended for fall bulb planting. Phosphorus also is crucial for producing flowers and you will sometimes see fertilizers with a high phosphorus content advertised as flower boosters.
  • Potassium is needed for overall plant health. It keeps the plants growing and aids their immune systems. Like nitrogen, potassium is also water soluble and needs to be replenished from time to time.
  • Trace elements: Besides the three primary nutrients, there are several trace elements that are necessary for good plant health, including calcium, magnesium, zinc, and molybdenum.

Why pH Matters

A lot of fuss is made over soil pH. In layman's terms, pH is a measure of the soil acidity or alkalinity. The scale goes from 1.0 to 14.0, with 7.0 being neutral. The lower the numbers go down from 7.0, the more acidic the soil. The higher they go above 7.0, the more alkaline the soil is. The reason soil pH matters is that nutrients in the soil are only available to plants if the soil pH is within a certain range. Many plants like a pH in the low acidic to a neutral range (6.2 to 6.8), but that's not true for all plants. Rhododendrons, heathers, and blueberries favor fairly acid soils, and lilacs and clematis will thrive in alkaline or even chalky soil. The only sure-fire way to know where your soil's pH falls is to have it tested. Keep in mind that it takes time to alter soil pH and your soil will tend to revert to its old pH over time, necessitating repeated treatment.

Why Soil Texture Matters

The ideal soil for growing plants holds moisture long enough for plants to absorb it, but not so long that the plant roots soak in standing water and drown or rot. It is also loose (friable) enough for the roots to get some air. "Soil texture" is the term used to describe how soil handles water and air. Although there are other factors at play, it is the size of the mineral particles in the soil that have the most impact on this.

Soils with very fairly large particles are described as sandy. Water, air and plant roots can move freely in sandy soils, sometimes too much so. At the other end of the spectrum is clay. Clay particles are so small they pack together tightly and leave little room for water, air, or roots. If you've ever tried to garden in baked clay you know it also leaves little room for a shovel blade. An easy test for soil texture is to make a ball of damp garden soil. If it breaks apart easily when you tap it, it's sandy. If you can press it between your thumb and finger and make a ribbon, it's clay.

Good garden soils are somewhere in between sand and clay. The ideal soil is a form called a sandy loam. Ideal soil should be light enough to allow for air and water movement, but it should also have some tilth—a kind of fine breadcrumb-like texture that occurs when there is plenty of organic matter in the soil.

Don't try to change your soil texture by adding sand to clay or vice versa. That is a recipe for cement. Some amendment recommendations for clay do include a portion of very fine sand, but there are better ways to change your soil texture.

When to Improve Garden Soil With Amendments

Garden soil should be amended when there is either a nutritional or pH deficit or texture problems that keep plants from growing well. The best time to amend garden soil is when a garden bed is first being established. In an ongoing garden, some form of amendment is an ongoing garden task, even if it's as simple as digging in some compost prior to each year's planting duties. And garden soils may require repeated amendment as the nutrients are consumed and the organic material breaks down.

Project Metrics

  • Working Time: About 1 hour to amend a 10 x 10 garden
  • Total Time: Some amendments will offer their benefits for 2 or 3 years before a reapplication is necessary. Fertilizers, where needed, typically need to be applied at least yearly.
  • Project Cost: Five pounds of garden lime cost about $15; five pounds of garden sulfur costs about $20 to $25. Fertilizer costs vary.

What You'll Need

Equipment/Tools

  • Garden shovel or tiller

Materials

  • Lime or sulfur (as needed)
  • Fertilizer
  • Organic material (such as compost, manure, or peat moss)

Instructions

  1. Evaluate Your Soil

    The only definitive way to know your soil is poor is to have it tested. Your local Cooperative Extension Service probably 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 give recommendations on what amendments to use, and in what quantities, to correct any deficiencies.

    A quick guesstimate of your soil's health can be made by looking at your plants' health. If they are thriving, don’t fix what isn’t broken. If your plants are languishing, yellowing or otherwise looking sickly, or you feel like you are forever feeding them, it would be worth testing your soil. It is best not to guess when it comes to amending your soil since it can be very hard to identify just what is wrong with it. What appears to be a nutritional deficiency calling for fertilizer, for example, may turn out to be caused by pH issues.

  2. Adjust pH (if Necessary)

    Soil pH is critical because plants cannot properly take up soil nutrients unless the acid/alkaline levels are in their ideal range. If a soil test shows that your pH is off, you will get a recommendation for adding either lime to raise the alkalinity, or sulfur to lower the soil pH. This is easy enough to do, and in an existing garden, it should be done in stages, so as not to shock the plants. Generally, it is recommended that you not add more than 5 pounds of lime or sulfur per 100 square feet of the existing garden. If you are creating a garden for the first time, you can go ahead and dump in the whole recommended amount as you prepare the area for new plants.

    Rake the lime or sulfur out over the garden, then dig it in thoroughly with a shovel or garden tiller.

    You should check the soil pH every couple of years, as the sulfur or lime will get consumed and need to be replenished. For vegetable gardens, for example, a slightly acidic soil works best, and lime may need to be reapplied every 2 to 3 years.

  3. Adjust Soil Texture With Organic Material

    Proper soil texture is essential to allow plant roots to take up moisture and air. Dense, "clayey" soils can remain too moist, causing roots to literally drown, while sandy soils may drain too quickly for roots to find and absorb moisture. The very best way to improve soil texture is by adding organic material, such as compost or peat moss. Organic matter is dead plant or animal material. There is always some organic matter in your soil, but usually not enough for a plant's needs. Decaying organic matter, or humus, will help give your soil tilth. It helps sandy soil by retaining water that would otherwise wash away and it corrects clay soil by making it looser so that air, water, and roots can penetrate. In all soils, it encourages beneficial microbial activity and it provides some nutritional benefits.

    Common forms of organic material are used to amend garden soil:

    • Compost makes an excellent amendment, and if you are composting your garden waste, it’s free.
    • Manure can often be obtained from local farms and stables. Manure should be composted and decomposed until it turns dark, crumbly and odorless. Fresh manure has too much ammonia in it and can burn your plants and offend your neighbors.
    • Peat moss is cheap and works well to loosen the soil. It is also very dusty. Wet it first to make it easier to work with.
    • You can even work grass clippings and other plant debris directly into the garden bed to decompose slowly. Be sure whatever you put down is free of seeds and has not been treated with pesticides or weed killers.
    • Cover crops ("green manure") are crops grown on unused soil with the intent of tilling them in and letting them decompose in the garden. The roots keep the soil loosened as they grow, 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.
    • Add Nutrients

    Plants require chemical nutrients, and this is where fertilizers come in. Many garden soils offer perfectly good nutrition without further feeding, especially if they contain lots of organic material. But if a soil test or poor plant performance indicates the need for fertilizer, you’ll have the choice of natural organic products or inorganic fertilizers that are synthetically produced. Inorganic fertilizer has some pluses in its favor. It is usually cheaper than organic fertilizer and it acts more quickly. However, it does nothing for the soil and in some cases actually damages the soil with its higher salt content. Inorganic fertilizers don’t actually amend the soil—they simply feed the plant. There have also been some studies that claim plants build up a resistance to inorganic fertilizers.

    Organic fertilizers are plant- or animal-based. They 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 period of feeding.

    Fertilizers vary in the type of nutrients they contain and in what ratio and quantity. A complete fertilizer is one that contains all three primary nutrients: nitrogen, phosphorus, and potassium. Different plants have different nutritional needs, but in most cases, a balanced complete fertilizer will be the type to use. You will need to learn how to read a fertilizer label in order to make proper choices for your plants. Your soil test report will also make recommendations on the type and quantity of fertilizer to use.

    You can also get supplemental nutrition from products such as manure and fish emulsion for nitrogen, bone meal for phosphorus, and wood ashes for potassium.

    Follow label directions for application of organic or inorganic (synthetic) fertilizer. Some granular forms are mixed into the soil, while water-soluble fertilizers are applied with a sprayer or watering can.

    Adjusting your soils pH, fertility, and texture to your plants' liking is the ultimate goal in making good soil through amending. All soil will benefit from the addition of organic matter. How well your soil incorporates that organic matter will determine how much supplemental feeding, if any, is necessary. Your plants will determine what your soil’s pH should be.

    Tips for Improving Soil

    • Adding compost or another organic material is the single most important way to amend soil. A yearly application of compost, for example, may eliminate the need for all other forms of amendments.
    • Plant-based composts are lower in salts than those containing decomposed animal manures. These composts are better for improving soil texture, although both do a good job of providing nutrients.
    • A surface mulch is not a true soil amendment, but it does serve that function if it is dug into the soil once it has broken down. For example, a thick layer of leafy matter applied as a mulch becomes an excellent amendment if it is dug down into the soil after it decays. An annual routine of digging in old organic mulch before applying a fresh layer makes for excellent ongoing garden soil.