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Soil Amendments for the Garden
submitted by Sarah Browning, UNL Extension Educator
Fall is the best time of year to improve the soil in your garden beds for several reasons. First, spring rains often hamper a gardener's ability to get into the garden and work the soil as needed to prepare for planting. Fall preparation gets your gardens into a "planting ready" state, so you can begin a new gardening season whenever spring temperatures are favorable, and take advantage of spring moisture, which may be particularly important for gardening success next spring, since soils are so dry following summer's hot, dry conditions. Adding soil amendments in fall also takes advantage of winter's repeated freezing and thawing of the soil, to gradually mix and incorporate them.
Changing Soil pH
Next, if you need to add sulfur or lime to change soil pH, fall application allows time for the chemical reactions to take place that are necessary for a pH change to occur. Sulfur is used on alkaline soils, lowering soil pH to a ideal range for plant growth, typically pH 6-7. Soil pH is measured using a logarithmic scale, so pH 7 is ten times more alkaline than pH 6. Below pH 5.5 or above 7.5, soil modification may be necessary to grow pH sensitive plants such as pin oak, river birch, rhododendrons or azaleas, because soil pH directly influences the availability of many nutrients. But gardeners can also work with the existing soil, and choose plants adaptable to higher pH, avoiding then need for pH adjustment in many cases. Many eastern Nebraska soils that have not been farmed are naturally pH 7.5 or above.
Elemental sulfur or aluminum sulfate are most commonly used to lower soil pH, however, large amounts of sulfur are required to make even a small change. For example, a loam soil with pH 7.5 requires 15-20 pounds of elemental sulfur per 1000 sq.ft. to reach soil pH 6.5. Modified soil will revert in time to it's original pH level, so it is often better to use pH adaptable plants in your landscape than to use plants with more strict pH requirements. Lime is used on acidic soil to raise pH. Land that has been farmed may be pH 5.5 or below, and could benefit from the addition of lime.
Always base your addition of sulfur or lime on soil test recommendations, rather adding products on a hunch.
Soil Amendments to Avoid
Some soil amendments are not recommended for Nebraska soils. For example, wood ashes are very alkaline, which isn't helpful when added to Nebraska's typical alkaline soil. Sand added to heavy Nebraska clay soil creates a texture similar to concrete, so don't do it! Gypsum is another soil amendment commonly discussed by homeowners to "sweeten" soil or lower soil pH, but in fact it does neither and is seldom a helpful amendment for Nebraska soils. Gypsum should only be used on soil with high levels of salt, where is can bind with the salt molecules and aid in moving them out of the soil profile.
If you're applying manure to provide nutrients or increase soil organic matter, fall applications are very important. Animal manures often contain microorganisms that are harmful to humans, such as Salmonella and E. coli. For that reason, the use of fresh manure is not recommended in the vegetable garden where microorganisms could contaminate food as it is grown and harvested. Fall manure applications give additional time for harmful microorganisms to die before a new crop is grown.
Composted or aged manure, which has been allowed to sit and begin to break down for at least a year, is a better choice for the vegetable garden. If fresh manure must be used, make sure it is not applied to the soil within 120 days of the next harvest. Horse, cow, sheep, or poultry manure is fine, but do not use swine manure because of the higher potential for contaminants.
So while our great fall weather continues, there is still time to improve your garden soil and ensure better growing conditions next year.
University of Nebraska-Lincoln Extension in Lancaster County is your on-line yard and garden educational resource. The information on this Web site is valid for residents of southeastern Nebraska. It may or may not apply in your area. If you live outside southeastern Nebraska, visit your local Extension office
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