How to Amend Soil for Better Flowering Shrubs

Why Soil Comes Before Fertilizer

To amend soil for flowering shrubs, test drainage and pH first, then spread 1 to 2 inches of compost over the whole bed and till it in about 8 inches deep before planting. Backfill each planting hole with the native soil you dug out. Correct pH only when a soil test calls for it, and go easy on nitrogen, which grows leaves instead of flowers. For shrubs already in the ground, the work shifts to mulch and surface feeding.

A sparse lilac or hydrangea that will not bloom invites a quick dose of fertilizer. Clemson’s Home and Garden Information Center points out that pale leaves, short shoot growth and decreased flowering can occur for various reasons, and a nutrient shortage is only one of them. Roots sitting in waterlogged clay, or in soil at the wrong pH, cannot use the nutrients that are already there. That is why the order matters: drainage, then pH, then organic matter, and fertilizer last.

Test Drainage and pH Before You Add Anything

Iowa State University Extension describes a drainage test that takes one afternoon. Dig a hole at least 12 inches deep and 4 to 12 inches wide. Fill it with water and let it drain. As soon as it empties, fill it again, measure the water depth with a ruler, and measure again after 15 minutes. Multiply the drop by 4 to get inches per hour.

Soil that drains 1 to 3 inches an hour is desirable for most plants. Less than 1 inch per hour is poor drainage, and more than 4 inches per hour is very well drained, which usually means sandy soil that dries fast.

For pH, send a sample to your state extension soil lab. Clemson advises testing before any planting project to learn both pH and nutrient availability, and its pH guidance is to follow the soil test recommendations for any lime or sulfur. Our guide to reading a soil test report walks through the numbers that come back.

Amend the Whole Bed, Not the Planting Hole

The common habit of mixing compost into the planting hole works against the shrub. Clemson’s planting guide states that the backfill should consist of only the loosened, unamended soil removed from the hole. When you are preparing a bed for more than one shrub, it says to incorporate organic amendments throughout the entire planting area instead.

The amount is smaller than most gardeners expect. Clemson sets the limit at 10 to 20 percent organic matter by total soil volume. For a bed prepared 8 inches deep, that is 1 to 2 inches of organic matter spread on the surface and tilled in well. Where the drainage test came back poor, Iowa State recommends more: compost spread 2 to 4 inches thick and worked into the top 6 to 12 inches.

Organic matter helps at both ends of the texture range. Colorado State University Extension explains that in clay soils it improves aggregation, porosity, aeration, drainage and rooting depth, while in sandy soils it increases water and nutrient holding capacity. Sand is not a fix for clay. Iowa State’s advice for poorly drained soil is to add organic matter and not sand, and Colorado State reports that adding gravel as an amendment makes drainage worse in many cases.

If the soil drains less than 1 inch per hour even after amending, plant higher. Iowa State suggests a raised bed or berm to create the conditions the plants need. For a single shrub in poorly drained or compacted soil, Clemson says to set the top of the root ball about 2 to 4 inches higher than the surrounding soil.

Choose the Right Organic Matter

Finished compost is the safe default. Three other materials need more care:

• Wood products. Colorado State warns that wood products can tie up nitrogen in the soil and cause nitrogen deficiency in plants. Keep wood chips and bark on the surface as mulch instead of tilling them in.
• Fresh manure. The same publication says fresh manure should never be used as a soil amendment or mulch.
• Manure-based composts. Products made with manure or biosolids are often very high in salts, so use them with caution.

Adjust pH Only When the Test Says So

Clemson gives pH 5.8 to 6.5 as the best range for most ornamentals. Acid-loving plants, a group that includes azaleas and rhododendrons, do best lower, at pH 5.0 to 5.5. At pH values above 6.5, phosphorus and most micronutrients become less available, which is why an azalea in limed soil can struggle even when the bed is fertile.

Lime raises pH. It reacts slowly in dry soil, and Clemson recommends applying it two to three months before planting so it has time to neutralize acidity.

Two products lower pH. Elemental sulfur works gradually, because soil bacteria have to convert it to sulfuric acid, and the conversion can take several months. Aluminum sulfate lowers pH as soon as it dissolves, but excessive applications may injure roots. With either one, follow the soil test recommendation and retest before you apply more.

Feed Lightly and Keep Nitrogen Low

Fertilizer is the last and smallest step. According to Clemson, mature trees and shrubs growing in favorable soil need little or no fertilizer. Both Clemson and NC State Extension note that too much nitrogen stimulates leafy shoot growth and can reduce flowering. For shrubs growing in a fertilized lawn, Clemson says to follow the turfgrass recommendations, which means the lawn’s feeding already covers them.

If a soil test shows a real need, Clemson prefers slow-release or controlled-release nitrogen, because of concerns about leaching and runoff. Spread it evenly out to the drip line, on mulched and bare ground alike. Plants absorb fertilizer while they are actively growing, typically from budbreak up to late summer and early fall. Let the soil test decide the grade: when the report does not recommend phosphorus, Clemson suggests a fertilizer with none, such as 30-0-10.

Improving Soil Around Established Shrubs

You cannot till around a shrub that is already rooted in, so improvement comes from the top. Clemson recommends 2 to 3 inches of natural, organic mulch over the planted area, kept 2 to 3 inches away from the trunk or stems. NC State describes keeping natural plant material on site as an effective, inexpensive way to enhance soil fertility, and suggests adding fallen leaves to planting beds instead of disposing of them. Our guide to choosing mulch for garden beds compares the materials.

For pH on an established bed, apply only the amount the soil test gives. Clemson’s advice is to test soil annually, so the next report shows how far the pH moved before you add more. If you are still picking plants for the bed, see our list of flowering shrubs for year-round structure. The same sequence of test, amend and mulch applies to a mixed border, including a pollinator garden planned for bloom from spring through fall.