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Amending and Fertilizing Soil
Amending Landscape Soils
It’s a good idea to amend your soil by adding organic matter to it before planting shrubs, but not trees, because trees usually grow best in native soils.
It may also need certain additives if it is too sour—acidic soils need garden lime—or sweet—alkaline soils need garden sulfur. Either condition binds up important nutrients with natural soil minerals, keeping them from reaching your plants’ roots.
Let’s discuss soil-texture and structure amendments first.
Amendments rich in organics such as well-rotted manure or compost enrich the soil. Most every type of organic matter improves soil, whether compost, composted animal manure, decomposed bark, or leaf mold. They also improve the soil’s physical structure.
In clay soils, adding organic material improves drainage and aeration. At the opposite end of soil texture range, in too-sandy soils such amendments increase the ability to hold moisture.
Till in organic amendments at a rate of 2–3 parts amendment to 7–8 parts native soil.
Soil amendments also increase the activity of beneficial microorganisms. They’re important to improved soil structure and its ability to hold and release nutrients, air, and water. All of these are critical to tree and shrub health.
A side benefit of organic amendments is that, besides their other benefits, they provide trace amounts of micronutrients and trace elements to the soil.
Some amendments—additives—also adjust soil acid-alkaline levels, measured on the pH scale of 3.5 to 9. Neutral soil is about 5.5 on the pH scale. Soils with lower pH are acidic, while those with higher pH are alkaline.
Always note the preferred pH range of the shrubs and trees you plant; we’ve included it in every GrownByYou plant description (See: Ornamental Trees], but you’ll also find it on plant tags on shrubs and trees in your garden retailer.
Add dolomitic limestone to highly acidic soil to raise its pH, or work sulfur into highly alkaline soils to lower the pH. In both cases, base the amount of limestone or sulfur you add on the recommendations provided by soil test results [see Testing Soil, pg. 33].
Fertilizers and Nutrients
The three key nutrients plants need are nitrogen, phosphorus, and potassium, called the three “macronutrients.” Trees and shrubs use nitrogen (N) for foliage growth, phosphorus (P) for root growth and flower development, and potassium (K) to make them more winter-hardy and drought tolerant.
Fertilizers with equal amounts of each are called “balanced fertilizers,” but some mixed fertilizers have unequal amounts or may omit one or more of the macronutrients. Always consult the product label to choose the correct fertilizer regardless of the purpose stated by its manufacturer on its package.
A fertilizer’s analysis is shown on its label, displayed as three numbers (for example, 10–10–10, 0–5–5, or 15–1–1).
The first number refers to the percentage of nitrogen (n) by weight that the fertilizer contains, the second number to the percentage of phosphorus (p), and the third number to the percentage of potassium (k). These numbers are more important to your shrubs than a label claim such as “Rose Food.”
Most soils contain the macronutrients in sufficient amounts for health trees and shrubs. If a soil test indicates that your soil is deficient in one or more of them, you’ll want to add an appropriate fertilizer to your landscape planting bed.
Select a fertilizer that matches your landscape’s needs based on your soil test or symptoms you see in your trees or shrubs, such as yellowing of foliage—a telltale for nitrogen deficiency— or reddish coloring—caused low potassium or too alkaline soil.
If you’re uncertain, select a slow-release, low-potency, balanced fertilizer and use it sparingly in your planting beds, mixing it deeply and thoroughly into the soil.