Soil and landscape fertility

Fertilizer recommendations… simplified

The purpose of this article is to provide some general guidelines for the fertilization and maintenance of landscape ornamentals. There are thousands of landscape plants and cultivars, as well as many different soil types. It would be impossible to provide the exact formulations for every possible combination. Therefore, I have tried to arrange common landscape plants together into several groups to simplify the process.

As any taxonomist will tell you, when trying to group together different plants or animals based on common characteristics there will always be some exceptions. However, the accompanying chart should serve as a good basic guideline for landscape fertility. The following paragraphs will briefly explain the soil classifications and how a soil test can help you fine-tune your program.

Soil classification and properties

Soils are made up of three components based on size. Those are sand, silt, and clay. Parent rock material, the climatic conditions, and other factors influence soil genesis and play a role in determining the type of soil present. For the purposes of this article we will again simplify soils into two basic types. Sandy soils that have high infiltration rates and low Cation Exchange Capacity (C.E.C.), and clay soils that have low infiltration rates and high C.E.C. These are at opposite ends of the spectrum and the user can use his or her soil test results to see where their soil actually falls.

Cation Exchange Capacity, or C.E.C. refers to the measured ability of a soil particle to attract and hold cations. These positively charged ions are the elemental forms of plant nutrients such as potassium, phosphorus, iron, manganese, calcium, magnesium, zinc, molybdenum, and so on. High C.E.C. soils have more negatively charged bonding sites and can hold larger quantities of plant nutrients than soils with low C.E.C. values. In essence high C.E.C. soils store plant nutrients that would otherwise be leached or volatilized into the atmosphere.

Infiltration rates of soil… explained

Infiltration rate refers to speed at which water moves into and through the soil. Clay soils have very small particles and therefore the pore spaces between the particles are very small. Water molecules have a natural affinity to these soil particles, and to themselves. As they wend through the tiny labyrinth of pore spaces they bond to the soil particles coating them. This attraction or capillary action resists the force of gravity pulling water down through the soil profile. Sandy soils have large pore spaces and less surface area. There is less affinity at work and water moves down through the pore space rapidly.

What you will learn from a soil test

Soil tests will typically provide information on available phosphorus, potassium, calcium, and magnesium. Optionally micronutrients such as iron and zinc, which are in the form of metallic ions, can be measured. In addition to available nutrients the soil test also typically provides soil acidity expressed as pH, Cation exchange capacity, and buffer capacity. Some labs also provide information on soil texture, percent organic matter and base saturation ratios. Most soil testing services provide crop specific information that helps the user to interpret the data. This can be in the form of fertility program recommendations or optimum fertility levels for the soil type and crop being grown. In the landscape we do not have a monoculture so the definition of “crop” encompasses all common landscape ornamentals, trees, shrubs, vines, and ground covers.

If soils have low C.E.C. fertilizer applications should be made more frequently using smaller amounts. In high C.E.C. soils one application is usually sufficient however some situations may call for spring and fall application. In all cases pH must be corrected in order for the plants to achieve maximum benefit. Use liming materials to raise pH and acidifying fertilizers and sulfur compounds to lower pH. Some fertilizers are specially formulated to lower pH as well.

Fertilizer guidelines for landscape ornamental plants, shrubs, trees

Plant Group Target pH LB N per season (1) ratio Recommendations
Shade trees 6.5 2 - 6
Use low rates for maintenance and higher rates for young trees where growth is desired, or for trees that are lacking vigor. For trees growing in turf areas increase the application of turf fertilizer in the root zone and avoid using products with systemic herbicides.
Deciduous shrubs 6.5 3 - 5
Low rates for maintenance and high rates for growth. Spread fertilizer uniformly under the canopy or uniformly throughout the bed for densely massed plantings. Do not pile fertilizer against stems or to wet foliage.
Broadleaf evergreens & dwarf conifers 5.5 - 6.0 1.5 - 3
Acidifying fertilizers may be warranted and products containing iron can improve color. Use lower rates than most other plants. To keep soil pH low spring and fall applications may be needed.
Ornamental grasses 6.5 - 6.8 2 - 4
Side dress individual clump type grasses using caution not to allow fertilizer to accumulate at the base of the stems. If lodging is a problem (lying over) use a 1:1 nitrogen to potassium product and reduce nitrogen to lower rates.
Small flowering trees & fruit trees 6.5 3 - 6
Spread evenly under the canopy and extending out 2-5 feet from drip line. Trees with abundant flower displays or fruit may benefit from post bloom application to replace lost energy. Do not fertilize during hot weather without adequate soil moisture or supplemental irrigation.
Annual flowers & ground covers 6.0 2 - 6
Broadcast through ground covers when foliage is dry using lower rates in shady conditions. For establishment of annuals incorporate pre-plant into soil. Where the season is extended follow up with top-dress applications 6-8 weeks after planting for maximum growth and flower production.
Tropical foliage plants & palms 6.5 2 - 8
Use ½ to ¼ lb N per 2 feet of trunk height applied 3–4 times during active growth. Lower rates in heavy soils or shorter growing seasons, higher rates in sandy soils or longer seasons. Supplemental manganese may be needed in sandy soils.

GreenView offers a complete line of Woodace fertilizers formulated specifically for Landscape Ornamental Plants, Shrubs and Trees.

Need to test your soil, kits are available from most garden centers and can be purchased from your local state extension agency. Find your local state extension agency .