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Fertilizing Trees and Shrubs

Tulips bloom in Brookgreen Gardens.
Tulips bloom in Brookgreen Gardens. The Sun News file

The application of fertilizer to trees and shrubs is not necessarily a routine to be followed automatically every spring and fall.

Most trees and shrubs do well with only one application of fertilizer per year in late winter or spring.

When trees and shrubs are surrounded by lawn the nutrients provided by regular turf fertilization often provide adequate fertilizer.

Keep in mind that fertilizer is not a cure for an improperly planted shrub or tree—typically one that is planted too deeply. It is not a treatment for poorly drained soil or soil compaction. Fertilizer is not a remedy for disease or insect infestation. Nor is it a cure for the stress caused by weather extremes. Plant problems should be identified and addressed before applying fertilizer.

Fertilizers are chemical or organic. Plants don’t know the difference. However, there are reasons to choose one or the other.

Chemical fertilizer consists primarily of nitrogen, potassium and phosphorus. It is lower in cost and bulk. It is also highly water soluble which means that the nutrients readily leach into ground water. That makes it a major contributor to ground water pollution.

Organic fertilizers are derived from plant or animal sources. The major nutrients become slowly available with the action of microorganisms. The fertilizer also provides various micronutrients. It adds organic material to the soil which helps it hold water and improve its structure. It is a good choice for sandy soils which tend to leach nutrients.

Nutrient deficiency is always worth a look when a plant fails to thrive or shows indications of a problem.

Accessibility of nutrients to plants works in association with soil pH. The pH level determines what nutrients are available to plants. At a pH of 6.5 most nutrients are in a form plants can use. When the pH is below 5.0 or above 7.0 plants may not be able to absorb certain nutrients, or they may take up so much of a nutrient that it becomes phytotoxic (poisonous). Think of pH as a silent killer in that it can starve or poison a plant. Simply fertilizing plants may not be the answer to giving plants necessary nutrients.

There are 13 mineral nutrients in the soil that plants need to grow. These minerals must be dissolved in water before plant roots can take them up. Soil pH affects their solubility and is therefore essential to the nutrients’ availability to plants. The minerals do not readily dissolve if the soil solution (water + soil) is too acid or too alkaline.

Signs of nutrient deficiency occur initially in either new or old leaves. Determine if old or young leaves are affected. Deficiency in the three primary elements, nitrogen, phosphorus, potassium shows up in the old leaves first.

When nitrogen is deficient leaves turn light green to yellow, and then brown. Lower leaves drop off first as they become necrotic. Growth is stunted and shoots develop poorly.

Phosphorus deficiency causes older dark green leaves to become purple. There is no chlorosis. Growth is stunted. Note: Highly acidic soil (low pH) greatly decreases phosphorus availability to plants.

With potassium deficiency leaf margins are first chlorotic, then brown and dry. Chlorotic spots appear on leaves and also turn necrotic. Plant growth is slow.

It is advisable to choose a fertilizer based on the results of a soil test. A soil test identifies the alkalinity or acidity of soil. It also tells us the existing nutrient levels. Not all plants thrive in the same pH range. If plants struggle in a certain spot, test the soil in that area. Take samples of garden soil 6 – 8 inches deep.

Submit soil samples in fall to effect proper changes for the next growing season. Test results will guide you with suggested lime and fertilizer application rates. Lime is only slightly soluble in water and slower acting than fertilizer. Consequently it may take two years to reach your optimum pH. Generally sandy soil should be tested every two to three years and clay soil every three to four years. For more information about soil testing google/search Clemson hgic 1652.

Reach Debbie Menchek, a Clemson Master Gardener, at dmgha3@aol.com.

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