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Applying fertilizer: Should you feed the soil or spray the plant?

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By Judy Koehly
Master Gardener
Our sandy soils cannot store nutrients for long periods of time, releasing them according to plant needs, as do loamy soils, so domesticated plants (including lawns, flowers, shrubs and vegetables) benefit from a careful periodic application of fertilizer. Gardeners add fertilizer either by applying it to the soil or by using a water-soluble product and spraying the foliage.
Soil application brings the best results most of the time. The purpose of the plant’s roots is to take up water and minerals, while the leaves are designed for photosynthesis.
There are times when foliar application is desired, but it should not become the standard method. The idea behind the practice of “leaf feeding” is that nutrients are immediately available so the plant response is faster. Responses might be faster in some cases, but it is not as complete or as long lasting when compared to soil applied fertilizer.
The function of roots and leaves is much different. Roots take up minerals and water from the soil, where they are transported to the leaves. Using energy from the sun, photosynthesis converts minerals to usable sugars. This food is then translocated back to other parts of the plant that needs it, including stems, fruit and roots.
Leaf feeding has many drawbacks compared to soil application. To avoid foliar burn, a weak solution of fertilizer must be used and then the product must be absorbed before being washed off the leaves by irrigation or rainwater. Plus, the leaves simply cannot assimilate all of the various nutrients, as well as those absorbed by the roots.
Fertilizers used for soil application are available in several forms, including dry granular, slow release pellets or soluble. The plant is not fussy about the type of fertilizer, only that it is applied in the right ratio and amount. It is important to use the results of your soil test to determine the ratio and amount and to carefully follow the directions on the fertilizer package for application method.
Liquid fertilizer is usually concentrated. They need to be mixed or dissolved in water before application to the intended plants. When properly diluted, you also negate the chances of fertilizer burn. Properly diluted liquid fertilizer is ideal for use of soft, sensitive and young plants.
Fertilizer for indoor plants, specialized food for seedlings, bonsai fertilizer, and orchid fertilizer are all water-soluble. Liquid fertilizer is easy to apply. Simply mix the required amount into a watering can or bucket filled with water (Tip: Add the fertilizer concentrate to the water). Water the plant with the mixture and remember that the fertilizer will be more easily absorbed if the soil is moist.
The instructions on granular or pellet fertilizer container will indicate the amount in ounces or cups that should be applied to a certain number of square feet. When actually applying the granular fertilizer, you can gauge an approximate square foot by pacing. Then sprinkle the full measuring cup evenly over the whole area that you gauged as the correct number of square feet.
When fertilizing trees or large shrubs, you need to sprinkle the fertilizer over the whole drip area, even if it happens to be over the grass. The roots of plants grow away from the trunk and the fine hair roots are usually those furthest away from the base of the tree or shrub. These fine hair roots are the ones responsible for feeding the tree or shrub. Be aware that heavy handfuls of fertilizer near the base of the tree or shrub can damage the root system by burning it.
Foliar application could be best used to supplement the basic soil application. Iron chlorosis, for example, can often be quickly corrected with a foliar spray of an iron solution rather than making a second fertilizer application to the soil. Leaf feeding can also be used to provide a quick, temporary green up of lawns, flowers or vegetables.
Send your gardening questions or comments to: Brunswick County Master Gardener Column, P.O. Box 109, Bolivia, NC 28422, or call 253-2610. Enclose a self-addressed stamped envelope if requesting information or a reply. Answers may be printed in this column.