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AUTHOR

Manjula Nathan
University of Missouri
Division of Plant Sciences
(573) 882-3250
nathanm@missouri.edu

Fertilizing Acid Loving Landscape Plants

Manjula Nathan
University of Missouri
(573) 882-3250
nathanm@missouri.edu

Published: May 1, 2010

When fertilizing your garden, you need to pay attention to the type of plants you are growing and treat them accordingly. While most of the plants that we grow in a garden prefers soil pH in the range of 6 -7, there are some plants that thrive well only in an acidic soil pH. These plants can only grow well and produce flowers and fruits in acidic soils and are referred to as acid loving plants. Some examples of popular acid loving shrubs commonly grown in landscapes are azaleas, rhododendrons, holly, butterfly bush, blue hydrangeas, camellias and heather. Some examples of acid loving trees include pin oaks, magnolia, dogwoods, and most conifers such as pines, spruce and yews. These acid loving plants prefers a soil pH of 4 – 5.5 for optimum growth. So the first step is to plant them in soils that are acidic (pH 4.0 -5.5), and to incorporate large quantities of peat or other organic matter into the soil during soil preparation for planting. If you are planting in a soil that is high in pH (greater than 6.5 to 7.0) you must make sure to acidify the soils before planting. Lime should never be applied to acid loving plants unless the soil test calls for it.

As first step soil should be tested and if the soil pH soil is higher than the optimum range (pH 4.0 to 5.5), it is desirable to acidify the soil before planting. Over time the soil pH will tend to increase, especially if the water supply used for irrigation is alkaline. Elemental sulfur, iron sulfate and aluminum sulfate are soil amendments that can be used to acidify soil.

Guidelines for lowering pH:

The growth and appearance of some perennial plants is optimized by a low soil pH. When the soil pH is greater than desired, finely ground elemental sulfur is recommended to lower the soil pH. A recommendation for sulfur depends on a target pH and the texture of the soil.

Sulfur reacts slowly with the soil and takes about three to six months to reduce the soil pH. Sulfur should be incorporated into the soil well in advance to planting. Iron sulfate too can be used in lowering soil pH. Iron sulfate reacts faster than elemental sulfur (within three to four weeks), yet it requires 4 to 5 times more material.

Guidelines for Fertilizer Applications:

The best option in choosing the right amount of fertilizer is to do soil testing and follow the recommendations. The other alternative is to apply a complete fertilizer like 10-10-10 once a year in early spring at the rate of 1 pound per 100 square feet. If your soil is well supplied with phosphorus and potassium, only nitrogen fertilizer is required. In this case, choose an acidifying nitrogen fertilizer like ammonium sulfate and apply at the rate of 1 pound per 100 square feet. There are some trade-named fertilizers available in the market for fertilizing acid loving plants. These products should be only used at recommended rates when required. Do not fertilize these plants after August 1. Fertilizing after this time may force growth during winter when the plants should be dormant.

Common Nutritional Problems:

The acid loving plants along with N, P, and K need minerals such as iron and manganese. In an acidic media the minerals like iron and manganese become soluble and become readily available to plants. The most common problem that develops in an acid loving plant is yellowing of leaves (chlorosis). This symptom is mistaken for nitrogen deficiency by home owners, where as the yellowing is actually due to iron deficiency.

The symptoms of iron deficiency appear as yellowing of new leaves at the tips of the branches, while mature leaves remain dark green. The veins of chlorotic leaves may be green. This deficiency can be temporarily corrected by one or two applications of iron sulfate sprayed on the foliage at the rate of 1 ounce per gallon of water. If necessary, a second application may be made after two weeks. Soil application of 1 to 2 pounds of iron sulfate per 100 square feet will have long term effects. Make sure to mix iron sulfate in soil and water the plants well after soil application. Iron chelates too can be used in correcting iron deficiency by applying at the rate of 1 ounce per 100 gallons of water and sprinkling over the plants and onto the soil.

In sandy soils, magnesium deficiency can become a problem with acid-loving plants. The symptoms appear as interveinal chlorosis (yellowing) first in older leaves and then moving to younger ones. The new shoots that form don’t look healthy. Only soil and plant tissue testing can help in identifying this deficiency, and in correcting the problem. Magnesium deficiency can be corrected by applying one half cup of magnesium sulfate (Epsom salt) per 100 square feet and by watering the plant well after application.

Table 1. Elemental Sulfur Application Rates to Lower Soil pH by one Unit
Soil Texture Amount of Elemental Sulfur (S) to Apply*
Area basis Volume basis
lb./100 sq. ft lb./1000 sq. ft lb./cubic yard
sand, loamy sand,
sandy loam
0.8 8.0 1.0
loam, silt loam 1.2 12.0 1.5
Clay loam, Clay 2.4 24.0 3.0
*Multiply by 44 to conver the rates in lb./1000 sq. ft. to lb./acre
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REVISED: December 5, 2011