Fertilizers – Too Little or Too Much

by Nanette Londeree, Master Rosarian

You’re giving your roses all the right things but they’re still not blooming like you’d hoped; the new foliage is a weird color green, and some of the lower leaves have fallen off. You don’t have to be Miss Marple to know something is amiss and certainly not typical for a healthy rose. Before you start piling on soil conditioner, Epsom salts, fish emulsion or other fertilizers, figure out what may be causing those unusual effects. Many different plant problems produce similar symptoms – and giving too much of a good thing might actually damage your plants.

In general, healthy soil with excellent drainage contains most of the nutrients your plants need to grow and thrive. Maintaining a balanced nutrient level in the soil – not too little or too much, is important for growing vigorous plants. Deficiencies of an individual nutrient can result when there is an inadequate amount of a specific nutrient in the soil or it is unavailable to the plant (most commonly due to incorrect soil pH or being tied up with other nutrients). Typical symptoms of nutrient deficiencies are reduced shoot growth and leaf size, leaf chlorosis (yellowing of leaf tissue due to a lack of chlorophyll), necrosis (death of plant tissue) and die-back.

A helpful diagnostic tool to help you determine what nutrient may be deficient is where the symptoms occur – on the older leaves or newer growth. Some nutrients can move easily through the plant (they’re mobile) while others cannot. If only the lower leaves are affected, then a mobile nutrient is the likely culprit. Conversely, if it’s the upper leaves showing the deficiency, the plant is likely deficient in an immobile nutrient, one that cannot move from older to newer leaves. The most common deficiencies in roses are nitrogen and iron. Nitrogen, along with phosphorus and potassium, can readily move through a plant while iron is immobile and cannot get to the newer growth. The table on the next page identifies nutrients mobility and where you would most likely observe symptoms.

In addition to problems associated with the lack of a nutrient, you can also observe symptoms if you’ve got too much of one. Nutrient toxicity, the accumulation of soluble salts in the soil, is not uncommon in the rose garden. The salts, generally chlorides, sulfates and nitrates, are prevalent in most water sources, fertilizers, manures and Epsom salts (magnesium sulfate). If they’re not leached (removed) from the soil, they can accumulate and interfere with water availability in the root zone, causing damage to foliage – marginal browning of leaves beginnings at the tip and proceeding to the base of the leaf along the edge of the leaf; it can also retard overall plant growth. Drip irrigation can exacerbate the problem since water may be delivered to only a portion of the roots, allowing the soil to dry out in spots where concentrated salts are present.

Roses are sensitive to salts but not all produce negative effects on the plant. They will tolerate relatively large amounts of sulfates while chlorides are especially injurious, especially as soil becomes drier. The treatment for correction of high salt accumulation is to remove them by flushing with water. To prevent accumulation of soluble salts, moderate the use of salt-containing materials and irrigate your plants thoroughly before and after fertilizing.

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