How to know when you are over-fertilizing your plants

Too much of a particular nutrient can damage your crop’s roots or leaves. Knowing the causes can help you diagnose and prevent it

by KEITH REID

Most of the time, I deal with questions about lack of soil fertility and how much nutrient is needed to correct it. Occasionally, however, the question is not one of deficiency, but of excess.

Fertilizer injury can be caused by adding too much of a particular nutrient or combination of nutrients, by having too high a concentration of nutrients near a sensitive plant part, or because the solubility of a particular nutrient has been increased by soil conditions. Diagnosing a nutrient toxicity is often difficult, because the damage is done before the symptoms show on the plant and then the concentrations have declined before any samples can be taken to pinpoint the problem. Fortunately, the effects of fertilizer injury seldom last more than one cropping season, but the damage they cause is quite real and should be avoided.

Fertilizer injury is most common where high-value crops are grown, and the grower is trying to squeeze the maximum yield out of each acre. I have also seen it, however, in lower-value commodities, where the fertilizer has been banded too close to the seed or an excess foliar application has been made.

Most of the time, fertilizer injury is due to what is called the salt effect, wherein the root of the plant is surrounded by a solution that is more concentrated than the solution in the plant cells. Water moves from areas of low salt concentration to areas of high salt concentration, so the effect is to pull water out of the root tissues into the soil. This is exactly the same process that occurs as cucumbers are turned into pickles, where the concentrated brine pulls water out of the cucumbers.

The effect on the tender root tissue (which is the part of the root that absorbs water and nutrients) is that the fine roots lose moisture and die. The appearance of these roots is shortened and darkened, looking as if someone had held a lit match to the roots. The common name for this injury is fertilizer burn, and the term is quite descriptive.
Fertilizer burn happens when there is too much soluble material in the soil for the volume of water to dilute it to safe levels. This means that sandy soils are more susceptible, because of their low water-holding capacity, and injury will show up most often in dry weather.

Banded fertilizer will create zones of high concentration, which can cause injury if they are too close to the seed. Pre-existing soil conditions, like high salts from an old manure pile or from a brine seep, will increase the risk of fertilizer injury. Broadleaf crops, particularly those with large seeds, are more sensitive to fertilizer injury than grass crops.

A secondary source of injury to the roots is the concentration of toxic material around the root. The most common material is ammonia, which can damage plant tissue when it is present in gas form. Occasionally, this will happen when the crop is planted too soon after a pre-plant application of anhydrous ammonia or if side-dress ammonia is applied too close to the row.

More often, the injury is the result of urea or di-ammonium phosphate banded too close to the seed. The appearance of the roots is the same as for salt injury. Frequently, the injury to the plant is a combination of salt and ammonia injury.

Injury to the roots impedes the ability of the plant to take up water or nutrients, so the visible symptoms of fertilizer injury are stunted growth, drought stress and sometimes nutrient deficiency symptoms.

Diagnosing fertilizer injury is best done by observing the roots, and by considering the fertilizer application rates, the soil type, and the weather conditions. Occasionally, a soil conductivity test will reveal high salt levels, but in most cases the soil solution has diluted the salts by the time the test is taken.

Leaves can be injured, too. Some micronutrients can be present in the soil at concentrations greater than the plant needs, but not high enough to injure the roots. In this case, the nutrients are carried in the plant sap to the leaves, where the water evaporates. This leaves a high concentration of these nutrients around the margins of the leaves, where eventually it kills the leaf tissue.

Marginal necrosis is most commonly a symptom of excess sodium or boron in the soil. It will occasionally show up on plants growing in very acid soils, from excess manganese that has been dissolved into the soil solution. In tree fruits, excess manganese will show up as “measles” in the bark.

Leaf injury can also be caused directly by foliar applications of nutrients. If the concentration of nutrients in the foliar spray is too high, it will burn the leaf tissue under the drops leaving a speckled appearance on the leaves.

Fertilizer injury can be easily prevented by only applying the nutrients that are needed, and by being watching the safe rates of application for banded fertilizers. BF

Keith Reid is soil fertility specialist with the Ontario Ministry of Agriculture, Food and Rural Affairs, based in Stratford. E-mail: keith.reid@ontario.ca