Overfertilization

Overfertilization of commercial pot or container-grown crops results in high concentrations of soluble salts in the potting medium, which are directly damaging to roots by slowing the net flow of water into the roots and indirectly by predisposing the plants to certain root diseases and damping-off.

Symptoms and Signs of Overfertilization

• Crust of fertilizer on soil surface
• Yellowing and wilting of lower leaves
• Browning leaf tips and margins
• Browned or blackened limp roots
• Defoliation
• Very slow or no growth
• Death of seedlings

Origin of the Problem

• Excessive amounts of soluble fertilizer added at one time.
• Application of soluble fertilizer several times with little or no leaching
• Use of excessive amounts of slow release fertilizers
• Improper use of slow release fertilizer in combination with soluble fertilizer
• Poor drainage of medium
• Excessive steaming of potting mix (too hot, too long)
• Growing plants under moisture conditions too dry for the fertilization rates employed
• Moving plants from high-fertilization, high-watering, fast-growing conditions to less rapid growing conditions without first leaching fertilizer that will not be needed by the plant in the new environment.

Measuring Soluble Salt Levels

A conductivity meter is used to measure how well a current flows through the solution. The higher the soluble salt concentration, the higher the reading will be. Readings are determined and reported in units of mhos x 10 to the -5/cm or mhos x 10 to the -3/cm (mmhos/cm), depending on the instrument used.

100 mhos x 10 to the -5/cm = 1.00 mhos x 10 to the -3/cm = 1.0 mS

The most accurate method of measuring soluble salt concentration is to first make a water saturated paste of the potting medium. The saturated paste is so wet that it glistens but not so wet that it puddles. After waiting and stirring intermittently for 1 hour, the liquid is then filtered off with a vacuum apparatus or squeezed out. The conductivity of the saturated paste extract is an accurate measure of the salts to which the roots are exposed. The most common method of measuring soluble salts is to take a certain weight of air-dry mix and add twice or five times that weight in water. Stir intermittently. Filter or squeeze off the water through cheesecloth. The salt solution in this liquid is more dilute but usually provides an adequate estimate of salt levels. The soil pH can also be measured using liquid from either method.

Acceptable conductivity levels vary from crop to crop (e.g., poinsettias and seedlings are sensitive to high salts while chrysanthemums and Zygocactus are relatively tolerant) and from potting medium to potting medium. In general the upper limit of acceptable readings are as follows:

Mixes containing soil -- 100 mhos x 10 to the -5 = 1.0 x 10 to the -3 (1:5 dilution) or 800-1000 mhos x 10 to the -5 = 8-10 mhos x 10 to the -3 (saturated paste) = 1.0 Ms

Soilless mixes (saturated paste or 1:5 dilution)-- 250 mhos x 10 to the -5 = 2.5 mhos x 10 to the -3 = 2.5 Ms

Management

• Purchase and routinely use a conductivity meter.
• Record which conductivity readings are consistently associated with high quality plants and which are associated with plant damage in your potting mix.
• Avoid overfertilizing by always adding enough liquid fertilizer so that 10% of what is added leaches out the bottom of the container.
• Do not use slow release fertilizer in combination with soluble fertilizer.
• Do not mix slow release fertilizer into the potting medium unless absolutely certain of using the proper amount.
• Double check the weight of soluble fertilizer being added to the injector or watering can.
• Leach excess fertilizer from plants prior to moving them to environments where they will have a slower growth rate or drier conditions.
• If a mix has been steamed too long or at too high a temperature, leach the mix before use as outlined below or allow it to "rest" for 2-3 weeks.

When salts are excessive due to a soluble fertilizer, leach. Apply plenty of tap water so that it pours out the bottom of the pot. Repeat this leaching once more 2-3 hours later or the next day.

Pots or containers
6" of water applied will reduce salts by 1/2
12" of water will reduce salts by 4/5
24" of water will reduce salts by 9/10

Benches
Apply 1/2 to 1 gal water per sq ft. Wait 1 hr. and repeat.

If salts are excessive because slow release fertilizer was used improperly, repot the plants or scrape some of the slow release from the medium surface.

If salts are excessive because of a combination of slow release fertilizer and soluble fertilizer, repot the plants.