Cultural

Yellow Leaves?
With above average rainfall this season many growers are facing plants with yellow leaves. Many things cause yellow leaves, not just a lack of fertilizer. What information can you gather to determine the cause(s) of the problem and the appropriate action to take?

Consider checking the pH and electrical conductivity (EC) which is a reading of the soluble salts from the fertilizer. Many growers check the EC and the pH to help troubleshoot a problem. However regular (every two weeks) monitoring of the EC and the pH can help identify a problem as it is happening which makes for a more confident decision when changes must be made. Monitoring strictly when trouble-shooting does provide information, but it is only a snapshot in time. Imagine how much more useful that information is when it has been collected over the production of that crop.

To illustrate:
Grower A in Black Gnat, KY has plants that aren't growing with pale leaves; the EC reading is 0.25.

Grower B in Monkey's Eyebrow, KY also has plants that aren't growing with pale leaves; the EC is 0.25.

Plants fertilized with controlled release fertilizers only should have an EC of 0.2 - 0.5 mmhos/cm.

Both growers applied a controlled release fertilizer in March and concluded that based on the fairly low reading to apply a second round of controlled release fertilizer in early July. Were they right?

Grower A used a 10-12 month release product. Had he/she been taking readings during the course of production he/she would have known that this fertilizer began at an EC of 0 and had worked its way up this latest reading at 0.25. Because of the upward trend and knowledge that a long-term product had been applied, the grower could have had confidence about not top-dressing. In addition, if this grower had been monitoring regularly he or she could have supplemented the controlled release fertilizer product with a liquid fertilizer during the early part of the season until the desirable soluble salts level was reached. Applying more controlled release fertilizer in July will likely eventually raise the soluble salts to a risky level, considering the upward trend the fertilizer orginally applied was experiencing. Cold sensitive growth could occur in the fall.

Grower B used a substrate with a high pH or added too much lime to the substrate. If the grower had been regularly checking the pH and EC he/she would have noticed that the pH was quite high making it difficult for the plants to take up certain nutrients even though they were available (albeit in a relatively low dose considering the soluble salts reading). More fertilizer won't fix this problem. Lowering the pH will fix this problem. Regular monitoring will also show if the measures taken to reduce the pH are working.

A form for tracking pH and EC readings and directions on how to take the readings are at:
http://www.ca.uky.edu/HLA/Dunwell/NLgetstart.html#Nutrient%20management

Another problem that can lead to yellow leaves is a root problem. With sustained wet weather this season, plants in poorly draining substrate could be experiencing root rot - simply from a lack of oxygen or from a pathogen or both. How can you tell if roots are less than healthy? Pull plants out of the pots and look at the roots. They should be firm and white or cream colored, not dark or mushy. Do this often enough to be comfortable recognizing what is normal for all plants grown. To know with certainty submit a plant sample including roots and soil to the UK Diagnostic Lab via your local County Extension Office.

Substrates with a high percentage of composted material tend to be higher in pH than peat or bark based substrates. A high percentage of compost or other decomposed organic matter or other very small sized substrate components often means excessive water retention. Some substrate components are more stable than others. It is important to measure many substrate qualities, including aeration porosity, if a substrate is being stored as it will change as the product ages.

Aeration porosity is a measurement of the percent air space in the substrate after the irrigation water or rainwater has drained. The optimum air space for woody plants is between 20 and 30%. Analysis of aeration porosity is easy. You need the container volume and the aeration pore volume.
Aeration porosity (%) = (Aeration pore volume/container volume)x100

1. Measure the container volume by sealing the drainage holes and filling the container with water to the level it would normally be filled to with substrate. Record the container volume.
2. Empty and dry the container and fill it with the substrate the plants will grow in. Slowly add water with a graduated cylinder or large measuring cup, until the mix is completely saturated. A very thin slick of water will appear on the surface when the substrate is saturated. Record the total volume of water added as “total pore volume.” This may take two hours when starting with new, dry substrate.
3. Remove the seals from the drain holes and collect the water that drains. This may take several hours. Do not tip the container on its side to allow more water to drain out. Measure the amount of drained water and record as “aeration pore volume.”

The three steps outlined above will also allow you to calculate total porosity.
Total porosity (%) = (Total pore volume/Container volume) X 100%

Keep in mind that inadequate root systems from excessive pruning, excessive fertilizer, or rotting can all lead to reduced nutrient uptake. While many confounding factors, in particular irrigation water pH and alkalinity, were eliminated from these examples the point is to make managment decisions with as much information as possible. Monitoring regularly will provide useful information.

Source: BMP Guide for Producing Container Grown Plants, Southern Nursery Association, and “Out- and-About” with Hannah, Container Production. Part 4: Media Monitoring, December, 1999.