Last time we explored the environmental and mechanical reasons for leaves turning yellow - especially this time of year. This week we will investigate the primary reason for leaves yellowing - nutrient deficiencies. Sometimes, however, too many nutrients can cause similar problems.
Nutrient deficiencies
Soil pH out of balance
Nutrients come in two forms - mobile ("translocatable") nutrients and immobile nutrients. The mobile nutrients are nitrogen, phosphorus, potassium and magnesium. That is, these nutrients can move quickly from the soil to the plant. The immobile nutrients are iron, sulfur, calcium, manganese, copper, zinc, boron, molybdenum, and chlorine. Their movement is determined by their electrical charge. If positive, the nutrients bind with the soil and become unavailable - an insoluble precipitate. The determining factor for this availability is the soil's pH.
Without a doubt, extremes in soil pH are the most common reason for yellow leaves. Although roses can survive a wide range of pH, they prefer their soil slightly acidic (6.5) to near neutral (7.1).
Other symptoms to look for:
- Yellow leaves only on the bottom of the shrub
- Irregular-shaped leaves
- Burned leaf edges
- Small blooms
Treatments:
- A quality soil test, performed at least bi-annually, will help determine not only the soil's pH but the available nutrients it contains.
- Check irrigation supply's pH
- Add organic soil amendments if the pH is too high.
- Add lime if the soil's pH is too low; autumn is the best time to apply.
Iron deficiency ("chlorosis")
Iron is one of the chief elements necessary for the production of chlorophyll - the molecule that makes green plants green. It also aids sugar burning enzymes which activate nitrogen fixation. And it regulates the respiration of the plant's cells. All-in-all, iron is an essential element to the rose.
Lack of iron (or its unavailability) is characterized by light yellowish-green leaves and dark green leaf ribs.
Other symptoms to look for:
- Stems may turn yellow
- Yellowed leaves appear near the top of new growth, but are generally distributed throughout the plant.
- Perform a soil test; correct soil pH to 7.0 or lower
- In iron-deficient soils, add bone meal or blood meal organic amendments, or
- Add iron sulfate or chelated iron liquid or granular inorganic amendments
Magnesium deficiency
Like iron, magnesium promotes chlorophyll formation and vital to the photosynthetic process necessary to produce dark green foliage. It also promotes healthy, disease-resistant plants. It is generally available in acidic soils.
A lack of magnesium is characterized almost identically with iron deficiency, but the yellow leaves with dark leaf ribs are generally found only at the bottom of the plant.
