Are there special considerations for container plants?
Written by
Nguyen Minh
Reviewed by
Prof. Charles Hartman, Ph.D.Container plants have special challenges related to phosphorus management as a result of narrow root zones and high leach rates". The container media \(as opposed to soil\) won't hold nutrients long. You should apply phosphorus at a concentration of 100-200 ppm on a weekly basis. Because the media has limited microbial activity, water-soluble formulas are best.
Application Protocol
- Weekly 100-200 ppm phosphorus solutions
- Water-soluble formulas like MAP or ammonium polyphosphate
- Drench soil until 10-20% runoff occurs
- Adjust frequency during cool/dormant periods
Deficiency Monitoring
- Check lower leaves weekly for purple stems/discoloration
- Measure new growth rates every 2 weeks
- Test runoff water pH monthly
- Document flowering/fruiting patterns
Soil Mix Considerations
- Avoid slow-release granules needing microbial breakdown
- Use coco coir/peat mixes for better retention
- Incorporate 20% compost for mild buffering
- Replace media completely every 2 seasons
Select your containers wisely to help reduce leaching problems. Porous terracotta is good but dries out quicker, requiring more frequent feeding; it also affects not only the days but your watering schedule throughout. Glazed ceramic retains moisture better. Always use pots with drainage holes to prevent salt from accumulating in your plant's soil. Group your containers by watering needs for simplicity.
Soil temperature has a significant impact on phosphorus uptake. Containers generally will heat up faster than soil in the ground. At soil temperatures above 85°F (29°C), root uptake of phosphorus is inhibited. It may be helpful to use light-colored pots and/or shade covers during very hot periods. Water your plants early in the morning to ensure the root zone stays cooler due to the moisture in the soil.
Keep an eye out for purple stems, which serve as your primary indicator of deficiency. This symptom will display more promptly in containers than in plants grown in the ground. At the first sign of purple stems, increase your applications to 200 ppm for a period of two weeks. After two weeks, watch the new growth for a return to normal green color as the plant recovers.
Avoid using slow-release fertilizers with container plants. Granules that require microbial degradation won't work, as you are limited to the microbes present in the containers. Furthermore, these are going to give you inconsistent feedings. Use foliar liquid feeds that you can control and manage effectively. With a liquid feed, you will avoid feast-or-famine cycles, which put your plants under stress.
Flood containers once a month to reduce salt levels that can build up. Water to the point of running off, applying more than necessary, but less than 1.0 EC level. Measure runoff pH once a week. A runoff pH above 7.0 indicates problems with alkalinity. Adjust feed solution pH levels to pH 5.8-6.2 to ensure phosphorus is available in the container.
Read the full article: Phosphorus Deficiency Plants: Signs and Solutions
