Why do some plants react poorly to certain compost types?
Written by
Liu Xiaohui
Reviewed by
Prof. Martin Thorne, Ph.D.Plants are not tolerant of incorrect compost. There are specific physiological needs for certain plants, with a difference in compost being particularly injurious. Blueberries suffer in alkaline compost, and cacti in compost retaining too much moisture. Heavy feeders, like tomatoes, do not thrive in weak compost. Mushroom compost, with its high salt content, is toxic to seedlings. If we studied the needs of plants and matched the compost to them, we would not have to contend with these evils.
pH Sensitivity
- Problem: Alkaline compost locks iron in acid-lovers
- Solution: Use ericaceous compost for blueberries
- Indicator: Yellow leaves with green veins
Salt Tolerance
- Problem: Mushroom compost salts burn seedlings
- Solution: Leach salts or use low-EC compost
- Indicator: Brown leaf edges and stunted growth
Moisture Needs
- Problem: Water-retentive mixes rot cactus roots
- Solution: Add 50% perlite to compost
- Indicator: Soft translucent stems
The need for acidic pH causes a total lockup of nutrients. Plants that prefer acidic conditions, such as Azaleas, require a pH of 4.5 to 5.5 to absorb iron. Alkaline compost, with a pH of over 6.5, binds the iron, making it unavailable for uptake. This causes interveinal chlorosis, where the leaves become yellow and the veins remain green. For sensitive species, always check the pH of the compost before using it.
Salt tolerance varies widely among plants. Mushroom compost contains soluble salts from gypsum additives. These salts cause dehydration of the seedling roots, resulting in burning. Mature vegetables are more resistant to higher concentrations of salts in solution. To remove excessive salinity, the compost may be leached with water before use. When salt tolerance is low, use other composts with low salt content, such as leaf mold compost.
Moisture preferences are rooted in the health of the roots. Desert plants, such as agave, require a quick-discharge compost, whereas a moisture-holding mix can choke their roots. Tropical plants prefer a moisture-holding compost. Match the texture of the compost to the nature of the plants to be used. If under-drainage is needed, use perlite; if moisture retention is necessary, use vermiculite.
Nutrient density influences plant response. Heavy feeders such as corn flourish in nitrogen-rich compost. Poor compost can starve plants of nutrients during the busy growing season. Use manure or blood for hungry plants. Light feeders, such as herbs, thrive in milder compost that is free from nutrient burn.
Read the full article: Choosing the Best Compost for Gardens
