Garden Word of the Day
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Foliar feeding refers to feeding plants by spraying nutrients on leaves and fruit.
Normally, plants absorb their mineral nutrients from the surrounding soil through the root system. Nutrients can also be absorbed through the stomata, tiny holes used for gas and moisture regulation, found on leaves and fruit.
Foliar feeding claims
Advertisements claim that foliar feeding is many times more efficient than soil feeding, that it cannot be used incorrectly, it promotes larger, sweeter crops, boosts a plant’s tolerance for heat and cold, increases pest and disease resistance, and even improves a plant’s internal circulation. Wouldn’t that be something? The number and diversity of these claims should raise a warning flag, and with good reason. Most of the claims about foliar feeding are false, but there are situations where foliar feeding is useful.
Foliar feeding research
The claims made about foliar feeding are based on research published in 1957 in which leaves and fruit were shown to be very efficient at absorbing tiny amounts of mineral nutrients in a lab setting. You can read the full report here.
Unlike nutrients absorbed through the root system and transported through the xylem, nutrients absorbed through leaf stomata are more likely to remain in nearby plant tissue. This is especially true for the ‘immobile’ nutrients, such as calcium and magnesium. According to the study, “Phosphorus, choline, sulfur, zinc, copper, manganese, iron, and molybdenum were intermediate [with regards to absorption] with decreasing mobility in the order given.” Potassium and sodium were shown to be the most readily absorbed and highly mobile nutrients.
Again, this research was conducted under laboratory conditions, not out in someone’s garden. As one might expect, results are very different in the field. There are, however, some cases where foliar feeding is a good thing.
Foliar feeding and alkaline soil
Nutrient absorption is helped or hindered by soil chemistry and electrical charges in the soil. One aspect of that chemistry is soil pH. Acidic soil has a pH of less than 7.0 and alkaline soil has a pH greater than 7.0. This is important because alkaline soil is slower to release metallic nutrients, such as iron and manganese. If your soil is deficient in these nutrients, foliar feeding can help in the short-term while you make long-term adjustments to your soil.
The downside to foliar feeding
Simply spraying fertilizer on your plants’ leaves is a good way to burn them. There are too many variables to make foliar feeding something you would want to do all the time with all your plants. Environmental conditions, species characteristics, developmental stages of the plants, varying thicknesses of plant cuticles, and the likelihood of stomata being open or not all contribute to a lot of wasted fertilizer and the potential for harm.
Foliar feeding case in point
For those of you who have been reading The Daily Garden for a while now, you may recall reading about how my first soil test, in 2015, reported extraordinary numbers for all nutrients except iron. This was due to over-fertilizing done by the previous owner. That imbalance made those abundant nutrients largely unavailable to my plants. Also, my soil pH at that time was 7.7 and the soil was badly compacted. Truth be told, it looked and felt like concrete.
At that time, nearly all the plants in my landscape were being damaged by fungal diseases (partly due to badly aimed sprinklers), aphids, borers, scale insects, and what looked like nutrient deficiencies. Of course, the automatic (and incorrect) response would have been to add more fertilizer. Thanks to my lab-based soil test, I had the information I needed to make better decisions.
This brings me to the most important aspect of foliar feeding: it is a temporary fix for a much larger soil problem.
Instead, select plants suited to your soil and microclimate, get your soil tested periodically, and remain skeptical about too-good-to-be-true advertisements.
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