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.
Secondary plant nutrients are calcium (Ca), magnesium (Mg), and sulfur (S).
The nutrients plants use the most are called primary nutrients. Nitrogen, potassium, and phosphorus are primary nutrients. On the other hand, only tiny amounts of boron, copper, iron, chloride, manganese, molybdenum, and zinc are needed. These micronutrients used to be called trace elements. In the middle are the secondary nutrients. Secondary nutrients rarely need to be supplemented, but they are very important to plant health.
Most soils already contain high enough levels of these secondary nutrients, but you don’t know for sure without a lab-based soil test. The effects of not enough or too much of any one nutrient can create a domino effect that is difficult to diagnose. Simply adding more fertilizer can often makes problems worse, rather than better.
Why are these secondary nutrients important and what are some signs of toxicity or deficiency? Let’s find out!
Plants use calcium to build strong cell walls, to move materials across cell membranes, to grow primary root systems, and to maintain the cation-anion balance. [Cations and anions are electrically charged atoms of minerals that plants use for food.] Optimal levels of calcium range from 1000 to 1500 parts per million (ppm).
Calcium is relatively immobile inside a plant. It takes a lot of water to move a calcium molecule around inside a plant. That’s why blossom end rot is more of an irrigation problem than a calcium deficiency problem. Calcium deficiencies, whether caused by real lack or insufficient irrigation, are rare in nature. When they do occur, they can cause bitter pit in apples, cavity spot in carrots, and leaf tip burn in several different plants.
Too much calcium is also rare, but it can interfere with the absorption of magnesium and potassium, causing deficiencies in those nutrients. Bottom line with calcium: irrigate adequately, regularly and consistently.
Magnesium is essential for plant health. Ideal levels of magnesium range from 50 to 120 ppm. Magnesium stabilizes cell membranes, making plants better able to withstand drought and sunburn. Magnesium is found in enzymes that plants use to metabolize carbohydrates. Most important, magnesium is contained in the chlorophyll molecules that convert the sun’s energy into food. This process, the Calvin Cycle, is what makes photosynthesis possible.
Too much magnesium in the soil makes it difficult for plants to absorb calcium and other nutrients, which can lead to blossom end rot, bronzing, and many other problems. This is a common problem in areas with alkaline soil. The opposite is true in areas with acidic soil.
Insufficient magnesium symptoms look like potassium toxicity symptoms. Older leaves, at the bottom of the plant, start turning brown, between and alongside the leaf veins, working upward through the plant. Magnesium deficiencies in stone fruits often start out as slightly brown areas along leaf edges (margins) that expand inward, causing cracking, necrosis, and leaf loss. Magnesium deficiency in California is extremely rare.
Plants use a surprising amount of sulfur. This secondary nutrient is used in making chlorophyll and certain proteins and enzymes. Sulfur is also part of the arrangement between legumes and rhizobia bacteria that allow them to make use of atmospheric nitrogen.
Sulfur deficiency is seen first in new growth. Leaves are pale and growth is spindly. If sulfur levels become toxic, leaves will be smaller than normal and have scorched edges. Sulfur is commonly used as an organic fungicide and to acidify the soil.
Do not use horticultural oil within 2-4 weeks of applying sulfur. Sulfur and horticultural oil are phytotoxic (poisonous to plants) when combined. Also, it is better to use fixed copper, rather than sulfur, on apricot trees.
Your plants may not need as much of these secondary nutrients, but they are just as important to plant health. Get a lab-based soil test to find out what is in your soil.
You can grow a surprising amount of food in your own yard. Ask me how!