If your soil is deficient in these nutrients, foliar feeding can help in the short-term

By Kate Russell

Can foliar feeding help your Morgan Hill garden plants grow bigger and be more productive?

Maybe, and maybe not.

Most people feed their plants by applying fertilizer to the soil. This works well, especially if you have soil test results in hand that tell you what your soil needs. Sometimes it doesn’t need anything and sometimes it contains too much. There is no way to know without a lab-based soil test. But say your soil test indicates that your soil is especially low in a nutrient. Can you just spray it on the plant?

Many claims are made about how well plants can absorb fertilizer through their leaves. This foliar feeding is said to promote larger, sweeter crops, boost a plant’s tolerance for heat and cold, increase pest and disease resistance, and even improve a plant’s internal circulation.

Wouldn’t that be something? The number and diversity of these claims alone should raise a warning flag. The claims about the benefits of foliar feeding are all based on research published in 1957.

This research demonstrated that leaves and fruit can be very efficient at absorbing small amounts of mineral nutrients through tiny holes in leaves and fruit, called stomata. This research was conducted under laboratory conditions, not out in someone’s garden. As one might expect, results are very different in the field. Most of the claims are false, but not all. Normally, plants absorb their mineral nutrients from the surrounding soil through the root system.

How easily those nutrients can be absorbed is a function of soil chemistry and soil pH. This is important because alkaline soil holds tightly to some nutrients, such as iron and manganese, and most Morgan Hill soil is alkaline.

If your soil is deficient in these nutrients, foliar feeding can help in the short-term while you make long-term adjustments. It can also help bring nutrients levels back into balance with each other.

The only way to know if soil has too few (or too many) plant nutrients is to submit a sample for a lab-based soil test. These affordable tests provide a wealth of information, so you can make informed decisions about what to add (or avoid).

Before spraying any type of fertilizer on your plants, you need to know this can burn leaves and damage fruit if done incorrectly. Be sure to read the label and follow the directions exactly, if you decide to use foliar feeding, which can be a temporary fix for a larger soil problem. Changing the nutrient content of soil is not a simple matter of adding fertilizer. These changes take time and informed decision making.

Kate Russell
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