Fertilizer Basics: Plant Micronutrients

The best long term way to keep your garden soil rich with the micronutrients it needs is by adding compost. The living things that go into compost — grass clippings, leaves, plant trimmings — already contain various amounts of micronutrients. Their presence in your compost guarantees that you’re returning those micronutrients to the soil.

But what if you can tell (pdf format), because of yellow leaves or other signs of weakness (or from your extension services soil test), that your soil is deficient in micronutrients? Your plants are well on their way and it’s too late to effectively amend the soil. What can be done to give them a quick boost full of the micronutrients they need?

The answer is foliar sprays. Plants are capable of taking in nourishment through their leaves as well as their roots. Foliar sprays work quickly to give your plants the micronutrients they need, and maybe just in time. Many foliar sprays are made of sea products: seaweed and/or fish emulsions. Some products are designed to give you just the specific micronutrients — say, iron — that you need. But seaweed products give you a range.

Spraying kelp and seaweed products is also generally a good idea once plants are established. By increasing your plants overall health, you help it fend off disease and insect pests. Just as nutritionalists suggest humans supplement zinc to keep the immune system up, a boost of micronutrients for your plants, including zinc, will increase their strength and resistance. Seaweed products also contain amino acids and enzymes that lead to vigorous growth and increased yields.

The seven micronutrients make up less than 1% of the dry weight of most plants. But they are crucial to your garden’s well-being. If you have access to detailed soil testing — most of the seven nutrients are included in the standard soil test offered by extension services — and know what micronutrients your soil lacks, then it’s wise to supplement your soil before planting with organic amendments such as gypsum to increase sulfur levels or borax to increase boron. But if you’re well into the season and you’re seeing yellow leaves or other signs of plant distress, then, by all means, give them a spray of kelp or seaweed extract.

If you’re lucky enough to live by the sea and have a source for seaweed, by all means add it to your compost heap. But be sure to hose it off first so that harmful salts will be removed. When your well-traveled Planet Natural Blogger lived on the beautiful Olympic Peninsula of Washington State, he would go down to the Strait in the fall after a storm and gather up a pickup load of seaweed that had been blown ashore. He’d hose it down or just let the rain do the work — we got up to 55 inches a year — and then add it to the compost heap. It also made a swell mulch but was kind of slick underfoot. But we attributed the vigor of our cool, damp garden to that composted seaweed and the micronutrients it supplied.