Macronutrients get most of the attention when feeding your garden, but the little guys are just as important to plant growth, health, and yield. Trace elements, or micronutrients, play critical roles in plant development. They are needed in very small quantities. Too little or too much of these elements can cause a great deal of harm to your garden. Three often overlooked micronutrients are cobalt, copper, and chromium.
Cobalt and Plant Growth
Cobalt is classified as an essential micronutrient and plays a critical role in the overall growth process of plants. Cobalt is necessary for the processes of stem growth, elongating the coleoptiles, and expanding leaf discs. It is a critical element needed for a plant to reach maturity and for healthy bud development.
It is a primary constituent of vitamin B12 and propionate. Vitamin B12 is essential for cell division during growth, while propionate serves as one of the prime energy sources during growth. In legumes, cobalt aids the plant’s ability to fix nitrogen from the atmosphere.
With such a critical role during plant growth, cobalt deficiencies should be corrected immediately. A cobalt deficiency will manifest as reduced or stunted plant growth and reduced seed germination. It may present as reddening of the leaves, stems, or petioles. In legumes, you may notice yellowing leaves or small root nodules.
Cobalt toxicity is more common than a deficiency and usually results from excess cobalt in irrigation water or excess nickel in the soil. High levels of cobalt create an iron deficiency so be on the lookout for symptoms of iron chlorosis. The leaves will turn yellow while the veins stay green, stunting the growth of the entire plant.
To ensure your plants are taking up the proper amount of cobalt, make sure your soil has a neutral pH and contains plenty of organic matter. If the deficiency is not corrected, then you can add a fertilizer that contains cobalt.
Copper in the Garden
Copper is another essential micronutrient necessary for plant growth. It facilitates respiration, photosynthesis, and plays a significant role in plant metabolism of carbohydrates and proteins. Copper affects the strength of cell walls as well as the sugar content and shelf life of fruit. It plays a part in several enzyme processes, including the formation of chlorophyll.
Some plants benefit from copper more than others. For example, sunflowers, lettuce, spinach, beets, onions, and tomatoes have higher copper requirements than cabbage, broccoli, cucumber, mint, peas, and potatoes. Copper is what gives onions their brown papery skin.
Copper is needed in very small quantities by plants. There is a thin line between copper deficiency and copper toxicity. It is important to note that under organic food laws, copper may not be added as a nutrient unless there is a documented soil or tissue deficiency. The normal range in a growing medium is 0.05-0.5 parts per million (ppm), and the normal range in most plant tissues is three to 10 ppm.
Copper deficiencies and toxicities are both rare, but they do happen under certain conditions. A deficiency will present itself as slow growth, chlorosis, reduced starch formation, delayed flowering, and sterility.
Seeds saved from plants with copper deficiency will have a hard time germinating. Legumes will not be able to fix nitrogen as well as they would normally, leading to chlorosis. The upper portion of broad-leafed plants will wilt and the top leaves will turn a bluish-green color.
Copper deficiencies occur most in sandy, alkaline soils. High alkalinity and waterlogged soil reduce the availability of copper to plants even if it is present in the soil. Organic matter that has not been broken down enough will tie up copper until it is fully decomposed. Excess zinc, nitrogen, and phosphorus also reduce copper uptake.
The most effective way to fix a copper deficiency is to fix your soil. Copper’s ideal pH range for proper uptake is 5.5 to 6.5. Be sure to improve your soil drainage by adding fully decomposed organic matter. You may also want to test your soil for excess zinc, nitrogen, and phosphorous. The quickest fix to correct this problem is to transplant your sick plants into a raised bed or container with proper drainage and pH, being careful not to damage the root system.
Copper toxicity can stunt the growth of your plants and restrict root growth by burning the root tips. It can reduce seed germination and lower iron availability. You may not catch copper toxicity at first because new growth will initially be greener than normal before plant health starts to decline and show signs of iron or other micronutrient deficiencies.
The most common causes of copper toxicity are excessive use of copper sulfate fungicides and industrial activity, such as mining, that releases copper into the soil.
Chromium and Plant Health
Although chromium is a trace element widely distributed in the soil, you should be aware of potential chromium toxicity in your garden. In low doses, chromium is an essential element in human and animal nutrition. However, large quantities can have devastating effects on humans, animals, and plants.
Although there are a couple of studies from the early 1900s that found it might be a stimulant for plant growth, there is an overwhelming amount of evidence that shows it does more harm than good for your garden.
Chromium is more of a health concern in modern times due to industrialization. Harmful forms of chromium are used in industry for plating, alloying, tanning animal hides, textile dyes and mordents, pigments, ceramic glazes, refractory bricks, and pressure-treated lumber.
Large amounts of chromium have been found in agricultural soils due to the use of organic wastes as fertilizers and the use of waste water for irrigation. Chromium does not degrade biologically and will remain stable for several months in the soil without changing its oxidation state.
The most toxic form of chromium, hexavalent chromium, is easily accumulated by plants. When too much chromium is absorbed, there will be reduced seed germination, chlorosis, photosynthetic impairment, and eventually plant death.
If you suspect a high concentration of chromium in your garden’s soil, it is a good idea to get it tested to be sure. The 2007 Canadian Soil Quality Guidelines for the Protection of Environmental and Human Health recommends soil should contain less than 64 milligrams per kilogram (mg/kg) of total chromium, and 0.4mg/kg of the form hexavalent chromium.
If you do have high levels of chromium in your soil, beneficial myccorhizae can ameliorate nutrient deficiencies caused by chromium toxicity. This also means you should dig the soil as little as possible so that mycorrhizal colonies can establish themselves. Your best bet would be not to grow in that soil at all. Instead, install raised beds with healthy soil or grow in containers.
Cobalt, copper, and chromium aren’t talked about much in the gardening world, but they can make or break your garden. Use this information to correct deficiencies and toxicities as soon as they occur for healthy, happy plants.