Micronutrients are nutrients which are needed by an organism to survive, but only in very small amounts, in contrast with macronutrients. Organisms across the planet have evolved some very specific nutritional needs which are met through a combination of macro and micronutrients. These nutrients are used by cells to create enzymes and perform a variety of functions which keep the organism as a whole functioning in a healthy way.
Plants and animals both need micronutrients. Some examples of human micronutrients are cobalt, copper, manganese, zinc, iron, and selenium. In some cases, consuming too much of a micronutrient can be hazardous, while consuming too little can lead to a nutritional deficiency and health problems for the organism. Many of these nutrients cannot be stored well, which means that the organism needs to get a small amount every day to meet its needs.
In the world of gardening, as long as plants are fed with the appropriate fertilizers, they should not suffer as a result of lack of micronutrients. Many fertilizers contain a complete blend, and plants can also access minerals in the soil. It is also advisable to test the soil to see which nutrients are already present, to avoid overloading plants with nutrients they do not use and cannot need. Soil testing can be done by taking samples and sending them to a lab.
If the soil in an area is unique, it may be necessary to blend a custom fertilizer to meet the needs of the plants. Once can do this by taking the soil profile and a profile of the needs of specific plants, and comparing them to identify nutrients which the plants need and cannot get from the soil. Gardening stores and nurseries sometimes sell micronutrients or blends of micronutrients formulated for specific soil types for the purpose of addressing specific nutritional deficiencies.
In the case of mammals, such as humans, livestock, and pets, micronutrients should be naturally absorbed by eating a balanced and healthy diet. However, feeding too much of the same thing, not eating a diverse diet, or radically changing a diet can lead to nutritional deficiencies. Some people address this by taking nutritional supplements which are designed to ensure that they get the micronutrients they need. In the case of animals, fortified feed can be used to provide complete nutrition if there are concerns about the nutritional balance available in regular food, or people can add diversity to the feeding schedule with things like vegetables and whole grains.