As a gardener, a basic understanding of plants' nutritional needs will help you tune into your garden's overall health. Plants give all kinds of warning signs when something isn't right, like yellowing leaves. Often, such symptoms can be traced back to nutrient imbalances in the soil.
There are 16 essential elements that, according to Missouri Botanical Garden, plants can't survive without. Carbon, hydrogen and oxygen are all major elements that plants need. Plants acquire these through water and air. The other 13 elements are considered plant nutrients and, according to Dr. Lois Berg Stack at The University of Maine Cooperative Extension, are provided to plants almost entirely through soil. Plant nutrients fall under two categories: macronutrients and micronutrients.
Macronutrients refer to nutrients that plants need in significant amounts. Macronutrients are further broken down into two subgroups: primary nutrients and secondary nutrients. Primary nutrients include nitrogen (N), phosphorus (P) and potassium (K) and are required by plants in larger quantities. Secondary nutrients include calcium (Ca), magnesium (Mg) and sulfur (S), which plants require in smaller amounts than primary nutrients. Since plants generally require macronutrients in greater amounts, these are the nutrients most likely to be deficient in soils for adequate plant health.
When thinking of the main nutrients plants need, which are nitrogen, phosphorus and potassium, remember the adage, "shoots, roots and overall health." Nitrogen is important for the development of a plant's shoots, meaning the leaves and stems. Phosphorus plays a major role in root development (as well as flower development). And according to the Center for Agroecology and Sustainable Food Systems at University of California, Santa Cruz, potassium helps to build a stronger, more disease-resistant plants.
The University of Missouri Extension lists micronutrients needed by plants as boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo) and zinc (Zn). Some sources, including Cornell University, consider nickel (Ni) and cobalt (Co) as micronutrients, as well. Generally, plants require micronutrients in very small amounts. As a result, it is less likely that your plants will suffer from a micronutrient deficiency.
The above images, provided by Karen McCarroll at Farmspeak, demonstrate the symptoms caused by nutrient deficiencies in plants. Do any of them look familiar? If you suspect a nutrient deficiency, get a soil test if possible to help confirm your suspicions. We don't recommend supplementing with isolated nutrients, even in organic form, unless you know your soil needs it. An overabundance of a nutrient can cause its own set of issues.
Tips for correcting nutrient imbalances
1. Check the pH.
Plant nutrients are most available at a soil pH of between 6 and 7, which is slightly acidic to neutral. Test the soil pH to see if it's out of range and may be contributing to the poor health of your plants.
2. Make sure you're watering appropriately.
Nutrient deficiencies may actually be a result of overwatering or underwatering. Overwatering can limit oxygen availability and result in poor root development, which reduces nutrient uptake. And underwatering limits the movement of nutrients through the soil, making them less available to plants.
3. Rotate your crops.
Different plants use different nutrients, and in different amounts. Rotating your crops year to year will help avoid nutrient depletion.
4. Plant a cover crop.
Cover crops, like white clover and winter rye, can be used in between growing seasons to rebuild the soil. Cover crops protect soil from erosion and provide organic material to dig into your soil to break down once you're ready to plant again.
5. Use compost.
The best way to ensure nutrient-dense soil is to add compost to your garden yearly. You can also make compost tea to use as fertilizer, which is loaded with micronutrients and beneficial soil organisms.