How to Spot (and Fix!) Common Nutrient Deficiency in Plants
You’ve chosen plants that thrive in the Denver metropolitan area: Check. You’ve been careful to give them not too much—or too little—water. Check. You’ve worked just enough compost into the soil to allow for drainage, aeration, and a healthy ecosystem of microbes and fungi. Check. You’re reasonably certain your plants haven’t been exposed to herbicides. Check.
But your garden’s just not thriving, which means it’s time to find out if you have a nutrient deficiency in your plants.
Here’s what you need to know about nutrient deficiency in plants in your garden, and how to address them.
Understanding essential garden nutrients
Soil nutrients work with—or against—one another, depending on how well they’re in balance. Nitrogen, phosphorus, and potassium are your garden’s nutrient rock stars, represented in most commercial plant mixes in the percentages following the N-P-K order. Therefore a product labeled as 10-5-5 contains 10% nitrogen, 5% phosphorus, and 5% potassium. Those numbers might change depending on the product’s concentration; for example, a common balanced fertilizer is 20-20-20.
Micronutrients—calcium, magnesium, iron, manganese, and others—are the “backup singers” in the band. Most packaged fertilizers contain these trace nutrients, and gardeners can purchase “standalone” organic and chemical amendments to fill in the gaps.
“I just need more fertilizer!” is the first assumption gardeners make when they notice stunted or discolored annuals, perennials, shrubs, or trees. They rush to their nearest garden center and stock up on steer manure, or raid their neighbor’s chicken coops for litter. But too much of a good thing is really, really bad for plants, even when that “good thing” is the most important nutrient for the garden.
- Not enough nitrogen: Mature leaves at the bottom of the plant turn yellow-green, eventually becoming dry and crumbly. Upper leaves turn pale. Stunted growth.
- Too much nitrogen: Shedding of healthy leaves, browned leaf edges, lower leaves yellowed and/or wilted.
- Good sources of nitrogen: Aged, thoroughly-composted manure, decaying matter from legume cover crops.
Phosphorus deficiencies usually occur in young plants before their roots have become sufficiently established to absorb nutrients, especially when cool Colorado spring temperatures haven’t triggered soil biome activity.
- Not enough phosphorus: Purple blotches on leaves, purple leaf veins, limited root growth.
- Too much phosphorus: The danger in overdosing your plants with phosphorus is hindering the absorption of important minerals. Look for symptoms found in iron and manganese deficiencies.
- Good sources of phosphorus: Bone meal, rock phosphate, bat guano, alfalfa meal.
Potassium triggers enzymes that create the building blocks for starch and protein production and the adenosine triphosphate (ATP), which is essential to photosynthesis. Potassium also helps regulate turgidity, the flow of water through the plant’s structure, and the water pressure within individual cells.
- Not enough potassium: Weak stems. Leaf edges will yellow, curl under, and become dry and brittle. Veins typically remain green. As with a (P) deficiency, purple discoloration may occur. Plants tend to wilt and wither in between routine waterings, as potassium helps protect plants against drought.
- Too much potassium: While surplus potassium itself won’t harm plants, an overabundance will negatively affect how the plant can absorb and use other essential nutrients. Look for symptoms associated with other nutrient deficiencies.
- Good sources of potassium: Kelp meal, hardwood ash, glauconite greensand
Plants small amounts of iron to build the cells required for photosynthesis. Iron’s what makes Colorado’s soil red, but that doesn’t mean your garden plants are getting the right amount of this micronutrient. Think, “iron, iron, everywhere, but not enough to uptake”. That’s because high soil pH—in other words, too much alkalinity—will prevent your plants from accessing its important mineral.
- Not enough iron: Poor fruit set, pale yellow, or even white leaves.
- Too much iron: Iron toxicity resembles nitrogen “burning” and other nutrient imbalances: Scorched and spotted leaves are most common.
- How to regulate iron: Lower your soil’s pH by adding sulfur or aluminum sulfate. If your water has high salt content, consider using rainwater, or water sources not connected to your water softening system. You rarely need to add iron, especially if you’re gardening in the Denver area. Plants are equipped with chelate reductase, an enzyme that makes the most of the existing iron in the soil.
Calcium’s primary function is creating strong cell walls.
- Not enough calcium: New growth exhibiting symptoms of calcium deficiencies can mimic those associated with herbicide residue poisoning: Stunted, curled leaves.
- Too much calcium: Calcium is alkaline, and calcium toxicity presents as a pH imbalance. It’s the leading cause of blossom end rot in vegetable plants.
- How to fix it: Amend your soil with lime or crushed, dried eggshells.
Manganese is essential to root cell formation, photosynthesis, respiration, and reproduction. Plants with manganese imbalances show symptoms that are nearly identical to those associated with too much (or too little) iron.
- How to fix it: Manganese quickly leaches from loose, sandy soil. Manganese sulfate or chelated manganese is best applied as a foliar spray. If you have too much manganese, reduce your soil’s acidity by adding lime and improving drainage.
Magnesium is important for producing chlorophyll, and it helps plants absorb potassium.
- Not enough magnesium: This might sound familiar: Yellowing or reddish blotches in between green veins. (Actually, magnesium deficiency can be quite pretty… until the plant fails to flower or set fruit.)
- Too much magnesium: Resembles calcium deficiencies, and can cause salt buildup in soils.
- How to fix it: Boost magnesium with foliar applications of diluted Epsom salts on mild days every few weeks can boost magnesium. Ease back on potassium additives, and add organic material to sandy soils. If you think you have magnesium toxicity in your garden, check for poor drainage.
Best practices for maintaining balanced soil nutrients
Prevention’s the best medicine for nutrient deficiency in plants. Plan to prep your garden well in advance of your actual planting dates so your plants have what they need to get established and have a healthy start.
Get a soil test. Because so many deficiency symptoms look alike, even seasoned gardeners need help making a diagnosis. Do-it-yourself kits can help get you on the right track, but did you know you can get comprehensive soil reports from your local agriculture extension? Denver’s Country Fair Garden Center offers soil collection kits from CSU extension complete with easy, detailed instructions so you can have accurate readings for mineral content, pH, and composition.
Plant species with similar nutrient, pH, sunlight, and irrigation needs. Some plants are notoriously heavy feeders, requiring a lot of nitrogen. Plants at different stages of growth and establishment need more of this, less of that.
Add organic content. Work two inches of well-aged compost into compacted soil to create a healthy ecosystem for the fungi, bacteria, beneficial organisms that help your plants access the nutrients they need. Compost also helps retain moisture and improve drainage, and as it breaks down, it adds nutrients to the soil at a rate that won’t cause sudden fluctuations in your soil chemistry.
Heavy, clay soils benefit from compost, but you may want to add garden sand to prevent water from pooling up and rotting roots. Poorly-drained soil accumulates salts and can throw other minerals off balance. Conversely, sandy soils leach out important nutrients and quickly dry plant roots. Organic material slows the flow.
Mulch your beds. Mulch helps create consistent soil temperatures and moisture, and can even reduce pest problems. Heat and drought stress inhibit nutrient uptake, and as we’ve discussed, can mimic symptoms of nutrient deficiencies. As organic mulches break down, they release nutrients to the soil.
Local gardening pros have the dirt on Colorado’s soil nutrients!
If you’re gardening in the Denver area, get advice from experts who know how to succeed in Colorado’s climate and native soils, as well as how to spot nutrient deficiency in plants. Contact us at Country Fair Garden Center for a soil test kit, and once you get the results, come back for recommendations on soil amendments that will put your garden in balance!