All About Micronutrients

While nitrogen, phosphorus, and potassium (N-P-K) are the A-list stars of the plant nutrition world, there’s a large supporting cast of essential nutrients that also play a critical role in plant health.

Plant nutrients fall into two major categories: macronutrients and micronutrients. We’ll describe these by using human nutrition as a handy comparison. 

Macronutrients are the main building blocks in plant nutrition, just like carbohydrates, protein and fats are for humans. It’s widely understood by gardeners that nitrogen (N), phosphorus (P) and potassium (K) play a similar role for plants. This N-P-K trio are known as primary micronutrients.

What’s often overlooked though, is that there are more essential macronutrients on the list; plants also need calcium (Ca), magnesium (Mg), and sulfur (S) in lesser quantities, from the soil around their roots. These elements are usually described as secondary macronutrients.

Plants also require carbon (C), hydrogen (H) and oxygen (O) (these are known as structural macronutrients), but because plants absorb them from the atmosphere and water, they don’t have any part to play in fertilisers. 

Micronutrients (AKA trace elements) are the essential minerals that plants extract from the soil. These dietary minerals, often compared to vitamins in human nutrition, are essential for plants to develop properly, plus resist diseases and environmental stress.

In human health, vitamins are essential organic compounds that our bodies can't produce on their own. We need to have vitamins in our food to maintain various bodily functions, like immune defence and cell repair. In much the same way, plants need micronutrients to support a range of vital processes, like photosynthesis, enzyme function, and growth.

Just like humans need a balanced diet to stay healthy, plants require a balance of macronutrients and micronutrients to grow and thrive. 

Nutrient Deficiencies: when the cupboard is empty

Although secondary macronutrients and micronutrients are only needed in tiny trace amounts, if a particular one of them isn’t available in the soil, it can cause significant problems - just as a vitamin deficiency in humans can cause long term health problems. Nutrient deficiencies can be caused (or aggravated) by poor soil quality, unbalanced fertilisers, lack of drainage, or excessive watering.

The soil environment can interact with nutrients in complex ways. For example, high or low soil pH can actually ‘lock up’ and deny important minerals to plants. This effect is exaggerated by extreme pH, so it’s always good to know what the natural pH level is in your garden. You can find out more about pH here.

How uptake of nutrients is affected by soil pH.

There’s a long-established concept in horticulture called ‘Liebig’s law of the minimum’. The idea is that the growth of plants is restricted by the nutrient that’s in the shortest supply. If any single nutrient is lacking, that nutrient acts as a handbrake on total growth.

It’s basically the plant version of the saying “a chain is only as strong as its weakest link”. It means you can throw lots of extra nutrients at a plant, but unless you increase the level of the one that’s missing, you won’t achieve any significant growth from all the others. We need to correct nutrient deficiencies to ensure plant health (and growth) reaches its full potential.

Fortunately, identifying symptoms of a nutrient deficiency is easier than you think. Most deficiencies have quite specific symptoms, so we can show you what to look out for.

Nutrients breakdown: What do they all do?

First, we’ll focus on the key micronutrients: iron (Fe), manganese (Mn), boron (B), copper (Cu), zinc (Zn) and molybdenum (Mo), which all play an important role in plant health. There are more micronutrients in the full list, but we’ll just look at the main ones. Here’s a quick breakdown of the jobs they do, how to identify deficiencies…and how to fix them.

Since the secondary macronutrients don’t get talked about very often, we’ll follow on with symptoms ID and remedies for them too. These are sulfur (S), calcium (Ca) and magnesium (Mg).

1. Iron (Fe)

Iron is vital for chlorophyll production and photosynthesis. It plays a crucial role in energy production inside plant cells.

Iron Deficiency Symptoms: Iron deficiency usually shows up as yellowing of the youngest leaves (chlorosis), especially on the veins. If there’s a shortage of iron, the plant can’t produce enough chlorophyll to stay green: severely affected leaves can turn completely white. This problem is common in camellias, azaleas and other acid-loving plants, as well as citrus, roses and some vegetables. It’s worth testing soil pH if you suspect an iron deficiency, as alkaline soil can lock up iron and prevent plants from absorbing it properly…testing will indicate if pH is the underlying problem. If it is, jump to the sulfur section for the fix!

Solution: Yates ‘Leaf Greener’ Iron Chelate is our go-to product to fix an iron deficiency. Applied as a foliar application, this delivers iron directly to the deficient leaves for faster results. Foliar applications add less iron into the soil, compared to traditional soil drench applications, which means less wastage and lower risk of soil toxicity.

If you aren't confident with your diagnosis, you can choose Yates Trace Element Chelates, which will cover all the possibilities. It supplies all the key elements needed for healthy plants (including iron), in carefully calibrated proportions to deliver a balanced result.

2. Zinc (Zn)

Plants use zinc to make plant hormones (especially auxins), which control growth and wound healing. Zinc also helps the plant regulate its metabolism.

Zinc deficiency in citrus, most obvious on older leaves

by Scot Nelson, via Flickr

Zinc Deficiency Symptoms: Zinc deficiency is indicated by small pale narrow-looking leaves, which can appear bunched together. The older leaves may develop yellowing (chlorosis) between the leaf veins. Other symptoms are branches with very few leaves, stunted growth, or poor flowering and fruiting. Both zinc and manganese deficiencies often occur in citrus trees.

Solution: Fortunately, there’s a simple remedy for a zinc deficiency: Yates ‘Citrus Cure’ Zinc and Manganese Chelate delivers zinc directly to the deficient leaves, as a foliar application. This reduces excess zinc additions into the soil (compared to traditional soil drenches), lowering the risk of soil toxicity.

If you aren't confident with your diagnosis, you can choose Yates Trace Element Chelates, which will cover all the possibilities. It supplies all the key elements needed for healthy plants (including zinc), in carefully calibrated proportions to deliver a balanced result. 

3. Manganese (Mn)

Manganese powers photosynthesis, by helping to split water molecules during the reaction with sunlight, that converts carbon dioxide and water into carbohydrates and oxygen. Manganese also helps plants absorb nitrogen.

Manganese deficiency in an orange. Young leaves are affected first.

Manganese Deficiency Symptoms: Shows up as mottled yellowing between the veins of young leaves, usually in early spring. The veins tend to remain green. In severe cases you may see browning (necrosis) at the leaf tips. A shortage of manganese looks similar to zinc deficiency symptoms, but you can tell them apart by looking at the age of the affected leaves (older leaves = zinc, younger leaves = manganese). It’s worth testing soil pH if you suspect a manganese deficiency, as alkaline soil can prevent plants from absorbing manganese properly. Testing will tell you if pH is the actual cause of the problem. If it is, refer to the sulfur section for a solution.

Solution: There’s a very easy fix for a manganese deficiency: Yates ‘Citrus Cure’ Zinc and Manganese Chelate delivers manganese directly to the deficient foliage. This reduces excess manganese additions into the soil, which lowers the risk of soil toxicity.

If you aren't confident with your diagnosis, you can choose Yates Trace Element Chelates, which will cover all the possibilities. It supplies all the key elements needed for healthy plants (including magnesium), in carefully calibrated proportions to deliver a balanced result. 

4. Copper (Cu)

Copper is essential for photosynthesis, respiration, and the formation of lignin, the substance that strengthens plant cell walls (and gives rigidity to bark and wood).

Copper Deficiency Symptoms: Plants can develop distorted growth, with younger leaves turning yellow, or browning off at the edges. The tips of young leaves can show signs of wilting, dieback or browning. Copper deficiency is more common in sandy or acidic soils than in clay soils.

Solution: Yates Copper Chelate is an easy-to-use liquid concentrate that corrects copper deficiency. It’s best applied as a foliar spray directly to the leaves, which reduces excess copper additions into the soil. This lowers the risk of soil toxicity.

If you aren't confident with your diagnosis, you can choose Yates Trace Element Chelates, which will cover all the possibilities. It supplies all the key elements needed for healthy plants (including copper), in carefully calibrated proportions to deliver a balanced result. 

5. Molybdenum (Mo)

Nitrogen is the nutrient plants need most, but they need to break it down in a 2-step process before it’s in a form they can use. Molybdenum is an important part of this process, which reduces nitrates into usable ammonia. Without molybdenum, plants can’t access essential nitrogen.

Molybdenum Deficiency Symptoms: Yellowing of the leaves, especially on older plants. It can also result in stunted growth and poor root development. Legumes can appear pale and yellow, as lack of molybdenum prevents their symbiotic root bacteria from doing their job of fixing nitrogen.

Solution: Our first choice to correct a molybdenum deficiency is Yates Trace Element Chelates. Because molybdenum is only required in tiny trace quantities, we’ve included it in our ‘all-purpose’ micronutrient solution. Yates Trace Element Chelates supplies all the key elements needed for healthy plants, in carefully calibrated proportions to deliver a balanced result. This is the one to use when you aren’t certain of your diagnosis, because it includes a comprehensive range of nutrients to cover all the possibilities.

6. Boron (B)

Boron is vitally important to plants. It supports the structure of cell walls and cell membranes, along with aiding the transport of sugars within the plant, plus root development and potassium uptake. Boron plays a big role in plant reproduction; it’s essential for pollen germination, seed development and hormone regulation.

Boron is especially important for legume crops, as it has a direct effect on their root nodules; these form homes for the symbiotic bacteria (Rhizobium) that do the actual work of fixing nitrogen for the plant. Insufficient boron = less nodules = nowhere for the helpful bacteria to live!

Boron Deficiency Symptoms: Low boron levels are relatively common in NZ soil. Boron deficiency can restrict the growth of developing seedlings and growing tips. Distorted, thickened or brittle leaves are common, along with interveinal chlorosis (yellowing between the leaf veins), stunted root development, and reduced or delayed flowering and fruit set, leading to poor yields.

Solution: Our go-to product for a boron deficiency is Yates Trace Element Chelates. Because boron is only required in tiny trace quantities, we’ve included it in our ‘all-purpose’ micronutrient solution. Yates Trace Element Chelates supplies all the key elements needed for healthy plants, in carefully calibrated proportions to deliver a balanced result. It's the one to use when you aren’t certain of your diagnosis, because it includes a comprehensive range of nutrients to cover all the possibilities.

7. Sulfur (S)

Sulfur plays a critical role in plant health. It’s a key ingredient of amino acids, which are the building blocks for proteins and enzymes…which in turn are involved in almost everything that happens in plant cells. Sulfur helps to form green chlorophyll, which absorbs sunlight to enable photosynthesis (that’s the process that lets plants convert sunlight into energy).

Sulfur deficiency in a currant bush, with the newest leaves yellowing first.

Sulfur Deficiency Symptoms: Yellowing of new and developing leaves. It’s easy to mistakenly diagnose this as a nitrogen deficiency, but the key difference in sulfur deficiency is that the younger leaves yellow first. Lack of sulfur can cause stunted (or very slow) growth, along with poor yields from vegies.

Solution: Yates Liquid Sulfur ‘Soil Acidifier’ is an excellent source of elemental sulfur, although it’s most often used as an easy way to reduce soil pH. Acidic soil is important for successfully growing acid-lovers like camellias, azaleas, rhododendrons, magnolias and blueberries. Yates Liquid Sulfur can also alleviate ‘lime induced chlorosis’, a condition that causes the leaves of acid-loving plants to yellow (chlorosis) when the soil is too alkaline for their liking. Unfortunately, high soil pH has the effect of locking up iron and manganese, making them unavailable to plants (as illustrated in the chart above).

If you aren't confident with your diagnosis, you can choose Yates Trace Element Chelates, which will cover all the possibilities. It supplies all the key elements needed for healthy plants (including sulfur), in carefully calibrated proportions to deliver a balanced result. 

8. Calcium (Ca)

Calcium is a must-have for plant growth and cell development. It plays a key role in strengthening cell walls, plus it allows plant cell membranes to absorb and pass on nutrients.

Importantly, calcium helps protect plants from harm: it’s a ‘secondary messenger molecule’ with a fascinating ability to trigger a response to outside stimuli (e.g. disease pathogens, frost, insects) by flooding into the fluid inside plant cells (cytosol). This activates a cascade of signals through the cells of the plant, that switches on defence genes that help the plant to withstand the stress. There’s a practical example of calcium’s effect on plant resistance; adding calcium is the classic method to combat blossom end rot in tomatoes.

Calcium deficiency in a strawberry plant.

Calcium Deficiency Symptoms: Stunted growth and burnt-looking dead tissue at leaf tips and edges are classic signs. Younger leaves can curl up or develop a cupped appearance. Calcium deficiency is usually more obvious on fruit than leaves: e.g. fruit can develop surface splitting, tomatoes and capsicums can develop blossom-end rot (dark, sunken-looking decay on the bottom of fruit). Bitter pit (horrible tasting pitted-looking brown spots) can appear on apples. Calcium-deficient lettuces are prone to tipburn (necrosis around the edges or tips of leaves).

Solution: Yates Liquid Lime & Dolomite is an effective and convenient method to add calcium and magnesium to soil, as you can be very precise about how much of each element you're adding. Yates Liquid Lime & Dolomite increases soil pH, so it's often used for jobs like turning hydrangeas pink.

Yates Thrive Natural Garden Lime is a ‘slow and steady’ solution, as lime contains a high level of calcium (Ca). Lime can take a year or more to correct a calcium deficiency in soil. Lime (made from crushed limestone) is often used to increase soil pH, or to improve the structure of heavy clay soil. Because lime naturally makes soil pH more alkaline, it’s good to test your soil pH so you have a baseline measurement, before you begin applying it.

Yates Thrive Natural Dolomite Lime will do a similar job, but in addition to calcium, it contains a generous amount of magnesium (Mg). Gypsum will also improve a calcium deficiency, without affecting soil pH.

9. Magnesium (Mg)

Magnesium does the biggest job of any trace element in activating enzymes that control energy transfer between plant cells (ATP synthesis). These enzymes enable plants to absorb phosphorus and move it to where it’s needed, plus they help to store the carbohydrates (sugars) that end up making your fruit taste delicious.

Magnesium is the central element that makes up a chlorophyll molecule - green chlorophyll is the substance that absorbs sunlight for the plant to convert into energy.

Magnesium deficiency in citrus, showing how the distinctive v-shaped green patch develops

Magnesium Deficiency Symptoms: Yellowing of mature leaves, but the yellowed leaves retain a distinctive residual green patch, shaped like an inverted V at the base of the leaf.

Solution: Yates Magnesium Chelate ‘Leaf Greener’ is an easy-to-use liquid concentrate that corrects magnesium deficiency. It’s best applied as a foliar spray directly to the leaves, which results in less magnesium entering the soil than with traditional Epsom salts. This lowers the risk of soil toxicity.

Alternatively, you can choose Yates Thrive Natural Dolomite Lime for ‘slow and steady’ results. Dolomite contains a substantial proportion of magnesium carbonate, plus a high level of calcium (Ca). Dolomite lime begins to react with soil immediately after application, but it can take 18 months or more to realise its full effect in soil. Apply in autumn and winter, repeating applications every 12 weeks. Because dolomite naturally makes soil pH more alkaline, it’s good to test your soil pH to get a baseline measurement, before you start using it.

If you aren't confident with your diagnosis, you can choose Yates Trace Element Chelates, which will cover all the possibilities. It supplies all the key elements needed for healthy plants (including magnesium), in carefully calibrated proportions to deliver a balanced result. 

Calcium deficiency, causing blossom end rot in tomatoes

Overdoing it: too much of a good thing

We’ve made the case above that trace elements are essential for plant growth…but it’s important to remember that plants only need tiny amounts of micronutrients, which explains why they’re called ‘trace’ elements.

There’s a very compelling reason to be cautious. Trace elements are an absolute necessity for plants in trace amounts; but in high concentrations they become toxic to plants and destructive to your soil.

An excess of a particular trace element can also interfere with the uptake of other trace elements, which leads to…you guessed it, a new deficiency!

Metallic trace elements like iron (Fe), zinc (Zn), manganese (Mn), copper (Cu) and molybdenum (Mo) are technically heavy metals. If used indiscriminately, these metals can contaminate soil; remediating contaminated soil is very difficult.

Calcium deficiency is the root cause of 'bitter pit' in apples

The good news: you absolutely can correct deficiencies in your garden safely, so your plants don’t need to suffer. The very simple rule of thumb for success is: Don’t over-do it. Making certain you don’t over-apply is easy…all you need to do is read (and follow) the label directions.

After you’ve applied a corrective product, check plant symptoms every few weeks to decide if further applications are needed. It’s best to correct deficiencies with small amounts, because you can keep re-doing small applications until the plant is happy. You can’t undo an application, so don’t be tempted to ‘go large’ to get it done in one go!

Apply Yates micronutrient products as a fine mist directly to leaves, but don’t spray to the point of runoff.

Yates micronutrient products are formulated for foliar application. Because they’re delivered directly to the deficient leaves, they work quickly, but a lot less of the trace element enters the soil than with traditional soil drench applications. The outcome is less wastage and a lower risk of soil toxicity.

What is a chelate?

Chelates are organic molecules that can chemically bind metal ions and hold on to them, to form complex compounds. When a chelate grabs a metal ion in a pincer-like grip, it prevents the metal from reacting to any other attractive substance it encounters. Otherwise, metallic trace elements have a habit of binding with other molecules and just sitting there, locked up in a form plants can’t take in. Chelates have a marvellous ability to make metallic trace elements soluble and bioavailable, basically improving their uptake by plants.

Yates micronutrient products are made with lignosulfonate chelates, which are biodegradable, water-soluble polymers derived from lignin, from trees (lignosulfonate chelates are a very useful byproduct of the pulp and paper industry).