Secondary Micronutrients That Are Crucial to Plant Health
Most growers have a handle on primary nutrients and macronutrients, but how is your understanding of the lesser known secondary and micronutrients?
Understanding essential plant nutrients and their role in the garden is the quest for every grower. To be a successful grower, knowing the difference between primary and secondary nutrients is as important as knowing why nutrients are categorized as macro and others as micro.
There are nutrients that are required by plants in larger quantities (referred to as macronutrients) and others (micronutrients) that are needed in trace or much smaller quantities.
Though many may assume all macronutrients are considered primary, there is yet another divide within this group—that of primary and secondary macronutrients (Fig. 1). Some of the essential nutrients come from the air and water, while the rest come from the soil or grow media and from fertilizers added to that media.
From Air and Water
|From Grow Media and Fertilizer|
The idea of certain nutrients being more important than others is misleading. Though the amount of these nutrients varies between plant genuses and species, these nutrients are all required to some degree for plant health and vigor.
As a crop’s yield increases, so is the uptake of almost all essential nutrients. We can look at leaf tissue analysis to get a better idea of how much of the essential nutrients a plant actually needs. This chart (Fig. 2) is reflective of a cereal crop similar to barley.
Plant Tissue Analysis / Growth Stage / Whole Plant
Nitrogen (N) %
1.75 - 3.0
3.0 - 4.0
Phosphorous (P) %
0.26 - 0.5
0.5 - 0.8
Potassium (K) %
1.5 - 3.0
3.0 - 5.0
Sulfur (S) %
0.15 - 0.40
0.40 - 0.8
Calcium (Ca) %
0.2 - 1.0
1.0 - 1.5
Magnesium (Mg) %
0.15 - 0.50
0.5 - 1.0
Zinc (Zn) ppm
15 - 70
70 – 150
Iron (Fe) ppm
20 - 250
250 – 500
Manganese (Mn) ppm
15 - 100
100 – 250
Chlorine (Cl) ppm
4.5 - 25
25 – 50
Cobalt (Co) ppm
2.5 - 25
25 – 50
Copper (Cu) ppm
3.7 - 25
25 – 50
Boron (B) ppm
5 - 25
25 – 75
Molybdenum (Mo) ppm
.03 - 5
5 – 10
Note: Requirements for macronutrients is listed in per cent, while micronutrients are ppm (or 1/10,000 per cent)
Source: Manitoba Provincial Soil Testing Laboratory, 1987.
When researching nutrient requirements, one will find many sources that have documented the primary macronutrients, but then the available information drops off for the others.
Most soils supply some of these “lesser” nutrients, but very often some are missing. With other grow media, these nutrients will likely be missing completely, which is no problem if the grower provides nutrients that make up for this. Be sure to read the label of your preferred nutrient products to make sure your plants are receiving what they need.
Since there is no shortage of information on primary macronutrients, let’s look at secondary macronutrients and what they do for a plant. These are sulfur, calcium, and magnesium.
Sulfur is integral to every living plant cell. It is required for synthesis of various amino proteins and acids as it is required during the process in which nitrate-nitrogen is converted to amino acids. Sulfur is a key nutrient for photosynthesis and crop winter hardiness. Obviously, sulfur is vital to plant health and yield.
Sulfur is supplied to plants from the soil by organic matter and minerals, but it is often present in insufficient quantities. Most of it is tied up in the soil itself and not available to the plants until converted by bacteria to sulfate.
During this mineralization process, acids are created in the soil and help bring down soil pH. This pH adjustment is very helpful in soils that typically face high pH issues. High pH will cause many nutrients to become bound to soil particles and remain unavailable for plant uptake. After this process, as a sulfate, it is mobile and is often leached through the soil and away from the plant.
When diagnosing malnutrition, sulfur deficiency and nitrogen deficiency are often confused. Symptoms of both deficiencies may appear as plants with poor growth, and often as a general yellowing of leaves.
Sulfur, then, is not mobile within the plant. With sulfur deficiency, yellowing symptoms often first appear in younger leaves, whereas with nitrogen deficiency, the yellowing appears on the older leaves first. In less severe situations, visual symptoms may not even be noticeable.
Calcium is often an overlooked essential nutrient, though it carries a very important load in plant growth. Soil fertility programs are developed for many high-yield and high-quality crops.
When calcium takes a back seat, not only does yield suffer, but plant health and many other factors are influenced. Peanut and tomato growers are probably the most likely exceptions to this problem as they generally emphasize good calcium nutrition for their crops.
Calcium improves the absorption of other essential nutrients by the root system, as well as facilitating their translocation within the plant. It can activate a number of plant growth enzymes and helps to convert nitrate taken up by the plant into forms needed to produce proteins. Calcium is vital for cell wall formation and healthy cell division as well as improving disease resistance.
In the soil, calcium can replace hydrogen ions on the soil surface when it is added during fertilization, and it can increase the soil water-holding capacity. Microorganisms need calcium to perform the process of converting crop residue into organic matter.
At the same time, calcium aids in releasing nutrients and helps enable nitrogen-fixing bacteria on the roots of legumes to capture atmospheric nitrogen and convert it into a form that the plant can utilize.
Magnesium is very important for plant health as it is key in a plant’s ability to produce and retain chlorophyll. The energy required for plant growth comes from photosynthesis and this process only occurs because of chlorophyll. Without chlorophyll, plants cannot manufacture food and life as we understand it would cease to exist.
Magnesium is also important for various plant enzyme systems. Stunted growth is often the result of magnesium deficiency. Magnesium is usually available naturally in most soils and is often overlooked as an important nutrient that may be needed for the health and vigor of a crop.
Magnesium becomes available within the soil by weathering processes on other natural minerals. Magnesium availability to plants is subject to soil pH.
Magnesium becomes subject to removal from soil when that soil is sandy or has a low pH (is acidic). Some crops will use and remove much more magnesium than others. Sugar beets and corn are two crops that use larger amounts of magnesium.
The ratio of calcium to magnesium availability is important. For soils with a cation exchange capacity (CEC) higher than about five ME per 100 grams, it is often a good idea to maintain the soil Ca:Mg ratio at about 10:1. For sandy soils (which leach more rapidly) having a CEC of five ME or less, it’s generally best to maintain the ratio at about 5:1.
Outdoor agriculture has long focused on the primary macronutrients to maintain and improve plant health and yield. This has generally been successful as soil naturally provides most of the essential nutrients, though there are exceptions in this practice due to climate and location.
With hydroponic growing, many grow mediums do not naturally contain secondary and micronutrients, so attention to these “lesser” nutrients is critical.
As our outdoor growing acreage continues to come under nutrient stress due to crop utilization, and as we grow more of our food using hydroponic methods, we will need to make certain our fertilizers contain a complete balance of all the essential nutrients that plants need.