As agriculture professionals, we know that there are more than a dozen nutrients that are essential to achieving optimal plant health. This includes three primary macronutrients (nitrogen, phosphorus and potassium), three secondary macronutrients (calcium, magnesium and sulfur), and eight micronutrients (iron, manganese, boron, molybdenum, copper, zinc, chlorine, and cobalt).
Maximum crop yields are only possible if these nutrients are supplied to the crop at appropriate timing and in the amount each plant needs for healthy development. Even though many of these nutrients are naturally occurring in the soil, they are not all naturally available to the plant in a soluble form that benefits the plant.
The following highlights the primary and secondary macronutrients and the essential micronutrients. It also looks at the key benefits each provides to the plants and what we can expect to see if the plants are experiencing specific nutrient deficiencies.
Primary Macronutrients (Nitrogen, Potassium and Phosphorus)
1. Nitrogen is an essential nutrient for plant growth, development and reproduction, and is part of every living cell. Nitrogen is vital because it is an important component of chlorophyll.
- Plants depend on chlorophyll to help them during photosynthesis, when they use the energy in sunlight to produce sugars from water and carbon dioxide. Essentially, if the plant lacks nitrogen it will not be able to utilize sunlight as an energy source.
- They also need nitrogen to produce proteins which are essential to their healthy development.
Healthy plants usually require and contain three to four percent nitrogen. Plants are best able to use nitrogen when it exists in the soil as ammonium (NH4+) and nitrate (NO3-). However, nitrogen in the soil predominately exists as organic nitrogen which is not immediately available to the plants.
The nitrogen cycle helps us understand how nitrogen converts from inorganic or plant soluble forms to organic forms of nitrogen. It also explains how nitrogen is lost from the soil predominantly through leaching and denitrification, which can increase nitrogen deficiencies in the crops.
- The most effective way to minimize or avoid nitrogen deficiencies is by applying nitrogen in the right amounts and the right form so it is readily available to the crop throughout its growing season.
- Different crops have different timing for their nitrogen requirements. With corn, for example, it will take up much of its nitrogen midsummer. On the other hand, wheat will use most its nitrogen in the spring to early summer.
What does nitrogen deficiency look like in plants?
A nitrogen-deficient plant is usually smaller with a greenish-yellow look and develops slowly, whereas a plant with adequate nitrogen will grow rapidly and produce large amounts of green foliage.
Nitrogen deficiencies start with the older leaves at the bottom of the plant and move through the plant. Sometimes the newer leaves may still be a lighter green.
2. Potassium is a critical component to advance and sustain plant growth and reproduction. This nutrient is also essential in helping plants fight against disease development.
Potassium also affects the plant’s size, shape and color.
- Potassium activates many of the enzymes involved in plant growth (up to 60 different ones). The enzymes facilitate chemical reactions that are essential to healthy plant development by physically changing the shape and exposing the chemical sites for enzyme reactions.
- This important macronutrient also helps with root growth and improves stomate activity which increases the plants tolerance to drought stress.
- Potassium helps with photosynthesis and food formation, improves the translocation of sugars and starches, increases protein content of plants, and helps reduce water loss and wilting.
What are some visible signs of Potassium deficiency?
Plants that are deficient in potassium are more vulnerable to extreme conditions including drought or excess water, extreme temperatures, pests, disease and nematode attacks.
One of the most common signs of potassium deficiency is the yellow sweltering along the leaf border. Potassium-deficient crops also have poorly developed roots, weak stems and growth.
3. Phosphorus is a critical macronutrient with a large impact on increasing yields. Improving phosphorus efficiency and availability for the plant is a conversation that we have discussed on several occasions on our LIFT blog.
- Phosphorus includes an important energy-producing molecule found in all living cells called Adenosine Triphosphate (ATP).
- This primary macronutrient gets a lot of attention because it is essential piece for a plant to complete its normal production cycle, as DNA and RNA are linked together by phosphorus bonds.
- Phosphorus is also important for the transport and utilization of the sugars produced in photosynthesis.
However, unlike nitrogen, phosphorus has extremely limited soil mobility.
- It consistently binds with positively charged elements in the soil causing it to become unavailable for plant uptake.
- Phosphorus tie-up means that much of the phosphorus in the soil is not available to the plants, so it can require the application of up to nine to ten pounds of phosphorus to increase soil test phosphorus by one pound.
The agriculture industry continues to work on increasing phosphorus availability to plants because of its importance to healthy plant development and maximizing crop yields and to reduce its movement to drainage and waterways. Overall, when a plant has adequate amounts of phosphorus, it experiences stimulated root development, increased stalk and stem strength, earlier crop maturity, and increased resistance to plant disease.
What are the visible signs of a phosphorus deficiency in plants?
Compared to nitrogen and potassium, a phosphorus deficiency can be more difficult to detect, and can vary across different crops. Early in the plant’s development, there may not be evident symptoms other than stunted growth. By the time visible signs start to appear later in the plant’s growing cycle, it can often be too late to sufficiently correct the deficiency. Visual symptoms that tend to occur later in the cycle are dark bluish-green leaves and purplish stems due to difficulty in proper utilization of sugars produced in photosynthesis.
Phosphorus deficiencies can be minimized by making sure the plant has adequate amounts of phosphorus throughout its growing cycle, since crop demand for energy and sugar formation and transport are needed from early on in seedling development through grain fill.
Secondary Macronutrients (Calcium, Magnesium and Sulfur)
Even though they may be labeled as “secondary” nutrients, calcium, magnesium and sulfur are equally important for plant nutrition as the primary macros. As crop yields increase and new products emerge within the agriculture industry, demand and methods for obtaining these secondary macronutrients are evolving. For example, growers used to rely on insecticides and fungicides as a source of sulfur. Now, most insecticides and fungicides are made without sulfur, so growers are increasing sulfur applications for their crops. Here’s a brief overview of the benefits each secondary nutrient provides the plants.
1. Calcium does not always get the credit it deserves. Found in every organism, this nutrient is vital as it turns crop residues into organic matter that releases nutrients, and improves soil aggregation and water holding capacity. Calcium also assists in the conversion of nitrogen into its soluble form so the plant can absorb it.
Calcium deficiencies most commonly occur in soils that are sandy, acidic and experience heavy rain or irrigation water. Common signs of a deficiency are reduced leaf growth or leaf tips sticking together.
2. Magnesium is the energizer bunny for plant growth. Crops require magnesium to capture the sun’s energy for growth and reproduction as it is a major constituent of the chlorophyll molecule involved in photosynthesis. The nutrient also helps carry phosphorus throughout the plant, and is necessary to create protein. Phosphorus uptake is not possible without magnesium.
Magnesium deficiencies affect the older leaves first and then spread to the younger leaves. Magnesium is recommended as a soil application to support the plants as they begin their active growth period. In some cases, it can also be foliar-applied to help minimize or correct developing deficiencies during plant growth.
3. Sulfur is important to every living cell, including plants. It is a key part of photosynthesis. It is often available in the soil’s organic matter and minerals, but can be deficient in many geographies or for a variety of today high-yield crop requirements for high yield crop production.
As most sulfur is tied up in the soil, the nutrient must be converted to sulfate (SO4 -2) by bacteria in the soil before it’s available for uptake by the plant.
Similar to the nitrate form of nitrogen, sulfate is mobile in the soil and can be leached out of the root zone with heavier rainfalls. In some cases, the sulfate may move back into the root zone as the water evaporates. However, in sandier or courser soils this is not as common.
What does a sulfur deficiency look like?
In terms of detecting deficiency, sulfur and nitrogen may often be confused for one another with similar symptoms of yellowing leaves and stunted growth. However, the major difference is that yellowing leaves with a sulfur deficiency first appear in younger leaves in contrast to older leaves, as is the case with a nitrogen deficiency.
The essential micronutrients include Iron (Fe), Zinc (Zn), Boron (B), Copper (Cu), Manganese (Mn), Molybdenum (Mo), Nickel (Ni) and Chloride (Cl).
- They are considered micros because they are needed in smaller amounts compared to macronutrients by the plant.
- Many micronutrients hold the key to how well the other nutrients are used; attribute to how well the plant develops and effects the total yield it will produce come harvest.
- They also help feed the microorganisms in the soil that perform important steps in various nutrient cycles of the growing process.
Zinc is an example of a micronutrient that is important to producing high-yielding crops as it is often seen as the most common micronutrient for limiting yields. Iron is another example of an important micronutrient as it is essential for chlorophyll development.
Crops and soil are prone to micronutrient deficiency in various ways. Soybeans that are deficient in iron can develop iron deficiency chlorosis (IDC). Alfalfa commonly experiences boron deficiencies. Wheat may be deficient in copper, and high-pH/sandy soils may be more prone to zinc deficiencies. For more information on micronutrients, check out “Role of Micronutrients in Efficient Crop Production,” by Purdue University.
Since all the essential nutrients work together, it’s important to consider the importance of each of them as individuals as well as how they relate to each other. If you find your crops experiencing a possible deficiency, a recommendation is to take plant and soil samples from both the affected and unaffected areas. A comparison of the results can then help to develop a clear and concise report of what is deficient and what actions are needed.
A good resource is the 4Rs of Nutrient Stewardship by the International Plant Nutrition Institute (IPNI). This decision-making guide uses the 4Rs method of: determining the Right Source, applying the Right Rate, applying at the Right Time, and applying in the Right Place.
It’s extremely beneficial to have a comprehensive nutrient plan in place to ensure that growers are getting the best return on their investment and utilizing the most effective application and use of nutrients throughout their plants’ growth cycle to get the best yield from the crop.