Meet the other macronutrients & learn why they matter

The “Big Three” get most of the attention when it comes to fertilizer.

October 19, 2023
3 mins
Dr. Jason Haegele
North American Agronomy Lead, ICL Growing Solutions
Christi Falen
Agronomy Technical Services Manager, ICL Growing Solutions
Dr. A.J. Foster
Agronomy Technical Services Manager, ICL

Secondary macronutrients, however, can be just as important as Nitrogen (N), Phosphorus (P) and Potassium (K) in some crops, and especially in potatoes. These nutrients make specific contributions that can maximize crop yield and are especially important to protect potato quality.

The secondary macronutrients that are most significant for potatoes are:

  • Magnesium (Mg). Mg is the central component in the chlorophyll molecule, making it critical for the plant to be able to photosynthesize and grow. It is also involved in the activation of many different enzymes.
  • Sulfur (S). S is vital for the formation of amino acids and proteins. It also plays a role in photosynthesis and improves N-use efficiency. S is critical for carbohydrate production, which is especially important for tuber production.
  • Calcium (Ca): In stressful environments, like those created by excessive heat or rain, Ca moves up through the root system and displaces growth hormones known as auxins. This, in turn, helps the plant regrow roots. Proper Ca levels can also help prevent internal browning and the development of hollow heart, a condition in which a star-shaped cavity forms in the middle of the tuber.


Quality Counts

Balance is key for any fertilizer program and that holds true for macronutrients. It’s also important to keep relationships and antagonism – when two nutrients may have a negative interaction – in mind. Individual nutrient levels should be viewed through the lens of overall crop nutrition.

Secondary macronutrients work in tandem with the Big Three and potato growers should give special consideration to K because of its effect on tuber quality and storability. K is essential for sugar and starch formation and increases plant vigor and disease resistance. Proper K levels also help minimize bruising.

Research has shown that increasing the amount of K during the growing season by applying it just after flowering gives an extra boost to tuber production.


Consider the Source

Several fertilizer sources can deliver secondary macronutrients, but one of the challenges is that most sources only supply one or two of these nutrients.

One that delivers Mg, S and Ca – plus K – is the mineral polyhalite. This naturally occurring mineral is mined exclusively by ICL off the coast of England and distributed worldwide as Polysulphate®. This fertilizer is unusual because it is not a blend of products; instead, it’s a complex mineral structure that contains K, Mg and Ca which are all associated with S in the sulfate form.

The main reason Polysulphate® is a great fertilizer for potatoes, as well as other crops including alfalfa, corn, soybeans, wheat, strawberries, and peppers is its release profile over the course of the season. It provides a season-long, gradual release of S that coincides with the uptake pattern for most annual crops. An advantage for potatoes is that Polysulphate® is a low chloride source of K, containing 2.4% chloride, compared to potash, which typically contains 45% to 47% chloride. It also has a low salt index which is important because, according to the National Institutes of Health, high salinity is a major global threat to agriculture, causing severe damage to crop production and productivity.


Field Trials

ICL has conducted field trials for Polysulphate® in the United States and Canada for several years and on many different crops. Potatoes have shown some of the greatest results and have been consistently responsive. That’s why we are really excited about the opportunity for this fertilizer for potatoes not only to increase yield, but also to improve quality.

Polysulphate® potato trials were conducted over four growing seasons in Minnesota, Michigan, North Carolina, Manitoba and Prince Edward Island. All trials showed a total yield increase over the control, ranging from 4.3 percent in Manitoba to 77 percent in North Carolina. Other trends observed through the trials included increased N-use efficiency, reduced hollow heart, and better fry quality.

Fertilizer practices vary widely across North America for potato production. Regardless of rainfed or irrigated production systems, and sources of secondary macronutrients such as gypsum and potassium magnesium sulfate, Polysulphate® has resulted in increased marketable yields. The ability to apply more nutrients with a single product has the potential to reduce fertilizer cost per acre or improve grower ROI depending on how Polysulphate is used in a dry fertilizer blend.

This is more evidence that nutrient source matters – it’s not just about the rate, it’s also about the availability of the nutrients that are within each fertilizer product. That has a significant impact on yield, and lower fertilizer costs per acre means a better return on investment to the grower.

Secondary macronutrients have an important function in yield and quality and must be taken into account in a balanced crop nutrition plan. When weighing fertilizer options, consider the availability of nutrients and how it will affect the bottom line.