Understanding Plant Nutrition: Fertilizers And Macronutrients

Understanding Plant Nutrition: Fertilizers And Macronutrients

When you select a water-soluble fertilizer, the primary goal should be to supply plants with a sufficient amount of essential plant nutrients for good growth and flowering. In this article, we will focus on macronutrients (nitrogen, phosphorus, potassium, calcium, magnesium and sulfur) supplied by water-soluble fertilizers. We will discuss macronutrient sources, fertilizer formulations and the application of fertilizer to the crop. In subsequent articles, we will discuss other aspects of fertilization including micronutrients sources and formulations and controlled-release fertilizer.

Fertilizer Formulations

Water-soluble fertilizers come in two types, either individual fertilizer salts or blended fertilizers. Fertilizer salts are chemicals containing nutrients that can dissolve into a water-soluble form that are needed for plant uptake. For example, potassium nitrate (KNO3) will dissolve into separate potassium ions and nitrate ions. Blended fertilizers are combinations of two or more fertilizer salts that supply several macronutrients. For example, 13-2-13 is a blend of calcium nitrate, magnesium nitrate, monoammonium phosphate and potassium nitrate, and so supplies nitrogen, phosphorus, potassium, calcium and magnesium.

When formulating blended fertilizers, there are eight water-soluble sources of nitrogen commonly used (Table 1), some of which only supply nitrogen, like urea and ammonium nitrate. However, for most other nutrients, the choices are limited. For example, calcium nitrate is the only form of water-soluble calcium. There is also typically only one source of potassium, potassium nitrate. Monoammonium phosphate (MAP) is the usual source of phosphorus. Magnesium is supplied by either magnesium sulfate or magnesium nitrate. Sulfur is supplied by ammonium sulfate or magnesium sulfate.

Because of limitations in the number of salts used to blend fertilizers, the ratio of macronutrients and their compatibility when mixed directly affects the formulation of the fertilizer, for example:

– Fertilizers that are high in phosphorus also tend to be high in ammoniacal nitrogen, because phosphorus is usually supplied as monoammonium phosphate.

– Fertilizers that contain calcium are also high in nitrate, because calcium nitrate is the only water-soluble source of calcium. In fact, all the commercially available fertilizer that contains calcium also has ammoniacal nitrogen levels of 25 percent or less of the total nitrogen.

– Calcium nitrate and monoammonium phosphate or monopotassium phosphate cannot be mixed in the same concentrated stock solution at high concentrations because insoluble calcium phosphate will form. However, the amount of calcium and phosphorus that can be mixed in the same stock tank can be increased by lowering the pH of the stock tank solution. Commercially available fertilizers that contain calcium and phosphorus tend to have low levels of phosphorus (i.e. 13-2-13-6 Ca-3 Mg) and will also contain a weak acid to lower the pH of the concentrated stock solution.

– Since calcium nitrate and magnesium sulfate are incompatible in the same stock tank, a fertilizer that contains calcium will use magnesium nitrate as the magnesium source. A fertilizer that contains magnesium without calcium will use magnesium sulfate as the magnesium source.

Nutrient Concentrations

Most fertilizer recommendations are given based on a concentration of nitrogen applied to a crop. In North America, that concentration is usually given in parts-per-million or ppm. One ppm is equivalent to 1 mg per liter. In other words, one liter (about 33 fluid ounces) of fertilizer solution with a concentration of 100 ppm N will contain 100 mg of nitrogen. Sometimes, concentrations are given in mMol of nitrogen. One mMol of nitrogen is equal to 14 ppm N.

In many cases, the concentration of the other macronutrients are either not known or are ignored. To calculate the concentration of calcium, magnesium or sulfur supplied by a blended fertilizer, you need to know the concentration of nitrogen in the fertilizer solution and the ratio of nitrogen to calcium, magnesium or sulfur that is listed under the “guaranteed analysis” on any fertilizer bag. For example, to calculate the concentration of calcium supplied by 13-2-13 (6 percent Ca) at 200 ppm N, you divide the percent of Ca by the percent of N, then multiply by the nitrogen concentration of the fertilizer solution.

So at 100 ppm N, you are also supplying about 92 ppm Ca.

An extra step is required to calculate the concentration of phosphorus or potassium. A fertilizer formula reports phosphorus as P2O5, not actual phosphorus (P), and potassium is reported as K2O, not actual potassium (K). To convert P2O5 to P, multiply the P2O5 value by 0.43, and to convert K2O to actual K, multiply the K2O value by 0.83. For example, using the equation above, the P2O5 and K2O values supplied by 13-2-13 at 200 ppm N would be 30 ppm and 200 ppm. This converts to an actual P concentration of 13 ppm P, and an actual K concentration of 166 ppm K.

Applying Fertilizers

Water-soluble fertilizers are typically applied using fertilizer injectors or proportioner. These devices add a concentrated fertilizer solution to the irrigation water at some ratio. For example, a 1:100 injector will add 1 gallon of concentrated fertilizer to 100 gallons of water. If the desired solution concentration coming out of the end of the hose is 100 ppm N, then the concentrated stock solution that the fertilizer injector is adding to the irrigation water has to have a concentration of 10,000 ppm N (or 100 times that of the desired diluted concentration).

The amount of fertilizer needed to make a concentrated stock solution is often listed on the fertilizer bag. If the information is not contained on the fertilizer bag, then calculate it using the formula given in Table 2.

Be careful about adding too much fertilizer to the stock tank or it may not all be soluble. The solubility of potassium nitrate, in particular, is greatly affected by water temperature. The lower the water temperature of the stock tank, the lower the solubility of any fertilizer made with potassium nitrate. If you are using well water (with a temperature of about 55ËšF) to dissolve the fertilizer or the ambient temperature in the greenhouse (and stock tank) is 60ËšF or lower, then it may not be possible to dissolve more than 2 to 3 pounds (48 to 64 ounces) per gallon of stock.

Another way to determine the concentration of fertilizer you are applying is to use the electrical conductivity (EC) of the fertilizer solution. For all fertilizers, there is a relationship between the concentration of nutrients and EC (Table 3). In most cases, the relationship is given between the concentration of nitrogen and the EC.

To determine the nitrogen concentration coming from the hose, two EC measurements must be taken: EC of the fertilizer solution and EC of the irrigation water (with no fertilizer). Because the values given in the EC chart are for the fertilizer mixed in pure water, the irrigation water EC must be subtracted from the fertilizer solution EC, for example, 20-10-20 with a solution EC of 1.2 and an irrigation water EC of 0.5. Subtract the solution EC (1.2) from the irrigation water EC (0.5) to get 0.7, which corresponds to a fertilizer concentration of about 100 ppm N.

Calculate ppm N from a 20-10-20 fertilizer solution with a total EC of 1.8 mS and an using irrigation water with an EC of 0.5 mS.

Using EC values to determine fertilizer concentrations has some limitations. EC values are generic measurements because they measure the conductivity of all the salts in the solution, not just the fertilizer. It is important to remember that the relationship between EC and nitrogen concentration is unique to that specific fertilizer salt or blend of salts in pure water. Never assume that all fertilizers have the same relationship between EC and ppm N.

Understanding how to fertilize your crop requires more than just selecting a fertilizer formulation off the shelf. You need to know what other nutrients are in the fertilizer, the relationship between the concentration of nitrogen and the other macronutrients, and how to supply them to the crop at a desired concentration. In next month’s article, we will discuss micronutrients.

Leave a Reply

7 comments on “Understanding Plant Nutrition: Fertilizers And Macronutrients

  1. 'So at 100 ppm N, you are also supplying about 92 ppm Ca.' I believe it should read '… at 200 ppm N…' 13-2-13 will deliver roughly 46 ppm Ca when mixed @ 100 ppm N. It will deliver 92 ppm @ 200 ppm N.

  2. What should be the ratio between Nitrogen,Phosperous, Calcium and Magnesium in a fertilizer mix to avoid interactions

Latest Stories
SunPatiens Sakata Medal of Excellence Feature

June 29, 2016

SunPatiens Earns The 2016 Medal Of Excellence In Market…

With its reputation for excellent garden performance from spring to frost, and as a problem-solver for Impatiens Downy Mildew, SunPatiens has become a “hero plant” that helps gardeners succeed.

Read More
Kelly Norris - feature

June 28, 2016

Kelly Norris: Three Variety Trends That Caught My Eye T…

My reflections on promising new plants, the realities of market penetration, and adding value along the supply chain.

Read More

June 28, 2016

10 Things You Need To Do Before And During Cultivate’16

From pre-planning to on-site networking and education, here are 10 things to consider before you depart for Cultivate’16.

Read More

June 28, 2016

How “Brexit” Will Affect The Horticulture Industry

The news this week of Great Britain’s departure from the European Union has sent shockwaves through many industries, including horticulture. Here’s a look at the potential effects of “Brexit” across the horticulture and floriculture industries.

Read More
iPad In Garden Center

June 27, 2016

Inspiring Summer Customers Without A Word Being Said

Now is the time of year to sell your customers what they like, rather than what they need. According to industry consultant Ian Baldwin, one way to do this is to use silent selling techniques and devices that help validate a purchase.

Read More
Oconomowoc

June 27, 2016

Fire Destroys Greenhouses At Oconomowoc Landscape Suppl…

The fire not only consumed the Wisconsin garden retailer's structures, but also major equipment like dump trucks. In all, the owners estimate $1.2 million in damage occurred.

Read More
Robert Conrad, Ball Seed Technology Research Manager

June 27, 2016

Ball Premier Lab Wins Medal Of Excellence Award For Its…

Over the past 30 years, the Ball Premier Lab has set the standard for high-quality seed in the industry, earning it Greenhouse Grower’s 2016 Medal Of Excellence for Industry Achievement, for its dedication to advancing seed and helping growers improve their margins.

Read More
Greenhouse Whitefly

June 26, 2016

Michigan State University Offers Tips On Whitefly Manag…

Whiteflies are making headlines in Florida, but they are found across the U.S. Michigan State experts say it’s important to know how to manage each type of whitefly.

Read More
Schaefer Exhaust Fan

June 26, 2016

What’s New In Greenhouse Cooling

Cooling and ventilation companies, as well as structures suppliers, are working to help growers manage efficiencies. Here are some of their latest innovations.

Read More
Dr Allan Armitage

June 25, 2016

Three Types Of Plant Consumers To Watch

There are three emerging groups of plant consumers that you should be targeting for plant sales in the future, according to Allan Armitage.

Read More
North Creek Nurseries lower trials

June 24, 2016

North Creek Nurseries Is A Finalist For 2016 Operation …

North Creek Nurseries in Landenberg, PA, is one of three finalists for Greenhouse Grower’s 2016 Operation Of The Year award and winner of the Excellence In Sustainable Production award for 2016.

Read More
Cal Poly Logo

June 24, 2016

Cal Poly Ag College Hosting Alumni Reception At Cultiva…

Alumni and friends of The Cal Poly College of Agriculture, Food and Environmental Sciences are invited to a reception that will be held Sunday, July 10.

Read More
Stocking spring plants at Petitti Garden Center FEATURE

June 23, 2016

How Growers Can Help Retailers Sell More In Spring

Fresh from a busy spring season, retailers share their wish list of things growers can do to make spring generate even more sales.

Read More
Houwelings Tomatoes

June 23, 2016

Houweling’s Tomatoes Has A New Corporate Head Grower

Arie van der Giessen, who has led the horticultural operations for many successful regional and national greenhouse operators in North America, will start his new role in September.

Read More
Vic Frey, Kurt Weiss Greenhouses Feature

June 23, 2016

Vic Frey Named A Finalist For 2016 Head Grower Of The Y…

Vic Frey, a finalist for the 2016 Head Grower of the Year and recipient of the 2016 Excellence In Production award, was always willing to trial and experiment with the latest technology. He loved the constant change in this industry, and turned every opportunity into a teaching moment.

Read More

June 21, 2016

Sign Up For The First-Ever Flower Run At Cultivate’16

When you pack your bags for Cultivate’16 in Columbus, OH, July 9-12, don’t forget your running shoes! There are still three weeks left to train for the first annual Luxflora Flower Run, which will kick off the start of Cultivate’16 on Sunday morning, July 10. The Flower Run celebrates Luxflora‘s mission to develop a visionary, influential floriculture network in which women leaders can create, inspire, and flourish. The 5k event is an opportunity for all floriculture professionals to come together to network, build relationships, and to have some healthy fun. Running is not required – this event is walker-friendly, as well. If you opt to be an observer rather than run/walk, then join your friends at the finish line to help shower the participants with flowers. Register yourself or your company’s team today, to join in on the fun. Free Flower Run tshirts will be available to participants, while supplies […]

Read More
BASF Orkestra Intrinsic

June 21, 2016

New Mode Of Action From BASF Offers Deeper Disease Cont…

When it comes to disease control, you need all the help you can get. BASF recently hosted growers, Extension personnel, and trade media to present its newest fungicide with two active ingredients, offering dual modes of action.

Read More

June 20, 2016

7 Garden Retailers Announced Closings This Month

Several established garden stores announced they were closing at the end of the 2016 season. Most of these retailers are decades old, including one that is closing after 133 years in business.

Read More
[gravityform id="35" title="false" description="false"]