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

8 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

  3. How do i measure these ingredients if I have made the four chemicals? I crushed eggs shells and epsom salt to make the calcium and magnesium sulfate. And I found an article to make the potassium nitrate. I know it seems like doing alot but i like the idea of recycling. I want to spray this homemade fertalizer on my garden so I need to know what the proper levels should be for them all and if they can all be put in water to be sprayed on my garden.

Latest Stories
employee-reviewing-shipping-list

February 21, 2018

Four Ways to Get Your Staff to Care About Quality

It can be a challenge to convince your employees to care as much as you do about your business and the quality of products you are providing. Here are four suggestions for making this happen.

Read More
Adult Thrips feature

February 20, 2018

Thrips Causing Headaches? New Research Shows Bio-based …

Dr. Rose Buitenhuis will present practical knowledge for growers to implement immediately during Biocontrols West Conference in San Diego, March 7-9.

Read More
Sedum-Pillow-Talk

February 20, 2018

12 New Poinsettias and Succulents for Holiday Sales, Cr…

Poinsettias and succulents are both in-demand products with today’s consumers. Here’s a look at some of the newer introductions available for retail sales in 2018 and 2019.

Read More
2016 Top 100 Growers With Drone

February 20, 2018

Top 100 Growers Survey for 2018 Is Now Open!

Operations with more than 400,000 square feet of environmentally controlled production are eligible to take the survey, which provides valuable market knowledge on how the nation's largest growers are innovating, sourcing labor, keeping up with demand, and setting trends.

Read More
Pepper-Mexican-Sunset

February 19, 2018

All-America Selections Names Latest Variety Winner, Ann…

All-America Selections (AAS), the 85-year-young non-profit plant trialing organization, had a very busy month of January.

Read More

February 18, 2018

Why Biochar Might Eventually Replace Peat Moss

In a recent study, researchers from the University of California, Davis investigated biochar as an alternative to peat moss.

Read More
Gotham Greens Atrium Style Greenhouse Chicago

February 17, 2018

Gotham Greens Building Second Greenhouse in Suburban Ch…

The new $12.5 million, 105,000-square-foot greenhouse will be near the company’s first Chicago greenhouse, and will be a free-standing structure.

Read More

February 16, 2018

New England GROWS Discontinues After 25 Years

After 25 years, New England GROWS — both the annual event and the organization behind the event — are discontinuing operations.

Read More

February 15, 2018

Boxwood Blight, Seed Your Future Among Topics Receiving…

The Horticultural Research Institute recently announced the 10 research projects that will receive funding in 2018 to investigate challenges and solutions in production, pest management, environmental stewardship, and business and marketing.

Read More
Weigela Czechmark (Spring Meadow Nursery)

February 14, 2018

Spring Meadow Nursery Furthers Its Mission of Giving Ba…

The Spring Meadow-Proven Winners Endowment Fund through the Horticultural Research Institute has now topped $800,000, with more scholarships coming in 2018.

Read More
Laura Drotleff

February 13, 2018

Don’t Let Your New Year’s Resolutions Fade …

Stuff got real in 2017, and it caused many of us to stop and reflect on the current state of the world, and how we can impact change — or at least improve our own situations. Life is too short, and we work too hard, to not use the annual renewal as an opportunity to take stock of our own personal lives, businesses, and careers, and make the necessary adjustments that will allow us to work smarter and live better. Even if you have already given up on your resolutions, perhaps you can think of January as a trial month and make February the month when you really get to work on your goals. And while I know it’s a difficult month to start on some of these changes, since you’re already immersed in the busy spring season, I promise that if you take some time to evaluate your reality […]

Read More

February 12, 2018

Calling All Designers: Enter the International Plantsca…

Applications are being accepted through April 13 for AmericanHort’s International Plantscape Awards, which recognize achievements in interior plantscape design, installation, creativity, renovation, and innovation.

Read More
Jose-Milan-Bayer

February 11, 2018

Bayer Has New Turf and Ornamentals Global Market Manage…

Jose Milan will be focused on helping growers deal with regulatory issues, while promoting the environmental benefits the ornamentals industry offers.

Read More
Dr. P. Allen Hammer, Dummen Orange

February 10, 2018

Applications for Dümmen Orange’s Dr. P. Allen Hammer Sc…

Named for Dr. P. Allen Hammer in acknowledgment of the contributions he has made to the floriculture industry, the scholarship is intended to extend Dr. Hammer’s legacy by supporting the next generation of floriculture students.

Read More
Hand-Crafted Stone Mushrooms (Stone-Age Creations)

February 9, 2018

Garden Retailers at TPIE Highlight Their Favorite Produ…

A team of garden retailers once again participated in the 2018 Tropical Plants Industry Expo Cool Product Awards.

Read More
Aron Hoff feature image

February 8, 2018

Who Will Be Our 2018 Head Grower of the Year and Operat…

The deadline for submitting nominations is fast approaching, so be sure to let us know who you feel is most deserving of these awards!

Read More
Oat Grass Banker System feature

February 8, 2018

Learn the Basics of Biocontrols for Controlled Environm…

Growers producing crops in protected structures, including ornamentals, vegetable crops, and cannabis, can learn about effective sanitation, banker plants, and buying and using beneficials and predatory insects at the Biocontrols USA West Conference in San Diego, March 3-7.

Read More
Amazon-Online-Plant-Store

February 7, 2018

The Latest on Amazon’s Online Plant Store

There is debate throughout the industry over how Amazon’s presence will affect growers who are looking to sell plants online.

Read More