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
Top 100 2016 Lighting Feature Image

July 24, 2016

Horticultural Lighting Conference To Take Place In Chic…

The one-day event is designed to provide cutting-edge information on the latest technologies and techniques impacting the advancement of the horticultural lighting market.

Read More

July 23, 2016

AmericanHort Kicks Off Cultivate’16 By Announcing Major…

Even though Cultivate’16 just wrapped up, it’s not too early to find out about the many changes you can expect for 2017.

Read More
Workers

July 23, 2016

5 Reasons To Invest In Employee Training

Training and developing your employees is critical to the future success of your organization and the horticulture industry at large. Here are five reasons why your employees are worth the effort.

Read More
Pleasant View Gardens Savor Edibles

July 22, 2016

Pleasant View Gardens Targets Millennials With Savor Ed…

Taking something as ubiquitous as vegetables and herbs and giving it a fresh new look is no easy task, but Pleasant View Gardens took on the challenge with extensive research and creative marketing.

Read More
Dummen New Columbus Office

July 21, 2016

Dümmen Orange Opens New North American Headquarters In …

The new office, which officially opened during Cultivate’16, supports the company’s vision to be integrated into a vibrant and inspiring community, and provides a bright, modern workspace for its approximately 30 local employees.

Read More
BeeSmart

July 21, 2016

How You Can Get Involved In Research

The Horticultural Research Institute (HRI) supports scientific research and students for the advancement of the horticultural industry.

Read More
Pollen Free Lisianthus

July 20, 2016

Sakata Seed Develops World’s First Pollen-Free Lisianth…

With no stamen to produce pollen, the new Lisianthus varieties have improved flower durability and the benefit of no scattered pollen that can cause staining of flowers and machines.

Read More
Floral Greenhouse at Vineland Research Centre

July 20, 2016

Research And Commercialization Unite At The New Vinelan…

The new, pre-scale commercial greenhouse facility at Vineland Research and Innovation Centre in Vineland Station, ON, will help accelerate the commercialization of innovative technologies and products coming out of research.

Read More

July 20, 2016

How To Prevent Greenhouse Heating Leaks

Ethylene from malfunctioning greenhouse heaters can lead to crop injury, while carbon monoxide can lead to worker illness. Know the symptoms and how to check greenhouse heaters to avoid these concerns.

Read More
Luxflora Flower Run

July 20, 2016

First-Ever Flower Run Was A Big Success, Luxflora Says

Kicking off Cultivate’16 on Sunday, July 10, Luxflora’s first annual 5K saw nearly 150 participants racing through the picturesque Scioto Audubon Park in Columbus under clear blue skies and perfect conditions. It was a great way to start the week, participants said. Congratulations to the winners of the first-ever Flower Run: First Place: Jacob Griffith Gardner Second Place: Mike Goyette Third Place: Jack Ford The top three winners — and all of the Flower Run participants — were showered with flower petals when they crossed the finish line, making for a colorful and fragrant finish to an invigorating experience. “Personally, I would consider it a grand success,” says Luxflora President Rebecca Lusk. “The overall mood was fantastic, from start to finish. There were many high fives and group hugs, numerous duo phone selfies and postings to social media, while participants enjoyed their bananas and bottled water donated by Experience Columbus. Most […]

Read More
Top 100 Breakfast Panel for 2016

July 19, 2016

Top 100 Growers Talk Labor, Automation, And Sustainabil…

More than 60 growers got up with the sun at Cultivate’16 to hear a four-person panel discuss hot topics such as labor, automation, and sustainability at Greenhouse Grower’s Top 100 Growers Breakfast, sponsored by BASF.

Read More

July 19, 2016

17 New Vegetable Intros For Greenhouse Production

New varieties of edibles introduced at 2016 California Spring Trials offer something for every type of gardener, from the beginning do-it-yourselfer to the hardcore, heirloom-only type. Check out some of the new varieties we saw at Spring Trials this year that will be available for retail in 2017.

Read More

July 19, 2016

A Look Back At Greenhouse Grower’s Evening Of Excellenc…

Greenhouse Grower held its annual Evening of Excellence event on Monday, July 11, during Cultivate’16. Check out the slideshow to see some of the highlights of the festivities. You’re sure to recognize a few familiar faces!

Read More
Cultivate 2016 Town Hall Panel

July 19, 2016

Town Hall Panel At Cultivate Focuses On Selling To A Ne…

Why are we still trying to sell our products in the 21st century using a model straight out of the 1980s? This was the main question raised during the annual Town Hall panel discussion that took place at Cultivate’16.

Read More

July 19, 2016

Do You Grow Young Plants? Only 4 Days Left To Take Our …

If your operation produces plugs or liners for wholesale growers, please take a few minutes to participate in Greenhouse Grower’s 2016 Young Plant Grower Survey. We know you are very busy and we value your time and input. This survey should only take a few minutes. Greenhouse Grower’s Young Plant Grower Survey has played a key role in building our Top 20 Young Plant Growers list over the years. The information helps us zero in on trends taking shape and the challenges you’re facing as young plant growers. If you have any questions about this survey or you are not the right contact for this at your operation, please email me at [email protected], or please forward the survey link to the appropriate person. We would like to wrap up this survey by July 25, so please take it soon! Thank you in advance for your participation. We value your opinion! » […]

Read More

July 19, 2016

Are You Bar Coding Your Plants Like Other Garden Retail…

We asked garden retailers how they bar code several plant categories. Take a look at our results and learn how in sync are you with your peers.

Read More
Sid Raisch

July 19, 2016

What Sid Raisch’s New Role At Bower & Branch …

The garden industry consultant answers questions about his new role in an e-commerce oriented company.

Read More

July 19, 2016

First Five Plants To Win 2017 All-America Selections An…

All-America Selections (AAS) has announced its first five winning plants for 2017. Three of the five are national winners, and two are regional winners. And of the five, one is an ornamental plant, while the other four are edibles. A panel of independent judges selected these plants after reviewing trial performances. AAS says all these plants are available for immediate purchase. Here’s the list of winners: Celosia ‘Asian Garden’ (2017 Ornamental From Seed National Winner). This celosia winning in North America’s trial sites makes it the first ever AAS Winner from Japanese breeding company Murakami Seed. The judges gave this entry high marks in the greenhouse for the good branching, almost bushy growth habit, and early-to-bloom flower spikes. In the garden, ‘Asian Garden’ celosia continued to bloom on sturdy stems, keeping the bright pink color all summer long, holding up even through some of the first frosts of the season. The AAS judges commented on […]

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