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
Chick Charms

April 29, 2016

Kelly Norris: Why The Plant Collector Market Is Set To …

In his latest column for Greenhouse Grower magazine, Kelly Norris says there are more plant collectors out there than we think, which opens the way for the gift plant market to explode.

Read More
Natureworks Monarch life cycle caterpillar FEATURE

April 29, 2016

Do Customers Really Care How Plants Are Grown?

The consumer uprising against neonicotinoids has roiled the industry over the past couple of years. In June 2013, someone applied pesticide to a tree in full bloom, using the product in an off-label manner. That misapplication killed tens of thousands of bees, capturing the attention of activists. A short three years later, that activism has led to policy changes for big chains like The Home Depot and Lowe’s. Several cities and towns across the country have banned the sale of neonicotinoids. All of this made me curious. How was all the publicity affecting consumer attitudes at local garden stores? Traditionally, customers have shown little interest in how flowering plants are grown, other than they like the idea that they are from a local source. They have been much more particular about food plants than they have ornamentals. So I sent questions out to a few retailers across the country, and […]

Read More
Pennisetum Fireworks

April 28, 2016

Why Ornamental Grasses Are Great For People In Condos A…

Allan Armitage says breeders need to do a better job of making growers, brokers, and garden centers aware of better ornamental grass cultivars for the increasingly shrinking garden space.

Read More

April 28, 2016

Holistic, Integrated Approach To Pest Control Rooted In…

Greenhouse growers have been practicing integrated pest management for decades, but it’s becoming increasingly more important with the continued scrutiny of conventional pest control by a number of “regulators” — government, retail, and consumers. I just returned from Meister Media Worldwide’s Biocontrols USA 2016 Conference, in Monterey, CA, at the beginning of March this year, which served 450 attendees and 50 exhibiting supplier companies. It’s clear from the presentations and the growing attendance at this specialized event — now only in its second year — that use of biocontrols in IPM will continue to be adopted widely, as more growers get past their personal hurdles of doubt and intimidation, and embrace a new way to approach pest and disease control. Many growers think of using biocontrols as an all-or-nothing approach, but ultimately, IPM is about balance. Growers will need to continue to focus on IPM, integrating chemistry with biology, because […]

Read More
Drip irrigated citrus liner

April 27, 2016

Unclog Drip Emitters In Your Greenhouse

This is the first article in a series of case studies designed to help growers reduce, remediate, and recycle irrigation water as part of a multi-state research grant (CleanWateR3.org).

Read More
Andropogon gerardii Blackhawks (Intrinsic Perennial Gardens)

April 27, 2016

Know Your Market When Choosing Ornamental Grasses

Growers have no shortage of choices in the ornamental grass market. Narrowing down the selection comes down to finding the right plant for the right purpose.

Read More
Fine Americas Website Feature Image

April 26, 2016

Fine Americas Offers A Digital Resource For Plant Growt…

The blog section of Fine America’s website is updated regularly, with input from both technical managers and independent researchers

Read More

April 26, 2016

“Bee-Friendly” Labels Matter To Plant Consumers, Accord…

Research at Michigan State University shows ornamental plant buyers understand and respond to bee-friendly production practices.

Read More
Cicada (Greg Hoover, Penn State)

April 26, 2016

Cicadas Set To Emerge In Several Eastern States This Sp…

While there’s no immediate cause for alarm, experts say the cicada’s egg-laying process can damage woody ornamentals and make them vulnerable to diseases.

Read More

April 26, 2016

How To Host A Spectacular Farm Dinner

Farm-to-table dinners are a great way to connect your customers with a love for nature and growing things. These three green industry companies have had a lot of success with their farm dinners. Find out what it takes to pull one of these dinners off successfully. Advice from Tangletown Garden’s Dean Englemann: You have to be certain you’re matching the ticket price to the experience, make sure you’re exceeding expectations. It can’t just be about the food. You have to deliver the experience. For us, we want to make sure there’s a lasting experience of connecting our food to families and dining. For lack of a better comparison, we want sitting in a field, eating food we grew and created, to be a religious experience. There’s almost a ceremonial aspect to these dinners. We’ve always thought that the shortest distance between the earth and people is the distance between the hand and the […]

Read More

April 26, 2016

Fun Display Ideas From California Spring Trials

In a year that was light on new introductions, plant breeders ramped up their display creativity. Garden retailers can find a lot of inspiration for their own stores here!  

Read More

April 26, 2016

12 Questions To Test If Your Store Is New-Customer Frie…

Consultant Ian Baldwin offers ways to help you look at your store with fresh eyes and how you can make it welcoming for new gardeners.

Read More
University of Florida Online Greenhouse Training Courses

April 25, 2016

University of Florida Offering Online Training Courses …

There will be five courses offered, with the first starting on May 30. Courses are available in both English and Spanish and range from beginner level to advanced education.

Read More
Bee on Bidens

April 25, 2016

Breeders Go The Distance To Bring New Plant Varieties T…

Breeders are meeting the demand for new varieties that perform and sell well by extending trialing to engage the grower community, using advanced technology, and encouraging international collaboration.

Read More
Parisitic Wasp Aphidius colemani

April 25, 2016

Plant Growth Regulator Use Can Affect Biological Pest C…

The use of plant growth regulators may negatively influence the outcome of biological control programs, according to researchers at North Carolina State University.

Read More
HGTV_2015CAST

April 24, 2016

9 Business Predictions Smart Brands Should Pay Attentio…

Andreas von der Heydt, Director of Kindle at Amazon, recently predicted what smart businesses will do in 2016 to strengthen their brands and promote their products.

Read More
North American greenhouse 1

April 23, 2016

Light Matters In Greenhouse Structures

The design of your greenhouse structure, in terms of light transmission, can have a powerful impact on your bottom-line profits.

Read More
Lin Schmale 1996

April 23, 2016

SAF’s Lin Schmale Offers Lessons From An Industry…

Schmale represented the floriculture industry on Capitol Hill for more than 20 years. The advocate has recently retired, and shares some valuable insights from her career.

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