Understanding Plant Nutrition: Fertilizers And Media pH

Understanding Plant Nutrition: Fertilizers And Media pH

Choosing fertilizers can be one of the most important decisions you can make for managing the media pH of container grown crops. It is therefore important to understand how fertilizers raise or lower media pH, which results largely from the form of fertilizer nitrogen (ammoniacal, nitrate or urea). This article will help you understand why fertilizers are classified as acidic or basic and how the reactions produced by the fertilizer affect media pH. 

You Can’t Measure Fertilizer Acidity With A pH Meter!

Here is a trick question: Which is the most acidic fertilizer in Table 1? The table shows the nitrogen (N):phosphorus (P2O5):potassium (K2O) ratio, the fertilizer label description in terms of potential acidity or basicity, along with the percent of nitrogen in the ammoniacal form (the rest of the nitrogen in these formulas is made up of nitrate). We prepared a solution of 200 parts per million of nitrogen (ppm N) in deionized water, and measured the pH of that solution. Acid solutions have low pH below 7, and basic solutions have high pH above 7.

Even though 15-0-15 has the lowest pH of the three solutions at 200 ppm N, it is actually classified as potentially “basic.” 20-10-20 is classified as a potentially “acidic” fertilizer, and 17-4-17 is classified as potentially “neutral.” How is this possible?

The effect that a fertilizer has on media pH is dependent on the reactions that take place after the fertilizer has been applied to the crop. This reaction is determined by the nutrients (especially nitrogen) contained in the fertilizer, rather than the pH of the fertilizer solution that you can measure with a pH meter.

Predicting Fertilizer Reactions

The potential acidity or basicity value (examples are shown in Table 1) is usually printed on the fertilizer label. This value is calculated from the “Pierre equation,” which was developed in the 1920s using field soils and fertilizers ranging from cow manure to the “new” synthetic fertilizers like ammonium nitrate. Values from the “Pierre equation” describe whether a fertilizer will generally raise media pH (acidic reaction) or lower media pH (basic reaction). Research to update this formula for container grown crops is currently underway at the University of Florida.

Units are given in terms of acidity or basicity in equivalent pounds of calcium carbonate (CaCO3, which is the main constituent of lime) per ton of fertilizer. For example, if 20-10-20 has a potential acidity of 429 pounds per ton, then the reaction produced by one ton of the fertilizer will neutralize 429 pounds of calcium carbonate. If 15-0-15 has a potential basicity of 420 pounds per ton, then the reaction produced by one ton of the fertilizer will be equivalent to 420 pounds of calcium carbonate.

The potential acidity or basicity should be interpreted as a general tendency of the fertilizer to raise or lower medium pH over time, because it is based on many assumptions that do not always apply to container-grown crops. When comparing specific fertilizers, use a broad range for determining which fertilizers will produce different reactions in the medium. A safe bet is that if the difference in the calcium carbonate equivalency of two fertilizers is more than 200 pounds per ton, then the reaction produced by the fertilizers will be different. If the difference is less than 200 pounds per ton, then consider the reaction produced by the fertilizers to be similar.

The Effect Of Nitrogen On Media pH

There are three types of nitrogen used in water-soluble fertilizers: ammoniacal nitrogen (NH4-N), nitrate-nitrogen (NO3-N) and urea (Figure 1).

Ammoniacal nitrogen is acidic (a mental reminder is that both words begin with the letter “A”). When ammoniacal nitrogen is taken up by roots, the plant can secrete an acidic H+ into the soil solution. The more H+ contained in the root media, the lower the media pH.

Urea is easily converted into ammonium in the substrate and therefore can be thought of as another source of acidic nitrogen.

In contrast, uptake of nitrate nitrogen increases substrate-pH because basic OH- or HCO3- are secreted by plant roots into the root media. Since OH- and HCO3- are bases, nitrate uptake therefore can cause the media-pH to increase.

Another important process is called nitrification. Several types of bacteria in container substrates convert ammonium to nitrate. Nitrification releases acidic H+ into the soil solution, causing the media pH to decrease.

Ammoniacal nitrogen is about three times stronger an acid than nitrate nitrogen is a base. For example, a fertilizer such as 17-4-17 has about 25 percent ammoniacal nitrogen and 75 percent nitrate nitrogen (1 NH4-N:3 NO3-N ratio), and the reaction produced by the 17-4-17 fertilizer tends to be neutral. With 40 percent of the nitrogen in the ammoniacal form and 60 percent in the nitrate form, 20-10-20 has an acidic overall effect (2 NH4-N:3 NO3-N ratio). In comparison, 15-0-15 contains only 13 percent of its nitrogen in the ammoniacal form and 87 percent in the nitrate form (0.5 NH4-N:3 NO3-N ratio). And the reaction produced by 15-0-15 tends to be basic.

The nitrogen in a fertilizer solution (measured in ppm N) has much more acid or base strength than the pH of that solution measured using a pH meter, as shown in Table 2.

For example, a solution with a pH of 5.0 would supply about 0.01 milliequivalents/liter of acidic hydrogen ions to the substrate. If all the 100 ppm ammonium-N were converted into nitrate-N through nitrification, the maximum amount of acidity produced would be 14.2 milliequivalents/liter of acidic hydrogen, or about 1,420 times more acidity than would be supplied by a solution with a pH of 5.0. Put another way, applying 100 ppm of ammoniacal nitrogen has the potential to supply the same amount of acidity as a solution with a pH of 1.8. The acidity produced by a solution with a pH of 5.0 would be equivalent to the nitrification of 0.14 ppm ammoniacal nitrogen (almost undetectable). Also, doubling the concentration of ammonium from 100 to 200 ppm N doubles the acidity.

While the effect that different nitrogen forms have on the substrate pH is much more complicated than this simple example, it does give you an idea why the nitrogen form of the fertilizer has a much greater effect on the substrate pH than does the solution pH.

Other Factors Affect Acidity And Basicity

- Nitrate nitrogen (NO3-N) can cause the substrate-pH to increase, but only if it is taken up by the plant. If plants are small, or stressed and not growing, nitrate has little influence on substrate pH.

- Nitrification of ammoniacal nitrogen (NH4-N) is inhibited by low substrate-pH (starting at around 5.5), low substrate temperature (less than 60ËšF or 15ËšC) and lack of oxygen through water-logging. Under these conditions, ammoniacal nitrogen is less acidic.

- Uptake of other positively charged nutrients such as potassium (K+), calcium (Ca2+) and magnesium (Mg2+) can also cause the secretion of acidic hydrogen ions (H+), similar to the uptake of ammoniacal nitrogen.

- Uptake of negatively charged nutrients such as phosphate (H2PO4-) or sulfate (SO42-) can cause the secretion of basic OH- or HCO3- ions, similar to nitrate nitrogen uptake.

- The effect nutrient uptake has on media pH will depend on the balance between the uptake of positively charged nutrients and negatively charged nutrients. If more positively charged nutrients are taken up, the net affect will be acidic. If more negatively charged nutrients are taken up, the net affect will be basic.

On a molecular basis, nitrogen is taken up more than other nutrients. Therefore, nitrogen form has a bigger effect on media pH than other nutrients.

The acidity or basicity value on the fertilizer bag is only a relative measurement of how the fertilizer formulation will affect media pH. Why is 20-10-20 more acidic than 15-0-15? Because 20-10-20 has more ammonium than 15-0-15. If media pH tends to drift up over time in your greenhouse crops, consider using a fertilizer higher in ammonium percentage, such as 20-10-20. 15-0-15 is more suitable if media pH tends to drift downward over time.

In next month’s article, we will discuss how fertilizer formulation will affect nutrient supply.

Leave a Reply

4 comments on “Understanding Plant Nutrition: Fertilizers And Media pH

  1. I want to grow with out soil but i dont have any exp.
    i am growing cucumber in soil in greenhouse.
    please advice me for esy way to grow with out soil?

  2. I want to grow with out soil but i dont have any exp.
    i am growing cucumber in soil in greenhouse.
    please advice me for esy way to grow with out soil?

More From Fertilization...

May 22, 2015

Nexus Greenhouses Is Optimistic For Expansion Into New Markets

Cheryl Longtin and Mike Porter, who own Nexus Corporation, say they were excited to attend the grand opening of Gotham Greens’ new structure atop the new Whole Foods grocery store in the Gowanus neighborhood in Brooklyn, N.Y., when it opened in December 2013. The project is just one example of some of the new and expanding markets that Nexus Corporation has expanded into over the past few years. Jeff Warschauer, vice president of sales for Nexus, says the company has enjoyed getting to know and working with the founders of Gotham Greens, Viraj Puri and Eric Haley, and Jennifer Nelkin Frymark, the chief agriculture officer, on their innovative approach to business. “They are very excited and work hard internally – just great people,” he says. “From our perspective, it’s great to see that excitement and vision. The employees there are happy and there’s no turnover; they’re only adding new people […]

Read More

May 20, 2015

2015 Farwest Show Announces Second Annual Equipment Innovation Day

The second annual Equipment Innovation Day will be Tuesday, Aug. 25, prior to the 2015 Farwest show, which will be August 27-29 in Portland, Ore. Equipment Innovation Day, which was enthusiastically received in 2014, offers a real-time opportunity to see new heavy and automated nursery equipment in action. The demonstrations take place in manufacturing and nursery settings, adding value to the showcase. Attendees will be able to talk with participating manufacturers and learn first-hand from innovative growers who use the equipment in daily operations. The day-long event will be held at the main manufacturing plant of GK Machines, Inc., Donald, Ore. Further demonstrations of field equipment will take place at the nearby nursery of A&R Spada Farms, LLC. Bus travel to and from the event is planned, starting at and returning to the Oregon Convention Center. Attendees are welcome to provide their own travel to and from the site. Preregistration is required. The cost […]

Read More
Bee On Flower

May 20, 2015

White House Task Force Releases Pollinator Health Strategy

An interagency Pollinator Health Task Force commissioned by President Obama released its “Strategy to Promote the Health of Honey Bees and Other Pollinators” on May 19. The strategy, released in accordance with the Presidential Memorandum issued last June, is accompanied by a Pollinator Research Action Plan, which outlines needs and priority actions to better understand pollinator losses and improve pollinator health. The recommended actions will be supported by a coordination of existing federal research efforts and accompanied by a request to Congress for additional resources to respond to losses in pollinator populations. Pages 47 through 52 specifically address pesticides and pollinators. The report calls out plant production, native plants, mosquito control and all urban uses in its Pollinator Action Plan. RISE (Responsible Industry for a Sound Environment) says it supports the goals of improving pollinator health and habitat contained in the White House Pollinator Task Force’s release of its National […]

Read More
Latest Stories

January 9, 2015

6 New Fertilizer Products For Healthy Plants

These five products add even more options for delivering nutrients to the root zone.

Read More

January 7, 2015

Fertilizers And The Future

As growers look for new ways to cut costs and conserve resources, fertilizer and equipment companies are offering products that strive to save water, reduce toxic runoff and keep chemicals out of the equation.

Read More

December 31, 2014

Gain Greater Control Of Fertilizer With Automated Ferti…

University researchers look at integrating irrigation and fertilization with the help of water sensors to reduce fertilizer treatments and improve application timing.

Read More

October 30, 2014

Basics & Beyond: Comparing Substrate Fertilizer Ame…

Cornell University researcher determines if substrate-incorporated slow-release fertilizers can be used to replace or reduce the need for liquid fertilizer for four spring crops.

Read More

July 24, 2014

Using Controlled Release Fertilizers To Produce Garden …

Researchers determined whether or not garden mums can be grown with controlled-release fertilizer, and if it reduces fertilizer leaching, as compared with water-soluble fertilizers.

Read More

March 14, 2014

New Foliage Pro Fertilizer Offers Complete Nutrition Pl…

Dyna-Gro Nutrition Solutions has developed a process it says is capable of keeping all 16 essential plant nutrients in solution form.

Read More

January 30, 2014

OASIS Grower Solutions Introduces New One-Bag Hydroponi…

The new 16-4-17 Hydroponic Fertilizer from OASIS Grower Solutions (OGS) is a one-bag solution that replaces two-part systems traditionally used by commercial hydroponic growers. It is specifically formulated for commercial hydroponic production of lettuce, herbs and vegetables.

Read More

December 30, 2013

Fertilizer Changes Growing Mix pH

When considering a fertilizer's influence on media pH, you need to know its acid or basic reaction.

Read More

December 30, 2013

Basics & Beyond: Fundamentals Of Phosphorus Nutriti…

Phosphorus is an essential element, after all.

Read More
Everris Liquid S.T.E.M.

December 30, 2013

New Fertilizers For 2014

New fertilizer products not only deliver optimum nutrition, they also provide for easier application and increased efficiency. Check out these new products to help your operation produce a healthy crop in 2014. Click through on the pages below.

Read More

December 18, 2013

Focus On Fertilizer: Micronutrients And Organics

New fertilizer products are focusing on micronutrients and providing efficient options for organic production.

Read More

August 27, 2013

BioWorks Adds EcoVita To The Verdanta Family Of Biofert…

EcoVita, a homogeneous granular organic fertilizer, has been added to Bioworks Inc.’s Verdanta biofertilizers product family. This fertilizer will be manufactured and supplied to BioWorks by DCM Corporation of Belgium, a producer of natural and organic-based fertilizers in Western Europe. EcoVita is suitable for a wide variety of crops with its gentle release curve including:• Organic fertilization as a base nutrition in potting mixes• Leafy vegetables• Fruiting vegetables (s a top dressing) • Roses and other ornamentals The new fertilizer offers long-lasting and continuous action for 75 to 100 days and contains organic phosphorus (5 percent P2O5) for fast rooting. Nutrients in EcoVita are gradually released by the soil microbes, in addition to producing humus for better rooting and less leaching. EcoVita is OMRI Listed, making it suitable for use in organic production. “We’re pleased to introduce EcoVita 7-5-10 as our organic NPK product to complement our organic products: K-Vita 2-0-20 […]

Read More

April 29, 2013

Rockwell Farms Introduces Ready-To-Pour Container Ferti…

Rockwell Farms has introduced Rockwell Farms Plant Food, a bottled liquid fertilizer that does not need to be diluted before use. Always looking for ways to help the consumer succeeed, Jason Roseman of Rockwell Farms says the operation is also always looking for ways to get consumers to come back and buy more plants and flowers. “We feel like fertilization is one of those things that can be very confusing,” Roseman says. “Not everyone does it, and not everyone knows what they’re supposed to do.” The solution: Rockwell Farms Plant Food. The formulation is 150 ppm of a 20-10-20 fertilizer and is sold in 24-ounce bottles. Rockwell recommends that one bottle be used to fertilize 1.5- to 3-gallon outdoor plant and flower containers every 21 days. The product’s signage shows a young patio gardener pouring the bottled fertilizer on a windowbox with the tagline, “Just pour on your way out […]

Read More

April 29, 2013

Plant Products To Be Purchased By MGS Horticultural And…

MGS Horticultural Inc., a full-service supplier of fertilizers, pest control products, seeds and substrates in North America, together with Haifa Chemicals, a global supplier of potassium nitrate for agriculture and industry, specialty plant nutrients and food phosphates, today announced a plan to purchase Canadian fertilizer and pesticides supplier Plant Products Co. Ltd. The deal is anticipated to close by the end of June 2013. MGS will acquire Plant Products’ Canadian distribution business, sales force and name. MGS plans to use both names (MGS Horticultural and Plant Products) in all communications going forward. MGS will maintain locations in Leamington, ON; Brampton, ON; Laval, QC; St. Hyacinthe, QC; and Detroit, Mich. As part of the deal, MGS has signed multi-year agreements with Haifa to maintain exclusive distribution of Plant-Prod Soluble Fertilizers, Acer Controlled Release Fertilizer, Stim-Root and potting soil premix fertilizers for distribution in Saskatchewan, Manitoba and Eastern Canada. “MGS is excited […]

Read More

March 19, 2013

Irrigation And Fertilizer Tips For New Vegetable Grower…

Compared to other variable costs, fertilizers are not the largest part of the budget in conventional greenhouse production. Nevertheless, over the course of the season, mistakes in fertilizer use can lead to significant damages or crop losses. That makes this an important topic for ornamental growers who are experimenting with growing vegetable crops. This article will emphasize major differences between fertilizers used in vegetable production and in ornamentals production. The different nutrition strategies, monitoring and water volume per plant will also be explained. Fertilizer In ornamental production, nutrients are delivered using various water-soluble fertilizers through a fertilizer injector, through the use of controlled-release fertilizers, or a combination of the two. There are numerous fertilizer mixes available with all the needed nutrients already included. The fertilization rate is often given in parts per million (ppm) of nitrogen (N), which is a way of expressing the fertilizer concentration. At younger stages, plants will […]

Read More

March 11, 2013

Daniels Plant Food Rebranded As Nature’s Source

Ball DPF has announced the launch of Nature’s Source, a rebranding in name and packaging for its seed extract-based plant fertilizer product Daniels Plant Food. The new brand will make its industry debut at California Spring Trials. “While we are proud of our heritage, we made this bold decision because we’re expanding sales to our existing market segments and entering new ones. It was a good opportunity to start with a fresh name and a modern look for all our products and packaging,” says Chance Finch, general manager for Ball DPF. “We wanted to make it obvious, beginning with our Nature’s Source brand name, to know our products are sourced from nature. Growers, contractors and gardeners can be confident that our effective and unique formulations remain unchanged. We’re excited about launching updated packaging for all our products, and especially the new ready-to-spray plant food for home gardeners.” The Nature’s Source brand […]

Read More

March 6, 2013

Everris Introduces E-Max Release Technology Coating Che…

Everris has introduced its new E-Max Release Technology, a proprietary coating chemistry for use on a wide variety of nutrient components that are incorporated into controlled-release fertilizer products. According to Chris Buchheit, marketing manager for Everris’ ornamental horticulture products, E-Max will help Osmocote and the company’s other existing brands deliver even better performance and value. “This coating will aid in the development of products that both complement and enhance our Osmocote portfolio and other fertilizer lines. It will increase Everris’ flexibility to create customized nutrition programs designed for horticulture growers,” Buchheit says. E-Max Release Technology is a durable, cutting-edge, reacted polymer coating for use on a variety of essential macro- and micronutrients. Nutrients coated with E-Max Technology are produced to the same performance standards as Osmocote. It will allow for continued expansion of the Everris portfolio, the ability to further enhance nutritional values and an efficient use of a grower’s […]

Read More

February 4, 2013

BioSafe Systems And Daniels Plant Food Company Will Par…

BioSafe Systems has entered into a strategic partnership with Daniels Plant Food Company, a wholly owned subsidiary of Ball Horticulture Inc. BioSafe and Daniels have worked closely together for the past two years and will now look to further develop liquid plant food serving both the conventional and organic markets. In conjunction, BioSafe Systems will be introducing its own branded line of plant food products focusing on turf, landscape and agricultural markets. “Liquid plant food is a natural progression for our company” says Rob Larose, CEO of BioSafe Systems. “It fits perfectly into our current line of green and sustainable products, and we are excited about partnering with Daniels.” Daniels manufactures and markets both conventional and organic liquid fertilizers, using botanical extracts to provide high-value nutrition to plants. BioSafe Systems develops and markets effective and sustainable products to a wide variety of industries, including fruit and vegetable production, turf and […]

Read More