Understanding Plant Nutrition: Fertilizers And Micronutrients

Understanding Plant Nutrition: Fertilizers And Micronutrients

Micronutrient (iron, manganese, zinc, copper, boron and molybdenum) nutrition is different from managing macronutrients, such as nitrogen, in three fundamental ways. First, the solubility and plant availability of micronutrients is affected by media pH to a much greater extent than is macronutrient solubility. Second, the difference between acceptable concentrations of micronutrients and concentrations that are either too low (deficiencies) or too high (toxicities) is small compared to a broader range for macronutrients. Finally, while most macronutrients are mobile within the plant, most micronutrients are immobile, and so a constant supply is needed for the duration of the crop or growth and plant appearance may be affected. In this article, we will focus on micronutrient sources, and how they are applied to a crop.

Preplant Sources

In soilless media, preplant sources of micronutrients are often added at mixing. In general, the sources in starter fertilizers can include both soluble forms (sulfates) and insoluble forms (oxides or fritted trace elements) (Table 1). Sometimes, water-soluble fertilizers or micronutrient sources are sprayed onto the media at mixing (Table 2). In general, micronutrients are incorporated in a root medium at low rates and therefore only represent a relatively small percentage of the total amount applied to a crop.

The one exception is often iron. Iron sulfate at rates up to 4 pounds per cubic yard (2.4 kg/m3) is sometimes added in a starter fertilizer to supply iron and to help keep the medium-pH low when using irrigation water that contains high concentrations of alkalinity. Depending on the rate used, the effect will probably not last for more than three to four weeks and may need to be reapplied.

There are several considerations for using iron sulfate for iron nutrition and acidification in a starter fertilizer. Iron sulfate oxidizes easily, and so it can be difficult to obtain consistent results. Never mix iron sulfate with limestone before incorporation, because the high pH of the lime can cause the iron sulfate to oxidize and become ineffective. Finally, if you are using both iron sulfate and lime in a mix, you may get better, more uniform pH control by leaving the iron sulfate out completely and reducing the lime incorporation rate.

Water-Soluble Sources

In soilless media culture, most micronutrients are applied to a crop after planting. The sources can include individual micronutrient sources blended together, commercially prepared micronutrient blends or the micronutrient package contained in a commercially prepared water-soluble fertilizer (Table 2).

The sources of water-soluble micronutrients come in two forms, inorganic salts (all micronutrients) or chelates (only iron, manganese, zinc and copper). Inorganic salts are materials that dissolve in water to form ions that are available to the plant. For example, iron sulfate will dissolve into separate iron (Fe) and sulfate (SO4) ions.

Chelates are organic molecules that envelop the ion and protect it from interacting with other ions in the soil solution. There are many chelating molecules available, but only three that are commonly used in horticulture: EDTA, DTPA and EDDHA. These abbreviations refer to the chemical structure of the organic molecule. In general, manganese, zinc and copper chelates are only found in the EDTA form. In comparison, there are three forms of iron chelate, FeEDTA, FeDTPA and FeEDDHA, although the most common form is FeEDTA.

With iron nutrition, the form of iron is very important. The three common chelated forms (iron-EDDHA, DTPA and EDTA) differ in their ability to hold onto the iron (and therefore keep iron soluble and available to plants) as the media pH increases. Between a media pH of 4.0 to 5.5, any form of iron will work (including iron sulfate) at supplying iron to the plant. However, as the media pH increases above 7.0, only the iron from Iron-EDDHA has high solubility. Research has shown that the ranking of iron forms from most effective to least effective at supplying iron at high media pH is Iron-EDDHA > Iron-DTPA > Iron-EDTA > Iron sulfate. If iron is applied in a form that is not soluble because of high media pH, then most of the nutrient will not be available to plants until media pH is lowered.

Commercially prepared water-soluble fertilizers usually contain micronutrients, with iron most commonly supplied as iron-EDTA. Manganese, zinc and copper will be supplied in the EDTA form for most N-P-K fertilizers (example are 20-10-20, 20-20-20 and 21-7-7), while sulfates are often used in formulas that contain calcium and magnesium (example is 13-2-13 or 15-5-15). Since the amount of sulfate supplied by the inorganic salts is very low compared with the calcium, there is no problem with precipitation between the sulfate and the calcium. In all formulas, boron and molybdenum are supplied as inorganic salts.

Applying Micronutrients

Commercially prepared water-soluble fertilizers contain micronutrients at one of two levels (Table 3). “General Purpose” formulas were designed when field soil was a primary component in container media. Because field soil often contained micronutrients, the levels contained in these fertilizer formulas were relatively low. Peat-lite formulas were designed for soilless media culture, and therefore have higher micronutrient levels (Table 3).

To calculate the concentration of micronutrients supplied by a blended fertilizer, you need to know the concentration of nitrogen in the fertilizer solution and the ratio of nitrogen to that micronutrient that is listed under the “Guaranteed analysis” on any fertilizer bag. For example, to calculate the concentration of iron supplied by a 20-10-20 peat-lite formula (0.1 percent Fe) at 200 ppm N, you divide the percent of Fe by the percent of N, then multiply by the nitrogen concentration of the fertilizer solution.

So at 200 ppm N, you are also supplying about 1 ppm Fe. If this 20-10-20 were a “General Purpose” formula, then percentage of Fe would be about 0.05 percent and the concentration of iron supplied by the fertilizer at 200 ppm N would be about 0.5 ppm Fe.

For growers who are adding micronutrients to the fertilizer, a “safe” concentration would be something similar to that supplied by a peat-lite water-soluble fertilizer at a concentration of 100 and 200 ppm N. Using iron as an example, that would correspond to a concentration of 0.5 to 1.0 ppm Fe from one of the water-soluble sources found in Table 2. If you are adding additional micronutrients to a water-soluble fertilizer, remember to take the micronutrients supplied by the water-soluble fertilizer into your calculation.

Acceptable pH Ranges For Crops

When an acceptable pH range of a crop is recommended, it is usually based on the use of a peat-lite fertilizer formulation applied at a moderate concentration of 100 to 200 ppm N. The micronutrient (and especially iron) form and concentration in that fertilizer greatly influences this acceptable pH range.

Take an iron-inefficient crop like petunias as an example. The acceptable pH range for petunias is often listed at 5.5 to 6.2. However, the acceptable pH range could be more narrow (and lower) if a “General Purpose” fertilizer formula (lower iron concentration) or a sulfate-based micronutrient formula (iron solubility issues) were used to ensure that enough iron is supplied. Alternatively, the acceptable pH range could be broader if the concentration of applied micronutrients was increased, or if the iron source was changed from FeEDTA to FeDTPA or FeEDDHA.

However, just increasing micronutrient concentrations and neglecting pH management also has risks. While that strategy may work for a single crop such as petunia, it can cause problems with other crops in your greenhouse. For example, higher iron concentrations increase the risk of iron toxicity in iron-efficient crops such as geraniums or marigolds. In next month’s article, we will discuss controlled-release fertilizers.


Leave a Reply

4 comments on “Understanding Plant Nutrition: Fertilizers And Micronutrients

  1. Dear Sir/Madam: It was very good topic for me and thanks for your helping people in this regards.if you can shear usage of it,(how and when) with seasonal calendar it will be better. thanks

More From Uncategorized...

April 17, 2015

Sakata Seed Uses California Spring Trials Display Plants To Give Back

Sakata Seed America is putting its post-CAST (California Spring Trials) plants and flowers to good use to support events in local California communities of Salinas and Morgan Hill. The plants, along with donations through Sakata's Charitable Giving Program, will support three fun-filled community events that promote healthy lifestyles and support the agricultural industry.

Read More
Hakonochloa macra Aureola v

April 17, 2015

Ornamental Grasses — A Few Thoughts

Grasses have been embraced by growers, landscape architects and retailers, and are an important component in wholesale and resale sales. Allan Armitage shares some popular grasses, one to avoid and a few to use with caution.

Read More

April 17, 2015

Allan Armitage’s Favorite Plants From Proven Winners, Syngenta And Danziger

Between visiting California Spring Trial giants like Proven Winners, Syngenta and Danziger, Allan Armitage saw a lot of great plants in one day. Despite the size of the challenge, Dr. Armitage finds a few favorites he thinks you should try.

Read More
Latest Stories

March 31, 2015

Manufacturers Are Taking Biologicals To The Next Level

Through acquisitions and new products, many crop protection companies are making firm commitments to the future of the biocontrols industry.

Read More
Aquaponics At Brogue Hydroponics

March 30, 2015

Aquaponics Is Making A Splash At Brogue Hydroponics

The owners of Brogue Hydroponics explain why they expanded into aquaponics, and how the shift has helped them uncover a new market opportunity.

Read More

February 11, 2015

Benchmarks: Find Your Game-Changer

Refuse to accept the status quo in your organization. Explore possibilities until you find what works best for your company.

Read More
Lychnis 'Petitie Jenny'

February 2, 2015

Variety Showcase: A Dainty Debutante

Lychnis flos-cuculi ‘Petite Jenny’ is a dwarf form of the popular Blooms of Bressingham® variety Lychnis ‘Jenny.’ This double-flowered little beauty was discovered by Paul Gooderham at Bressingham in a patch of its parent, ‘Jenny.’ ‘Petite Jenny’ produces masses of large, fluffy, double flowers like ‘Jenny,’ but with a more diminutive habit. The species goes by the common name “ragged robin,” but there’s nothing unkempt about ‘Petite Jenny’. An easy-to-grow, tidy little plant, it is suitable for any sunny or lightly shaded border and is also a good container candidate. Garden Performance Mass of bright pink double blooms beginning in mid-May. Sterile blooms provide a longer flowering period Erect, leafy flowering stems Forms compact, bushy clumps just 14” tall with an 18” spread Attracts bees, butterflies, hummingbirds and is deer-resistant Ideal for containers, front of borders, informal cottage gardens and as cut flowers’ Sun or part shade USDA Hardiness Zones 5 to […]

Read More

January 20, 2015

Register Now For Biocontrols 2015 Conference & Trad…

The first-ever conference, sponsored by Meister Media Worldwide and the Biopesticide Industry Alliance, March 3-5, offers hands-on, real-world technical advice on biocontrols to help you craft a “softer” pest management program focused on resistance management, MRLs and results.

Read More

October 6, 2014

Fighting Fall Foliar Disease: The Best Defense Is A Goo…

Managing foliar diseases starts with evaluating the greenhouse environment. Virginia Brubaker, GGS Pro Technical support supervisor for Griffin Greenhouse Supplies, shares some ways you can defend against fall foliar diseases.

Read More

July 11, 2014

Highlights From The 2013 USDA Floriculture Crops Summar…

The U.S. Department of Agriculture released its 2013 Floriculture Crops Summary in June. Here are some of the main findings.

Read More

June 16, 2014

Pine Wood Chips As An Alternative To Perlite In Greenho…

This is the third article of the four article series highlighting the production and use of pine wood chips as aggregates in greenhouse substrates. We found growers do not need to adjust their production practices when 20% pine wood chips are utilized as a perlite replacement.

Read More

June 4, 2014

Hortilux To Service Philips Indal Supplemental Lighting…

Hortilux Schréder (Hortilux) will be undertaking the warranty and service work for existing Philips Indal supplemental lighting systems for greenhouse horticulture. To deliver this service, Hortilux is taking over the entire stock of parts and production tools from Philips Indal Horticulture, so Hortilux is able to provide the full range of service and maintenance on these supplemental lighting systems, even after they are no longer covered by warranty. In North America, this will fall under the domain of PL Light Systems Canada Inc., a division of Dool Industries and a sister-company of Hortilux. In North America, PL Light Systems will contact the existing Philips Indal horticultural clients and work with them to quantify their service and maintenance needs. PL Light Systems offers a wide range of service and maintenance services, including on site supplemental lighting measurements, lamp and reflector measurements, trouble-shooting and repair maintenance and automated reflector cleaning services. These […]

Read More

May 29, 2014

Country Raisin’s Joins HGTV HOME Plant Collection…

The HGTV HOME Plant Collection announced Country Raisin's as a new annuals grower for 2014.

Read More

April 24, 2014

ePlantSource Announces Five New Partnerships

ePlantSource has announced the addition of five new suppliers to the list of partner companies who provide their products on ePlantSource.com. The additional partners will offer customers new products as well as expanded options for programs and locations.

Read More

April 16, 2014

Update: House Approves Bill To Raise ACA Full-Time Defi…

The U.S. House of Representatives passed legislation on April 3 that would raise the definition of full-time employment under the Affordable Care Act (ACA) to 40 hours per week.

Read More

March 27, 2014

Amidst Drought, Solar Desalination Allows California Wa…

California's Panoche Water District has implemented a solar desalination facility in response to the need for cost-effective solutions that would ensure the availability of sustainable water in the vital agricultural region. The facility, located in Firebaugh, Calif., utilizes renewable desalination technology from WaterFX.

Read More

March 14, 2014

Adaptive Plastics Announces Laurie Stribling Will Move …

Adaptive Plastics, Inc. has announced the promotion of Laurie Stribling to a newly created outside sales position. In her new position, Stribling will drive sales of Solexx greenhouse covering to commercial greenhouse and nursery growers.

Read More

March 14, 2014

BASF Empress Intrinsic Brand Fungicide Approved For Use…

BASF Empress Intrinsic brand fungicide has received supplemental labeling from the U.S. Environmental Protection Agency (EPA) for use on herbaceous and woody plants in greenhouse, nursery container and field production in New York.

Read More
Nicotiana 'Starlight Avatar' from Bioglow is the world's first light-producing plant.

January 31, 2014

Auction For ‘Starlight Avatar’ Went Live To…

BioGlow's auction went live on January 31 for nicotiana 'Starlight Avatar,' the world's first light producing plant.

Read More

January 29, 2014

U.S. Farm Bill Passed In The House After Years Of Indus…

The Farm Bill passed in the House of Representatives on January 29, and could be voted on as early as January 30 in the Senate.

Read More

January 2, 2014

MSU To Host 2014 Independent Plant Breeders Conference,…

If you want an opportunity to learn from and network with other plant breeders, new product development and market gurus and intellectual property experts, consider attending the 2014 Independent Plant Breeders Conference, October 30 to November 2, 2014. Held at the Amway Grand Plaza in Grand Rapids, Mich., educational sessions will fill the first two days, focused on helping independent breeders be successful, from technical aspects of breeding through product development and marketing. The final day will include tours of Western Michigan nurseries, garden centers and horticultural landmarks. Sessions will cover topics including: Basic and advanced plant breeding techniques Managing a breeding program Bringing selections to market Intellectual property management Market trends A formal agenda, list of sessions and registration details will be released in the spring. Meanwhile, if you have questions, contact Michigan State University’s Ryan Warner. Find information on hotel accommodations here. Independent Plant Breeders & Students of […]

Read More