Fertilizer know-how can grow profits

Features - Production

Experts explain the variables associated with crop fertility.

February 25, 2014

Wouldn’t it be nice to have a one-stop solution for achieving greenhouse crop fertility? Kind of like one-stop shopping. Of course, it doesn’t work that way. Greenhouse managers have a multitude of variables to consider when choosing a fertilizer program. These variables include the crop grown, stage of development, indoor and outdoor environmental conditions, water analysis, potential for nutrient leaching, and the rate and amount of fertilizer used to grow the crop.

Failure to hone in on these variables may not lead to total crop loss (although it could), but more likely a failure to maximize growth and the aesthetic value of the plant, which leads to increased labor and energy costs and subsequent loss of profits in an industry already struggling with tight profit margins.

The following article is intended to help you understand the role of major and minor nutrients on good crop growth, discuss the interplay of the variables mentioned above, and introduce some new, and perhaps new to you, plant food products on the market, including organics.

The “fertilizer pyramid”

Few things operate in a vacuum and fertilizer is no exception. Dennis Crum, director of growing operations at Four Star Greenhouse in Carleton, Mich., says there are three components of a crop fertility program—fertilizer, soil, and water—that can be compared to the food pyramids we know from our school days. It is imperative that greenhouse growers analyze these components closely and regularly in the greenhouse to develop a viable fertility program.

Understanding ppm and fertilizer injectors

Rates of fertilization are often given in parts per million (ppm) of nitrogen (N). For example, it is often recommended that 150 to 250 ppm N be applied in irrigation water on a “constant feed” basis for fertilizing many floricultural crops. If a f20-20-20 ferrtilizer with 20 percent nitrogen, 13½ ounces are needed to make 100 gallons of a 200 ppm N solution, whereas with a 15-15-15 fertilizer containing 15 percent nitrogen, 18 ounces are required to make 100 gallons of a 200 ppm solution.

Fertilizer stock solutions are mixed according to the fertilizer injector ratio. Each injector will deliver a certain amount of stock solution for each increment of irrigation water that passes through the injector. For example, a 1:100 injector will deliver 100 gallons of diluted fertilizer solution for each gallon of concentrated stock solution. A 1:200 injector will deliver 200 gallons of diluted fertilizer for each gallon of concentrated stock solution (or 100 gallons of diluted solution per half gallon of stock). If both injectors were to deliver 200 ppm of nitrogen from the same fertilizer, the stock solution for the 1:200 injector would have to be twice as concentrated as the one for the 1:100 injector.

The injector ratio determines the concentration of the stock solution that is needed to deliver a particular rate of fertilization. Some injectors (Hozon, Smith Measuremix) have a fixed (nonadjustable) injector ratio whereas other injectors (Anderson, Dosmatic, Dosatron, M-P Mixer Proportioner) have adjustable ratios. Many growers prefer injectors with adjustable ratios so that different fertilizer rates can be applied to crops with different nutrient requirements.

Source: UMass Amherst Greenhouse Crops and Floriculture Program.

Crum uses Proven Winners’ fertilizers that address pH and alkalinity issues in greenhouse water supplies. These products are available to all growers and work particularly well with Proven Winners’ plants, particularly its Supertunia petunia and calibrochoa hybrids, which have specific pH and fertility needs.

“It’s a service to our customers,” says Crum of getting into the retail fertilizer business. He says growers often send water samples to Quality Analytical Laboratories (QAL) in Panama City, Fla., that make their way back to Crum. From there he can evaluate the test results and can match up Proven Winners fertilizer blends to the grower’s water analysis or custom blend a product for them to use.

A test of water alkalinity rather than pH provides a better picture of water analysis, says Crum. For instance, a low pH reading doesn’t necessarily mean the alkalinity levels in the water are okay. He says the “sweet spot” for alkalinity in a water analysis is around 90; alkalinity tests can be taken with simple water test strips.

In regards to soil, or greenhouse substrates, Crum says it is important to know your growing media, what’s been added to the growing medium, if anything, and what will subsequently need to be added to your fertilizer program, based on the composition of the substrate.

Extension specialists at UMass Amherst have a few more variables that should be considered in a crop fertility program and include:

Fertilizer type: ratio of ammonium to nitrate-N, trace element charge, content of calcium and magnesium, and potential acidity.

Frequency of application: how frequently are you watering? Crum says some growers don’t realize that as the weather is warming outside you’re watering more, which not only means you’re applying more fertilizer, but also means if your alkalinity is high, you’re watering plants with high alkaline water which can skew your fertilizer program.

Leaching fraction: plants leach nutrients more as the volume of water increases, which means fewer nutrients are available for plants; less watering means less leaching, but more potential for salt build-up.

Plant growth rate and environmental conditions: floriculture crops generally use more nutrients during rapid growth. Too much of a good thing, i.e., nitrogen, during low growth periods leads to salt buildup.

Fertilizer rate: while fertilizer rate, as expressed by parts per million (ppm), is normally the main focus of growers, extension specialists at UMass advise that growers consider the other factors listed above to get a better picture of how much fertilizer to apply at any given time.

Don’t forget the micronutrients

During my first year managing a commercial greenhouse I was proud of the terrific germination that took place with a couple hundred flats of petunias we had started that spring; I was equally dismayed when they all turned as yellow as the petals on a sunflower. Luckily, a university extension specialist was on hand. He pointed out that while I was applying adequate amounts of nitrogen, which normally prevents chlorosis, or yellowing of foliage, I was ignoring an important micronutrient which is essential for many greenhouse crops. The fertilizer I was using simply lacked iron.

Growers are advised to research a crop carefully, particularly a new crop, to determine what micronutrients might be required and the variables that can affect nutrient efficiency.

“There are so many variables,” says Mark Jeffries, vice president of Masterblend International, a maker of greenhouse fertilizers in Morris, IL.  “When you get into a jam it could be because light levels or your water has changed.”

This interplay of variables greatly affects the amount and type of fertilizers growers will add to injectors. Jeffries says the best way to find out what is needed to correct a problem is to have a plant tissue test done, which Masterblend will do for growers via Micro Macro International (MMI) in Athens, Ga. In regards to fertilizer delivery, it is equally important to test the solution coming out of the injectors.

“I frequently encounter nutrient- starved plants and when we trace back the problem it turns out the injector was not working as intended,” says Neil Mattson, associate professor and floriculture extension specialist Department of Horticulture Cornell University. Crum says they test their water daily during propagation.

Bring on the organics?

Providing organic solutions to crop fertility issues in the greenhouse has been a tough sell, given the short timespan crops are grown under glass and some of the logistics of using organic products in a greenhouse. Yet, there is a recognized need for fertilizers that are not only sustainable in terms of how they are acquired, but also plant food that deals with issues of nutrient leaching and ground water contamination.

There are some new products on the market from New York-based Bioworks Inc. that are worth a look for greenhouse growers of annual bedding plants, vegetable transplants, herbs, and perennials. These organic fertilizers may not provide a one-stop or all inclusive solution to these problems, but they do show particular promise for growers of edibles and herbs.

“A lot of conventional growers wouldn’t be interested unless they went to a more sustainable product or a product that will decrease leaching,” says John Francis, technical services manager at BioWorks, Inc.

“The backbone of all of our products is the organic components that provide a slower release of nutrients,” he adds.

Bioworks offers a line of slow-release liquid and granular fertilizers for greenhouse crops under the Verdanta. For example, Verdanta® PL-2 , 2-0-6 can be used as an organic liquid feed to supplement potassium for any crop, according to Francis. It has a slow salt index which means lower electrical conductivity in the growing medium. Francis says Bioworks’ products also stimulate microbial activity in the root zone. The organic material, which includes sugar beet cane juice, isn’t composted before they go into the formulation, which increases the biological diversity of the growing material.

Verdanta OFE Foliar is beneficial for plants like mums, geraniums and petunias that can go off color due to iron chlorosis. It is a chelated iron foliar spray (3-0-0 +3 percent iron) derived from both organic and mineral sources, according to Francis. It contains urea to help move iron into plant leaf tissue and contributes to greening of the leaves. The secret ingredient is 3 percent seaweed extract, which is used as a foliar spray.

One concern greenhouse growers have in regards to using organics is the consistency of nutrient analysis from one batch of fertilizer to another. BioWorks products carry the patented MINIGRAN designation, meaning that its products are guaranteed to contain the same nutrient analysis from bag to bag.


With the number of variables involved in growing a successful crop, greenhouse growers have to be part CSI investigator and part horticulturist.

“The most important element is the one that is missing,” says Jeffries.


Neil Moran is a horticulturist and freelance writer based in Sault Ste. Marie, Mich.