Aerating soil or media allows for good plant growth, and regular cultivation of the soil and growing container allows the roots to draw in needed oxygen. Likewise, growing media is selected based on a certain level of porosity. The result is apparent in healthier plants. This knowledge can be applied to growing under cover, whether you’re planting in peat-based products or in water, i.e., hydroponics.

Dissolved oxygen (DO), which is measured in mg/l as a percent of saturation or in parts per million, is influenced by the temperature and salinity of the water, says Kurt Becker, director of commercial products sales and marketing at Dramm. Cold water holds more dissolved oxygen than warm water. Clean water also has a better capacity of holding dissolved oxygen than suboptimal water that contains mineral deposits and other undesired material from irrigation equipment or from the source, e.g., a pond or reservoir.

Growers can measure the oxygen in irrigation systems, substrates and hydroponics with a dissolved oxygen meter. Various companies are also developing technology in a range of price points that will increase the amount of oxygen reaching the plants, which can be the ticket for better root development and prevention of diseases, such as Pythium.

Hydroponics vs. container production

“There are two different situations, [the first] is growing in a container with peat or a coir substrate,” says Dr. Paul R. Fisher, professor and extension specialist, floriculture, University of Florida. “We chose those substrates to have large air pores in them. Oxygen moves through air much quicker than it does through water. If you’re a tomato grower growing in pots, focus on not overwatering and having enough air porosity in your growing medium providing oxygen to your roots.”

The other growing situation is hydroponics, Fisher says. “In hydroponics where the roots are bathed in water, it is very important that there is enough oxygen dissolved in the water because that’s the only way roots are going to get oxygen for their healthy growth and respiration,” he says.

“What you want to do in hydroponics is, through air movement or injecting air into water, try to keep the oxygen level close to the natural saturation point of water, which is going to be somewhere around eight to nine parts per liter,” he says.

In hydroponics, growers are working with warm water. Compared to cooler water, warm water doesn’t hold as much oxygen, and root and bacterial respiration occurs at a higher rate, Fisher says. Plant pathologists have found that pathogens such as Pythium are more likely to start affecting roots when oxygen levels are low.

Paying attention to dissolved oxygen

It appears not all produce growers are manipulating dissolved oxygen to their benefit, particularly those using peat-based products.

“We saw incidences where customers were having growth problems and they had nothing to attribute it to — they just had issues,” Becker says. “It turned out in many incidences they had really low dissolved oxygen in their water, but nobody was paying any attention to it.”

Becker draws a comparison between DO and when, 50 years ago, growers started taking notice of electrical conductivity (EC) in the soil. “We knew if we added fertilizer, it helped the plant grow. Optimizing EC, though, has resulted in huge strides in plant productivity,” he says.

The same can be said for pH. Adjusting the pH to the right level will result in better uptake of nutrients and a healthier plant. Becker says similar gains will occur if you measure the dissolved oxygen in your water and make the necessary adjustments.

“Now we understand that dissolved oxygen is the third measurement that people haven’t been paying attention to,” Becker says.

Many growers are unaware of the level of oxygen present in their irrigation water or that raising the level of oxygen in the water will produce a plant with a better root system, Becker says.  

“There is still a need for education,” Becker says. “There are a lot of people who don’t understand it. Those growing in water understand it, the hydroponic bath guys get it, as do those growing cut flowers.”

Becker says that when O₂ is low, which is fairly common with growers, plants can grow less and have reduced cation exchange.

Improving root growth through electrolysis

Relatively inexpensive oxidation systems can deliver oxygen to the root zone of plants with the potential to increase root mass and foster better growth. In one university study this premise was put to the test using technology from a company called O₂ Grow, which delivers 100 percent oxygen to irrigation water via a process called electrolysis. Unlike an air bubble system where large air bubbles rise to the surface and escape, the O₂ Grow system injects tiny hydrogen molecules and oxygen that quickly dissolve, into the water.

“We had been noticing poor root development in two of our hydroponic settings,” says Dr. Sonja Maki, assistant professor in the Department of Plant and Earth Sciences at the University of Wisconsin-River Falls. “We were trying to improve plant growth in a vertical greenwall and an NFT system; we noticed the DO was pretty low at 5mg/l. We tried adding air stones and it didn’t help too much.”

Maki says she conducted an experiment with an undergraduate student, Taylor North, using crown peas grown in an NFT system in a greenhouse. “We chose the crown pea — a determinate plant — because they also grow wide at the top and produce a lot of flowers,” she says. They conducted experiments using a control group and the O₂ Grow technology and noticed a marked difference in growth in the experimental plants.

“The main takeaway is after we grew the crown pea in the experimental group, we noticed better root growth and taller plants. They also branched significantly more with the oxygen,” Maki says. The roots of the plants in the control group were “brownish and the lateral roots terminated prematurely,” she says, while those in the experimental group had longer roots and better weight. Using a three in one meter that also measures pH and EC, she says they achieved about 8 mg/l of DO in the water the plants were growing in. She acknowledged that there are other ways to achieve higher levels of DO in the water. However, this system offered a relatively low-cost method of improving root development and overall growth of a plant.

“I would say if they’re interested in exploring how it will benefit root health they might want to try it,” Maki says. “I’m just working with one cultivar, one that is not used commercially. They might want to try it on the cultivars they use.”

Ozone in water

Ozone (O₃) is a potent and efficient oxidizer when added to an irrigation system. It keeps the pipes clean by oxidizing organic material and biofilm in the water. What does it have to do with dissolved oxygen? Clean pipes place less demand on oxygen, which helps maintain higher levels of DO. More importantly for growers, ozone is 13 times more soluble in water than O₂, allowing higher levels of oxygen to be dissolved in water. According to Becker, higher levels of dissolved oxygen can be maintained in an irrigation system by adding O₃.

To conclude, adding dissolved oxygen and ozone could be the ticket for better root development and shorter cropping time. Meters are relatively inexpensive but the technology to deliver these gases, particularly ozone, can be expensive. Growers must look at the cost of the various units and determine which ones will deliver the best return on investment for their operation.

Neil is a horticulturist and freelance writer. greenindustrywriter.com

The passage of the Food Safety Modernization Act (FSMA) brought with it some sweeping changes and requirements for nearly every facility that grows, processes, transports or distribute foods. And produce growers — field or greenhouse — are no exception. In fact, it may be the produce growers who have faced the greatest adjustment, since the rule marked the first time that science-based minimum standards were established for this segment of the food industry.

As specifically stated in the Summary of FSMA’s Produce Safety Rule: “The rule sets forth procedures, processes, and practices that minimize the risk of serious adverse health consequences or death, including those reasonably necessary to prevent the introduction of known or reasonably foreseeable biological hazards into or onto produce and to provide reasonable assurances that the produce is not adulterated on account of such hazards.” But because no two facilities are identical — growing the exact same produce under the exact same conditions — individual operations need to consider their circumstances to address relevant hazards and assess the risk.

Thus, FDA goes on to say in the rule that “one way to do this is through the voluntary use of farm-specific operational assessments and food safety plans.” These assessments, it says, “can help farms identify and take measures that may be prudent for their individual operations to prevent the introduction of known or reasonably foreseeable hazards, including any non-biological hazards.”

As a foundational element of FSMA’s Preventive Controls rule, the food safety plan is essentially the written documentation of a food facility’s identification of hazards and development of preventive controls based on the assessment of their risk. So while produce growers need not go to the extent of developing a full food safety plan with all its requirements and intricacies, the application of its hazard identification and risk analysis may be — to apply the words of FDA — “prudent.”

Chart courtesy of FDA

So how does one go about applying this prudence? A good start would be to follow the example of FDA itself, which used a four-step process to inform the development of the standards of the Produce Safety Rule in its qualitative risk assessment on produce contamination. The process focuses on the application of a hazard analysis, which, as used by FDA, is defined as “the process of collecting and evaluating information on hazards and the conditions leading to their presence to determine which hazards are significant for food safety and therefore should be addressed.”

Hazard identification and risk analysis. While FDA’s process included hazard identification, hazard characterization, exposure assessment and risk characterization, produce growers can use a somewhat simpler version combing two of these and adding the management step, as follows:

1. Identify the hazard. Identify any potential biological, chemical and physical hazards, such as those directly related to each specific produce item and process, any biological amendments used and hazards that could be introduced from the environment are ascertained. These should include any “known or reasonably foreseeable hazards” capable of causing adverse health effects and can be determined through a brainstorming session, facility historical data, as well as research into relevant sources (scientific literature, outbreak data, government or industry food safety guidance documents, historical data for similar products, etc.). Some additional considerations should include:
• Intended use and distribution (e.g. direct to retail, or will it undergo further processing?)
• Past experience with the produce, such as product testing results and consumer complaints
• Pathogens known to be associated with specific produce or biological amendments used
• The condition, function and design of the greenhouse that could cause contamination
• Processing activities and equipment used to make the product (e.g., ease of cleaning, potential for damage, cross contamination of allergens, etc.)
• Sanitary conditions of the greenhouse and employee hygiene
2. Characterize the hazard and assess exposure. The hazard characterization is essentially a dose-response assessment. That is, it is the probability of a microbiological hazard occurring to the level (dose) that it could cause an adverse health effect (response). It provides a qualitative description of the adverse effects of hazards that may result from consumption of contaminated produce. Similarly, exposure is the level of contamination in the food related to the quantity of the food consumed. A variety of factors should be considered in evaluating exposure, such as the likelihood of produce contamination from water, soil amendments, animals, workers, equipment and buildings. For example, the table on page 30 shows FDA’s evaluation of the relative likelihood of produce becoming contaminated with pathogens of public health concern from soil amendments.
3. Manage the risk. With the hazards identified, assessed and prioritized, produce growers can more efficiently and effectively manage the associated risk. It is important to remember, though, that risk management is not a one-time activity. Rather, potential hazards should be regularly re-assessed and the management plan re-worked and updated as needed for control.

The Produce Safety and Prevent Controls rules of FSMA are separate and distinct, but there are aspects that overlap as recommendations, if not requirements. As FDA states, in full, “Although we are not requiring that covered farms conduct operational assessments or develop food safety plans, we continue to believe that such assessment can help farms identify and take measures that may be prudent for their individual operations to prevent the introduction of known or reasonably foreseeable hazards.”

Lisa is the editor of Quality Assurance & Food Safety (QA) magazine. llupo@gie.net

Chart: Joachim Nachmansohn

Fertilizers are the most central crop input in a greenhouse, with the possible exception of water in irrigated systems. Fertilizers also comprise the biggest portion of the input costs. Yet, of all inputs, they’re often the one that has the biggest losses.

It’s therefore very important to have a correct understanding of fertilizers and the soil-water-plant-system, and a relevant approach to fertilizer management. This can be achieved by focusing on these three aspects: (1) how to think about fertilizers, (2) which to use and (3) how to execute your fertilization plan.

The principles I’m about to explain are universal for all plant production systems, such as soil-based, soilless or hydroponic systems. However, as the application of the principles have some differences between the systems, I will give examples as I explain.

How to think

Before we do anything, we need to know what we should do, and preferably why. This can be divided up in three sub-categories: what the plant needs, how much it needs and how to make it available to the crop.

What are the plants’ needs?

First of all, all plants need all essential plant nutrients. Always. There seem to be confusion regarding this among some producers and fertilizer suppliers. And the demand for the nutrients come in certain ratios, which are often quite similar for horticultural crops like vegetables and fruit (see table).

How big is the need?

The main difference in fertilizer demand between crops is based on their capacity to assimilate sunlight, i.e. the growth rate. Fast-growing crops need more fertilizers than slow-growing ones. Note that it’s not the size of the plant that matters but the capacity to form new plant tissue. The crop takes up the nutrients from the soil (or the soilless environment) in the form of a flow over time. The rate of this flow must at least be enough to sustain the growth rate and development of the crop. Otherwise, we will see inhibited growth or even deficiency symptoms.

Plant availability of the nutrients?

The plant nutrients are only available to the plant when they’re dissolved in the water accessible in the root zone, regardless of if the system is soil-based, soilless or hydroponic culture. So plant availability is determined by the supply of soluble fertilizers, and in soil-based systems, by the rate by which the microbes decompose the organic material, which in turn frees up nutrients that were locked up in the soil, in a process call mineralization. There must always be enough nutrients dissolved in the soil or growing media water to sustain a sufficient uptake rate by the plant.

Another important parameter for nutrient availability is pH. From the perspective of plant nutrition, an optimal soil pH is around 5.5. It’s preferable to try to enable a pH between 5.0-6.0 in the root zone. Outside of that range, some nutrients become unavailable to the crop. Even though some plants are adapted to soils with higher pH, and are able to acquire required nutrients, the optimal range generally remains the same in all systems.

Chart: Joachim Nachmansohn

What fertilizer products should we use?

What kind of product to use is a matter of what kind of production system is in mind, but in the thought process we should start with a complete fertilizer product that supplies all the essential nutrients. Then we scale back the need to supply some of these nutrients depending on what, and how much, the soil or the system can supply them. The irrigation water also contains naturally occurring nutrients that should be accounted for.

This does not imply that you must necessarily fertilize with all nutrients. You must simply ensure that there is access to all of them in the production system. If you have a good and fertile soil there is rarely the need to supply all micronutrients, as they can be attained by the crop from the soil. The strongest contrast to this would be a soilless hydroponics system, in which case you really must supply all nutrients via the fertilizers.

Regardless of system, nitrogen-phosphorus-potassium (NPK) fertilization is virtually always necessary in order to attain optimized yield levels. I would even argue that the other macronutrients (magnesium, calcium and sulfur) need to be supplied via the fertilizers, if high productivity is to be maintained. Even in systems where the soil is very fertile, it’s unlikely that it can sustain a sufficient flow of these nutrients that matches the demand, when aiming for high yield levels via different inputs.

For the micronutrients, it’s usually not as much a matter of their presence in the system as it is an accessibility issue, in which the pH is the most important parameter. The best way to manage the pH in the root zone is via the fertilizer. N is supplied to the plant either in the form of ammonium or nitrate. Ammonium lowers the pH in the root zone in conjunction with nutrient uptake. So by using a fertilizer with an ammonium/nitrate-ratio adjusted for your conditions, you assure that the nutrients present in the root zone can be taken up by the plant (see table).

In order to scale back according to the above reasoning, testing of both soil and water is necessary. However, don’t rely too much on the test results. They serve as a very good starting point, but always look at the crop response, as other factors can affect the outcome.

Note that it can often be worthwhile to use a complete fertilizer, even though the soil might have a high fertility level, or even if you are able to make your own mix for soilless and hydroponics culture. Especially a product that provides the micronutrients in chelated forms that will remain accessible to the crop, and that has the above-mentioned ammonium/nitrate-ratio. Then you guarantee the supply of all nutrients, almost regardless of soil conditions or other parameters in soilless systems.

How should we do it?

Once you have decided which products to use, you need to supply them in the best way possible. This means to supply on demand over time. The only way to do this with precision is via high-frequency fertigation. Optimally, nutrients can be supplied at each irrigation, but if that’s not possible the doses need to be adjusted. Depending on limitations in the production system, you can scale back the precision level of the on-demand supply, in which case you need to supply more nutrients in advance. If you need to scale all the way back to one, or only a few fertilizer applications, for the whole season, I would recommend a controlled-release fertilizer that follows the need of the crop.

Joachim works internationally as a fertilizer and soil & water management expert. Through his company Nachmansohn Consulting & Co. he, among other things, works with spreading accurate knowledge and necessary skills on how to improve fertilizer use for increased profitability. His unique specialty is Demand-Driven Fertilization. joachimnachmansohn.com; consulting@joachimnachmansohn.com

In this issue, we’re once again trading some of the greenhouse photos for headshots as we honor seven people who are moving the produce industry forward, whether it’s promoting fruits and veggies on Capitol Hill or finding ways to produce better vegetable transplants in vertical farms. In our second annual People in Produce section, we feature an entrepreneur with a taste for unique crops, a scientist attuned to consumers’ sensory needs, a community-focused grower in the Arctic and more. Read about these interesting, successful members of the produce industry.

This month we’re also wrapping up our Urban Agriculture series with a feature on the economics of urban ag by Robin G. Brumfield and Charlotte Singer, both from Rutgers University. If you missed any of the other articles in this series, you can find them here. And if you’d like to improve your marketing, don’t miss Leslie Halleck's column this month about how interacting with your local area can strengthen both your relationships and your brands.

Christopher Currey takes a close look at tomatoes, the most popular crop among our readers, this month in the first part of a two-part Hydroponic Production Primer. He addresses some of the basics such as varieties, production systems, propagation and young plant production, and more.

For fertilizer best practices and the benefits of root zone oxygenation, you won’t want to miss the articles here and here that address these topics. Then, food safety expert Lisa Lupo offers advice on how to align hazards with risk when you’re developing your food safety plan, in order to remain compliant with the Food Safety Modernization Act (FSMA).

Finally, if you attended Cultivate’18 this past July, you may have seen the new vertical farm that popped up at the Greater Columbus Convention Center. Learn more about it and how it came to be in this month’s Consumer Corner.

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