Switching To Organic Fertilizer

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Basil seedlings, Control and 3 organic fertilizer treatments

Basil seedlings treated with organic fertilizers four weeks after sowing. Left to right: Control, no fertilizer; Sustane 8-4-4; Worm Power vermicompost extract drenches; Sustane plus vermicompost extract.

Converting an operation to using organic fertilizer is a more complex process than simply using a new brand. There are several factors a grower needs to consider and plan for before making the decision to change.

Most growers who make the switch, says Neil Mattson, associate professor of horticulture at Cornell University, do so because the market demand is there for it to make sense financially.

If you are thinking of growing organically, here are the issues you need to consider, according to Mattson.

It’s Easier For Short Term Crops

Mattson says those growers who change from using conventional to organic fertilizers, who have had to make the biggest adjustment, are usually the ones producing plants with a longer production cycle.

“For growers who are producing vegetable seedlings or transplants, the change to organic fertilizers has not been too difficult,” he says. “For growers producing vegetables hydroponically or growing longer-term crops that will be in a greenhouse for several months, the transition is more difficult.”

Look For The Right Fertilizer

Mattson has done numerous research studies comparing the differences between conventional and organic fertilizers and says he has not yet found an organic fertilizer that offers the same amount of control and flexibility as conventional fertilizers.

“Growers have to adapt their cultural practices to the type of organic fertilizer choices that they have available to them,” he says.

One of the difficulties for growers of long-term organic crops is making sure plants are supplied with enough nutrients.

Also, some organic fertilizer sources are high in salts, which can accumulate and can have a negative impact and damage the plants over time.”

Mattson says another concern with organic fertilizers is the potential for nutrient imbalances. For many organic fertilizers, some quantity of all nutrients are being supplied. It can be difficult to pick and choose a fertilizer or complementary fertilizers to supply exactly what the plants need and nothing else.

“With conventional fertilizers, growers can supply specific elements in the ratio required by the plants,” he says. “With organic fertilizers, if a deficiency develops, such as an iron deficiency, there are not a lot of individual nutrient tools available to increase the rate of that element.”

Match The Fertilizer To The Crop

Growers who have been successful using organic products have been able to determine the right use for them in their production systems, Mattson says.

The most successful growers of short-term vegetable or bedding transplants use organic fertilizers, added to their potting mix ahead of time.

“These are usually granular poultry-based fertilizers, such as microSTART and Sustane or a blend of several components such as EcoVita,” he says. “They are quite high in the nutrients they supply, in regard to nitrogen, phosphorus and potassium.”

Mattson has been successful incorporating these granular fertilizers (microSTART60 Plus 7-2-2l Sustane 8-4-4 and EcoVita 7-5-10) into the potting mix for vegetable transplants like tomatoes. He found a rate of 0.64 pounds of nitrogen per cubic yard worked well.

The granular fertilizers performed well for several weeks before their effectiveness eventually dwindled, he says.

“That’s when a grower basically has two choices. He can either topdress with more of the granular fertilizer or supplement with a liquid fertilizer.”

Organic fertilizers tend to be a little more expensive than their mineral counterparts, but Mattson says granular formulations have become a cost effective way for growers to apply organic fertilizers.

Consider How Water Impacts Fertilizer

Changing to organic fertilizers for a long-term vegetable crop would be similar to using them for a long-term ornamental crop, Mattson says.

“For the first few months, a grower would have to reapply organic fertilizer several times,” he says.

The most difficult production method would probably be growing vegetables hydroponically, because the grower is has to rely solely on the hydroponic solution to deliver nutrients.

Mattson recommends vegetable growers use a bag culture with a large reservoir, such as a 5-gallon bag.

“This would help to supply a larger reservoir of nutrients,” he says. “That’s going to get more nutrients to the plants. Also, granular organic fertilizers could be applied to build up the fertility in the potting mix.”

The potting mix would also act as buffer for pH and salts. A grower could monitor the substrate and leach if necessary to keep the salt levels under control.

“If it was a true hydroponic production setup and the grower was collecting and recycling the irrigation water, the salt levels could buildup relatively quickly,” Mattson says. “The alternative would be to dump out the fertilizer solution and start all over again.”

A grower using a hydroponic solution to fertilize would have to monitor his water for salt buildup.

“Many growers monitor pH and salts manually at least once a day and make the appropriate adjustments,” he says.

Sensors can also be problematic, since they can change over time.

“They should occasionally do some manual monitoring to make sure that the pH hasn’t changed,” Mattson says. “Those sensors may need to be recalibrated.”

Water alkalinity can cause the pH to change, and water supplies in the Midwest and West often have high alkalinity.

“Water alkalinity includes dissolved carbonates and bicarbonates in the water,” he says. “The pH of the water is going to naturally rise over time as the carbonates build up, whether growers are using grow bags or a hydroponic solution.”

Another thing that can affect the pH is the form of nitrogen fertilizer. As plants take up ammonium nitrogen, the pH is going to decrease. As plants take up the nitrate form of nitrogen, the pH is going to increase.

“With conventional fertilizers, we can closely predict what is going to happen with the pH because we know exactly the ratio of ammonium and nitrate,” Mattson says. “For example, a 21-5-20 fertilizer consists of 60 percent nitrate nitrogen and 40 percent ammoniacal nitrogen. That tends to be a slightly acidic fertilizer.”

Most of the nitrogen is not in the nitrate or ammonium forms of organic fertilizer, rather, the organic material needs to be digested by microbes to release the nitrogen, Mattson explains.

“That results in ammonium, which the microbes can further convert to nitrate,” he says. “With organic fertilizer, the microbial process is necessary to create ammonium and nitrate forms of nitrogen.”

Temperature Affects Performance

Organic fertilizers are dependent on microbes to break them down into nitrogen forms the plants can use. Mattson says the nutrient availability of organic fertilizers is temperature dependent.

“If a crop is being grown at cool temperatures, it is going to be very difficult for the organic fertilizer to supply enough nutrients,” he says. “Whereas with the conventional fertilizers, nitrates are soluble in water and are ready to be taken up by the roots.”

Mattson conducted an experiment with tomato transplants that were grown for six weeks with either organic or conventional fertilizers at 50ºF, 60ºF and 70ºF.

“The organic fertilizers performed well at 60ºF and 70ºF, but they did not perform very well at 50ºF,” he says. “Tomato plants, in general, don’t grow very fast at 50ºF, but at 50ºF the plants that received conventional fertilizer were twice the size of the organic-fertilized plants. When the air temperature is cool, the microbes are still going to be less active in converting nitrogen to forms that the plants can use.”

David Kuack (dkuack@gmail.com) is a freelance technical writer in Fort Worth, Texas.
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