Originally published in Issue 6
Switching from an inorganic to organic fertilizer is not a simple process. Growers have to be willing to put in the time and effort to ensure the changeover is successful.
The goal of any fertility program, regardless of whether it is with organic or traditional inorganic fertilizers, is to ensure that all of the essential plant nutrients are present in appropriate amounts. Kansas State University horticulture professor Kim Williams said whether the nutrients are injected into the irrigation water or pre-planted in the growing medium, ensuring that proper levels of all essential nutrients are provided to the plants “can be surprisingly tricky.”
“When we say adequate amounts or think about nutrient balances, how much of each nutrient should be present was originally based on how much of each nutrient ends up in the plant,” she said. “For example, nitrogen and potassium both make up about 4 percent of the dry weight of most plants. Therefore they are typically provided in relatively high quantities in fertilizer programs. That’s why there are general purpose formulations like 20-10-20 and 15-2-15. Thinking about nutrient balances in terms of conventional fertilizers, those ratios are applying about the same amount of potassium and nitrogen, which is quite common. There are some exceptions. There are some crops, including cyclamen and Rieger begonia (Begonia x hiemalis), that grow better with higher potassium levels than nitrogen. But in general, for conventional fertilizers, the ratio of one part nitrogen to one part potassium generally holds. This is a challenge for growers trying to use organic fertilizers because most of these fertilizers don’t provide combinations of essential nutrients in the same ratios that growers are accustomed to with inorganic nutrient formulations.”
Knowing what you’ve got
Williams said for commercial organic fertilizers it’s common to see nutrient formulations that don’t contain equal amounts of nitrogen and potassium.
“Sometimes phosphorus can be very high,” she said. “Sometimes potassium needs to be supplemented depending on the nutrient source. Research was conducted at Kansas State with a soybean seed extract fertilizer with a formulation of 10-4-3. When applications were made based on the typical nitrogen range no disorders developed from having a potassium imbalance that would be expected to be seen with conventional fertilizers.”
Williams said whether growers use organic or inorganic fertilizers the place to start is to make sure that all the nutrients are present and in adequate amounts—not too much or too little.
“In the past growers using general all-purpose fertilizers may have tended to over fertilize with nitrogen and potassium in order to deliver enough micronutrients like iron,” she said. “ They may have thought an improvement in growth was coming from the additional amounts of N-P-K when in fact it was coming from additional amounts of micronutrients.
“I worked with a grower who was using a soluble organic fertilizer to feed her crops, but she reported symptoms of iron deficiency developed on her petunias and calibrachoa towards the end of the production season. I thought the problem was related to upwards pH drift associated with the high alkalinity well water she was using for irrigation. For a couple years we worked on optimizing the root medium pH. After visiting her operation and doing some in-house nutrient testing, we discovered the problem didn’t have anything to do with pH. It was a result of the organic fertilizer she was using not having enough iron to meet the plants’ needs. When she added an iron supplement, the plants quickly re-greened and there was an improvement in growth.”
Williams said growers need to make sure that all of the essential nutrients are in the fertilizer they are using and that they are going to be available in adequate amounts. Their availability can be affected by pH and other root medium characteristics.
Williams said another challenge facing growers using organic fertilizers is ensuring that they are receiving a consistent product from batch to batch.
“Being able to rely on a fertilizer to give the same results from one batch to the next is critical,” she said. “This is especially true for growers who invest the time and effort trying to understand how a fertilizer is going to deliver nutrients to their crops and how it integrates into their production systems.
“Many fertilizer companies are working to manufacture consistent products by tightening up their standards for what the nutrient analyses of the raw input products have to be. This hasn’t always been the case in the past. For example, the nutrient content of one batch of fish emulsion could be widely different to the next. There are now companies producing organic fertilizers that start with either animal or plant inputs that have done a good job of tightening up their protocols so that they are delivering consistent products from one batch to another. There is a difference between a company that is using whatever raw products are coming out of a food processing facility, drying it, grinding it up and packaging it. That’s taking a waste product and using it as fertilizer. It is not actually engineering a fertilizer in the sense that the manufacturer is carefully monitoring and controlling the raw inputs that go into it so that the end-product performs in a consistent way for growers.”
Williams said there are a lot of aspects of water quality that come into play in terms of interacting with any fertilizer.
“With organic fertilizers a grower is often adding a high salt load to the solution,” she said. “That can be an issue if a grower already has high levels of salt in his water supply, whether that is coming from alkalinity in the form of calcium, magnesium, sodium carbonate or other water contaminants. It is more difficult to use organic fertilizers because you don’t have as much wiggle room in terms of extra salts before the total salt level affects plant growth.”
Williams said in the case of conventional inorganic fertilizers they are relatively pure so when a grower uses potassium nitrate that is what primarily is applied.
“In general, the electrical conductivity (EC) for an organic fertilizer tends to be higher because there are so many other ions in the fertilizer,” she said. “Some of these ions are plant essential and many of them aren’t. Sodium is one of the ions that is sometimes found in too high levels in many organic fertilizers.”
Williams said water quality becomes more critical the longer the production cycle is for a crop.
“Water quality is going to be much more critical for a grower producing tomatoes hydroponically,” she said. “Trying to maintain plants over a period of months in a rockwool slab is going to require more water quality monitoring than short term crops like basil or greens.
Williams said growers who are considering making a change from conventional inorganic fertilizers to organic fertilizers should start small and trial short-term crops.
“Growers should be prepared to spend more time doing nutrient and pH monitoring so that they have more information than just pH and EC,” she said. “I would encourage growers as they are collecting this data to plot nutrient levels over time. Growers should essentially determine the nutrient release pattern from the addition of a given rate of the fertilizer at a given temperature. Monitoring and recording the temperature will allow growers to know what to expect regarding the release pattern when they expand the size of the crop.”
Williams suggests that for the first 48 hours of using an organic fertilizer in a recirculating solution, growers should sample the fertilizer solution twice a day so that they can understand how the pH and EC are changing. “Our experience is that for the first couple of days there is tremendous shift in the pH and EC when organic fertilizers are applied because of the microbial activity,” she said. “Growers should collect recirculating solution samples that can be sent off to an analytical lab to see what nutrients are available. For a hydroponic grower making the switch from conventional inorganic fertilizers to organic fertilizers, sending a sample in once a week for analysis isn’t enough. This grower would need to do it more frequently until he really learns what to expect from a given organic nutrient source.”
For more: Kim Williams, Kansas State University, Department of Horticulture, Forestry and Recreation Resources; (785) 532-1434; email@example.com.
David Kuack is a freelance technical writer in Fort Worth, Texas; firstname.lastname@example.org.