EDMONTON – One of the most common problems customers have with greenhouse fertilizers for bedding plants and vegetables is water quality, says Thom Rypien of Westgro Horticultural Supplies in Calgary.
“Just because the water is potable doesn’t mean it’s going to be good for plant growth,” Rypien told the recent Alberta Horticultural Congress in Edmonton.
“Two key things are pH and bicarbonates,” he said.
“My first question is always, ‘how is it for washing clothes?’ If it causes a lot of suds, you’re probably going to have high sodium and the water is almost untreatable for plant growth. Sodium levels over 100 ppm, bedding plants are not going to like it very much.”
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Because most of the water Rypien sees is hard, he deals mostly with high bicarbonate levels.
“We like to get to around the six mark for pH, with bicarbonates around 100. You still want to have some bicarbonates for a buffer. If you didn’t have that, your pH could start to yo-yo all over the place.”
A bicarbonate level of only 150 can be reduced with phosphoric acid. Nitric or sulfuric acid works with higher levels because phosphoric acid might deliver excess phosphorus.
Rypien said commercial greenhouse fertilizers are generally soluble, but there are different degrees of solubility.
He recommends putting 250 to 500 grams per litre into the water. After that, the fertilizers will not dissolve. Hot water should be used with calcium-based fertilizers.
“Your fertilizer will drop the temperature anywhere from 11 to 16 degrees, depending on the composition of the fertilizer. If you’re using cold tap water and trying to dissolve it, it could take quite a while.”
In some cases, dumping fertilizer into water and stirring it will simply put the fertilizer granules in suspension and result in a sludge at the bottom. He recommended shaking it and said he mixes it in a concentrate pail, adding 10 litres of hot water and pouring it back and forth between two pails. By doing that, he can see when it’s finally dissolved.
Greenhouse fertilizer is basically a salt, which can either be a carrier or the actual nutrient. Because they’re salt-based, when dissolved in water they produce an electrical conductivity, some more than others.
“20-20-20 has a lower salt index than 28-8-20,” Rypien said.
“If you have a high EC in your water, you may want to lean towards a fertilizer with a lower salt index so it doesn’t push your EC up to a really dangerous level. Conductivity charts are available for all fertilizers that will tell you exactly their salt index or EC value, at a set part per million.”
Rypien said fertilizer can also affect pH. Ammonium, phosphates and sulfate are acidic while calcium, magnesium, nitrates and potassium tend to be more alkaline.
“If you’re not monitoring your pH, every year we see iron chlorosis in petunias because pH has been pushed up with increased calcium feeds.”
Certain fertilizer elements are incompatible. Because vegetable growers feed high levels of calcium, they have to watch for phosphate and sulfate incompatibility. Calcium will also react with high levels of bicarbonates and producers may see precipitate forming.
“Magnesium can also react with phosphates and bicarbonates in your water. It forms a precipitate. Things like zinc, manganese will react with phosphates as well, so you want to keep those things separate, especially in a vegetable feed program, as we’re feeding higher rates.”
Rypien said there are new fertilizer formulations that will keep calcium and magnesium together and soluble.
“A lot of people were using 20-8-20, alternating with 14-0-14. The 14-0-14 had calcium and magnesium, but you didn’t want to have it with the 20-8-20 because of the phosphates,” he said.
For an early plug feed program, a 12-2-14 mix that contains calcium and magnesium is ideal.
“There’s also 17-5-17, which is a general feed program containing calcium and magnesium. It comes as a bag within a bag. They can’t pack the two together because it reacts and cakes, but once you dissolve it, it’s fine.”
Nutrient imbalances can cause certain growing problems. Rypien said there’s a strong three-to-one ratio between calcium and magnesium. If they’re not in that ratio, they counterbalance each other. Foliar feeding is one way to identify this type of problem.
“If you find there’s a nutrient imbalance, like with iron, spraying with iron chelate at about half the recommended rate you’d give as a feed program will show up quickly in the tissue. It’s a quick way to see if I need to correct my feed program.”
Fertilizer injectors add a concentrated solution of fertilizer into the water line to get the proper rate into solution. Rypien said most set their ratios at one to 100.
To ensure proper operation, he recommended pre-filters before the water goes through the injector and backflow prevention on the injector.
“If you shut the injector off, the piston is in the open position and you have a little drop in the hose, it will siphon out your concentrate and it can go back into your water source. You don’t want your water to be contaminated with high levels of ammonia fertilizer, especially if you have young children.”
