EDMONTON – In the greenhouse industry, where plants are typically grown in soil-free conditions, nutrient management programs are high science. The same plant can have a range of feeding programs, depending on the stage of growth and the short-term objectives of the grower.
“(With bedding plants), the early stage, we want the plant to establish, so we go with a high phosphorus feed,” said Mohyuddin Mirza at the recent Alberta Horticulture Congress in Edmonton.
“Once the plant is established, we want to grow them. Before selling, we want to harden them. So those are the three stages in most cases.”
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Mirza, a greenhouse industry development specialist with Alberta Agriculture, said plants need about 500 kilograms of water to move through their system, to make one kg of dry matter.
“Micro elements are needed in small amounts, but if a nutrient like molybdenum is short in the fertilizer program, then nitrate accumulation takes place and it’s not synthesized further,” Mirza said.
“Molybdenum is vital, converting nitrate into ammonium. If there’s not enough molybdenum, your leaf becomes dark green and nitrogen levels will jump, because it’s not being used up. You want to give plants an optimum level: not excessive and not deficient.”
When preparing a fertility program, the time of year plays an important role in choosing nutrients. Mirza said when a plant takes up the nitrate form of nitrogen, it must then be converted into ammonium nitrogen. This process uses a lot of energy.
“In winter, under low light conditions, Sept. 30 to end of March, you’d like to use fertilizer that’s at least 90 percent nitrate nitrogen. In summer, with high light conditions, you could go with more ammonium nitrogen. With ammonium nitrogen, your plant will grow more leaves. The plant will grow far more vegetatively.”
Nitrate nitrogen such as calcium nitrate or potassium nitrate works best under low light conditions while more ammonium should be used under high light conditions. Fertilizers such as
20-20-20 or 20-10-20 include ammonium nitrogen and are good for bedding plants that are started about the middle of March.
Nutrient deficiencies can be tricky to diagnose. Magnesium and iron deficiencies in bedding plants have similar symptoms: green veins and yellowing between the veins. Mirza said when the symptoms are on the younger leaves, that’s generally an iron deficiency, compared to a magnesium deficiency when found in older leaves.
“Manganese toxicity can occur if you’re using too much sawdust in the growing media. Trees accumulate a lot of manganese as part of their hardening process,” he said.
“Copper gives shelf life to chlorophyll. With the new growing media, copper is tied up quite a bit, both in peat-based media and coconut fibre. Normal rates for copper feeding are higher than before, about 0.25 ppm.”
Some elements, such as silicon, are not necessary for plant growth, but can have other benefits. Mirza said potassium silicate goes into the cell wall with calcium, which can make plants resistant to powdery mildew.
“It’s not essential, but it adds as a biomanagement tool.”
Mirza said certain micronutrients can affect the quality of the final product. For example, higher rates of potassium can produce more vitamin C in tomatoes.
“Fertilizer programs are based on high potash for tomatoes, cucumbers and peppers. Peppers need high boron, more than one part per million, (a level) that will kill bedding plants. But when the fruit is being formed, they need high levels of boron.”
Withholding specific nutrients is another way to manage fertility in certain crops.
“If you want to withhold a fertilizer to control growth, you must understand the principle. Withhold nitrogen for bud set in tree seedlings or to set tubers for seed potatoes in the greenhouse. When the vegetative growth is done, a little bit of stolons are coming, you withhold nitrogen for two or three weeks and they start producing tubers.
Tree seedlings, once they reach vegetative growth and they’re close to bud set, you withhold nitrogen and they start setting bud.”
“Phosphorus can be withheld when growing plugs.
Nutrient concentration isn’t the only problem that must be monitored to prevent growth problems.
“The pH of the growing media can have an effect on fertilizer uptake. With more alkalinity, the uptake of iron and manganese is low. More acidity, the uptake is four times higher, where edge burning will occur,” Mirza said.
“With a root-zone oxygen deficiency, the uptake of trace elements is reduced. Many times, the iron deficiency looks like there’s a root rot condition, because iron and copper are taken up by the first two millimetres of the root tip.”
