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As the repotting season continues, you might be thinking about switching up your soil and fertilizer. But with so many brands and options available, how do you know which is the best for your bonsai?
NBF Co-President Jack Sustic spoke with us about the myriad choices for the second installment of our Bonsai Basics blog series. Sustic said there isn’t a panacea to rely upon – if you ask 10 people what their preferred mix is, you’ll get different answers from each.
“Bonsai soil is no different than Grandma’s biscuit recipe – all the grandmas have a different one and it is better than the other grandmas’ recipes,” he said.
The type of soil you need first depends on the tree you’re working on. Sustic said deciduous trees typically like soils that retain moisture well, while evergreens like pines and junipers prefer slightly drier mixtures. But don’t be mistaken – using a drier mixture doesn’t mean you should adjust the amount of water you’re giving the tree because the soil types absorb water differently.
“If you put an evergreen in a mix that’s extra dry and you don’t water it that much, it could be dangerous for the tree,” Sustic said. “If we have a deciduous tree in a mix that retains water pretty well and we have a conifer in a mix that drains really well, I end up watering them the same.”
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The chemistry of it all
Soils are amalgamations of nutrients and materials like sand or bark. Some bonsai artists buy premixed soil formulas and others forge their own.
Sustic usually prefers to mix his own from local ingredients, first sifting the finer particles from a store-bought potting mix, then adding pine bark and a combination of filter sand and pumice in ratios dependent on the type of tree that needs soil. The more sand and pumice you add to a mix, the faster water will drain from the soil, Sustic said.
“The goal here is to have good drainage, and I don't want the real fine stuff in the potting mix to clog up the drainage holes,” he said. “Maybe I'll have one part sand in a deciduous mix, and I’ll add two parts for the pine because it likes to be a little drier.”
But Sustic said someone new to bonsai who is only repotting once every two or three years would find buying a bag of soil the easiest method. Akadama, a type of clay soil from Japan, is the best performing premixed bonsai soil and will last years for people with small bonsai, like shohin.
When determining which soil will suit your bonsai best, Sustic’s main piece of advice is simply to avoid any heavy mixes. Sifting out fine particles from premixed soil is a must to ensure that your soil drains well and keeps your bonsai healthy, he said.
“Sand doesn’t hold nutrients very well at all,” Sustic said. “If you planted a tree in sand, you would have to fertilize it a lot because it doesn’t hold the nutrients. Potting mix and pine bark hold a lot of nutrients.”
The nitty gritty of fertilization
Fertilization techniques also range far and wide.
Sustic uses a combination of fertilizers, switching up his materials with the seasons. He tosses rapeseed fertilizer pellets on his evergreens in the winter. But he fertilizes with Miracid, a liquid that he mixes with water, once deciduous trees have budded out in the spring and summer to help make up for the high pH levels in his water.
Some people use organic fertilizers, like cakes or a fish emulsion, to prevent salt from building up in their soil, which can happen with commercial fertilizer and is dangerous for your trees.
“Salt can inhibit the tree's uptake of water, so the tree can’t transpire as fast,” he said. “The most common sign of salt damage is burned leaf margins.”
But Sustic said incorporating the horticulture technique of fertigation, or mixing fertilizer into water and flushing the soil with each watering, eases his worries about salt build up. He added that commercial brand fertilizer, like Miracid or Peters, immediately provides nutrition for bonsai, while organic mixes can take longer to produce an effect. Sustic often cuts the suggested measurement in half to avoid excessive fertilization and uncontrollable growth.
“If they say one tablespoon per gallon I use half a tablespoon per gallon,” he said. “I want them to be healthy but not growing too fast.”
Now that you’re equipped with soil and fertilizer knowledge, you’re ready to help your bonsai grow strong. Share your progress by tagging us on Facebook, Instagram or Twitter!