Choosing an organic fertilizer

By David Kuack
Growers looking to make the switch from traditional
inorganic fertilizers to organic fertilizers might feel a bit overwhelmed when
considering the number of options available. Kansas State University
horticulture professor Kim Williams said growers looking to use organic
fertilizers should start out trialing a small number of plants to avoid
sizeable losses. She also recommends that growers start with a commercially
blended organic fertilizer rather than a straight product like manure.
“It’s going to take some practice, some trialing, so
growers don’t burn their plants because that is another transitional change
compared to conventional starter nutrient charges,” Williams said. “Where a lot
of growers can get into trouble is incorporating manure or composted manure
into organic substrates as a starter nutrient charge. That is something that I
would not recommend doing for the novice organic grower. What sometimes happens
is high levels from the manure burn young seedlings. Fresh and incompletely
composted manure can also introduce disease problems and/or weed seeds into the
production system as well.”
Williams said that there can also be a lot of variability
in nutrient availability from products like bone and blood meal depending on
the source and their particle size.
“Different sources of bone meal for example are going to
have significantly different nutrient release rates,” she said. “This can
result in an excessive nutrient release too early in the production cycle. If
the grower is not expecting this, he can lose a lot of plants depending on
their stage of growth. Seedlings and plugs are much more sensitive than older
plants to high salts. It really does take some practice with a new nutrient
“If a grower uses a commercially blended product as
opposed to manure from his neighbor’s horse farm, he is going to avoid
potential problems. I would recommend a commercially prepared organic product
like composted and processed turkey litter that is not going to contribute
disease problems.
Williams said growers should start with an organic
nutrient source that has a known N-P2O5-K2O
ratio and then trial it at a few different rates with a small number of plants
before using it extensively.
Supplying enough
Williams said that growers who incorporate a preplant
organic fertilizer into their growing mix still need to provide additional
fertilizer to the production system.
“I have yet to use an organic nutrient source that didn’t
provide an initial nutrient release that was very significant,” she said.
“Nutrient release then tends to tail down quite a bit depending on the rate of
application, temperature and media moisture content, all of which influence
microbial breakdown of the organic material and thus nutrient release.
“This can be a challenge for growers. There is an initial
spike of nutrients released so growers have to protect against that by reducing
the rate of what they are adding as a preplant. But then their crops are going
to need supplemental nutrition later on when the soluble fraction of the
organic fertilizer is used up.”
Research conducted by Williams showed that organic
fertilizers bring a lot of microorganism populations with them into the
production system. It is these beneficial microbial populations that are then
contributing to the nutrient conversion.
“In our experience so far with organic fertilizers it has
not been necessary to do additional microbial inoculations when organic
fertilizers are used,” she said. “Growers who apply a preplant dry organic
fertilizer will start to get microbial activity even before applying a
supplemental liquid organic fertilizer. Growers who incorporate a preplant
fertilizer are providing microorganisms with a food source of carbon so that
the populations can really take off much more quickly than if they applied
conventional inorganic fertilizers. However, the disadvantage with the organic
fertilizers is that they are less predictable for growers. Managing nutrition
is more challenging since the growers are relying on microbial release of
nutrients from the organic sources.”
Williams said some growing media manufacturers are
producing organic potting mixes into which an organic fertilizer is
incorporated as a starter nutrient charge.
“Media manufacturers put in a preplant organic fertilizer
to provide a starter charge,” she said. “Then the growers can decide on a
schedule with supplemental soluble feed.”
Dry and liquid
Growers have the choice of both solid and liquid
formulations of organic fertilizers. Williams said many growers are applying a
dry preplant amendment to their potting mixes.
“Some growers have chosen to incorporate solid forms of
certain fertilizers such as bone meal that have a smaller nitrogen component or
something like feather meal that is going to take some time to break down.” she
said. “These growers then supplement with a liquid organic fertilizer.”
Williams said that organic fertilizer manufacturers are
making products that are more convenient to use and more consistent in their
nutrient formulations.
“There are some fertilizers that have more complete
nutrient formulations. These are the result of digested or fermented
combinations of a number of organic materials,” she said. “These types are
really the only way that a grower can get a somewhat balanced N-P2O5-K2O
ratio in a fertilizer.”
Williams said most of the commercial organic fertilizers
are very shelf stable. Because the formulations are so concentrated, microbial
populations cannot grow in them.
“When growers start to dilute these fertilizers in a
stock tank is when other micro-organisms can come in and start feeding on the
nutrients before the fertilizer solution is applied to the crops,” she said.
“When a fertilizer is taken out of the concentrated container and diluted down
in a stock tank is when organisms can grow in it. Growers have said that within
a couple of days organisms will be growing in the stock tank.”
Williams said to avoid problems with organic fertilizers
going bad, most growers mix up just enough tank solution to feed the plants
they want to fertilize that day.
“That’s also a disadvantage with organic fertilizers.
With inorganic fertilizers a grower can make up a month’s worth of stock
solution and let it sit there and not be concerned with it going bad,” she
said. “With organic fertilizers a grower has to mix up just enough to feed his
plants and if he has to do that every few days that’s a lot more work.”
Williams said another thing growers should consider when
choosing a soluble organic fertilizer is whether it can be used with their
current injector equipment.
“The grower needs to ask the question, “What fits into my
liquid fertilization system in terms of how I mix up liquid fertilizer and how
I apply it to my crop?” Some injectors will handle organic fertilizers better
than others. Some fertilizers can clog injector systems.”
Williams said growers using conventional hydroponic
production systems generally tend to use fertilizers that contain very little
ammoniacal nitrogen compared to nitrate nitrogen. The reason is that the plants
can run into ammonium toxicity problems.
“A hydroponic system doesn’t have the buffering capacity
of a growing medium that has some cation exchange capacity,” she said. “The
substrate would hold some of the ammonium and not make it available for plant
“If ammonium is in solution then it’s available for the
plant roots to take up. Ammonium can’t be stored in the plant cells like
nitrate can. If plants are absorbing too much nitrate, that’s not really a
problem because the excess can be stored in the vacuoles of the plants’ cells
and it’s not going to cause toxicity problems.
“When plants absorb ammonium they need to assimilate it
right away, their cells can’t store it. If the plants can’t use the ammonium
then it’s going to damage the cells and burn the plants.”
Williams is starting a hydroponic research project to
determine the differences in growing butterhead lettuce with organic and
inorganic fertilizers and different types of microbial inoculums.
For more: Kim
Williams, Kansas State University, Department of Horticulture, Forestry &
Recreation Resources, (785) 532-1434;

David Kuack is a freelance technical writer in Fort
Worth, Texas; our corporate website at