Propagation/Greenhouse ManagementUnit 1.3 | Part 1 – 123D. Supplemental Fertility in the Greenhouse
- Conditions where supplemental fertility may be necessary or useful
Although organic mixes ideally contain all the fertility needed to sustain steady,
uninterrupted plant growth, there may be times where additional inputs are necessary
a) To compensate for poor quality, nutrient-deficient ingredients, especially immature or
older, poorly stored compost
b) To promote biological activity and nutrient release by supplying nitrogen to the soil
microbial population; microbes use supplemental nitrogen to facilitate the release of
plant-based nutrients from the soil mix
c) To alleviate stress, especially in cell-type containers when plants are past optimal
transplant stage, have become root bound, or are showing signs that previously
available nutrients have been exhausted
d) To stimulate growth, such as when the grower needs to accelerate plant growth for a
specific plant-out date or when seedlings have been contracted for sale and it is clear
that they will not reach salable size quickly enough
- Potential concerns when using supplemental fertility
a) Water soluble nutrients such as the nitrogen in fish emulsion can easily be leached
out of growing containers and potentially enter local waterways if irrigation is poorly
managed and vegetative buffer strips are not present to preserve water quality (see
Supplement 2, Conserving Water and Protecting Water Quality)
b) Inputs can be expensive, and need to be available on site so that fertility issues can be
quickly addressed
c) Supplemental fertility typically requires substantial additional labor for application
d) Additional application equipment is required; in some cases, additional filtration is
needed if being applied through drip or fine-nozzled spray systems
e) Excess nitrogen application can promote highly nitrogenous, pest-susceptible growth,
which may then lead to using more inputs to control pests
f) Overreliance on highly soluble nutrient inputs mimics the conventional mindset of
feeding plants directly, with readily available ingredients, rather than building soil
health and biology to promote nutrient release and pathogen resistance
- Ways to apply supplemental fertility
a) Inputs can be blended into soil mixes at time of mix making. This common strategy
requires advance knowledge of need for additional fertility and is most useful with
medium- to slower-acting meals and powdered ingredients.
b) Powdered, granular, and pelletized ingredients can be “top dressed” on the surface of
container soils. This can be effective with fast- to medium-acting inputs, depending on
the crop life cycle and extent of immediate need.
c) Water soluble inputs are commonly applied as a soil drench as part of a regular
irrigation set, thus becoming a “fertigation.” This is a particularly useful, quick-fix
approach to address immediate nutrient deficiencies or to rapidly increase the rate of
plant growth using readily available, water-soluble nutrients, delivered with irrigation
directly to the root zone.
d) Water-soluble nutrients can also be delivered as foliar sprays, specifically directed at the
leaf undersides where stomata are concentrated to maximize uptake potential
- Commonly used supplemental fertilizers for soil drenches and foliar application (see
Resources section for sources of OMRI-/NOP-certified supplemental fertilizers)
a) Fish emulsions and soluble fish powders for N-P-K (2-5N-2-4P-0-2K) are regularly used
for both soil and foliar applications. They provide immediately available nutrients to
support growth.
Lecture 4: Soil Media, Fertility, & Container Formats