Organic fertility in nutrient film technique
(NFT) hydroponic systems is notoriously
challenging. One aspect of Jason
Nelson’s research at Kansas State University
(KSU) was to evaluate hydroponic lettuce
production with both organic and conventional
inorganic nutrient sources. He
produced butterhead lettuce ‘Rex’ in NFT
troughs. The organic fertilizer treatments
were based on Kimitec soluble organic fertilizers
(www.kimitec.es/en), and we tweaked
the recipe over three experiments. Some
of our best growth, as shown in Figure 1,
occurred when using Bombardier (8-0-
0), Espartan (2.7-3.0-2.6), and Caos (10.5
percent Ca), all at 0.7 ml/L recirculating
solution; and Tunda (micronutrients) at 0.3
ml/L. Potassium was delivered from the
organic salt potassium magnesium sulfate
dissolved at a rate of 0.02 g/L. This mix
resulted in an EC of 1.9 dS/m. The inorganic
fertilizer regime was derived from
fertilizer salts and contained about 150 ppm
nitrate-N and 40 ppm ammoniacal-N. In
other studies at KSU, we have had good
quality lettuce growth in an NFT system
using fish emulsion as the primary nutrient
source, though this is messy, smelly and difficult
to manage based on recirculating solution
EC and pH.
A big difference between organic and
inorganic fertilizers is the form of nitrogen
— the nutrients in organic fertilizers are
typically components of complex molecules
like proteins that need to be broken down
into their constituent elements before they
are absorbed by plants. That process goes
from organic molecules to ammonium to
nitrate, thanks to the activity of microorganisms
in the system. So inorganic fertilizers
have a lot more nitrate-nitrogen,