RESEARCH: HYDROPONICS |
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The
Crop Physiology Laboratory has used hydroponic plant culture
for over two decades to precisely control the root-zone environment.
Our work includes the development of unique nutrient solution
recipes for specific crops; the development of procedures to
control nutrients in recirculating hydroponic culture; the effect
of ammonium/nitrate ratios on growth and yield; and the testing
of organic buffers to stabilize pH.
In February 2003, Bruce Bugbee traveled to New Zealand to
present a keynote address on hydroponics at the South Pacific Soilless
Culture Conference (www.spscc.org). He gave the following
talk:
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NUTRIENT
SOLUTION RECIPES (Updated 04.04.05):
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AMMONIUM / NITRATE RATIOS Two
misperceptions exist regarding the effects of ammonium on plant
growth: 1) High levels of ammonium are toxic to plants; and
2) a 30/70 mix of ammonium/nitrate promotes plant growth compared
to 100% nitrate. Our studies show that neither of these
widely held perceptions is correct. When pH is controlled,
plants grow equally well on 30 to 80% ammonium, as on zero ammonium
(100% nitrate). At least in wheat, there is no beneficial
effect of a mixed ammonium/nitrate ratio – provided that reflective
barriers around the edge of the plots prevent the effect of
side lighting.
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CLICK ON THE TITLES TO VIEW ABSTRACTS:
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FOR MORE HYDROPONIC RESEARCH, CLICK ON THESE TITLES TO VIEW ABSTRACTS: |
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Nitrogen
Dynamics in Advanced Life Support
Dawn Muhlestein, T.
Hooten, J. Norton, and B. Bugbee
American Society for Gravitational
and Space Biology
Nov. 1999; Seattle, WA
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ABSTRACT
Conversion of
NH4+ to NO3- in advance life support systems can be difficult. The
ability to supply NH4+ directly to the plants can eliminate the need
for a nitrifying bioreactor. Many plant physiology textbooks indicate
that NH4+ is toxic to plants, but we now know that this may not be
true if pH is rigorously controlled. However the long term effects of
high NH4+/NO3- uptake ratios are poorly understood. In four studies,
two cultivars of wheat were grown to maturity with NH4+/NO3- ratios
from 0 to 0.85 in recirculating hydroponic solution. In the third and
fourth studies, NH4+ was supplied as either (NH4)2SO4, NH4Cl or both.
Contrary to conventional wisdom, there was no beneficial effect of
supplying 25% of the N as NH4+ compared to nitrate control. The high
NH4+ treatment (85% NH4+) reduced seed yield by 20% in the first two
studies, but yield was not reduced in the third and fourth studies.
Chloride and sulfate were equally effective as counterbalancing ions
for NH4+. Nitrification potential was measured in the fourth study to
estimate NH4+ conversion to NO3-. Potential nitrification could
account for a maximum of only 0.2% of N in plants taken up over the
entire life cycle. Studies are currently being conducted using
inoculation and at pH 5.8 and 7.0 to quantify the potential for
nitrification in NH4+-based hydroponic solutions.
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Is
Nitrate Necessary to Biological Life Support?
Dawn Muhlestein, T.
Hooten, R. Koenig,
P. Grossl, and B. Bugbee
- 1999
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ABSTRACT Urea is 85% of the recycled
nitrogen in a life support system. Urea is quickly converted to
NH4+ but nitrification to NO3- is
difficult. Supplying NH4+ directly to plants eliminates the need
for a nitrifying bioreactor. Most plant physiology textbooks
indicate that NH4+ is toxic to plants, but we now know that this
may not be true if pH is rigorously controlled. However, the
long-term effects of high NH4+/NO3- uptake ratios are poorly
understood. In four studies, two cultivars of wheat were grown to
maturity with NH4+/NO3- ratios from 0 to 0.85 in recirculating
hydroponic solution. In the third and fourth studies, NH4+ was
supplied as (NH4)2SO4, NH4Cl, or both. Contrary to conventional
wisdom, there was no beneficial effect of supplying 25% of the N
as NH4+ compared to a nitrate control. The high
NH4+ treatment
(85% NH4+) reduced seed yield by 20% in the first two studies, but
yield was not reduced in the third and fourth studies. Increasing
calcium and potassium supply in the nutrient solution appears to
be critical to ameliorating the detrimental effects of NH4+. Seed
protein concentration was increased from 17 to 22% at the highest
NH4+ level. These studies indicate that it may be possible to
eliminate the need to recycle N as NO3- in regenerative life
support systems.
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Nitrogen
Dynamics in ALS: Growing Wheat with 80% Ammonium
Dawn Muhlestein, T.
Hooton, J. Norton, and B. Bugbee
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Recycled nitrogen in Advanced Life Support (ALS) will be predominately
NH4+. Conversion of NH4+ to
NO3- in bioreactors can be difficult.
Nitrogen is the only nutrient absorbed by plants as a cation (NH4+) or
an anion (NO3-). High ratios of NH4+/NO3- are considered toxic for
three reasons: (1) Excess acidification of the rhizosphere (2) Induced
Ca2+, K+, and Mg2+ deficiencies and (3) Root carbon skeleton
deficiencies.
Koenig and Pan (1996) reported that increased NH4+ supply increased
yield in soil with supplemental Cl-. The Cl- also increased calcium
uptake. This may be due to improved charge balance facilitating
increased uptake of Ca2+. However, it is not clear if other anions
(e.g. SO4-) might substitute for Cl-.
Nitrifying microorganisms convert NH4+ to NO3-. Padgett and Leonard
(1993) reported significant nitrification in NH4+-based hydroponic
systems. However, Allison and Prosser (1993) found that nitrifying
bacteria occur optimally within pH 7.0-8.5 in liquid media, and
hydroponic solutions are typically controlled between pH 5 and 6.
Surface attached nitrifiers can maintain activities at lower pH than
suspended cells. Root surfaces in hydroponics could provide the
surface necessary for nitrification to occur at lower pH's.
In four studies, two cultivars of wheat were grown to maturity with
NH4+/NO3- ratios from 0 to 0.85 in recirculating hydroponic solution.
In the third and fourth studies, NH4+ was supplied as either
(NH4)2SO4, NH4Cl, or both.
Contrary to conventional wisdom, there was no beneficial effect of
supplying 25% of the N as NH4+ compared to a nitrate control. The high
NH4+ treatment (85% NH4+) reduced seed yield by 20% in the first two
studies, but yield was not reduced in the third and fourth studies.
Chloride and sulfate were equally effective as counterbalancing ions
for NH4+. Increased NH4+ ratio also increased protein content in
seeds. Nitrification potential was measured in the fourth study to
estimate NH4+ conversion to NO3-. Potential nitrification could
account for a maximum of only 0.2% of N in plants taken up over the
entire life cycle.
Studies are currently being conducted using inoculation and at pH 5.8
and 7.0 to quantify the potential for nitrification in NH4+-based
hydroponic solutions.
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ABSTRACT
Containerized plant growth is common in controlled environment
research and the greenhouse industry. Because of the small root-zones
of containers, water availability changes rapidly over short time
intervals. To overcome this, irrigation is often excessive causing
leaching. Environmental concerns has prompted studies on improved
irrigation efficiency. Tensiometers equipped with low tension (fast
response) ceramic cups and pressure transducers were used to measure
and control water in a peat:perlite mix. Plants were watered with a
dilute nutrient solution when the water potential of the media
decreased to either –5 or –20 kPa. Irrigation continued for ten
seconds or until the water potential of the media was greater than the
setpoint. Other researchers have used this technique to control
watering but have compared effects of watering setpoints through fresh
and dry mass at the end of the study. A better technique is to make
real-time measurements of plant growth. We measured whole plant
transpiration rate, leaf temperature, and leaf expansion as indicators
of water availability. Transpiration was measured using a whole-plant
open gas-exchange chamber, leaf temperature with a infrared
thermometer, and leaf expansion with a digital camera. Controlling
irrigation with a tensiometer minimized leaching without affecting
plant growth.
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Carrot Cultivar Evaluation:
Soilless Media vs. Hydroponics
Derek Pinnock and B.
Bugbee
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Nine cultivars of carrots were
grown in a growth chamber. Each cultivar was grown both in
hydroponic and soil-less media root-zone for sixty days. Three 30L
tubs were used for each root-zone treatment. Three cultivars were
planted in each tub, initially at 180 plants m-2
then thinned to 90 plants m-2 on day 45.
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