Supplying crop nutrition
through fertigation
ISSUE
NUMBER:
2002/12
Fertigation is the process of supplying fertiliser or nutrients to crops in the irrigation water.
Traditionally, fertigation was used mainly by seedling, punnet, potted colour and foliage crop
producers. However, the opportunity to use fertigation in a greater variety of nursery crops is
explored in this Nursery Paper.This Nursery Paper should be read in conjunction with the two
accompanying Nursery Papers;‘Water quality and nursery crop nutrition’, issue number 2002/11,
and ‘Supplying crop nutrition through controlled release fertilisers’, issue number 2002/13.
Providing plant nutrients through fertigation
ranges from the occasional use of nitrogen-based
fertilisers in irrigation water, to supplementing
controlled-release fertilisers through to
complete liquid feed or hydroponic programs.
Plant nutrients
Plant nutritional elements can be separated into
two different categories – macro-elements and
micro-elements. Macro-elements are used in
greater quantities than micro-elements. All of
these elements should be supplied to the crop
in quantities that resemble the level of uptake
by the plants.
Apart from nitrogen, which usually coincides
with a higher rate than potassium for various
reasons (including nitrogen drawdown), the
relative use of nutrients by plants is detailed in
Table 1.
Micro-elements used at high rates often cause
extensive and long lasting toxicities. Toxicities
caused by iron are rare. However, those caused
by manganese, zinc, boron and copper occur
relatively frequently in containerised plant
production. (See Nursery Paper ‘Are trace
elements a waste of money?’ issue number
1998/05.)
Potassium is needed at levels about nine to 10
times higher than magnesium, the least utilised
of the macro-elements, but about 300 times
higher than iron, the most utilised of the microelements, and about 8000 times higher than
molybdenum.
Types of fertigation
Fertigation can be done with every irrigation from
the start to the finish of the crop or intermittently.
The nutrient concentration should be reduced
when fertigation is continuous. Algal problems,
however, can be extreme where continuous
fertigation occurs using overhead irrigation.
Most woody plant nurseries favour interrupted
fertigation. This usually means three days per
week or even once per week. Concentrations
are adjusted accordingly. In other situations the
fertigation may be turned on for part of the
crop cycle only. Similarly, one nutrient can be
occasionally added (eg, calcium, iron or
magnesium) to or deleted (usually nitrogen)
from the program.
© NGIA – The Nursery Papers – Issue no 2002/12 – Page 1
Dual fertiliser systems
Fertigation works well in conjunction with controlled
release and other fertilisers. It is recommended that
most of the micro-elements be applied to the growing
media before potting as this decreases the complexity
and expense of the fertilisation program.
The type of growing media, including the pH and
how the pH changes over time, the use of any
liming materials and the concentrations and range
of pre-mixed micro-elements and CRFs are all
considerations in determining a fertigation
program.
For example, supplying 50-70% of the microelement requirement of a crop through CRFs can
avoid wastage, especially if fertigation via overhead
irrigation is used to supply the other nutritional
requirements. This reduces the complexity of and
retains the fine-tuning and corrective ability of
fertigation while lessening the risk of excessive
release from CRFs during hot weather.
Formulation of nutrient solutions
There are several ways of expressing nutrient
concentrations.The most common one is parts per
million (ppm). One millilitre (mL) of water weighs
one gram (g), and one litre (L) weighs one kilogram
(kg).Therefore, 1g of a substance dissolved in 1000L
of water will produce a 1ppm solution. One mg of
a substance dissolved in 1L will also produce a
1ppm solution.
Drip and sub-irrigation systems can result in
significantly less wastage in terms of minimal water
usage and loss of fertilisers when compared to
overhead sprinkler systems, (see Nursery Paper
‘Reducing nutrient leaching from pots’, issue
number 1996/09.
Effects of the growing media
The nitrogen drawdown characteristic of growing
media significantly influences the amount of
nitrogen that needs to be supplied by fertilisers (see
Nursery Paper ‘Improving nitrogen management in
wood-waste-based potting mixes’, issue number
1996/11).The lower the Nitrogen Drawdown Index
(NDI), the more nitrogen is required.
Table 1: The relative use of nutritional elements by plants
Macro-elements
Micro-elements
1. potassium (K)
7. iron (Fe)
2. nitrogen (N)
8. manganese (Mn)
3. calcium (Ca)
9. zinc (Zn)
4. phosphorus (P)
10. boron (B)
5. sulphur (S)
11. copper (Cu)
6. magnesium (Mg)
12. molybdenum (Mo)
Table 2: A and B nutrient stock solution compatibility
Stock solution A Stock solution B
Calcium nitrate
monammonium phosphate (MAP)*
iron EDTA
potassium dihydrogen phosphate (MKP)
potassium nitrate
magnesium nitrate
ammonium nitrate
ammonium sulphate*
magnesium sulphate
sodium borate (borax)
copper sulphate
zinc sulphate
manganese sulphate
sodium molybdate
* - only use to lower pH, as necessary
Therefore if you see a reference to a 200ppm
nitrogen solution it means 200mg of dissolved in 1L
or 200g dissolved in 1000L of water. Note that
other elements are present in a compound. For
example, only 35% of ammonium nitrate (nitram) is
nitrogen. Therefore, in the above examples, it is
important to add the required amount, either
200mg or 200g, of actual nitrogen, and not 200mg
or 200g of nitram.
Using injection pumps
The calculations above apply to the
concentrations of solutions applied directly to the
plants. However, if a proportional injector is used
to add nutrient to irrigation water, then stock
solutions at higher than end-use concentrations
need to be prepared.
Useful injection rates provided by proportioning
pumps are in the range of one part of stock
solution to 200-500 parts of irrigation water. The
lower the injection rate, the larger the volume of
the stock tank required, so it is desirable to have
injection rates as high as possible.There has to be a
compromise, however, due to the solubility limits
of the fertilisers added to the stock solution. If a
proportional injection pump was set at an injection
ratio of 1:200 then the stock solution would need
to be 200 times more concentrated than the end
concentration.
When stock solutions are prepared at
concentrations in excess of 10 times the application
rate, incompatibilities can become a problem. For
example, calcium nitrate will combine with
Page 2 – © NGIA – The Nursery Papers – Issue no 2002/12
phosphates and sulphates to
produce relatively insoluble
precipitates. In this case you may
resort to using A and B stock
solutions where each is pumped
separately by dual injectors into
the irrigation water and diluted
before they mix and form
precipitates, see table 2.
Alternatively, you can inject the
solutions at different times of the
day or on different days or
purchase specially formulated
mixes that contain the required
elements in a more compatible
form or revert to using dedicated
tanks for each solution.
It is vitally important to regularly
check
pH
and
electrical
conductivity (EC) levels of fertigation solutions as well as the
potting mix. A and B solutions will
have different pH characteristics
and providing they are wellbalanced in terms of nutrients,
each can be used at greater
frequencies to increase or decrease
the pH of the potting mix.
Table 3: Composition, solubility and pH reaction of fertilisers
Fertiliser
Composition
(%w/w)
Macroelements
Ammonium nitrate NH4NO3
Ammonium sulphate* (NH4)2SO4
Calcium nitrate Ca(NO3)2.4H20
Calcium Nitrate Ca(NO3)24H20
Magnesium nitrate Mg(NO3)2.6H20
Magnesium sulphate MgSO4.7H20
Monammonium phosphate
(MAP) NH4H2PO4
Potassium dihydrogen phosphate
(MKP) KH2PO4
Potassium nitrate KNO3
Solubility at PH reaction
15ºC(g/L)
35N
21N, 24S
20Ca, 14N
17Ca, 12N
9.5Mg, 10N
10Mg, 13S
1183
706
1430
1200
750
710
Acidic
Acidic
Alkaline
Alkaline
Alkaline
Neutral
12N, 26P
227
Acidic
29K, 23P
39K, 14N
330
316
Acidic
Alkaline
53Mo
430
NR
11B
26Cu
14Fe
23Mn
40Mo
900
300
400
900
440
NR
NR
NR
NR
NR
Microelements***
Ammonium molybdate
(NH4)6M024.H20
Sodium borate (borax)
Na2B407.10H20
Copper sulphate CuSO4.5H20
Iron EDTA
Manganese sulphate MnSO4.5H20
Sodium molybdate Na2Mo04.2H20
* Only required in small amounts for occasional pH adjustment
** Only in small amounts for occasional Mg boost, pH and/or EC adjustments
*** Only needed in small amounts (if at all) so only purchase small quantities as needed.
Table 4: Electrical Conductivity (EC) contributions made by different
macro-element fertilisers
Fertiliser
EC (ds/m) of a 100ppm solution
of the nominated element
Ammonium nitrate NG4N03
Ammonium sulphate* (NH4)2SO4
Calcium nitrate Ca(NO3)2.4H20
Magnesium nitrate ** Mg(NO3)2.6H20
Magnesium sulphate MgSO4.7H20
Monammonium phosphate (MAP)
NH4H2PO4
Potassium nitrate KNO3
0.51
0.96
0.59
0.97
0.84
Formulation
There is a wide range of readymixed soluble fertilisers available
that
have
nitrogen
(N),
phosphorous (P) and potassium
(K) ratios suitable for nursery crop production.
Many of these also contain useful levels of other
nutrients. Some do not contain magnesium or
calcium, which may need to be supplied either
alternately or through a dual injector system.
Well-formulated growing media should contain
sufficient micro-elements if pH remains between
5.3-6.5. A complete fertigation program will need
most, or all of, the chemicals listed in Table 1. Note
that sodium and chloride-based fertilisers can cause
toxicity problems in certain situations.
Different fertilisers have different pH characteristics. They also have different ECs at the same
concentrations. Therefore it is possible to manage
(N)
(N),0.84 (S)
(Ca), 0.84 (N)
(Mg), 0.85 (N)
(Mg), 0.64 (S)
0.23K, 0.29P
0.34K, 0.96N
the pH and EC effects by choosing different sources
of fertiliser, see Tables 3 and 4.
Feeding rates and recipes
Some basic guidelines for fertigation nutrient rates
are suggested in Table 5. However, in producing
this table, a number of assumptions have been
made, including:
• Either continuous fertigation with no CRFs or
three times a week fertigation applications with
50% manufacturer’s rates of CRFs.
• Containerised production using soilless media
with a low level of nitrogen drawdown.
• No added trace elements.
• Irrigation through sprinklers or well-monitored
(for EC changes) trickle irrigation.
© NGIA – The Nursery Papers – Issue no 2002/12 – Page 3
Table 5: Feeding solutions for low N-drawdown soilless media in mg/l (=ppm)
N Drawdown
N
P
K
Ca
Mg
S
Fe
Mn
>0.7
150
20-30
200
120
30
30
1.0
0.5
0.5 to 0.7
200
30-50
200
150
30
30
1.2
0.5
Zn
0.2
0.2
B
0.2
0.2
Cu
.03
.05
Mo
.03
.03
Note:
1. If the potting mix meets the Australian Standard in micro-element composition and pH is maintained with the suitable range, there is
usually no need to add micro-elements to the fertigation program if the crop cycle is less than seven to eight months.
2. If composted pinebark or peat-based media are used and dolomite and limestone have been used to correct pH to 6-6.5 and sodium,
potassium and ammonium levels are acceptable, then there is probably no need to add magnesium and calcium to the fertigation
program if the crop cycle is less than five to six months.
3. In some cases the potassium (K) in CRFs will be sufficient to meet the needs of the crop. However, it may still need boosting at
transplanting to provide for demand until the CRF source becomes available.
Leaching
Fertigants are easily leached but are just as easily
reapplied. However, prolonged rainfall causes
problems and heavy irrigations reduce the
effectiveness of infrequent fertigations. After
rainfall, it is usually necessary to increase fertigation
rates or frequencies for long enough to restore
nutrients.
In terms of waste-water contamination, the more
competent fertigation programs may produce less
acute problems than controlled release applications
in subtropical and tropical situations.
NPK ratios
The rate of N used depends on the nitrogen
drawdown of the growing media. Nitrogen
drawdown characteristics will be related to the
amount of available phosphorus and the availability
of bacterial foodstuffs in the basal mix ingredients
and temperature. In terms of fertigated rates of N,
there can be as much as a three times increase in
nitrogen needs between a low drawdown and a
high drawdown mix.
In a very low nitrogen drawdown growing media,
such as a peat-based mix, the K/N ratio suggested
is 1.3-1.5. In the better pinebark media the K/N
ration may need to be closer to 0.8-1.0. In poorly
composted sawdust-based media the K/N ratio
may need to be 0.3-0.4 for much or all of the crop
cycle.
The P/N ratio used in most situations need not be
higher than 0.3, and this includes media with
reasonably high nitrogen drawdown. For
Australian Standards media receiving overhead
fertigation three times a week, 0.15-0.3 has been
suitable for a wide range of outdoor and
greenhouse crops grown over short and longer
term crop cycles.
The bottom line
The selection of a good nutritional program for
your nursery will depend on knowing the
nutritional requirements of your crops, the quality
of your water and growing media characteristics
as well as an understanding of chemical
interactions that occur relation to plant nutrients.
The best approach includes gaining analytical
information on your water and growing media
from a competent water/media analysis
laboratory, evaluating nutritional program
options and careful on-site experimentation.
Further reading
Bodman, K & Sharman, K, 1993. Container Media
Management. Queensland Nursery Industry
Association. Brisbane,Australia
Cresswell, G & Huett, D, 1996. Managing Nursery
Runoff – Techniques to reduce nutrient leaching
from pots. Horticultural Research and Development
Corporation, and NSW Agriculture.
Handreck, K, 1998. Controlled-release fertilisers:
more efficient than fertigation.
Australian Horticulture, February 1998, pp102-105.
Huett, D & Gogel, B, 1999. Longevities and
nitrogen, phosphorus and potassium release
patterns of polymer coated controlled release
fertilisers at 33˚C and 40˚C. Proceedings of 1999
NIAN state conference, Ballina, NSW.
Acknowledgements
The information in this Nursery Paper was supplied
by Keith Bodman,WA Horticulture and Environmental
Science Skills Centre.Input was also received from Dr
Geoff Cresswell, Cresswell Horticultural Services.
Page 4 – © NGIA – The Nursery Papers – Issue no 2002/12