Sambungan:
Fertigation
Fertigation is the application of water-soluble fertilizers through irrigation water. When plants
are actively growing, nutrients are usually applied with every irrigation (constant liquid
fertilization – CLF). The benefit of CLF is that the irrigation and fertilization level is matched
with the plant growth rate. The faster plants grow and dry out, the more often they will be
watered and fertilized, and the slower plants grow and dry out, the less often they will be
fertilized and watered.
Many factors impact how quickly plants utilize nutrients. When growing plugs, seedlings, or
cuttings, the root system area is quite small. When they are transplanted into larger pots, the
media may not dry out as quickly as the plants use the nutrients, so they may be temporarily
nutrient deficient. Similarly, on cloudy, cool days, when plants are not transpiring as much, they
may utilize the nutrients present in the media, but not the water. In situations like these, the
grower has a couple of choices. The first is to use a fertilizer drench with a high fertilizer rate;
the plants gain the nutrients they need, without excess water. The second is to initially irrigate
the plants with a higher CLF rate, then revert to the level that will be used for the rest of the
season.
The fertilizer concentration chosen depends on a number of factors. The most important of these
are the species grown, irrigation frequency and leaching fraction. Season (higher fertigation
rates in summer vs. winter), media (high cation exchange capacity = low fertigation rates), plant
growth stage (higher rates at the beginning of the production cycle, low to none at the end of the
production cycle) are also variables that influence plant nutrient requirements.
Three basic categories that are generally used to divide species are heavy, medium, and light
feeders. Heavy feeders, including poinsettia (Euphorbia), chysanthemum, and Abutilon, require
high levels of nutrients. Most bedding plants need only medium nutrition levels. Many potted
flowering crops like gloxinia (Sinningia) and African violets (Saintpaulia) are considered light
feeders. Growers usually grow a wide variety of plants with a wide range of nutritional needs.
The easiest way to deal with this is to tailor your irrigation patterns to the crops needs. Set the
fertilization rate for the species that is heaviest feeder, and then combine fertigation and clear
water irrigation for the other crops in order to give them their ideal nutrition level. The lower the
nutrition required, the more frequently clear water should be used to irrigate the plants. An
alternative is to set the fertilization rate at the level needed by the lightest feeder, and then
supplement those species that are heavier feeders with controlled release fertilizers incorporated
into the media or applied as topdressing.
Irrigation frequency and leaching fraction (the proportion of water that runs out of the pot
compared with what goes in) interrelate to impact the fertilizer concentration needed to keep
nutrient amounts at ideal levels. The more frequently plants are irrigated with high leaching
fractions, the higher the fertilizer concentrations necessary in order for plants to have adequate
nutrient levels in the media. If using a low-leaching fraction regime, beware of fertilizer salts
accumulating in the growing media.
If the media used does not have micronutrients pre-incorporated into it, the grower will need to
use a water-soluble micronutrient mix. They usually include zinc, manganese, copper, and iron
as sulfates, oxides, or chelates. Molybdenum is usually included in mixes as molybdate and
boron as borax. Media pH is best kept acidic (less than 7.0) in order for sulfates and oxides of
micronutrients to be available for plant use. If pH is already above neutral, chelated
micronutrients can be used. pH does not generally affect the availability of boron and
molybdenum like it does zinc, manganese, copper, and iron. Chelates are large organic
molecules that bind zinc, manganese, copper, and zinc, and prevent them from becoming
insoluble compounds. Plants are able to absorb these micronutrients from the chelates because
the chelates are soluble. Chelated micronutrients are more expensive than the sulfate or oxide
forms, and are generally used only in areas with high pH irrigation water or if specific
micronutrient problems occur.
Mixing Water Soluble Fertilizers
Most greenhouses use injector ratios between 1:15 and 1:200. Injectors are used to inject
specific amounts of concentrated fertilizer solution (stock) into irrigation water. For example, a
grower could mix a given number of ounces of fertilizer into a 5-gal. bucket, then the injector
would input one part of the fertilizer stock solution per 200 parts of irrigation water that goes
through the system in order to gain the desired concentration of 100, 150 or 200 ppm N (parts
per million Nitrogen). Some injector systems are variable; in that, the grower can change the
ratio injected to match the desired nutrient application rate. EC is often used to monitor fertilizer
levels to ensure the injector is working properly and inputting the correct ratio of stock solution
to irrigation water. To monitor the EC, you would collect a sample of the fertigation water and
test it with an EC meter. The EC reading would tell you if the ratio is correct. This information
is often printed directly on the bag of water soluble fertilizer, from the amount of concentrate to
add to the stock tank to the expected EC of the mixed solution as it is applied to the plants.
Injectors. Several types of injectors are available for use; they include Venturi suction devices,
water motor-controlled injectors, and water meter-controlled injectors. The type chosen will be
dependant on price, portability and the amount of fertilized irrigation water needed.
The cheapest, most easily portable and smallest injectors are Hozon or Syphonex. They are
Venturi suction devices and have a low ratio that varies from 1:12 to 1:16. Most fertilizer
manufacturers base calculations on an average ratio of 1:15. They are most useful for small
operations, due to the frequency with which the stock bucket needs to be refilled. The ratio
injected varies with water flow rate and between individual units of the same brand. These are
not appropriate for use when precise fertilizer application rates are needed.
Water motor-controlled injectors insert fertilizer solution into the waterline by means of a piston
that moves in response to the water flow. An example of this type of injector is a Dosmatic.
.
This category of injectors is moderately priced and one type is portable. These are well suited to
medium sized operations. Their dilution ratio is adjustable.
Water meter-controlled injectors inject the stock solution into the water stream by means of
electric or water powered pumps. Examples of this type of injector are Anderson, Fert-o-Ject.
and Dosatron.
. Water meter-controlled injectors are expensive, have variable dilution ratios,
and are generally fixed in one location. They are ideal for large greenhouse operations that
utilize large amounts of fertilizer and irrigation water.
Pre-mixed vs. Mixed Fertilizers. As with media, fertilizers can come premixed or as separate
components that the grower would mix. Operation size, crops grown, and cost are factors that
can influence the grower’s choice whether to buy premixed or individual fertilizer components.
Pre-mixed fertilizers are more convenient and do not require extra labor to mix. They are more
expensive, but this expense is often offset by the time saved in labor needed for mixing and
convenience provided to the smaller grower.
Larger operations are often justified in mixing their own fertilizers, in that as the quantity they
are buying increases, the cheaper the product becomes. In addition, larger operations often have
a wide variety of crops growing, making specialized fertilizer mixes more necessary to meet the
different nutritional needs of the crops. A mishap in mixing nutrient components, while
detrimental to a small grower, is more easily absorbed by a larger operation, since the specialized
fertilizer would be applied only to small number of crops, and not to all the crops the grower is
growing. A mishap with fertilizers can rapidly result in crop damage, especially when
micronutrients are being applied.
Developing a Nutritional Program
The first step in developing a nutrition program is to determine the crops that are to be grown.
Once the crop type is determined, the next step is to research the crop, and determine the optimal
pH and EC range, along with the type and rate of fertilizer that will provide the crop with the
nutrition level it requires. Also to be taken into consideration is the type of medium used to
grow the crop. For crops grown in a soil-based medium, the optimal pH range is usually
between 6.2 and 6.8; for crops grown in soilless media, the optimal pH range is usually between
5.4 and 6.0. The final step is to determine how to apply both N and K. Most growers choose fertigation.
Some growers, however, choose to combine controlled release fertilizers and fertigation. Often
this combination produces the best looking crops.
Selecting a Fertilizer
A wide variety of fertilizers are available for floriculture and greenhouse crops; there are even
some specific to poinsettias and geraniums. These fertilizers may be satisfactory for use under
normal production conditions, but any number of factors can change their effectiveness. The
main goal of fertilizing is to provide adequate, not excessive, nutrient levels to a crop.
Monitoring the pH and EC helps determine whether the fertilizer levels are within the acceptable
range. Five factors influence the effectiveness of any fertility program.
The first is acidic vs. basic fertilizer. This is where researching the crop and determining the
optimal pH and EC range become important. Knowing this range allows the grower to determine
whether a acidic or basic fertilizer will be the most effective. In general, a high proportion of
NO3-to NH4+ will cause pH to increase, while a fertilizer with a high ration of NH4_+ to NO3 will
cause the pH to decrease. Also impacting the choice of acidic or basic fertilizer is the alkalinity
level of the irrigation water sources. Is an acidic fertilizer required to neutralize the alkalinity of
the irrigation water, or is the water pure and in the need of a basic fertilizer to help buffer the
media pH?
The second factor is the correct P level to apply. Most crops need only 5 to 10 ppm P for
satisfactory growth. Application of excess P to crops will not help them maintain their desirable
compact form. Utilization of fertilizers with low P levels, for example a 15-2-20 or 14-5-38, will
provide adequate N and K without excess P.
The third factor is whether the fertilizer supplies all essential elements. Oftentimes adequate Ca,
Mg, and S are supplied by irrigation water. However, in many areas of the southeastern U.S.,
adequate levels are not supplied, so a fertilizer supplement must be used. Between 50 to 100
ppm Ca and 25 to 50 ppm Mg are sufficient when provided to the plants by constant liquid feed.
The fourth factor to consider is the Mg: Ca: K ratio. Fertilization rates where K is supplied in
excess of 200 ppm often results in K having an antagonistic effect on Ca and Mg uptake by the
plant. To avoid antagonistic interactions, it is usually best to apply the nutrients Mg: Ca: K in
ppm ratios of 1: 2: 4.
The last factor is the availability of micronutrients. Most fertilizers supply their own array of
micronutrients in levels adequate for good plant growth. Always be sure to check the bag label,
to ensure that micronutrients are provided by your fertilizer. It is also a good idea to know
whether the crop you are growing needs larger levels of a particular micronutrient. Poinsettias,
for example, require higher Mo levels for optimum growth.
No single fertilizer may be able to provide all the nutrients, both micro- and macronutrients, you
need, at the levels you need them for every crop you grow. A way to get around this is to rotate
fertilizers. Find an array of fertilizers that will supply the nutrients you need. Rotate the use of
basic and acidic fertilizers so media pH will remain in the ideal range. To ensure that your
nutrition program is on track, it is important to monitor media pH and EC. By monitoring both
pH and EC every two weeks, growers are able to determine trends, whether toward deficiency or
toxicity, and can correct them before symptoms are visible on the plants.
Fertilizer Formulations
Controlled-release fertilizers.
Water soluble fertilizers.
Monitoring Nutrition
A vital part of successfully growing a saleable crop is monitoring its nutritional status. Factors
such as temperature, irrigation frequency, leaching fraction, water quality, media nutrient content
and plant growth rate combine to make each crop distinctive from others. The simplest manner
of monitoring is to visually inspect the crop; this is best done by an experienced grower.
However, visual monitoring, while helpful, does not yield the type of information helpful to
growers in the long term. By the time visual nutrient deficiency symptoms have been noticed,
crop growth will have been reduced and overall crop quality will not be as high. The easiest way
to monitor the nutritional status of your crop is by testing the media pH and EC.
pH and EC testing
pH and EC testing of media tracks changes of pH and EC over time using pH and EC meters. It
is important that media be tested every two weeks. This enables the grower to discover a minor
problem and fix it before it becomes a major problem. If you have 15 benches of one crop, at
least five plants from each bench should be tested. The easiest way to do this is the pour-thru
method. The pour-thru method utilizes leachate from the pots to determine pH and EC.
The first step is to thoroughly water the plants. One hour later, with your five randomly selected
plants from each bench, pour approximately ¼ to ½ cup distilled water (50 to 100 ml) on each
pot. There should be collection trays under each pot, so leachate from distilled water pour-thru
can be saved. Once the water is poured on and collected, a meter (available from greenhouse
supply firms, ranging from $50 to several hundred dollars) should be inserted into the leachate,
and the pH and EC recorded on charts. See the example chart given on the next page that can be
customized for individual growers.
Much more information on the pour-thru method of root zone media management can be found
at the North Carolina State University floriculture web site:
www.ces.ncsu.edu/depts/hort/floriculture/crop/crop_PTS.htm Other recommended resources for greenhouse managers:
Reed, D. (ed.) 1996. Water, media, and nutrition for greenhouse crops. Ball Publishing.
Compact but thorough compendium of information and advice from top university and industry
experts. An absolute must! (www.ballbookshelf.com]
Whipker, B.E., J.M. Dole, T.J. Cavins, J.L. Gibson, W.C. Fonteno, P.V. Nelson, D.S. Pitchay,
and D.A. Bailey. 2001. Plant root zone management. NC Comm. Flower Growers Assoc.,
Raleigh, NC. Another important tool for the greenhouse grower who wants to stay on top of
crop nutritional status. (http://www.nccfga.org/pubs)
Much of the information in the above publication can also be found online at
www.ces.ncsu.edu/depts/hort/floriculture/crop/crop_PTS.htm See Ch 14C for additional fertilizer and injector resources.
References used in writing this chapter:
Dole, J.M. and H.F. Wilkins. 1999. Floriculture: Principles and species. Prentice Hall: New
Jersey.
Raven, P.H., R.F. Evert, and S.E. Eichhorn. 1999. Biology of Plants. 6th Ed. Freeman and Worth:
New York.
Salisbury, F.B. and C. Ross. 1992. Plant Physiology. 4th Ed. Wadsworth: California.
Whipker, B.E., J.M. Dole, T.J. Cavins, J.L. Gibson, W.C. Fonteno, P.V. Nelson, D.S. Pitchay,
and D.A. Bailey. 2001. Plant root zone management. NC Comm. Flower Growers Assoc.,
(Text Box comment Table of Contents)
Raleigh, NC.