Fertigation
The Basics of Injecting Fertilizer for Field-Grown Tomatoes
Fertigation refers to injecting fertilizer into an irrigation system for field crops. This is accomplished in drip (trickle) by using some type of injector to meter the concentrated fertilizer solution into the irrigation system. The basics of the system design are outlined in this publication.
A theory behind why fertigation has become the state of the art in vegetable nutrition is that nutrients can be applied to plants in the correct dosage and at the time appropriate for a specific stage of plant growth. When plants receive conventional preplant fertilizer and then two (or more) sidedressings, they get a larger dosage of fertilizer than they require at the time it is applied. Between applications there may be a deficiency of fertilizer.
With fertigation, plants can receive small amounts of fertilizer early in the crop's season, when plants are vegetative. The dosage is increased as fruit load and nutrient demands grow, and then decreased as plants approach the end of the crop's cycle. This gives plants the needed amounts of fertilizer throughout the growth cycle, rather than a few large doses.
Timing
Fertilizer can be provided in different frequencies--daily, every other day, several times each week, or weekly, depending on irrigation needs, soil type, and other factors. For Mississippi conditions, once a week should be adequate. On very sandy soils, more frequent fertigation might be necessary. Don't hesitate to irrigate if irrigation is needed other than during the fertigation.
Rates
The amount of fertilizer to apply is recommended in terms of pounds per acre per day, week, or whatever application increment is selected. The amount used can vary during the growing season, starting off low, increasing as plants set fruit, and then declining toward maturity. With tomatoes, for example, nitrogen use might be in the neighborhood of 7 pounds per acre per week early in the crop, 10 pounds per acre per week as plants approach fruit set, and 14 pounds per acre per week when plants have the heaviest load of fruit. In the last 2 weeks, the rate can be reduced to a lower level. This could be applied once a week, or more frequently (7 lb/a/week = 1 lb/a/day), however it best fits into cultural practices in Mississippi conditions.
Fertilizer Choices
Typically, nitrogen (N), potassium (K), or both are injected. Do not inject phosphorus (P). Since phosphorus does not move much in the soil, it is best to incorporate all of it before planting.
All fertilizer sources must be highly soluble. It is difficult or impossible to unclog drip irrigation tubing once you have clogged it with insoluble fertilizers, algae, or sand.
Solid nitrogen sources include calcium nitrate, ammonium nitrate, potassium nitrate, and others. Potassium sources are usually potassium nitrate or potassium chloride. Table 1 has the elemental breakdown of these fertilizers.
Commercially prepared liquid fertilizers for injection are also acceptable. These are usually combinations of N and K and include 4-0-8, 6-0-6, 7-0-7, 10-0-10, and others. Higher grade liquid fertilizers are preferred to lower grade liquid fertilizers, since more actual fertilizer and less water is being purchased. If soil test results indicate low K, liquid fertilizers such as 7-0-7, 8-0-8, or 10-0-10 are acceptable. In those situations where soil K is already high, straight N can be injected in commercially prepared solutions made from ammonium nitrate and urea. These include 19-0-0, 28-0-0, and others. Base fertilization amounts on soil test and crop requirements.
Table 1
Nutrient composition
of individual fertilizers commonly used in fertigation
(elemental forms of P and K are shown in parentheses)
| Fertilizer | % Nutrient composition | pH* |
| Ammonium nitrate | 34% N | A |
| Calcium nitrate | 15.5% N, 19% Ca | B |
| Diammonium phosphate | 16% N, 46% P2O5 (20.1% P) | A |
| Monopotassium phosphate (MKP) | 52%
P2O5 (22.7% P), 34% K2O (28.2% K) | B |
| Nitrate of soda potash | 15% N, 14% K2O (11.6% K) | B |
| Potassium chloride (muriate of potash) | 60% K2O (49.8% K) | N |
| Potassium nitrate | 13.75% N, 44.5% K2O, (36.9% K) | B |
| Sodium nitrate | 16% N | |
| Urea | 46% N | B |
Solubility Limits
Solubility limit refers to how much of a fertilizer can be dissolved in a certain amount of water. If this amount is exceeded, the fertilizer will precipitate, which is commonly called "salting out." It is important that fertilizers completely dissolve; otherwise, they will settle out in the tank, and plants will not receive the full dose. Also, undissolved fertilizer can clog irrigation tubing and emitters. Table 2 shows the solubility limit of some fertilizers in 100 gallons of cold water. Putting more than these amounts of fertilizer in this volume of cold water will result in some fertilizers' not being completely dissolved.
The temperature of the water will have an influence on how much a fertilizer grade will dissolve. Therefore, the time of year can help determine how much fertilizer will dissolve as well. In cooler weather, such as very early spring or late fall, less fertilizer will stay in solution in the cooler water.
The most limiting solubility in the listed fertilizers is potassium nitrate. However, if growers remember that not more than about 1 pound per gallon of potassium nitrate should be dissolved, there will not be a problem. Solubility beyond the limits listed in Table 2 can be achieved by continuous agitation (mechanical paddle or recirculating pump) or by warming the water and keeping it warm.
Table
2.
Solubility limits of fertilizers
| Fertilizer | Pound per 100 gallons |
| Ammonium nitrate | 984 |
| Calcium nitrate | 851 |
| Diammonium phosphate (DAP) | 358 |
| Potassium chloride | 290 |
| Potassium nitrate | 108 |
| Sodium nitrate | 608 |
| Urea | 651 |
Calculating Amount of Fertilizer
To determine how
many pounds of a fertilizer to use for a certain application rate, use
the following formula:
Pounds of fertilizer = [(pounds per acre desired) divided by (percent
of element in fertilizer)] x 100
Example:
If a grower wants 7 pounds per acre of nitrogen each week and is
using potassium nitrate (13.75% N),
Pounds of fertilizer = [7 lb/a) divided by (13.75%)] x 100 = 51 pounds
Equipment
The irrigation system consists of a main line, sub-main lines (or headers), feeder tubes, and drip tubes. Drip tubes, or drip tape, are generally 8 to 10 mil thick for single-year use on vegetables. Headers often consist of vinyl lay flat tubing (blue or black) or PVC pipe. The "lay flat" can be driven over with a tractor, so it does not need to be buried. The drip tubing is rated in gallons per hour per foot, or 100 feet, at a specific design pressure. For example: 24 gallons per hour per 100 feet, when operated at 8 psi pressure.
For any fertigation system, the basic components, in addition to the irrigation tubing, include a fertilizer tank, an injector, a filter, a pressure gauge, check valves, and a pressure regulator. A good filter is essential for any drip irrigation system to work. Inject the fertilizer solution into the line in front of the filter to be certain any undissolved solids are removed before they enter the rest of the irrigation system.
Application Technique |
When
fertilizer is being injected into a drip irrigation system, there
are certain precautions a producer should take, including:
|
Fertilizer Metering Devices
(Injectors)
Many types of injector pumps are available. It is not necessary to use a complicated or expensive injector to obtain good results. Positive displacement pumps are precise and operate on an external power source such as electricity (120 volt AC or 12 volt battery), an internal combustion engine, or water power. The other types of pumps work on differential pressure rather than positive displacement.
Positive displacement pumps are piston pumps or diaphragm pumps. Once a piston pump is calibrated to a given rate, it is accurate, but it has surfaces that might be exposed to corrosion, and it must be stopped to change calibration. Diaphragm pumps, on the other hand, usually are made of a chemically resistant material. They are accurate and can be adjusted as they run. These pumps inject at a constant rate regardless of flow or pressure changes in the system.
There are also proportioner injectors that sense the rate of flow and adjust the injection rate as the flow rate changes. These pumps do not require an outside power source, and they work well in nurseries or greenhouses. One possible disadvantage is that these injectors require some pressure to operate, and pressure changes in the system might alter the rate of injection, which might or might not be proportional to the desired rate.
The venturi bypass is simple and relatively low cost. It works from differential pressure in the system (usually 20 percent) from one side of the device to the other. Since the injection rate depends on the pressure differential, any pressure fluctuations in the system change the injection rate.
Positive displacement injection pumps give better control of injection rates and are preferable to venturi or pressure differential devices. The injection pump should be sized for maximum amount of fertilizer to be injected at any time during the season, and so the fertigation process can be completed in less time than will be required to meet the irrigation needs of the crop.
Backflow Prevention
Depending on the water source, different backflow devices might be required. Also, if the system is going to be used to inject pesticides, certain EPA backflow requirements must be met. Basic protection for fertigation systems should be a check valve between the water source and the injection point. If a positive displacement pump is being used, then install a check valve on the injection line between the pump and the injection point, and the system should be interlocked so that if the pump water supply goes off, the injector goes off as well. If a hydraulic driven injector is being used, the only protection that can be used is the check valve in the main line between the water source and the injector.
There should be a low pressure drain and vacuum breaker between the injection line and the water source to prevent seepage back into the water system when nothing is running. The low pressure drain should be discharged at least 20 feet from any water supply source and protected from draining toward it.
Injection Point Location
The injection point must be between the check valve and the filter. To allow adequate time for the fertilizer solution to mix uniformly, use at least 25 feet of line between the injection point and the filter system. It should also pass through at least two 90-degree turns to insure adequate time for thorough mixing and for any precipitant to come out in front of the filter. This will give more uniformity to the fertilizer that each plant receives and decrease the danger of a precipitant's plugging the system.
Calibration
You must know three factors before you can calibrate. These are (1) the system flow rate, (2) the injection pump flow rate, and (3) system pass-through time.
The system flow rate is the flow rate of the system per unit time, usually expressed in gallons per minute or gallons per hour. Injection rate is the amount of material desired to be injected per unit time. The system pass-through time is the time it takes for all of the water in the system to be totally replaced with new water at operating pressure. Pass-through time could also be expressed as the amount of time it takes for fertilizer injected at the pump to reach the farthest point in the system from the pump. Pass-through time is important to make sure the system runs long enough to inject the desired amount of material or that the injection rate is long enough to fill the system completely and be flushed out before the system shuts off.
The easiest way to calibrate an injector is to use a graduated cylinder, in ounces or milliliters, and actually put the injector suction in the cylinder that is filled to a known volume with the material that will actually be injected. Time the injection cycle for one minute or any known time. Then convert what has been used in the cylinder in one minute to the desired rate per hour, or whatever the time may be.
The following example might help you with some of these calculations. The length of time for the irrigation cycle is based on the assumption that tomatoes need 1.5 inches of water per week. If they are already getting enough water from natural rainfall, then the length of time can be based on how long it takes to move the fertilizer from the concentrate tank to the field.
Example:
A grower has 5 acres of tomatoes, with 3,000 plants per acre. The grower wants to irrigate 1 acre at a time, putting out 1.5 inches per acre and 7 pounds of actual N with the irrigation. The drip tape being used is rated at 24 gallons per hour per 100 feet of tubing at 8 psi.
The fertilizer the grower will use is potassium nitrate (13.75% N). It should be noted that potassium nitrate has a solubility limit of 108 pounds per 100 gallons of water, or about 1 pound per gallon. In other words, using cold water, not more than about 1 pound will dissolve in each gallon of water.
- Calculate how much
potassium nitrate is needed to get 7 pounds of actual N. To do this,
divide the pounds of N needed by the percent N in the fertilizer (and
multiply times 100):
Lb N needed / percent of N in fertilizer x 100 = pounds of fertilizer required (7 / 13.75 x 100 = 51)
Therefore, 51 pounds of potassium nitrate are needed to get 7 pounds of actual N. Keeping the solubility limit in mind, this 51 pounds will dissolve in 50 gallons of water. - Determine how many
row feet will be fertigated. This will be the same as the number of
feet of drip tubing. To do this, divide the number of square feet in
1 acre (43,560) by the spacing between rows. If the tomatoes have 6
feet between rows, this is calculated as:
Square feet in 1 acre / spacing between rows = linear feet of drip tubing (43.560 / 6 = 7,260)
Therefore, there are 7,260 feet of drip tubing in one acre. This can also be represented as 72.6 100-foot sections (7,260 / 100 = 72.6) per acre. - Calculate how many
gallons per hour are needed to irrigate the field. Drip tubing is capable
of delivering 24 gallons per 100 feet of tubing per hour. Therefore,
to determine how much water can be delivered to an acre of tomatoes,
multiply the 72.6 (100-foot sections) times 24 gallons of water per
100-foot section per hour:
Number of 100-foot sections x 24 gallons per 100-foot section per hour = total flow per hour (72.6 x 24 = 1,742 gal/hr)
The tubing in 1 acre can deliver 1,742 gallons in 1 hour. This is equivalent to 29 gallons per minute (1,742 gallons per hour / 60 minutes per hour = 29 gallons per minute). - How long do you
need to run the system to deliver the amount of water required? An acre-inch
is equivalent to 27,000 gallons of water. Once acre-inch per hour is
equivalent to 450 gallons per minute (27,000 / 60). So, the flow rate
in gallons per minute (gpm) divided by 450 gpm will give you the number
of acre-inches being pumped or applied.
The applied tubing requires 29 gpm, or 0.0644 acre-inches per hour (29 / 450). For tomatoes , you want to apply 1.5 inches of water per week. Therefore, divide the flow rate in acre-inches per hour into the desired application to determine the total pumping time:
1.5 inches / 0.0644 acre inches per hour = 23.4 hours of operation.
This is how many hours per week it takes to apply 1.5 inches of water. - The injection time is determined by catching a volume of material from the injector pump over a known period of time. Assume 26.7 ounces of material are pumped by the injector in one minute. The fertilizer is mixed in 75 gallons of water. Therefore, the injector can put out 1,602 ounces per hour (26.7 ounces per minute x 60 minutes per hour), which is equal to an injection rate of 12.5 gallons per hour (1,602 ounces per hour / 128 ounces per gallon). The 75 gallons of fertilizer solution, then, will take 6 hours of injection time (75 gallons / 12.5 gallons per hour = 6).
- To inject for a
6-hour period, the system should probably run about 8 hours minimum.
If an irrigation time of 24 hours is too long, then the irrigation sets
could be 8-hour-long runs three times per week (23.4 hours to apply
1.5 inches / 8 hours per set = 3 sets per week of 0.5 inches each).
Note: check the injection pressure to make sure it is greater than system running pressure. The injector may need to be between the pressure regulators and the filter if injection pressure is less than water source pressure.
The examples are based on 1.5 inches of water per acre for tomatoes. However, tomatoes do not form a complete crop canopy at maturity. The row spacing is too wide for a 100-percent crop canopy to develop. If 1.5 inches per acre are required to meet demand for a full canopied crop of tomatoes, then the full 1.5 inches may not be needed, given the space between rows, since the canopy does not cover the full acre.
Assuming an 18-inch walkway between 6-foot rows, the actual plant canopy coverage would only be 4.5 feet of the 6 feet, or 75 percent. Thus, to apply an equivalent rate of 1.5 inches per acre would take only 75 percent of this, or 1.125 inches of water. This can be done, since the irrigation system covers only the shaded areas under the plants and the entire acre is not irrigated. Time of year may also affect water use.
Preplant Fertilizer Considerations
Growers can apply some of the required fertilizer (according to soil test recommendation) preplant incorporated into the bed or banded along the side of the row. This is usually 20 to 33 percent of the total N and K and all of the P. Alternatively, all of the N and K fertilizer can be applied via fertigation. Either method is acceptable. If phosphorus is recommended by the soil test, apply it preplant. If you are using a preplant fertilizer application, delay fertigation until about 2 weeks after plants are transplanted, since they will have enough fertilizer to start vegetative growth. However, if no preplant fertilizer is used, begin fertigation right after planting.
Recommendation for a Tomato Fertigation Schedule
To determine how much fertilizer to feed into the irrigation system, first decide how much N and K the crop will need for the entire season. For tomatoes, the usual recommendation in Mississippi is 120 pounds of N, P2O5, and K2O. This should be adjusted according to the soil test results.
The second step is to decide how many weeks the crop is likely to be grown. A Mississippi spring tomato crop is probably in the ground about 14 weeks. The weekly feeding, then, is 120 / 14 or about 8.5 pounds per week. This amount is adjusted by two other factors. First, if you use a preplant fertilizer, the total needs to be reduced by this amount. For example, if 20 percent of the N and K is applied preplant (24 pounds each), then only 96 pounds will be applied by way of fertigation (80 percent x 120 = 96).
This time, since the preplant fertilizer will supply needs for the first 2 weeks of the 14-week cycle, the fertigation is delayed for 2 weeks. With 12 weeks remaining for fertigation, the weekly application is 8 pounds (96 / 12 = 8).
The second factor that influences the weekly application is the stage of growth of the crop. As mentioned, less is needed earlier, while more is needed as the fruit load increases. Using the example, for the first couple of weeks, 6 or 7 pounds per acre should be enough. As the load increases, apply 8 to 10 pounds per acre.
Suggested schedules for fertigating an acre of transplanted tomatoes in Mississippi are shown in Tables 3 and 4. Table 3 assumes that all of the N and K are supplied through the fertigation system. If the soil test shows that less of either is needed, adjust the numbers in the table accordingly.
Table 4 shows numbers for a field of tomatoes in which 20 percent of the recommended fertilizer has been applied preplant in the bed. If the actual cropping season is longer or shorter than that shown in the tables, the numbers need to be adjusted.
Table 3
Suggested fertigation schedule for transplanted tomatoes in Mississippi, using all fertilizers via fertigation (14-week schedule)*
Total lb/a Growth stage No. of weeks fertigated Injection rate
(lb/a/week)Total injected at stage (lb) N K2O vegetative 2 6 12 bloom 3 8 24 120 120 fruit set 7 10 70 fruit set ended 1 8 8 maturation 1 6 6 Totals 14 120
Table 4.
Suggested fertigation schedule for transplanted tomatoes in Mississippi, using 20 percent of N and K2O preplant (12-week schedule)*
*Note: phosphorus fertilizer is not mentioned in these tables because all phosphorus should be applied preplant and not fertigated.
Total lb/a Growth stage No. of weeks fertigated Injection rate
(lb/a/week)Total injected at stage (lb) N K2O vegetative 2 0 0 bloom 3 7 21 96 96 fruit set 7 9 63 fruit set ended 1 7 7 maturation 1 5 5 Totals 12 96
By Dr. Richard G. Snyder, Extension Vegetable Specialist, and James G. Thomas, Leader, Extension Agriculture Engineering
Publication 2037
Extension Service of Mississippi State University, cooperating with U.S.
Department of Agriculture. Published in furtherance of Acts of Congress,
May 8 and June 30, 1914. Ronald A. Brown, Director
Copyright by Mississippi State University. All rights reserved.
This document may be copied and distributed for nonprofit educational purposes provided that credit is given to the Mississippi State University Extension Service.
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