by Mark E. Affleck

Fertilizer injection, application of plant nutrients through an irrigation system, has proven effective in avo­cado groves throug~out southern California. This "fertigation" princi­ple has spawned the proliferation of systems small and large as the avoca­do grower strives to increase his cost efficiency and fertilizing effectiveness.

The flow of irrigation water is the key to fertigation, and injected fertili­zer, generally inorganic material, can be disseminated through a sprinkler or drip system. Based on a direct wat­er mix, the primary advantages over traditional application methods are labor savings, efficient utilization of fertilizer and general ease of opera­tion.

Fertilizers used through injection systems must be totally water soluble and available in various formulas to provide a continuous, balanced nu­trient application. The ideal injection program contains frequent, diluted fertilizer concentrations applied con­tinuously. 26 AVOCADO GROWER

The Systems Fertilizer forms used in injection

systems are liquid or powder/granule varieties. Emitter clogging can be­come an occasional problem with dry forms, but is less of a threat when certain liquids are used. In dry forms the coating on the prill (pellet) can cause clogging at the filter, although recent fertilizer developments have reduced this problem Significantly.

For most growers, a pressure dif­ferential system is frequently used where fertilizer is injected into high pressure water lines. Initially, the ap­plicator is loaded with the measured amount of fertilizer and the irrigation system is engaged and checked. At this juncture, a pressure differential is created which injects the fertilizer into the system.

Valves are situated on these injec­tion tanks to create the differential be­tween incoming and outgoing water which draws fertilizer into the system. When fertilizer has passed through, the applicator should be shut down

for 15 to 30 minutes before the entire system is shut off, according to avo­cado grower Frank Koch. This flush­es the mild nitric acid solution through the system, allowing the brass and bronze parts to escape cor­rosive damage.

For larger groves, a more elabor­ate system which includes pumps and sophisticated tanks and equip­ment is desirable. Automatic timing and back-flushing features are also available with these units. The cost of such systems is quite high though, and when viewed from a small grow­er's perspective, may not even be necessary. Less than 10 acres would; according to Grangetto's Agricultural Supply Pest Advisor Gordon Archibald, warrant the use of pres­sure differential systems whereas larger acreage justifies the more elab­orate sytem. Injection Pros and Cons

Since the cost of manual fertiliza­MAY 1979

This shot accurately captures a complete, sophisticated injection system. The tanks in the foreground house the fertilizer and the units below are the injection system components.

tion is prohibitive in many cases, ferti­lizer injection can decrease labor ex­penditures dramatically. Traditional broadcasting methods like aerial ap­plication, tractor spreading and hand distribution become costly from the labor viewpoint. If grove terrain is rough or rocky, the labor problem is even more pronounced.

Steve White, president of Cal Fla­vor, counters the labor savings the­ory, however, by stressing, "If you have inefficient labor, then sure it's expensive--but if you have produc­tive labor, it pays off in the end. If it's a good investment, I go with the labor-­if not, I save it."

E rtigation efficiency of operation has a myriad of varying factors. If fer­tilizer remains on the soil surface and does not penetrate immediately, for example, nutrient levels may be re­duced. The closed system capability of fertilizer injection can be an asset in this situation since nutrients are ap­plied onto the soil's surface, minimiz­ing above ground influences.

Human error in calculating the amount of fertilizer applied can result in dosage problems. On the positive side, a closed injection system can be beneficial by providing even, unob­structed distribution--but conversely, miscalculation of ingredients can cause serious dosage complications. Application

Component quality of the injector parts and arrangement of the fertilizer system will correlate directly with the operation's success. Even water dis­tribution to each tree is very impor­tant.

This accuracy of application is en­hanced by fertilizer injection. Fertiliz­er is applied at a precise rate on a per­acre basis, and spread evenly throughout an avocado grove. Jim McDonald, owner of McDonald Agri­science, qualifies the even distribu­MAY 1979

tion theory, however, expressing cau­tion regarding pressure regulation. "If the irrigation system is not perfect-1y regulated throughout, you can get unequal distribution of water and consequently unequal distribution of fertilizer. " The problem of fertilizer overdose or area omission is para­mount in this situation, according to McDonald.

White claims the system operator must thoroughly understand the in­jection unit, which, he adds, should be used in conjunction with a well­engineered irrigation system. The potential difficulty here, White at­tests, is the threat of a one-year-old tree receiving the same amount of water as a 1 O-year -old tree. "This is a major problem in a lot of plantings in this area (north San Diego County) where the differences haven't been compensated for."

If the injection system is not entire­ly leak-free, fertilizer will be lost and wasted. In addition, the possibility of tree damage is heightened if leaks are substantial and fertilizer concentrates around a few trees, contends McDonald.

To ensure even and effective ap­plication, McDonald believes in ferti­lizer injection through sprinklers. "If you have young trees, a slow-release fertilizer like IBDU can be used which requires just two applications annual­ly. Each time the sprinklers are acti­vated, a small amount of fertilizer will dissolve and permeate the soil."

White supports this viewpoint, echoing confidence in the slow-re­lease products because they benefit young trees and reduce the fertilizer bum risk.

McDonald considers this program one of the most accurate methods of metering fertilizer to trees, since each portion is weighed prior to applica­tion, confirming equal doses for each tree. Slow-release fertilizers last from five to six months, providing a steady, continual feeding each time the irri­gation system is in operation.

Another accuracy problem, McDonald contends, is the pressure­related difficulty associated with

sprinkler application of fertilizer. Cer­tain areas within each water district will experience fluctuations in the water pressure from area to area. On higher ground where frequent pres­sure variations are prevalent, water pressure cannot be maintained dur­ing peak demand periods. The con­cern in this situation, according to McDonald, is the inability to operate the same number of sprinklers simul­taneously in the mid-summer months as opposed to, for example, the spring season.

C onflicting discussion throughout the avocado industry focuses on the ability of trees to receive fertilizer. Some maintain trees will pick up ferti­lizer more effectively from aerial application during certain periods of the year than they will through fertili­zer injection. Proponents of this view claim alternate methods of fertiliZa­tion can be employed to supplement the injection program when these conditions arise. Backflow Prevention

Water districts require a backflow prevention device on injection sys­tems to alleviate the potentiality of fertilizer backing into the main water source. John Collins, general manger of the Vista Irrigation District, claims the backflow requirement is neces­sary to "protect our system."

If something were to malfunction during fertilizer injection that dropped the pressure on the district side of the line, fertilizer would be in­gested, contaminating domestic wat­er, according to Collins.

A California law under the health department requires either an air va­cuum arrangement or, for fertiliZing purposes, a double-check valve as­sembly. In the Vista Irrigation District, this valve device is installed by the department and assessed a slight monthly charge for quarterly mainte­nance.

Injection system manufacturer Ed Ellerbe claims a safety switch is also available for injection irrigation units to avert backflow hazards in the event water pressure dips drastically. The switch automatically shuts down the

AVOCADO GROWER 27

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unit as the water line pressure drops. While district requirements may

vary throughout the state, Collins be­lieves each area has some form of backflow prevention. "Generally, you'll find most districts have a pro­gram/that requires installation, testing and maintenance," Collins ex­plained. Tank Considerations

Fertilizer tank capacity generally ranges from five to 500 gallons. Ferti­gation from one to five acres will, as an average, require a tank in the 50-300 gallon class, depending on grove needs. (Financial considerations are important in tank selection since price breaks on fertilizer are available in the larger units of 300 gallons or more. ) The five- to II-acre grove will generally require a slightly larger tank, and the extremely large groves may need still larger equipment. A situational decision should be made regarding tank size based on the varying characteristics of each grow­ing area and the financial objectives of the grower.

One important consideration, however, is the on-site storage of fer­tilizer in the holding tanks. If large amounts of fertilizer are stored in an

Joe Waters of Fallbrook Fertilizer makes final preparations on an electric propor­tioner unit prior to fertilization. Note the fertilizer storage tank in the background.

MAY 1979

isolated grove, the risk of vandalism or theft in the absentee situation is significantly increased. If the grove is managed by a grove service, they will, generally, warehouse and store fertilizer before transporting precise amounts to the site as the program dictates.

A storage shed with a lock and pro­tective roof over the water head unit can provide adequate security for the grower who stores large amounts of fertilizer in his grove.

Most systems are pressure rated for 150-300 pounds per square inch. If a sudden pressure surge should occur in the line, a gasket could conceiva­bly blow at this time. However, Archibald claims the problem would likely occur elsewhere in the system. System Beneflcials

Applying fertilizer through drip sys­tems to the tree's root concentration area is one major advantage of the in­jection system, according to Archibald. ''The fertilizer is more apt to reach the root zone through the drip system as opposed to scattering it on the ground." With drip irrigation a more "controlled feeding" is obtained.

Ease of application, the ability to fertiIize at any time based on each particular situation, is a primary ad­vantage of the injection mode of ferti­lization. This method allows the avo­cado grower to ascertain his grove re­quirements and respond accordingly through application of injected fertili­zers.

While a balanced approach to fer­tilizing is generally agreed as most ef­fective, fertigation can maximize re­turns and increase cost efficiency. Each situation will, however, require insightful analysis by growers and grove managers to detennine the most effective and cost-efficient ferti­lizing program for each specific con­dition.

MAY 1979

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