ANHYDROUS AMMONIA APPLICATION RATE ERRORS R. W. Weber, R. D. Grisso, C. A. Shapiro, W. L. Kranz, J. L. Schinstock

ABSTRACT. The application rates of 61 anhydrous ammonia applicators were measured to determine their application accuracy. Thirty percent of the operators underapplied anhydrous ammonia while 34% overapplied. The principle distinction between applicators was the use of "controllers" or "regulators." The application rate errors of 17 controllers and 44 regulators were significantly different. Fifty-nine percent of the controllers and 27% of the regulators had acceptable application rate errors. Thus, it was concluded that controllers were more accurate than regulators. Keywords. Agricultural chemicals, Anhydrous ammonia, Applicators, Controllers, Field measurement, Nitrogen fertilization, Regulators.

Nitrogen (N) is an essential element for growing crops. Approximately 65% of the commercial N is annually supplied to Nebraska agriculture as anhydrous ammonia (NH3). This makes up 682 G (751,000 tons) of NH3 applied to Nebraska cropland (Nebraska Department of Agriculture, 1992).

Nitrate contaminated wells have been associated with agricultural practices (Exner and Spalding, 1990). To help protect the groundwater, improved N management and precise application techniques are needed. Current best management practices used in production agriculture to help protect groundwater from N contamination include: (1) selecting a realistic yield goal, (2) accounting for all N-sources that contribute to meeting crop requirements, (3) determining the N-source that best fits the farming practice, and (4) managing irrigation water applications to minimize leaching of water and N beyond the crop root zone.

Nitrogen application rates depend on the crop species, soil texture, the amount of rainfall and/or irrigation expected, the time of application, and the amount of N available from other sources (left over in soil, expected from irrigation, applied as a starter, etc.). Typically, N-rates range from 112 to 224 kg-N/ha (100 to 200 lb-N/acre) for corn, 67 to 112 kg-N/ha (60 to

Article was submitted for publication in February 1994; reviewed and approved for publication by the Power and Machinery Div. of ASAE in September 1994. Presented as ASAE Paper No. 93-1548.

Contribution from the Depts. of Biological Systems Engineering and Agronomy. Journal Series No. 10568, Agricultural Research Division, University of Nebraska, Lincoln. Mention of trade and company names are for the benefit of the reader and do not infer endorsement or preferential treatment of the products by the University of Nebraska, Lincoln.

The authors are Robert W. Weber, ASAE Member Engineer, Graduate Student, Robert D. Grisso, ASAE Member Engineer, Extension Engineer and Associate Professor, Biological Systems Engineering Dept. University of Nebraska, Lincoln; Charles A. Shapiro, Extension Soil Specialist and Associate Professor, William L. Kranz, ASAE Member Engineer, Extension Irrigation Specialist and Assistant Professor, Northeast Research and Extension Center, Concord, Nebr.; and Jack L. Schinstock, ASAE Member Engineer, Professor, Biological Systems Engineering Dept., University of Nebraska, Lincoln.

100 lb-N/acre) for grain sorghum, and 28 to 56 kg-N/ha (25 to 50 lb-N/acre) for wheat. Anhydrous ammonia is typically applied for corn during preplant (early spring), sidedress (late spring), or post harvest (fall) operations. Application during these times is often dependent on soil conditions and the total area an operator needs to treat. For wheat and grain sorghum, NH3 is usually applied during pre-plant operations.

Research has focused on improving N-rate selection procedures and accounting for all nitrogen credits with less emphasis placed on improving application practices and equipment. As producers fine-tune their application rates, they become more interested in reducing the risk of uneven application.

The objective of this project was to determine the application rate errors of farm operators with their anhydrous ammonia application equipment and to identify some of the factors affecting application errors.

LITERATURE REVIEW Anhydrous ammonia used in agriculture is referred to as

a fertilizer grade. By definition, NH3 contains a minimum of 99.5% ammonia, of which 82% is N, 0.2 to 0.5% is water, and a maximum of five parts per million (ppm) oil (Agri-Chemicals, 1985). The boiling point of NH3 is -33 C (-28 F) at atmospheric pressure. Therefore NH3 is typically transported and stored in a pressure tested tank that is able to withstand pressures of 2760 kPa (400 psi).

Concerns about N's affect on the environment have been raised because of increasing detection of nitrates in the groundwater. Once ammonium is converted to nitrate it becomes susceptible to leaching into the groundwater. As agriculture has increased its use of N fertilizers, nitrate level increases have been detected in domestic wells. Therefore, concerns for environmental issues have caused the researcher to redirect their efforts on N-rate selection procedures. Madison and Brunet (1985) noted that concentrations of nitrate less than 3 ppm are indicative of contamination by human activities. They estimated that nationally 20% of all wells exceed this concentration, with 6% exceeding the national safe drinking limit of 10 ppm. Nielsen and Lee (1986) noted that wells containing

Applied Engineering in Agriculture

VOL. 11(2):211-217 1995 American Society of Agricultural Engineers 0883-8542 / 95 /1102-0211 211

> 3 ppm occurred in 29% of the counties across the United States.

Injection application method is defined, by ASAE Standard S327.1 (1987) as "the mechanical placement of a chemical beneath the soil surface with a minimum of mixing or stirring of the soil, as with an injection blade or knife." Anhydrous ammonia application below the surface does require more energy to place the material than broadcast or banded applications (Hoeft and Siemens, 1975), but NH3 requires the least amount of total energy for fertilizer production, transportation, and application as compared to other forms of nitrogen fertilizer.

Quirin and Wells (1968) gave seven criteria that should be considered when designing NH3 application equipment: (1) the physical and chemical characteristics of the soil, (2) the farm and field size, (3) the crops grown, (4) soil and crop conditions present during different seasons of the year, (5) the penetration depth to minimize losses of NH3 to the atmosphere, (6) adequate tool bar strength to allow the closest spaced knives to be pulled through the soil without incurring stress breakage and minimize compaction, and (7) the potential for combined operations.

Metering systems for NH3 are different from other chemical applicators because a pump is not required to create flow. Flow is created due to the nurse tank pressure forcing NH3 to flow out of the discharge tubes when the valve is opened. There are two types of metering devices commonly used with NH3 application equipment. These are pressure regulated orifices, "regulators", and feed back control systems, "controllers".

Regulators maintain a constant pressure drop across a metering orifice. Changing the size of the orifice regulates NH3 flow rates. Manufacturer's calibrate regulators by measuring the flow rate at a known pressure. Since regulators meter NH3 in the gas/liquid phase, they need to compensate for the gas portion. Anhydrous ammonia gas is formed when the pressure in the transfer hose is lowered by fluid friction. The gas weighs much less than the liquid. According to John Blue (1981), "as the pressure of ammonia is lowered, as by friction in a flowing line, gas is evolved, which weighs much less than the liquid, and the density is less so die flow rate through a given size meter orifice is reduced".

A controller measures the ground speed and NH3 flow rate, and compares a computed application rate to an intended application rate. After comparing, me controller adjusts a servo-valve to change the flow rate. Most controllers will use a heat exchanger to condense most of the vapor before entering the flow meter. This permits liquid to pass through the flow meter which increases the accuracy of measurement.

Liquid pesticide and NH3 calibration techniques are very similar. The "known area" method is a common technique used by producers and fertilizer dealers. The operator applies material over a "known area" and the volume or weight applied is recorded to calculate an application rate. The advantage of this technique is its accuracy. Disadvantages are the large quantities of NH3 needed and time required for accurate calibration. If an error exists, it is difficult to correct the area completed during the calibration process.

Another method, used by Moraghan (1980) and Gomes and Loynachan (1984), was to place the individual

injection knives into tared plastic buckets containing water, then discharge NH3 for 60 to 120 s. Weight differences were used to determine the amount discharged and the flow rate. By measuring the application speed and swath width, the application rate in kg-N/ha (lb-N/acre) can be calculated. This method also determines the knife-to-knife distribution across a swath width. Advantages are that the method can be performed before going to the field, and takes less NH3 than the "known area" method. Disadvantages are the potential danger to the operator and the technique is not conducted under field conditions.

Numerous surveys in the United States have investigated the application accuracy of pesticide and chemical sprayers (Rider and Dickey, 1982; Hofman and Hauck, 1983; Ozkans, 1987; Grisso et al., 1988; Wolak, 1989; Varner et al., 1990). These surveys found that only 25 to 35% of the applicators applied pesticides within 5% of their intended application rate. According to these studies, most of the errors were attributed to calibration and tank mix errors. No similar data have been collected for NH3 applicators.

When calibrating liquid or granular application equipment, it is possible to substitute a nontoxic material (i.e., water or blank carrier) during calibration. Calibration procedures for liquid and granular application equipment are well documented and personal safety equipment is relatively inexpensive. However, calibration procedures for NH3 applicators are not well established. Currently, mere is no safe material with the same properties as NH3 that can be used to calibrate NH3 application equipment. Thus, NH3 application equipment may be less likely to be calibrated. To minimize application accuracy problems, "insurance N" is often applied to guarantee that a minimum amount of N has been applied over die treated area. This "insurance N" can be achieved by setting me metering system 10 to 20% greater than what is actually needed (Shapiro et al., 1992).

METHODS An instrumented NH3 nurse tank was developed to

measure application errors. Each observation from the instrumented tank included an estimation of die error for a particular combination of NH3 applicator, tractor, and operator.

A survey of the equipment, operator, and management techniques was taken to determine what factors were associated with application rate errors. The survey was grouped into four areas: operator information, equipment information, management practices, and site information. Operator information included questions about the operator's age, experience, and education. Equipment information covered the applicator equipment, width, tractor, and metering system. The management practices section inquired about equipment maintenance, calibration mediods, information used to determine application rate, and NH3 purchase decisions. Site information included crop residue cover, ambient air temperature, and soil texture.

A data acquisition system was composed of five sensors mounted on a 3970 L (1000 gal) NH3 nurse tank to record weight of NH3 applied, travel speed, travel distance, NH3 pressure, and temperature. Sensor measurements were recorded using a datalogging system. Details concerning

212 APPLIED ENGINEERING IN AGRICULTURE

sensors, their accuracy, and the data acquisition system were reported by Weber et al. (1993).

Variation of the application rate attributed to the data acquisition system was calculated using the Taylor's expansion series given in Mood et al. (1974). Using the average amount applied and area covered from 61 applicators and the variation determined from calibration of the sensors (Weber, 1993), the system was able to record the application rate of NH3 applicators within 3.7 kg-N/ha (2.4 lb-N/acre). The coefficient of variation (CV) for application rate was calculated to be 3.2%.

Variables recorded by the data acquisition system were summarized by sample averages, sample standard deviations, and CV. Sample averages were used as point estimators. Standard deviations were used to calculate confidence intervals. Coefficients of variation were used as indices of the uniformity of observations. Travel distance and implement widths were used to calculate the total area covered by the NH3 applicator during the test.

Application rate error was the overall error associated with the application of NH3. Application rate and travel speed errors were determined by comparing the intended application rate and speed with the recorded values. The application rate and speed errors were calculated by:

Measured - Intended , e = x 100

Intended (1)

The measured application rate of nitrogen was determined by measuring the amount of NH3 applied (calculated from the weight difference between the beginning and end of the test) dividing by the area covered during the application and multiplying by 0.82 (ratio of actual N in NH3). The measured speed was the average speed recorded during the test.

Discharge rate error could result from inaccurately setting the NH3 metering device, the metering device malfunction or restrictions to flow in the system. The discharge rate error was not measured but was calculated by (Weber, 1993):

(100% + eA R)x(l00% + es) 100

-100% (2)

where eNR = discharge rate error (%) eAR = application rate error (%) e s = speed error (%) To determine which type of metering system was more

accurate, a criterion was established to identify if the error was acceptable. The acceptance criterion used in this study was 5.6 kg-N/ha (5 lb-N/acre) up to an application rate of 112 kg-N/ha (100 lb-N/acre) and 5% criterion for 112 kg-N/ha (100 lb-N/acre) and above. The justifications for using this criterion were that below 112 kg-N/ha (100 lb-N/acre) a 5% error became too restrictive for the operator to set some types of metering systems, and for the limited capability of the metering systems to operate effectively at low flow rates. Typically, the metering systems are less accurate at the low range of its operating capacity. For example, the Hiniker Company (1984) has a delivery system designed to work within the range of

340 to 2724 kg-N/h (750 to 6000 lb-N/h). They state that "lower flow rates are measurable to about 136 kg-N/h (300 lb-N/h) with reduced accuracy".

Above 112 kg-N/ha (100 lb-N/acre) a flat application rate difference becomes too restrictive for the equipment to maintain an accurate speed throughout the application. It becomes restrictive in that the margin of speed error decreases to maintain the 5.6 kg-N/ha (5 lb-N/acre). The limited speed range is too difficult for equipment to maintain during application.

The criterion was expanded to a 11.2 kg-N/ha (10 lb-N/acre) under 112 kg-N/ha (100 lb-N/acre) and 10% for 112 kg-N/ha (100 lb-N/acre) and above. This enables another boundary region for comparison.

A t-test and F-test were conducted to determine if differences exist between "controller" and "regulator" metering systems. These statistical analyses were conducted on application rate errors and application rate differences. A chi-square analysis identified other survey factors that were strongly associated with application rate errors.

RESULTS AND DISCUSSION Sixty-one applicators were observed to determine the

application rate error of NH3. The sample was composed of 44 regulators and 17 controllers. Sixty-eight percent of the regulators were manufactured by Continental while Hiniker manufactured 65% of the controllers sampled. The intended application rate for most regulators (63%) was less than 125 kg-N/ha (107 lb-N/acre) and 41% of the controllers were used in applications over 170 kg-N/ha (151 lb-N/acre). Both metering systems were used across a wide range of intended application rates. The intended application rate was significantly associated with metering system type. Controllers were used predominantly for higher application rates when compared to regulators.

A majority of the applicators were observed (96%) during the sidedress and preplant periods. Sixty-one percent of the regulators were sampled during the sidedress period, and 82% of the controllers were observed during preplant. This may be a reason the controllers sampled had a significantly higher intended application rate. Greater amounts of N are typically applied in preplant operations whereas sidedress applications split the N application between starter fertilizer applied during or before planting and at sidedress.

Seventy-seven percent of all applicators were owned by the operator. All controllers in the sample were privately owned. Controllers require additional equipment be placed in the tractor cab for programming and monitoring and thus limited their use on rental units. Sixty-eight percent of the regulators observed were privately owned.

Maintenance of metering systems include replacement and/or cleaning of metering parts. Sixty percent of the metering systems had not been serviced during the last two years or the operator did not know when the servicing was performed. Half of the regulator system operators did not know when the units were serviced last. Sixty percent of controller metering systems were serviced within the last two years, but some operators (12%) did not know the last time the metering system had been serviced.

Calibration methods were broken into three primary methods: "weight" which uses the known area method with

VOL. 11(2):211-217 213

Table 1. Statistical information on application rate error Metering Type Controllers Regulators

All

No. 17 44

61

Median Max. S.D.(%) (%) (%)

7.4 3.2 25.0 16.0 -3.3 46.9

14.3 -0.4 46.9

Min. (%) -4.7

-41.0

-41.0

the weight recorded from weigh scales; "percent" method uses the known area method by estimating weight from the percent fill gauge on the nurse tank; "formula" uses the manufacturer's formula only. Eighty-eight percent of the operators using of the controllers used the "weight" method. Those using regulators used all three calibration methods equally. There was a significant association between calibration method and metering system type. However, all of the calibration techniques had about the same application accuracy.

APPLICATION RATE ERRORS The sample was composed of 44 regulators and

17 controllers (table 1). Figure 1 shows the distributions of the two different metering systems. The controllers had a uni-modal distribution that was skewed to the right. Controllers had a tendency to overapply with one system applying in excess of 25% more than intended.

Table 1 shows that the application rate error range for regulators was wider than the controllers. A larger standard deviation (16%) indicates regulators were less precise at metering than controllers with a 7.4% standard deviation. Figure 1 suggests that regulators were less precise than controllers.

A statistical F-test determined if "controllers" and "regulators" variances were different. The probability that the two variances were equal was 0.002, which means the variances were significantly different. The t-test showed a significant difference [P(Test statistic > t) = 0.04] between types of metering systems.

APPLICATION RATE DIFFERENCE Controllers had an average application rate difference

(a difference between the measured and intended application rate) of 6.2 kg-N/ha (5.5 lb-N/acre) while the regulators averaged -2.6 kg-N/ha (-2.3 lb-N/acre). Again,

H Controllers p ]^ Regulators

& 9? p $> # . ^ A o S " (P ? P

Application Error (%) Figure 1-AppIication error distributions for regulators and controller metering systems.

o

C 20%

cr

LL

Controllers Regulators

Application Rate Difference (kg-N/ha) Figure 2-Application rate difference distributions for controller and regulator metering systems.

there was a visual difference between the standard deviation, range, and distribution (fig. 2). The controller deviations were smaller than the regulators, showing a more precise distribution (table 2). The mean and median were identical for the regulators. Figure 2 shows that controller systems have a tendency to overapply.

According to statistical F-test analysis, the variances of the "controllers" and "regulators" metering systems were the same [P(Test statistic > F) = 0.14] for application rate differences. Assuming the variances were equal, the population means were compared. The t-test analysis indicated that a significant difference [P(Test statistic > t) = 0.0688] exists between the metering systems.

From the statistical tests conducted, it was concluded that "controllers" and "regulators" metering systems were different at the 0.10 alpha-level.

ACCEPTANCE CRITERION Using the acceptance criterion, 36% of all applicators

were doing an acceptable application while 30% of the applicators underapplied NH3 and 34% overapplied. The regulators (fig. 3) had 27% within the acceptance region, while controllers had 59% (fig. 4). Below 112 kg-N/ha (100 lb-N/acre), 33% of the controllers were acceptable, and 35% of the regulators were acceptable. For 112 kg-N/ha (100 lb-N/acre) and above, 64% of the controllers were acceptable compared to 21% of the regulators. The regulators had application errors that were unacceptable both over and under the acceptance region; while the controllers had application error only above the acceptable region. Using the acceptance criterion, it was concluded that the controller metering systems were more accurate than regulator metering systems.

If the boundary was expanded to 11.2 kg-N/ha (10 lb-N/acre) and 10%, 82% of the controllers and 59% of the regulators were applying within the expanded

Table 2. Statistical information on application rate difference Metering Type Controllers Regulators

All

No. 17 44

61

SX). Median Max. Min. (kg-N/ha) (kg-N/ha) (kg-N/ha) (kg-N/ha)

12.9 3.9 46.2 -9.5 18.0 -2.6 31.5 -57.4

17.1 -0.4 46.2 -57.4

214 APPLIED ENGINEERING IN AGRICULTURE

CO

CD

| 150 d DL < a

CD 100

Unacceptable Error O

Acceptable Error

Acceptance Criterion

Expanded

O ps //Q

_y%6_

O Q '

A>6 O

o

o

6/*

-J

A

O / yr

8

i

50 100 150 200 250 Intended App. Rate (kg-N/ha)

Figure 3-Measured vs. intended application rate and acceptance region for the regulator metering systems. Those outside the solid lines did not meet the acceptance criterion.

large discharge rate error (> 5%). When the chi-square analysis was separated by the type of metering system, the discharge rate error was strongly associated with controllers but not regulators. Controllers had a higher percentage with acceptable application rate error and small discharge rate error (< 5%). Conversely, applicators using controllers in the unacceptable region had large discharge rate error (> 5%). Thus, a controller with an unacceptable application rate error had a very high probability that the metering system malfunctioned, was assembled incorrectly, or information was incorrectly entered into the control console. Application rate errors did not show an association with discharge rate error for regulators. Although, over half of the applicators that had unacceptable error also had large discharge rate error (> 5%).

The chi-square analysis showed that to have a high probability of having an acceptable application rate error, a controller metering system operating within a 5% discharge rate error was needed.

boundary. Eighteen percent of the controllers and 23% of the regulators overapplied the expanded boundary, while 23% of the regulators underapplied.

INFLUENCE OF OTHER FACTORS The chi-square analysis showed that only two variables

were strongly associated with application rate error. These variables were "type of metering system" and "discharge rate error". None of the other variables tested were found significant (0.10 alpha-level), but frequency tables and discussion of these survey factors can be found in Weber (1993). Metering systems were strongly associated because a higher percentage of controllers occurred in the acceptable region than what the chi-square analysis expected. Regulators had a higher percentage occurring in the unacceptable region, but were close to the expected chi-square values.

Discharge rate errors were divided into < 5% and > 5% error regions. Systems with acceptable application rate error had significantly smaller (< 5%) discharge rate error. Applicators with an unacceptable application rate error had

CO JZ 2 200 D) CD

| 150 Q. Q. <

CO CD

Unacceptable Error O

Acceptable Error

Acceptance Criterion

Expanded

7^

50 100 150 200 250

Intended App. Rate (kg-N/ha)

Figure 4-Measured vs. intended application rate and acceptance region for the controller metering systems. Those outside the solid lines did not meet the acceptance criterion.

SPEED AND DISCHARGE RATE ERRORS Speed and discharge rate error can influence the

application rate error. These errors were evaluated based on the type of metering system. Controllers measure the speed and flow rate and changes the flow rate to maintain near the operators intended application rate. Since it was assumed that regulators do not change discharge rates significantly during application, a speed error would have a direct effect on application rate error.

The average speed error for regulators was - 2.8%. This would mean, on the average, the applicator's speed was slower than intended and should result in overapplication. However, the average discharge rate error was - 4.6% which is an underapplication. Indicating the regulators was set too low.

For regulators the speed and discharge rate error distributions were uni-modal and skewed to the right (fig. 5). The discharge rate error range was approximately twice the speed error range. The discharge rate error was influenced by outliers that may have been due to mechanical factors such as stuck valves, worn holes in pressure regulating diaphragms, operator being unfamiliar with the metering system, or the operator selecting the

Speed Error

Discharge Rate Error

I I I I I

.?

wrong setting. Variability in discharge error may have been due to the large gradients on the regulators setting dial. Most regulated metering systems have gradients of 113.5 kg-N/h (250 lb-N/h) making precise adjustments difficult. The discharge rate error was a function of both human and equipment error.

The speed error distribution for both metering systems had a small standard deviation. This can be attributed to the increase in technology for measuring speed such as the use of radar, ultrasonic, or nondrive wheel sensors. The outlier (fig. 5) for speed error is probably due to a calibration error of the applicator's speed sensors.

Controllers had an average discharge rate error of 4.3%. Figure 6 shows that the controllers were functioning well in responding to the speed variation throughout the field.

If the speed CV during a field observation was over 10%, the variation could have a significant effect on the NH3 application. The speed CV is directly related to the application distribution across the field. Regulator systems try to maintain a constant output of material flow, therefore a 10% variation in speed may lead to a variation in application rate across the field. Controllers adjust the flow for the variation in speed, but a high variability in speed may still cause nonuniform application. This nonuniform application may be caused by the response time for the controller. Sixty-five percent of all applicators had speed CVs below 10%.

CONCLUSIONS Sixty-one applicators were observed to determine the

application rate error of NH3. The two types of metering systems observed were "controllers" and "regulators". Seventeen controllers was significantly different from the 44 regulators.

The acceptance criterion for application rate error was 5.6 kg-N/ha (5 lb-N/acre) for application rates below 112.1 kg-N/ha (100 lb-N/acre) and 5% above application rates of 112.1 kg-N/ha (100 lb-N/acre). From this acceptance criterion, the 17 controllers had 59% within the acceptance region while 44 regulators had 27%. All controllers outside the acceptance region overapplied, while regulators had 32% overapplied and 41% under-applied. From the acceptance criterion and the difference in

H 1 1 1 1 1-

Speed Error Discharge Rate Error

S? f> > !$> # > V \ o V Jl? # *>

Nielsen, E. G. and L. K. Lee. 1986. The magnitude and costs of groundwater contamination from agricultural chemicals: A national perspective. Washington, D.C.: USDA-Econ. Res. Serv., Nat. Res. Econ. Kiv., Staff Rept. AGES70318.

Ozkans, H. E. 1987. Sprayer performance evaluation with microcomputers. Applied Engineering in Agriculture 3(1): 36-41.

Quirin, N. L. and W. Wells. 1968. Anhydrous ammonia equipment. In Proc. ofAgric. Workshops on Anhydrous Ammonia, 15-16,25 July. Lincoln, Nebr.

Rider, A. R. and E. C. Dickey. 1982. Field evaluation of calibration for pesticide application equipment. Transactions of theASAE 25(2):258-260.

Shapiro, C. A., R. W. Weber, W. L. Kranz and R. D. Grisso. 1992. In-field determination of anhydrous ammonia applicators in Nebraska. In Proc. North Central Extension!Industry Soil Fertility Conf., St. Louis, Mo.

Varner, D. L., R. D. Grisso and R. C. Shearman. 1990. Calibration of golf course pesticide applicators. Applied Engineering in Agriculture 6(4):405-411.

Weber, R. W. 1993. Accuracy of anhydrous ammonia application. Unpub. M.S. thesis, Biological Systems Engineering Dept., Univ. of Nebraska, Lincoln.

Weber, R. W., R. D. Grisso, W. L. Kranz, C. A. Shapiro and J. L. Schinstock. 1993. Instrumenting a nurse tank for monitoring anhydrous ammonia application. Computers and Electronics in Agriculture 9(3): 133-142.

Wolak, F. J. 1989. Pesticide application survey in South Carolina. Applied Engineering in Agriculture 5(4):514-516.

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