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Agronomic Services

Dairy Lagoon Sampling Study

by Robin J. Watson, NCDA&CS Regional Agronomist


The cost of agitating a dairy lagoon and the need for specialized agitating equipment often prevents farmers from obtaining a lagoon analysis prior to field application. Most farmers do not agitate and sample their lagoons until they are ready to apply the waste to the field. Application strategies are thus based not on the nutrient content of the applied waste, but on the results of the previous lagoon sampling. Because nutrient levels can change drastically each time a lagoon is emptied, the risk of mismanagement is great. This study outlines a practical and accurate way to sample lagoons without agitating them. A comparison of agitated and nonagitated samples revealed no statistical difference in measurements of P, K, Ca, S, Fe, Mn, Zn, Cu, and B. Although results for Mg and N differed statistically, the differences were not large enough to affect recommended application rates.

Basing application rates on the analysis of agitated lagoon samples may represent a management ideal, but cost and equipment constraints often make such analyses impractical for dairy farmers, most of whom agitate and sample their lagoon only when they are preparing to empty it. As a result, unanalyzed wastes are frequently applied to the land. Many farmers based their application rates on the previous lagoon sampling—a less-than-ideal circumstance given that the nutrient content of the lagoon tends to alter each time it is emptied.

Farmers need an accurate pre-agitation sampling procedure capable of providing analytical data before wastes are applied. The purpose of this study was to determine the accuracy of one such procedure.

Materials and Methods

Lagoons consisted of hay, manure, water, urine, baling twine, and other materials. Some lagoons had a thick crust while others were primarily liquid with little floating debris. All pre-agitation samples were taken fewer than 30 days before emptying the lagoon. These samples were compared with post-agitation samples taken from the upper third of the lagoon. After agitation, the consistency in the middle and lower thirds of the lagoon was also evaluated.

Sampling materials consisted of one 1.5-inch piece of PVC pipe, 10 feet long, with a ¼-inch hole drilled in one end; a piece of baling twine; a liter-sized heavy plastic drink bottle with the top portion cut away; a plastic bucket; and a quart-sized plastic container.

The sampling apparatus was constructed by threading baling twine through drilled openings in the PVC pipe and securing the loose ends to each side of the lower half of the drink bottle. The space between the PVC pipe and the top of the drink bottle was about 12 inches. A plastic bucket was used to collect individual samples for mixing. Once the samples were thoroughly mixed, portions of the mixture were placed in the quart-sized plastic container.

Lagoon samples were obtained by lowering the sampling apparatus into the lagoon, thereby pushing the cup through any top crust that might be present. Approximately 12 seconds were allowed for the contents to equilibrate. The cup was then pulled out of the lagoon, and the contents were poured into a bucket. No concern was given to any foreign debris present during sampling. This method was repeated in five other randomly selected locations around the lagoon. Precautions were taken to avoid sampling near the unloading ramp. The contents were then mixed, and a portion was poured into the collection bottle.

Samples were analyzed, using standard waste-analysis procedures, by the Plant, Waste, and Solution laboratory of the North Carolina Department of Agriculture's Agronomic Division. Tests were performed on 12 lagoons. Analytical determinations were made for the primary nutrients—N, P, and K; secondary nutrients—Ca, Mg, and S; and micronutrients—Fe, Mn, Zn, Cu, and B.

Results and Discussion

For N and Mg, differences in the pre- and post-mix samples were significant at the 5% level using a paired-sample t test. Differences in P, K, Ca, Fe, Mn, S, Zn, Cu, and B were not significant. Pre- and post-mix concentrations for N, P, and K are shown in Figures 1–3. The data indicate that the wide variability in nutrient content between various lagoons. The need for laboratory analysis to determine application rates is clear. Figure 4 indicates pre- and post-mix N% from lagoon 9 at various depths; there is no significant difference between depths.

Figure 1. Bar graph of nitrogen in lagoon samples.
Figure 1. Nitrogen in lagoon samples.

Figure 2. Bar graph of phosphorus in lagoon samples.
Figure 2. Phosphorus in lagoon samples.

Figure 3. Bar graph of potassium in lagoon samples.
Figure 3. Potassium in lagoon samples.

Figure 4. Bar graph of sampling depth and nitrogen in lagoon 9.
Figure 4. Influence of sampling depth on percent nitrogen in lagoon 9.

Summary and Conclusions

Agitation affects N and Mg statistically, but it does not alter management recommendations. Nitrogen concentrations for pre- and post-mix samples varied an average of 5.1 lbs/1000 gallons of liquid waste. Pre- and post-mix concentrations for all other elements were identical. These findings suggest that the pre-mix sampling procedure described above provides an accurate alternative to post-mix sampling. It is desirable to take samples within 30 days of emptying the lagoon. This would ensure that the analytical results would be received in adequate time.


I would like to express my sincere appreciation to Mr. Ed Chapman of Pumping and Spreading Service, Inc., the NCDA Agronomic Division staff, and the individual dairy farmers that made this study possible.

Watson RJ. 1994. Dairy lagoon sampling study. In: Kelley JA, editor. Proceedings of the 37th annual meeting of the Soil Science Society of North Carolina; 1994 Jan 18–19; Raleigh (NC). Raleigh (NC): Soil Science Society of NC. p 51–6.


Last Update July 9, 2007


NCDA&CS Agronomic Services Division, Colleen M. Hudak-Wise, Ph.D., Director
Mailing Address: 1040 Mail Service Center, Raleigh NC 27699-1040
Physical Address: 4300 Reedy Creek Road, Raleigh NC 27607-6465
Phone: (919) 733-2655; FAX: (919) 733-2837

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