*By: Saqib Mukhtar: Assistant Professor and Extension Agricultural Engineering Specialist Waste Management; The Texas A&M University System*

Effluent from animal manure and wastewater impoundments is often applied to field crops and pastures using big gun nozzles and sprinkler systems. This effluent contains nitrogen (N), phosphorus (P), potassium (K) and micronutrients essential for plant growth.

When managed properly, irrigation of crops with effluent reduces a producer’s reliance on commercial fertilizers and helps protect surface and groundwater quality. To use effluent efficiently while avoiding over irrigating it on crop and pasture land, producers must know:

- The N, P and K needs of the crop, based on a realistic goal for yields.
- The N, P and K in the soil available to the plants before irrigation.
- The amounts of N, P and K in the effluent that will be irrigated.

To determine the N, P and K contents in their soils, producers should collect soil samples and have them tested. The soil test gives estimates on whether the soil contains enough N, P and K to produce the targeted yield of a crop. Producers should also have the effluent tested for concentrations of N, P and K (fertilizer value).

Sampling procedures for soil and effluent are explained in detail in Extension publication L-5175, “Managing Crop Nutrients Through Soil, Manure and Effluent Testing.”

Table 1 lists the number of gallons of effluent that are applied by big gun nozzles or sprinkler systems, based on various wetted areas (acres) and at depths of 1/4 inch to 2 inches.

To estimate the depth of the application, use a plastic rain gauge or a straight-sided container (a coffee can, for example) marked with gradations of 0.25, 0.50, 0.75, 1, 1.25, 1.50, 1.75 and 2 inches on the inside with a permanent marker.

For a better estimate of irrigation depth, place several gauges equally spaced over the wetted area. Once you know the total application volume and its N, P, and K contents, you can calculate the total amounts of these nutrients applied (see example).

Knowing how much N, P and K are in the effluent will help you target your effluent applications and volumes to meet your yield goals. It will also help you avoid applying too much of the most restrictive nutrient, such as P, required by a nutrient use plan.

For diameters or areas not listed in Table 1, the volume of effluent for a given depth can be calculated using the following formulas. Some conversion factors are provided in Table 2.

For a known diameter, V = 0.489 x d x D2 .

For a known area, V= 27,152 x A x d.

V = volume of effluent in gallons.

D = diameter covered by the big gun in feet.

d = depth of effluent in inches.

A = Area covered by a sprinkler system in acres.

### Example

A farm uses big gun nozzles that cover a diameter of 240 feet. (Or if it were using a sprinkler system, the area covered is 1.04 acre.) The effluent is applied to a depth of 1 inch.

The laboratory test for the dairy lagoon effluent shows:

N = 0.03 percent

P = 0.01 percent

K = 0.04 percent

Use these steps to figure out how much of each nutrient was applied to the soil.

**Find the amount of area covered by the big gun or sprinkler system.**

In this example, the diameter covered by a big gun is 240 feet (the area covered by the sprinkler system is 1.04 acres).

**Using a rain gauge or coffee can, determine the depth at which the effluent was applied.**

In this example, the depth is 1 inch.** **

**Check Table 1 to find out the total volume applied to the area at the effluent depth.**

Table 1 shows that at an effluent depth of 1 inch, the total volume applied at the big-gun diameter of 240 feet (or the sprinkler area of 1.04 acres) is 28,199 gallons.

**Have the effluent tested to find out the N, P and K percentages in the effluent.**

The percentages for this example: N = 0.03 percent P = 0.01 percent K = 0.04 percent

**Use the formula and the percentages from the laboratory tests to calculate the total nitrogen (N), phosphorus (P) and potassium (K) in the effluent.**

For this example:

N (lb) = % N x volume applied x 0.0834

Hence, N = 0.03 x 28,199 x 0.0834 = 70.6 pounds

P (lb) = % P x volume applied x 0.0834

Hence, P = 0.01 x 28,199 x 0.0834 = 23.5 pounds

K (lb) = % K x volume applied x 0.0834

Hence, K = 0.04 x 28,199 x 0.0834 = 94.1 pounds Where 0.0834 is a constant multiplier.

**Convert P (phosphorus) to P2 O5 , (phosphate), which is the form of phosphorus that is sold in commercial fertilizers.**

The conversion formula from Table 2 shows that P2 O5 = P x 2.29

Therefore, in this example, P2 O5 = 23.5 x 2.29 = 53.8 pounds 7

**Convert K (potassium) to K2 O (potash), which is the form of potassium sold in commercial fertilizers.**

From Table 2, K2 O = 1.2 x K

Therefore, for this example, K2 O = 1.2 x 94.1 = 112.9 pounds

According to these calculations, the farmer in this example has applied these nutrients to the crop:

70.6 pounds of nitrogen

53.8 pounds of phosphate

112.9 pounds of potash

It is estimated that nearly 75 percent of the total nitrogen is available during the crop growing season, while the other 25 percent is being mineralized and will be available for the next year’s crop. Therefore, 75 percent of 70.6 (0.75 x 70.6) = 53 pounds.

Also, as much as 33 percent of this available nitrogen may be lost to the atmosphere in the form of ammonia during surface application of effluent with big gun or a sprinkler system. So, reduce the amount of nitrogen available by an additional 33 percent. The reduction from 53 pounds of available nitrogen (1 – 0.33 x 53) = 35 pounds. Hence, the plant-available nutrients in the effluent are:

N = 35 pounds

P2O5 = 54 pounds

K2O = 113 pounds

## Take credit

Once you know the amount of each nutrient you have applied via the effluent, you can subtract it from the amount that the soil test indicated is needed by the crop to achieve the desired yield. The amount needed minus the amount already applied is the amount of nutrients you need to apply using commercial fertilizer.

Using this procedure will prevent you from applying too much fertilizer, wasting money and harming the environment.

**Download a printer-friendly version of this publication: Using Animal Manure and Wastewater for Crops and Pastures: Know and Take Credit for your N, P, and K**

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