Fertigate for soil health and profit – Part 9: Examples for the calculation of injection rates

by Frikkie Koegelenberg (Pr ENG) and Gert Conradie (Pr Eng)

In the previous entry of this series we examined some examples of the calculations of fertiliser injection rates in static irrigation systems. In this edition we shall discuss the application of fertiliser in moving irrigation systems such as pivots and moving big gun sprinklers.

The application of fertiliser for a moving system must be adapted to the revolution speed. The revolution speed must be physically determined in the field under wet and dry conditions and at different spots in the field if the slope differs. To prevent leaf-burn in moving systems, it can generally be accepted that fertiliser concentrates of 0,2% in the irrigation water is safe. The following must be kept in mind when fertigation is done with moving systems:

•The storage tank for the fertiliser mixture must be large enough to keep enough of the mixture for an entire revolution.

•An agitator pump must agitate the mixture continuously during the application process.

•The injection pump must be switched off after a revolution is completed, but the pivot must still irrigate until the system is flushed clean.

•The calculation for the application by the end sprinkler of a pivot that switches on and off from time to time, is very difficult and can only be done effectively by means of manual control.

•A big gun or canon sprayer is very wind sensitive and should not be used for fertigation during windy conditions.

Example 3 (Wilson, 1996)

The flow rate of a pivot with a length of 340 m and an overhang of 10 m, is 160 m3/h. The discharge of an end sprinkler that irrigates continuously, is 40 m3/h and irrigates a radius of 45 m. The pivot’s revolution speed was measured as 1,7 m/min. 50 kg of nitrogen/ha must be applied 3 times at the beginning of the irrigation season. The fertiliser advisor suggests urea in granular form.


Area under irrigation

Total wetting radius (r)

= 340 m + 10 m = 395 m

Area = π r2

= π × 3952

= 49 ha

Quantity of fertiliser to be applied:

According to the fertiliser advisor, urea has the following properties:

– 46% N

– solubility

= 108 g/100 g H2O by 20°C

= 1,08 kg/ℓ

Total quantity of fertiliser required

= 109 kg/ha × 49 ha

= 5 341 kg

Size of storage tank required:

A tank of 5 000 ℓ is available, therefore the concentration of the mixture is 1,07 kg/ℓ.

Fertigation tanks can be linked into the system to automate the process. Photo:

Revolution time of pivot:

Circumference of circle

= 2 π r = 2 482,8 m

Injection time:

From equation 1:

Example 4 (Wilson, 1996)

A big gun with a sprinkler that delivers 56,7 m3/h, is used to irrigate a maize field of 400 × 365 m. The wetted diameter of the sprinkler is 105 m. The strip spacing is 60 m and the measured running speed of the machine is 15 m/h. The farmer wants to apply liquid ammonia nitrate (21%) through one application of 50 kg nitrogen/ha during spring.


Area under irrigation

Area = strip spacing × length of the strip

= 60 m × 400 m

= 2,4 ha

Quantity of fertiliser to be applied:

According to the fertiliser advisor, ammonia nitrate has the following properties:

– 21% N

– density = 1,27 kg/ℓ

Total quantity of fertiliser required:

= quantity ammonia nitrate/ha x area

= 187 ℓ/ha × 2,45 ha

= 458 ℓ

The storage tank should be able to hold at least 500 ℓ of the solution.

Application time:

The big gun must irrigate the entire 400 m.

The length of the supply pipe

= strip length – wetted radius of sprinkler

= 400 m – 105/2 m

= 347,5 m

Injection rate:

Concentration of fertiliser mixture in the system:

This concludes our series on fertigation. Hopefully you will be able to use this information to produce higher yields.

Next month we shall start with a series on compost and composting systems. We thank the ARC Agricultural Engineering in South Africa who made the information on fertigation available to the readers of ProAgri Zambia. Visit for more information.

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Irrigation made easy – Part 3: Water distribution