Frikkie Koegelenberg Pr Eng and Gert Conradie Pr Eng

The manner in which the injection rate is calculated is drastically influenced by the type of irrigation system used. With static systems such as micro and sprinkler irrigation systems, the injection rate is fixed, and the quantity of fertiliser required is calculated and applied according to time.

The quantity dissolved in the tank is therefore the quantity applied on a specific area. If different sizes of blocks are treated with the same injection equipment, the fertiliser must be applied proportionally by an injection pump.

With moving systems such as pivots and water cannons, the application varies and fertiliser can only be applied proportionally. With the proportional method, the injection rate is very important, because it determines the quantity of fertiliser applied to a specific area. When calibrating the injection pump, the speed of the system and the area on which the fertiliser mixture is applied, must be thoroughly considered.

Legend of equasion variables:

Static systems

Example 1 (Wilson, 1996)

A permanent sprinkler system with a sprinkler spacing of 18 × 12 m, and sprinkler delivery rate of 1 750 ℓ/h (Dr) must be used to apply 50 kg/ha of nitrogen in the form of water-soluble calcium nitrate in a fruit orchard. The sandy loam soil’s easily available soil water capacity is 150 mm/m. The nitrogen must be applied to a depth of 300 mm. The effective root depth is 1,0 m. Twenty sprinklers (S) irrigate simultaneously and the standing time is 12 hours.

Answer

(A) Area irrigated per setup

20 sprinklers × 18 × 12 m = 0,43 ha

(Qk) Quantity of fertiliser applied

Calcium nitrate has the following properties, according to fertiliser experts:

-15,5% N (c)

-solubility

= 120 g N/100 g H2O by 20°C

= 1,2 kg/ℓ (K)

To apply 50 kg/ha nitrogen (g), the following quantity of calcium nitrate is required:

(Q) Total quantity of fertiliser required per setup

With a solubility of 1,2 kg/ℓ, the minimum quantity of water required will be:

A tank with a capacity of at least 125 litres, to provide for the increase in volume as a result of the mixing process, is required.

(IT) Time required for fertiliser injection

For a sprinkler system, it is recommended that at least 60 minutes injection time (T) is used to ensure that the fertiliser is applied as uniformly as possible.

Injection rate

Concentration of the solution

A concentration of < 1% is usually required. The calculated value below therefore complies with the guideline.

Depth of infiltration in the soil

The required depth to apply nitrogen, is 300 mm.

The application of calcium nitrate must begin after 6,45 hours of irrigation standing time has elapsed and must be applied for 1 hour. The irrigation continues after this for 5,55 hours, which means that the fertiliser has penetrated to a depth of 300 mm into the soil.

Example 2 (Wilson, 1996)

A drip irrigation system is used to irrigate a strawberry field. A liquid fertiliser solution (nitrogen and phosphates) must be used. The spacing of the strawberries is 1,2 × 0,3 m. The length of the rows is 90 m (300 plants per row). The discharge of the drippers is 2 ℓ/h and the dripper spacing is 0,6 m.

Each irrigation block consists of 25 rows. Standing time per block is 1 hour. The producer wishes to apply 1 g of nitrogen and 1,5 g phosphate per plant in five equal parts, weekly. Ammonia polyphosphate is recommended by his fertiliser advisor as a liquid fertiliser solution.

Answer

Number of plants per block

= 25 rows × 300 plants per row

= 7 500 plants

Amount of fertiliser to be applied with each application

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

• 10% N

• 15% P

• density = 1,37 kg/ℓ

Amount of the type of fertiliser required:

Approximately 11 litres of the fertiliser solution are required.

Injection rate

The fertiliser must preferably be applied for a 20 minute period, 30 minutes after the irrigation has begun. This will ensure that the system is at pressure before ap-plication, and secondly, sufficient flushing time will be available after application.

The maximum concentration of the fertiliser mixture must preferably not exceed 1%.

The concentration of the fertiliser mixture in the system is:

The injection rate is therefore acceptable because it is smaller than 1%.

We thank the ARC Agricultural Engineering in South Africa who made the information on fertigation available to the readers of ProAgri Zambia. Next month we shall discuss the application of fertilisers of moving systems. Visit www.arc.agric.za for more information.