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Calculation of Percentages

Suppose we had known that 256 ha. of the 640 ha area was under maintenance and we wished to know the percent under maintenance, then:

256 ha. = ?% of 640 ha.

Here we know the product and a factor, so we divide the product by the known factor:

256ha / 640ha = 0.4%

but the factor is wanted as a percent,

therefore 0.4 x 100 = 40% under maintenance.

If we know that 40% of the area was under maintenance and that this amounted to 256ha, we could find the total hectares of that area.

40% of the area = 256ha.

Again we know the product and a factor, so divide:

256 / 0.40 = 640 ha.

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Chemical Rates and Quantities

1. To determine the amount of active ingredient (a.i.) needed to mix in the spray tank.

No. of litres or kg =

No. of ha .to be sprayed x kg active ingredient required per ha.
kg active ingredient per litre or per kg

2. To determine the amount of pesticide needed to mix a spray containing a certain percentage of the active ingredient.

No. Of litres or kg =

Litres (L) of spray desired x % of active ingredient wanted
kg active ingredient per litre or per kg x 100

3. To determine the percentage active ingredient in a spray mixture.

Percent (%) =

kg or L of concentrate used (not just the a.i.) x % a.i. in concentrate
Litres of spray x 1kg (weight of water/litre)

4. To determine the amount of pesticide needed to mix a dust with a given percent active ingredient.

kg of material =

active ingredient wanted x kg of mixed dust wanted
% active ingredient in pesticide used

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Fertiliser Application and Actual Nutrient Formula

Potassium sulphate (0-0-50) is expressed as K2O which is 83% (0.83)K. To convert from K2O to K, use the formula:

K = %KO x (0.83),

and to find out how many kg of 0-0-50 are needed to provide 9.4 kg of K, divide 1 by 0.83: 1/0.83 = 1.2 kg, then multiply 9.4 x 1.2 = 11.3 kg of 0-0-50 contain 9.4 kg K.

How many kg of an 18-6-12 fertiliser are required to fertilise a 7,500 m2 green at the rate of 2 kg N per 1,000 m2.

The first step is to determine how many kg of the 18-6-12 are needed to get 1 kg N. Since the 18 is a percentage (0.18) it is divided into 1. 1/0.18 = 5.6

This means that 5.6kg of 18-6-12 contains 1 kg of N

Your answer may be checked by multiplying the answer, (5.6) by the percent N (0.18).

5.6 x 0.18 = 1.0 kg N

Since 5.6 kg of 18-6-12 will provide 1 kg N, then 2 x 5.6 kg = 11.2kg will provide 2 kg N.

Calibrating a Boom Spray Unit

Your final calibration result will rely on three factors:

1. Speed of travel
2. Operating pressure
3. Nozzle size

1. When you have arrived at a comfortable travel speed, taking into consideration the area you will be spraying, time how long it takes to cover 100 metres (say 40 seconds).

2. Select a set of nozzles and pump pressure (most modern spray units have guidelines for both in accompanying hand books).

3. With clean water, run the spray boom with the selected nozzles and measure the width of the sprayed area (say 4 metres).

4. While the boom is running, slip a measuring jug under a nozzle and hold it there for the amount of time it took you to travel the 100m (40 sec. in our example). Do this with all the nozzles on the boom and record the respective volumes in litres. This will also show up any variation in flow from nozzle-to-nozzle. By adding the volumes together, you will obtain the overall total volume of water used in that time.

e.g

Nozzle 1	1.81 litre
Nozzle 2	1.20
Nozzle 3	1.22
Nozzle 4	1.21
Nozzle 5	1.91
	6.00

There should not be much variation in water used from each of the nozzles. If there is, check the filters, hoses and nozzles for blockages. Also check for wear in the nozzles.

Multiply the total volume of water used by 100, divide it by the effective spray width to obtain the rate per hectare.

e.g. (6 x 100) / 4 = 150 litres per hectare

Thus, to spray 1 hectare at our designated speed, pressure and nozzle selection, we need 150 litres of spray mixture.

If your area to spray is smaller, or larger, multiply the spray rate by the area factor.

e.g (a)	or (b)
1/2 hectare	2 1/2 hectare
150 x 0.5	150 x 2.5
= 75 litre	= 375 litre