This article explains how to convert ‘Rate per Hectare’ into ‘Rate per 100L’ (or per 15L, or any other water volume).
When spraying with a boom, several variable factors (spray nozzles, pump flow rate, driving speed, foliage density, operator perception of what’s ‘damp’ enough, etc), all combine to mean that one bloke may use say 80L of water to cover a hectare, and the next bloke 120L.
But the different volume of water they use doesn’t matter, as long as the correct amount of the actual chemical is added to the tank. Bloke A will add 2L of, for example, GrassMate to his 80L of water and spray the lot onto one hectare. Bloke B will add 2L of GrassMate to 120L of water, and spray the lot onto one hectare.
In both cases, the hectare receives the correct 2L of GrassMate. The different water amount doesn't really matter at all.
Most people already know what volume of water it takes them with their boom gear to cover a given area at the desired degree of ‘wetness’. And that desired wetness is generally to the point of the leaf surface being visibly damp, but not soaking. Just short of the point of run-off is about right.
Handgun & Knapsack
But people who are doing area-type spraying with a handgun or a knapsack don’t generally know how much water it takes for them to spray a given area. That's because those types of equipment are generally used for spot spraying, not area spraying.
It certainly can be done: spraying an area with a knapsack or a handgun. Quite a few of our customers have done it quite satisfactorily. However, they must know in advance how much area they will cover with, for example, a 15L spray tank, to get that desirable degree of wetness.
The only way to find out is to conduct a small trial first, using just plain water (no chemical) via your knapsack or handgun. Spray a given area, for example 25 x 20 metres, and see how much water you used, OR spray a given amount of water, for example 5 litres, and see how much area it covered. Either one will do.
Let’s say you sprayed 5L of water and it covered exactly 25 x 20 metres. That’s 500 square metres, which is 5% of a hectare, or a 20th of a hectare. If the published rate per hectare is 2L of chemical, then the rate per 5L of water according to that personal calibration trial would be one 20th of 2L, which is 100ml.
For a 15L knapsack, you’d therefore use 300ml of the chemical (there being three times 5L in 15L). That will give you the correct 2L per hectare.
That's quite a few figures, but the good news is that you need only do it the once. When you know your personal area coverage for a given volume of water, you can easily calculate the correct amount of any chemical that has a ‘Rate per Hectare’ given in the directions.