You can opt to spread in one pass or two overlapping passes at half the rate. Two passes is likely to give a better result. Either way you will need to calibrate the spreader accordingly.
The Cresco 20SW spreader has a numerical scale, the setting is dependent on the granule size and density. The fertilisers that I supply are virtually all the same consistency so once you have calibrated the spreader you won't have to change it again. I use a setting of 9 on the scale provided. This delivers about 15 to 16 gsm and I always do a double pass so the toatal amount applied is 30 to 32 gsm.. You may need to make small adjustments because your walking speed may differ from mine.
For other spreaders you will need to calibrate the spreader. Here are two ways to calibrate - I have tried both but prefer the second - the lazy way!
The scientific way.
To calibrate you need a length of polythene or tarpaulin about 4 metres by 10 or 15 metres. A 10 metre tape measure. A set of hanging scales and bucket.
1/. Weigh 5kg of fertiliser into bucket.
2/, Pull lever on and walk at normal speed for 10 metres over polythene then immediately switch off.
3/. Tip remaining fertiliser from spreader into bucket and weigh to establish how much fertiliser has be used.
4/. Measure spread width.
5/. Calculate application rate: Weight used divided by area covered (Spread width x 10 metres). So if 500 grammes was used and the spread width was 10 metres the result would be 500 divided by 30 = 16.67 gsm. You want a rate of 16 gsm so adjust the lever setting accordingly. You can then re-run the test at the new setting. When you have finished you can retrieve the fertiliser from the polythene.
The lazy way
Weigh 5kg of fertiliser into a bucket and pour into the hopper. With a setting well below medium walk the spreader the length of one lawn and get an assistant to measure the spread width. Now calculate the area covered. This is 32 metres (the length of one lawn) x the spread width. If the spread width was 3 metres, the area covered is 32 x 3 = 96 sqm. Tip out the remaining fertiliser from the hopper into the bucket and weigh again to see how much was used. If 2kg was used the application rate was 2000 grammes divided by 96 sqm = 20.83 gsm. Adjust the setting on the spreader accordingly and repeat the process but move one spread width across the lawn so that you don't double dose the lawn. Very soon you will have arrived at the correct setting and shouldn't have to make further changes. You may end up with a slightly uneven application where you started but if you approach this with a cautious setting to begin with no harm will be done.
I find this is the easiest way and is more accurate than the scientific way because the area covered is so much larger.
Calibrating the Brilliant Broadcast Sprayer
You need to judge your speed so that the tank runs dry at the right time. It takes 10 minutes for the sprayer to deliver 50 litres If you do a double pass over a full size law, you need to cover the length of the lawn in 37.5 seconds (I aim for 35) but test this with water first.
To apply accurately you should use bout markers. These can be anything from bamboo canes to flower pots. I use electric fence posts which have sharp spikes and are easy to use. They are available online for about £10 for ten. At croquet clubs it is a good idea to have permanent bout markers, these can be painted on boundary boards (see the picture). The markers are set 10 feet (3 metres) apart for the Cresco spreader and the 12 feet (3.65m)for the Brilliant Sprayer.
Spreading with a Cresco 20SW
With a spreader, the first pass is marker to marker and the second pass is gap to gap which ensures an even overlap. Then spread the 4 yard wide strips at the North and South boundaries. For the second pass repeat the process but instead of going gap to gap, you go marker to marker thus overlapping the previous application. Spread the 4 yard strips at the North and South and you will be left with two 1.5 yard strips down the East and West boundaries. Set the 20SW border slide so that it only spreads on the right hand side and apply to those strips. With some products the spread can be more to the right than the left, this is easily compensated by aiming slightly left of the bout markers.
Spraying with a Brilliant Sprayer
Bout markers should be placed at 4 yard intervals. I recommend two overlapping passes similar to above. The first pass is gap to gap and the scond in marker to marker. At the East and West boundaries a half bout should be sprayed by turning the flow valve down. I usually keep the sprayer going at the turns, it does mean that there is a measure of overspraying but it is a lot easier and you don't have to do the end strips separately. On that basis, there are 16 lengths of the lawn plus two lengths at half flow. The exact time per length to spray 50 litres is 35 seconds end to end - the sprayer delivers 50 litres in 10 minutes. It is best to practice with water first.
WARNING It is important to rinse the sprayer thoroughly after use. Be sure to pump plenty of clean water through the pipes and pump so that no product is left in the sprayer. This is particularly important if soluble iron is used because it can recrystallise and damge the pump.