I'd start a week before each spray timing to give you time to get the results back and then add stuff into the tank.
Be very cautious of just single tests and even one years tests aren't really that good, you really need multiple years of results all taken at similar times through the years to be able to build a true picture of what you need or not.
Remember a tissue test is only a snap shot of that plant on that day test the same plant a week later after a warm spell ect and you'll most likely get a different result.
It took longer than a week to get my test results back from NRM. Usually 10-14 days. I did mine 2 weeks before a planned spray pass to allow enough time to get the results back & buy what I needed to add. Avoid posting samples on Thursday or Friday as they could be sat sweating in a mail sorting office all weekend.
On high pH soils, boron, copper & potash always came up as low. Whether they actually reduce yield is another matter. As Robbie says, you will start to get a longer term picture build up through repeated testing.
It’s up to you. Some do it before every planned spray pass and again with grain samples at harvest. I just did a few samples of each crop on different soil types during stem extension, more for a comparison than anything else.
If you’re a member of a buying group it’s worth asking them if they have an account. An individual gets charged a different rate in some labs.
Arable Conversations: Alternative markets and new cash crops 11.30 – 12.30 on 29th June 2021
Join us at Arable Scotland on 29 June at 11.30 am for an interactive ‘Arable Conversation’ that will discuss ‘Alternative markets and new cash crops’. The session will cover the seasons market potential and the current rollercoaster grain prices. New crops being trialled in Scotland, opening new doors and opportunities for the cereals sector will also feature. Chris Leslie from AHDB, will lead the discussion and be joined by the following panellists.