Best time of the year to soil sample
My advice is to test for soil nutrients at the time of the year when nutrient levels are likely to be at their lowest. In temperate climates in which there is some precipitation during the summer, then this is an autumn timing. Autumn is a good time to test for another reason. When you can do soil tests in the autumn, that gives all winter to make a nutrient plan for the next year.
In temperate climates without much precipitation during the summer, but with precipitation during a winter when grass is not growing rapidly, the soil nutrient levels will be the lowest at the end of the rainy season. Places with a Mediterranean climate have such growing conditions, and in these places the best time to soil sample is at the end of the rainy season.
For locations where the grass grows all year, the best time to sample is at the end of the rainy season. If you want to test twice a year (that’s rarely necessary), then I’d test at the end of the rainy season and at the end of the dry season.
For locations where the grass grows all year and where there are no distinct wet and dry seasons, I recommend doing soil tests at the start of the busiest season for golf or sports play—test immediately before the period of heaviest use.
Those recommendations above are for soil nutrient testing. What about total organic material (OM246) testing?
I recommend doing OM246 testing at the start or at the end of the peak growing season. I prefer autumn in temperate climates, because that timing allows plenty of time to plan the sand topdressing and cultivation plan for the upcoming year.