Not always relevant as you may be getting soil from a garden centre at the same time you could be out in the back of nowhere or wanting to use your own soil. Identifying your soil types and the types of plants/crops they can be used for will improve your chance of good yields. At the same time doing a bit of research also means you can adapt the soil to the purpose of the types of plants you want to grow by adding in whatever nutrients the soil could do with being added if any.
Testing soil is necessary and easy to do, doing it at the beginning also means you can adapt it quickly without plant disruption and more importantly won’t suffer with failed plants due to the wrong type of soil used for them. This is why soil testing is the first step in your container garden and you will need to know that it has the right nutrients for certain plants to flourish.
This is also a good way to monitor your soil for its nutrients, the soil will need Nitrogen, Phosphorus and Potassium to continue providing healthy plants. Where possible take the test at the end or a few months before the growing season (plan to plant) to correct the soil problems. Also just as important the soil should be dry as false results could be given otherwise.
There are various test kits available and some are easier to use than others. I would advise checking reviews on places like Amazon to see how other people found them. Some kits are also designated to only test specific nutrients.
Now as this is a container garden and not a general household garden or field I would say try and select soil from areas that you know have good nutrients already (as obviously you don’t want to be repairing an entire field with nutrients when your only needing your container soil improved).
Grab a bucket and a shovel and head to where your looking to select your soil from. Dig around 7” deep with 6 holes and take a shovel of dirt from each hole dug. When dug mix the samples together and remove 2 cups from your test sample.
Then read the guidelines given to you by the test kit purchased.
Soil test completed you are no doubt going to have some results at the same time going to be looking at transferring your soil to containers. The problems your likely to face are below, some of them it would be easier to select soil elsewhere but pretty much everything can be dealt with.
High PH: Mix some sulpher into the soil.
Low PH: Mix some lime into the soil.
Low Nitrogen: Mix nitrogen rich fertilizers into the soil.
High Nitrogen: Water well, and avoid adding fertilizers for three to six months.
Low Phosphorus: Mix super phosphate and bone meal into gardening area.
High Phosphorus: By growing more plants will aid in absorbing extra phosphorus
Now there is also the chance of the figures being out simply because there are other layers of soil below the sample level that could have affected the results. If using the field these are relevant but as we are taking the soil away for planting they won’t be causing anymore disruption in future but will also be a reason why you monitor the soil on a regular basis to get its levels right.