I would like to know if there is a maximum amount of compost that can be safely added to garden soil. I am particularly interested in transplanting rhubarb.
Your question on how much compost is enough is an interesting one because it raises the question of the kind of soil nutrients that are found in different types of compost, and their effects on soil and on the plants growing in it. For a general discussion of the many important benefits of compost, here is a guide produced by the Toronto Master Gardeners: https://www.torontomastergardeners.ca/gardeningguides/compost-for-your-organic-garden-a-toronto-master-gardeners-guide-2/. Most home compost bins will contain compost comprised of plant materials only; commercial products are often in the form of composted manure. The nitrogen-phosphorus-potassium balance found in plant-based composts is considered to be just about right for ornamental plants and vegetables, whereas the exclusive use of composted manure may be too high in phosphorus. One garden writer describes the reasons for this here: https://www.gardenmyths.com/compost-is-it-poisoning-your-garden/. For a useful list of soil amendments for organic gardeners, here is another guide produced by the Toronto Master Gardeners: https://126.96.36.199/~torontom/wp-content/uploads/2012/07/Improving-Your-Soil-Organically.pdf
The Compost Council of Canada’s website is another great source of information. It suggests adding one inch of finished compost each spring and fall as part of your ongoing garden maintenance routine.
Now to the specifics of your rhubarb question: dividing or transplanting rhubarb should be done in very late fall, and compost will help to give your transplanted rhubarb a good start. To move your rhubarb plant, loosen the soil about 6 inches around the crown, and to a depth of about 8 inches, so that the roots are protected when you gently lift the entire plant. At this point you can divide it into smaller clumps if that is what is needed. You may add an inch of compost directly into the new planting site to increase soil nutrients. A layer of compost or mulch of about 2 inches around the new planting will help to retain moisture in the soil. Make sure your compost is not touching the crown of the plant. Your transplanted or divided rhubarb may take a couple of years to re-establish itself before giving you a regular harvest.