Mychorrhizal fungi

Last year, I bought some soluble myco fungi in a box.  I'd never heard of it before.  I used it as directed to water the plants in the old GH border when planting out and when removing the old plants in autumn I was pleased to see lots of white fungi strands within the  soil, so the fungi was thriving, hopefully doing its job.    I will need to improve the border soil in the new GH but I'm loath to use the horse  manure I have, as it's very seedy.  I'll probably make manure tea with it and use it that way.  I have my own compost to incorporate as well and will give the plants a dose of fungi.
My query is, do I have to repeat treatment of the fungi each year or is it a one time thing? Should I dose the old GH borders at planting a second time?