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Top soil, organic matter and advice for planting new plants

TheGardenerFromMarsTheGardenerFromMars Posts: 65
in Plants
I've read various guidance about adding organic matter to the planting hole when you're planting new plants (compost or manure), but if the soil is in particularly bad condition can you fill the hole with new topsoil, and if you did, would you still need to add some compost or manure to the hole when planting?

How important is it to adjust the amount of organic matter that you add to the planting hole for different types of plants? For example, if a plant likes moist soil should you add a lot more compost or manure to the hole when planting?

If a plant likes good drainage should you add sand or horticultural grit to the hole?

And for plants like salvias and hebes that apparently do quite well on poor soil, how poor is 'poor'? Would you ever add organic matter to the soil for these plants?

Finally, how do you decide when to and when not to apply products like Rootgrow to the hole?

That's quite a lot of questions, but I'll be doing some planting soon and want to understand these things to give the plants the best start possible!

Posts

  • JennyJJennyJ Posts: 10,576
    edited August 2022
    It's better to improve a wider area rather than just a planting hole, whether it's organic matter or grit or new topsoil that you're adding.The reason is that in dry sandy soil like mine, the plant's roots won't venture out of the improved pocket of soil which is effectively like keeping it in a pot and it'll suffer eventually, and with clay soil you're creating a sump where water will collect, like a pot with no drainage holes.
    The jury is out on rootgrow, but if you use it, it's for shrubs/trees, and you dust it directly onto the roots of the plant when you take it out of it's pot, not into the soil. I've only used it for trees and larger shrubs, never for small stuff like salvias and hebes. They've done fine without it.
    I do improve the soil with organic matter whenever I'm replanting an area but I don't add more if I'm putting in an individual plant. My soil is poor, dry and sandy with plenty of grit and pebbles of all sizes in it so I never add any more grit or sand. I add organic mulch on top of the soil in spring if I have enough available. Usually just a few small areas each year.

    Doncaster, South Yorkshire. Soil type: sandy, well-drained
  • TheGardenerFromMarsTheGardenerFromMars Posts: 65
    Thanks!
  • Arthur1Arthur1 Posts: 542
    Plants want to grow. There is a theory that if you mollycoddle them too much when planting, they will be less tough when established. For instance, trees will often establish more quickly if planted as small trees into unimproved soil.
    Personally, I would look at improving my garden soil in general with regular top dressings of organic matter. You don't need to replace all the soil in a planting hole. In fact you might be creating a sump.
  • LoxleyLoxley Posts: 5,698
    edited August 2022
    I agree with Kate - improve your soil by adding OM to the surface, either as a mulch or lightly worked in. You can create pockets of anaerobic soil if you add too much OM to the bottom of planting holes on clay soil. OM will also break down over time, which may cause your plant to sink if it's sitting on too much of it! As Jenny says, if you want to improve the soil by digging in OM, it's usually better to do it over a wider area.
    "What is hateful to you, do not do to your neighbour". 
  • TheGardenerFromMarsTheGardenerFromMars Posts: 65
    I think I've got the idea re. organic matter, but garden centres also sell bags of top soil - would you ever use that to replace the existing top soil when planting?
  • LoxleyLoxley Posts: 5,698
    Only if the existing topsoil was really terrible or non-existent. I've used bagged topsoil for raising levels, filling holes, and (mixed with MPC) for filling pots .
    "What is hateful to you, do not do to your neighbour". 
  • Arthur1Arthur1 Posts: 542
    Would be very expensive, and not environmentally friendly, to replace topsoil with bought bagged soil. Much better to work with what you have, improve it as best you can and pick 'right plant  right place'.
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