Why do you need to add magnesium and lime is your soil deficient in both?

Working out the right amounts would be easier if we also knew your soil pH--lower pH means adding more lime. Dolomitic lime would give you a twofer, since it contains both ingedients. The pH should be listed on whatever soil analysis urged you to add lime and magnesium--along with the recommended amounts, usually!

You need to answer MG question first. You should do a pH test but if you think that your soil has been regularly manured over many years perhaps a light liming in the spring wouldn't do too much harm. If you are growing Rhodos and their ilk then leave well alone. As far as magnesium is concerned I would only use it on individual shrubs that are yellowing but only in spring as it will be washed away by winter rains.

Hmm, I'm very curious about this - why do you think you need to add both these elements to your soil, has someone told you you should? What plants do you want to grow?

I'm particularly curious because most (though not all) parts of Kent have a high level of chalk in the soil, you can find lumps of it when you dig, which usually means its pretty alkaline - so lime wouldn't be a good idea.

This advice came from the fruit farmer nextdoor.

Yes, I see the question's been altered to include that info, Steragram.

Frankly, there are all kinds of reasons why your fruit trees (whatever they are, you haven't said) 'haven't been good this year' (you haven't said in what way they haven't been good either).

I would not recommend you go adding these elements to the soil without first working out what else might have gone wrong to cause the problems (rainfall or lack of it, poor pruning, poor temperatures, drought like conditions, infection or infestation, lack of pollination, not enough flowers, whatever) and if nothing is apparent, then having soil samples taken and tested to see if that could be the problem.

Fruit farmers grow a lot of fruit, far more than your average punter - for that reason, and because its a commercial crop, they will supplement (having taken soil samples first) because the soil needs it in order to get good crops. This may not be the case for the average gardener though, because your soil is not constantly being 'drained' or stripped of particular elements because of what you're growing. How long have you lived where you are now - what were the crops like last year, what type of fruiting trees are they?

Very interesting...I dont know, but i know that our farmer adds magnesium every year for his crops. I suppose this is because he is practising monoculture which presumeably depleats the soil. I wait to hear further comments with interest, as we have very poorly performing fruit trees in our slightly acidic soil here.

The only reason to add Lime around apple trees would be if you were having problems with Bitter pit caused by a Calcium deficiency so adding Calcium carbonate would , in theory, help.
As Bamboo says, more information is need to be able to help properly.