I regularly mix commercial amendments into my garden soil. Nevertheless I've noticed the older parts of my garden are less productive. New soil I've broken in always grows better veggies. This is quite unexpected.
Does anyone know a reason for this?
Thanks
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applestar
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Interesting observation. 
I suspect this would depend on your gardening practices -- do you till? Do you use chemical pesticides, herbicides, fungicides? Do you use crop rotation and plant different crop in different designated parts of your garden, moving the crop from one to another year to year?
It would also depend on what was originally there where you have broken in what you call "new soil"
I'm just speculating, but I guess some people would say it's because where the "new soil" is had been left farrow to build up into layers of topsoil composed of decomposed organic matter (roots, vegetation, leaves) with active soilfoodweb -- living microbes and macrobes on up to insects and animals.
Depending on your gardening practices, all of this will be reduced or destroyed and no amount of store-bought amendments will replenish or rebuild those critical living elements. The ground could become compacted, lack of mulching and ground-protecting layers could dry out the soil and change the texture and other properties that are best for growing, etc.
Without the biodiversity of fauna, existing population of pests could flourish while predators decline.
If you don't rotate your crops, disease causing fungi, bacteria, or viruses specific to a crop can survive or even build in the soil to infect the same crop grown in the following season. Some diseases don't show up with spectacular symptoms but merely saps vigor from the plants, though sometimes symptoms can be observed in the roots.
I suspect this would depend on your gardening practices -- do you till? Do you use chemical pesticides, herbicides, fungicides? Do you use crop rotation and plant different crop in different designated parts of your garden, moving the crop from one to another year to year?It would also depend on what was originally there where you have broken in what you call "new soil"
I'm just speculating, but I guess some people would say it's because where the "new soil" is had been left farrow to build up into layers of topsoil composed of decomposed organic matter (roots, vegetation, leaves) with active soilfoodweb -- living microbes and macrobes on up to insects and animals.
Depending on your gardening practices, all of this will be reduced or destroyed and no amount of store-bought amendments will replenish or rebuild those critical living elements. The ground could become compacted, lack of mulching and ground-protecting layers could dry out the soil and change the texture and other properties that are best for growing, etc.
Without the biodiversity of fauna, existing population of pests could flourish while predators decline.
If you don't rotate your crops, disease causing fungi, bacteria, or viruses specific to a crop can survive or even build in the soil to infect the same crop grown in the following season. Some diseases don't show up with spectacular symptoms but merely saps vigor from the plants, though sometimes symptoms can be observed in the roots.
Do a soil test. It may be that in the older parts of your garden you need to adjust the pH or you are over/under fertilizing.
It is best to use compost from a variety of sources so maybe you have been using the same amendments for years so you are adding the same thing and maybe missing the same things. I admit, I am guilty of this too, but I am trying to remedy this by alternating the amendments I use and using different brands. I also do have to solarize once in a while to control nematodes and other problems with the soil. I have planted cover crops and scavenger crops to try to balance things out.
Once every 5 years or so, I will get another soil test and check out how I am doing. If my soil gets blacker and I can see organic matter in it and the plants grow well, then I know I am on the right track. I haven't needed to add anything except nitrogen for years because my phosphorus is so high. I do get some phosphorus from the organics, that cannot be helped, I just avoid adding any more. After I plant a high nitrogen crop like corn, I plant a scavenger broccoli or mustard cabbage to scavenge the excess nitrogen.
It is best to use compost from a variety of sources so maybe you have been using the same amendments for years so you are adding the same thing and maybe missing the same things. I admit, I am guilty of this too, but I am trying to remedy this by alternating the amendments I use and using different brands. I also do have to solarize once in a while to control nematodes and other problems with the soil. I have planted cover crops and scavenger crops to try to balance things out.
Once every 5 years or so, I will get another soil test and check out how I am doing. If my soil gets blacker and I can see organic matter in it and the plants grow well, then I know I am on the right track. I haven't needed to add anything except nitrogen for years because my phosphorus is so high. I do get some phosphorus from the organics, that cannot be helped, I just avoid adding any more. After I plant a high nitrogen crop like corn, I plant a scavenger broccoli or mustard cabbage to scavenge the excess nitrogen.