I have some garden soil that has decaying spruce needles mixed through it. What should I do to the soil in order to grow vegetables in it normally?
Suspect over-acidity... check the pH and add lime if needed. Is the soil good otherwise? Spruces (at least around here in New Hampshire) are often in pretty sterile soil that needs amending with other, sweeter organic material, like composted manure or other compost.
When there's too much uncomposted "brown stuff" (leaves, etc.) in the soil, it will rob nitrogen in order to digest all the carbon. So consider adding nitrogen one way or another.
If the dog has also been urinating in the area a lot, it can mess up the soil. Apply gypsum for that.
Does the soil drain well? Is there hard pan underneath it?
Two potential problems:
Spruce (and other conifer) needles are acidic, and having them mixed into the garden will make your soil acidic. If your soil pH is too low (acidic), vegetables will have a hard time taking up the nutrients they need. This can cause the yellowing you mentioned in a comment. You can get a pH test kit at your local garden center.
At a 48:1 C:N ratio, spruce needles are somewhat high carbon, and they're small, so they have a relatively large surface area. If you have a large amount in your soil, the microbes that will break down the needles will "steal" nitrogen to do their job. If your vegetables aren't getting enough nitrogen, they will turn yellow.
To fix these problems:
- Following a pH test, add lime in the recommended amount to bring the pH up to about 6.0-6.5 (this is good for many common vegetables).
- Add nitrogen.
- Short term: dried blood or poultry manure are fast acting, organic sources of nitrogen. If you're buying bagged products, read the label and follow directions for how much to apply. If you've got a ton of needles in the soil, use the "heavy feeding" directions, and if it's not that much use the "light feeding directions". If in doubt, follow the light feeding directions, wait two weeks and reapply if needed.
- Longer term: if you build up the organic matter content of your soil, you'll have a reserve of nitrogen so that you don't need to rely so much on short term additions. To do this, add about 1" of compost or well rotted manure and dig it into the top 4-8" of soil (the deeper the better). In subsequent years, when you've built up the organic matter and vegetables are growing well, you can back this off to ½" or so.
You didn't mention whether this is an existing garden that's had vegetables growing in it successfully prior to being sabotaged by the neighbors dog. If it is new, I'd recommend getting a professional soil test done (nutrient, pH, and organic matter %). It should cost around $15; if you're in the US your county extension office will be able to tell you how to take a sample and where to send it. The test results will tell you how much lime you need as well as how much and what kind of fertilizer to add.