This is Part 3 of 3! See bottom of post for links to parts 2 and 3.
I do not have direct personal experience planting my own tomatoes in an in-ground garden, but know many people who have them so most of this information is what I’ve gleaned from talking to them and researching online. I have a couple of in-ground boulevard gardens in my front yard and have had great success growing garlic and sunflowers in them, and some success with potatoes and squash, but overall the good quality soil does not go very deep before one hits pure clay. I’m working at building up the soil quality but for the time being it seems to be a better place to grow smaller plants with fairly shallow roots.
Note that an in-ground garden does not need to be a huge, half-yard production – it can be as simple as digging out the grass along the sunny side of your garage or something and replacing it with soil (I have relatives who grew the largest tomato plants I’ve ever seen in a tiny dirt strip along the garage).
I’m not sure if there is any scientific basis for this or if it’s just my anecdotal experience, but it seems that tomato plants, when planted directly in the ground will get bigger than their raised bed or potted counterparts. This may or may not translate to higher production, and maybe is more dependent on the variety grown, but I’ve never had raised bed harvests even close to what some of my friends and family with in-ground gardens have seen.
A large issue that my friends with this type of garden complain about is how weedy they can get, much more so than raised beds (though you can remedy this with mulch!). Depending on the placement and soil quality, your garden may also not be as well drained as you would like. You may need to use more fertilizer in a larger garden to ensure the actual tomato plants get what they need rather than the weeds, and some may be lost to runoff.
(Photo is courtesy of City of Saskatoon Healthy Yards Program)
Unless you are going to dig out whatever soil exists and completely replace it, you’re somewhat at the whim of your natural soil quality and need to work to build up its health over time. This is not a bad thing – your natural soil might be healthy and already high in nutrients and earthworm activity which will be beneficial for your plants. I am of the understanding that rototilling, something my parents would pay someone every year to do for their garden, is actually bad for the soil and causes it to distribute weeds and lose nutrients. If you have an in-ground garden it’s apparentely better to just amend by top dressing and eventually, over time your soil will naturally aerate.
In-ground gardens do seem to hold water a bit better than raised beds, and the tomato roots will have the opportunity to go a lot deeper, so you may not need to water as much – but this can be dependent on where the garden is situated, how much sun it gets, how much clay is in the soil, etc.
Pros of in-ground gardens: More space for plants and roots to spread, possibly higher production, may retain water better, higher natural nutrients, can use high volume fertilizers
Cons of in-ground gardens: Will get weedy if you don’t mulch, soil takes longer to warm up in spring, likely to be compacted, may get standing water, nutrients may be lost to runoff or stolen by weeds/grass
As always, I’m not being comprehensive, so if you want to read more on the subject, here are a couple other posts regarding the advantages and disadvantages of in-ground gardens (in comparison to raised beds).