If you have heavy clay or mostly sand, it will help. If you already have a loamy soil with good organic matter content, it will be mildly detrimental. The peat moss in most potting soils crusts up when it's dry, repels water and inhibits germination. It's also a magnet for cats to crap in if you have that issue. I would mix it thoroughly with the native soil.

Thanks for the pictures. You will certainly need to use every conceivable water saving method, that much is clear. The area of your future house looks very dry indeed. The closest experience I have is gardening in central florida where although there is no shortage of water as a yearly average, there is a quite dry season during the cooler part of the year, the sky is very clear so the sun is bright and hot and the sandy "soil" dries out very dry. There is a also a lot of drying wind that is very hard on food plants and young fruit trees. I suspect there will be several critical factors for you, presuming that the main issue, water, is available. Windbreaks will be super important, and the easiest thing would be to build the house with walls that enclose a sheltered garden. Second, you likely have far too much sun for most annual food crops, so if that sheltered area can have shade from about 1 or 2 in the afternoon might help a lot. With such clear sky and high sun there the morning will probably be enough. In extreme environments the majority of the heat stress on a plant that is not well-adapted will be from the west. Equally important will be crop choices, and that is where it's especially unlikely anyone on this forum will be able to give good info. As mentioned earlier, there must be people in the region who will know everything. We all know of some dry hot climate tree crops: notably date palm and almond, maybe pomegranate, because they are popular foods in the west, but I am pretty sure that there are a number of leguminious trees as well that will grow in those conditions. There must also be some annual type and maybe perennial type greens and other vegetables. I would think it logical that any of the common food crops that originated in the damper and/or cooler climates (which is most, by far) will be a waste of your time and resources.

It makes no difference if the potting soil is for indoor use or for outdoor use - the principles are the same. You want durability, a good textural component and fast drainage coupled with a degree of moisture retention. Sounds more complicated than it is :-) I doubt you will find what you need with just sand and compost. The Container Gardening forum has a very long running thread that discusses container soil mixes and the physics behind them. The author is one of the most knowledgeable in his field. There are two mixes in particular that are referenced. The first - known as the 5-1-1 - is used frequently for houseplants and more temporary or seasonal containers. The second - the gritty mix - is used for more long term plantings or plants that require exceptional drainage like lavender, succulents and cacti. You'll notice that peat is used in moderation in one and not at all in the other. And nowhere is sand or compost used - they do not have either the particle size or durability to be good long term container soil components. The 5:1:1 mix: 5 parts pine bark fines (partially composted fines are best) 1 part sphagnum peat (not reed or sedge peat please) 1-2 parts perlite garden lime (or gypsum in some cases) controlled release fertilizer (if preferred) Gritty mix: 1 part uncomposted screened pine or fir bark (1/8-1/4') 1 part screened Turface 1 part crushed Gran-I-Grit (grower size) or #2 cherrystone 1 Tbsp gypsum per gallon of soil (eliminate if your fertilizer has Ca) CRF (if desired) I do encourage you to review this thread and at least read through Al's first post. You will greatly increase your understanding of what makes a good container soil and how best to care of containerized plants. Here is a link that might be useful: Container soils and water movement thread

I'm also thinking this bed has a lack of nitrogen. I hope the infusion of the organic matter will help that. Good call on the shallow root tendencies of garlic colin3. I did sift about 15 gallons of grow media out of this bed - collected a lot of worms and what ever large stuff as well - and placed the media over another small bed. The large stuff and worms essentially give me a third bin in my basement now. The sifted media was really granular to almost the point of being pelleted, consistently sized and small, black, and smelled great albeit a little dry. I'll experiment with pocket / trench feeding blended foods next year both in containers and my larger beds. I'm sure I'll seed the gardens with red wigglers at least from cocoons when I use the VC from my indoor bins next spring. The only thing that still worries me is settling as the newly added mater breaks down and reduces in volume.