I know what you mean about sandy soil. Mine grows bahir grass ok, its not much good for the cattle but it holds thru the dry and kicks on after rain. To grow veg is another disaster altogether. One BIG advantage we have on sand is that its dead easy to dig and it turns to loam very well. Here's my "instructable" about using Chooks and free veg to make soil in a hurry.
Dig out a level trench in a straight line and as much on contour as you can, 2 - 3 shovels deep and 1m (36") wide ... or if you have short legs whatever you can straddle - this will be soon be your veg bed. Anything less than 10 m long is a bit wimpy, but length is up to you, has more to do with your landscape and gardening habits. Stack the dirt on each side, lay in some cardboard (protection) and lay plastic over it forming a 15 - 20cm shallow pool. Any plastic works it will not degrade underground, joins made by double folds work well. . Find some PVC pipe a bit longer than your trench is deep - 2" plus - bigger diameter is better. This - or they ... will be used to fill your trench with water and also tell you when you need to "water". Drill lots of little holes drilled for 15cm at one end. Wrap the end with shadecloth a few times so the plastic does not get damages by the pipe, and sand in the trench doesn't fill up the pipe. Pack around the pipes with small stones, then fill up the "plastic pool" you have made with your sand. You should still be at least one shovel depth below the surface! Use a slow hose to fill the trench. Make a note of how fast your water is flowing and how long it takes to fill. If you have badly behaved sand it might resist the flow of water thru it. Just keep an eye on it. If the water does not move through the sand fast enough you'll need to put round gravel along the bottom of your next trench, or just slow your hose to suit the water flow. You want to see water pooling evenly along the length of the trench. You can use a bit of rag twirled around a stick to measure how much water is in your trench pool at any time. Efficient watering without miles of tubes and bits that get clogged or leak. Nothing will evaporate and weed seeds willnot get a nice watering from the top to grow. Now fill 5cm more of your trench with your lifeless sand and any organic stuff. We dont want roots to go to CHOOSE>>
2 Choices: With or without chooks - WITHOUT. (Bad idea but I understand)
Now you are going to kick start your soil by growing your own compost. Throw in lots of seeds of anything that produces a lot of leaf growth. All the better if they are legumes. They grow fast because they have the perfect water qty available all the time. Water is constantly being wicked up through the sand. When they grow to a really thick carpet 15 cm high (about 4 weeks in summer) throw more seeds around and then throw in more sandy stuff. about another 2". You dont have to fully cover the old plants - they will keep the earth shaded a bit and keep out unwelcome competition.- of sand - Note you are still way below the old soil level. Good. Repeat this exercise adding whatever else you can find. Dont weed unless you have a really obnoxious plant growing. Put some old sacks and gloves in the car. Anytime you pass a field of cattle grab a sack or two of pats. Drown it in water for a week or three - smashing it up is good. That kills the weed seeds and makes a great, easy to add fertilizer. Over the non winter seasons you will have at least 4 crops of "compost". If you can put in a few inches of cow pats over winter, you'll feed all the little elves and fairies who will help you big time in the spring. Of course make your own piles of compost with whatever wlse you can get. But remember, animal dung is gonna have to show up sooner or later for you to get the best soil. Alternative ... worms, but ... not here, not now. Here's a tip. Never pull out the roots. I know its hard to resist, but as you keep growing stuff, use what grows above ground but leave the underground stuff to the fungi and soil biota. Just use a sharp blade to cut off the stuff you harvest, add some new compost and then sow again.
WITH CHOOKS and green grocers waste. Make a few roly poly hen hoops.
Over your trench, make a low slung half round hoop house. So that will be about 50 - 60 cm high and you can make it permanently shaped and easily ROLLED OVER by adding cross bits to the bottom. Cover sides and bottom with strong chicken wire. Dont skimp. Electrical conduit, Poly irrigation pipe with bamboo inside, whatever you can find, and make sure it has a bottom. We dont want chooks scratching up the sand of the trench or bad dudes getting your chooks. Add a longways center pole and one halfway up the side. Cut 40cm feeding flaps with bits of wood attached as weights. This will be your moveable chicken coop. you'll toss in HEAPS of old veg you collect from green grocers, through the flaps, cover one end with a bit of old vinyl sign and add an easy to get at removeable egg laying box - again with a lock down flap so only you can get the eggs.
The chooks will mascerate your veg, you will need to give them nothing more than some shell grit. Add chopped straw if its all getting a bit wet, but you really want a MESS. Overpopulate the enclosure. After everything is very messy - depends on population density, move them to a new Roly Poly hen House over more trench and repeat. Roll away the old coop, toss in a little of your sandy soil, and sow seeds. When the plants are young and juicy send in your chooks again. They will eat and scratch out your plants, make great soil and every time you add a little more sand, the water holding capacity of your soil will be huge and it will be very fertile. You can do this forever harvesting only eggs, or you can grow a real crop, leaving the remnant for chooks. HINT : Quit while you are ahead. Don't grow crops past the first flush of fruit. Leave the rest for the chooks. All crops get bugs as they get older.
My digging into sand has shown you can really move a lot quickly. Ponding water underground is easy. Watering by a letting a trickling hose hose fill your "ponds" over night, is something you may only have to do once a month even in summer. In a short time you will have worms queueing to get in.