I accidently learned this while I was in my rush to create garden beds. I had too little compost and manure available, actually none to spare, but I had the energy and time to create beds. I was eager to grow. So I just went ahead and planted anything I had on hand. Being a somewhat cheap person, I didn't want to waste expensive seeds on unimproved soil, so I planted cheap livestock feed and plant cuttings from around the farm. That meant, racehorse oats and sweet potato cuttings.
Quite honestly I was surprised to see the change in the soil structure. When I later went back to "harvest" the greenery to feed the rabbits, I took a handpick to the ground to loosen it for its first "real crop", the soil looked visably improved. It was "lighter", more friable. It had worms. It was no longer hard. Oh, the rocks were still there and needed removing. But it was far easier than if I hadn't grown something there first. Funny thing, but just having the grass and weeds growing in place didn't improve the soil. But the oats and sweet potato combination did.
Later, in other spots, I've used taro starts. I applied a grass clipping mulch around the taro starts and let them grow pretty much ignored. The taro fared mediocre, not producing much in the way of a corm, or even leaves. But I got small plants that I could harvest for livestock food. Once again, I found that the soil structure had improved, although not as significantly as with the sweet potatoes. No soil amendments were used other than the grass mulch to keep the weeds down around the baby taro, and I didn't follow that up with more mulch later. But the soil was more friable, hosted worms, was retaining moisture. In this case, the mulch was a contributing factor.....or possibly the primary factor.
While I still consider the addition of compost to be my number one soil improving strategy, the simple act of growing an initial crop (even if it grows poorly) is my number two best strategy. Over time I've refined my "first crop" to just sweet potato cuttings. I don't usually see much in the way of tubers being produced....small ones, if any. But the vines are all harvestable for livestock fodder. I always have sweet potato cuttings available and it only costs me my labor to put the crop in.
So now, anytime I open new ground, I immediately plant sweet potato cuttings. I space them close, 6"-8" apart in all directions. I let them grow for 4-6 months before I harvest the vines by either cutting them & feeding them to the rabbits, or letting the bigger livestock graze them off. Then I'll use a pick of some sort to rake through the ground removing some of the bigger rocks. Now I have a choice......
1- leave the tubers in place and allow the sweets to go back. And repeat the harvesting process in 4 months. At this stage, the soil will be as friable as it will get without more work. So I'll go remove rocks, tubers, add various soil amendments, then plant a crop.
2- harvest the tubers, add soil amendments, and go right to using the area for crop production.
Either way, I'm a winner.....the soil is looking better, although option #1 makes for nicer garden soil faster.
I haven't experimented with using other types of nurse crops yet. Since I have abundant sweet potato vines, that's what I use. But I'd venture a guess to say that other plants would work too. Once I have an over supply of homegrown bean seeds, I think I'll try using beans and see what happens to the soil. Will it look as good after beans as it does with sweet potatoes? Or perhaps it will nicer. Who knows.