River sand is basically devoid of microbial life. The river washed away most of the aerobic microbes and fungi and what was left died when the river dried up.
Compost is the best way to replenish the microbes but then you have to feed them. They eat sugar from plant roots and protein from dead things that might die there and fall on the surface. You can supplement protein with any ground up grain. I like corn meal, but whatever you can get cheap works. Ground up soy, mung, coffee (used, of course), rice, oat, flax, wheat, milo, alfalfa, cottonseed, or any other ground up seed, bean, or nut you can find works well. Ground feather, hair, and leather is also good and lasts a long time but take a long time to work. The application rate for all these is 10-20 pounds of meal per 1,000 square feet (5-10 kilos per 100 square meters).
Once you get the microbes well established in the sand, the sand will actually turn black and mucky, like great soil. But that won't happen unless you use an organic, protein based fertilizer. Synthetic fertilizers will guarantee that you maintain pure sand as long as you use it because they provide no protein for the microbes to grow.