If you're going to add only one amendment, make it kelp meal, says Bill Wolf, president of the board of directors of the Organic Materials Review Institute. Kelp or seaweed is a great source of trace minerals and growth elements. Adding seaweed triggers an increase in microbial activity and fuels the decay cycle of organic matter in the soil. It doesn't take much to get things cooking. According to Wolf, there is one overlooked ingredient in soil health.
Air is the driving force in stimulating soil life.
Microorganisms, earthworms, biotic activity, all rely on a good supply of air. That's the reason for the standard caution against working the soil too early in the year. Tilling wet soil will destroy its structure, compact the soil, and squeeze out the air. When you compress the air out of the soil, you kill everything in it. Don't overtill, and don't even walk on it when it's wet.