We reccomend a soil science textbook anda biology book (though you have an excellent biologist at hand!), especially Ages of Gaia, by James Lovelock, 1988 and Soil Science Simplified, by Milo Harpstead, Thomas Sauer and William Bennett, illustrated by Mary Bratz, first edition: 1980...as well as any of the other books on our suggested reading page.

Based on the limited data you have provided, a quick and dirty answer might be to explain that in a standard (non-Active Fallow) garden or farm, there is usually a deficiency of many nutrients, including nitrogen and phosphorus. Nitrogen is usually formed by living things: it is a good estimate of the amount of organic matter in the soil. Nitrogen is the building block of protein, and all living cells have protein. Nitrogen is also evidence of niterbacteria, or organisms that take the vast amounts of nitrogen in the atmosphere and mineralize it in the soil. Where there is life, there is sufficient nitrogen to survive. Tillage improves nitrogen content (see research by Goss, et al.) by improving the quality of life for microorganisms. Phosphorus is similar: DNA requires P.