Why natural fertilizers?   Conventional fertilizers frequently pollute waterways with excessive nitrogen and phosphorous, are are petroleum based.   Conventional fertilizers can be detrimental to soil organisms.  

For optimal soil health, and to reduce reliance on fertilizers, it is wise to compost plant residues and kitchen scraps, and rotate plants to different areas, with nitrogen fixing legumes being alternated periodically to increase nitrogen.   Rotating crops also reduces pest and disease problems.   And make your garden a haven for natural predatory insects, birds and bats.