Achieve the proper soil pH. A very high or very low soil pH may result in plant nutrient deficiency or toxicity. A pH value of 7 is neutral; microbial activity is greatest and plant roots absorb/access nutrients best when the pH is in the 5.5 to 7 range.
Add organic matter to your soil. It improves structure, slowly releases nutrients, and increases beneficial microbial activity. (NOTE: It is virtually impossible to know the nutrient content of aged manure.)
Know Your N-P-K
Plants’ primary nutrients are nitrogen (N), phosphorus (P), and potassium (K). These are available in chemical/synthetic (nonorganic) fertilizers (on the package, the numbers of each nutrient indicate the percentage of net weight contained) or as organic additives suggested here.
Nitrogen (N) promotes strong leaf and stem growth and dark green color, such as desired in broccoli, cabbage, greens and lettuce, and herbs. Add aged manure to the soil and apply alfalfa meal or fish or blood meal to increase available nitrogen.
Phosphorus (P) promotes root and early plant growth, including setting blossoms and developing fruit, and seed formation; it’s important for cucumbers, peppers, squash, tomatoes—any edible that develops after a flower has been pollinated. Add (fast-acting) bonemeal or (slow-release) rock phosphate to increase phosphorus.
Potassium (K) promotes plant root vigor and disease and stress resistance and enhances flavor; it’s vital for carrots, radishes, turnips, and onions and garlic. Add green sand, wood ashes, gypsum, or kelp to increase potassium.
Avoid applying excess chemical/synthetic fertilizer. It can damage roots and/or reduce the availability of other elements. It is virtually impossible to overdo organic fertilizers. Plants can not distinguish between synthetic and organic fertilizers.