Preparing Soil for Planting

NPK Fertilizers, Soil Amendments, and Soil Fixes

By The Old Farmer's Almanac

When it comes to a healthy garden, soil preparation matters.

All vegetables need soil that’s rich in nutrients. Some soil needs a helping hand. Here are tips on building healthy soil:

Test Your Soil

  • Test your soil. Results will reveal its pH, phosphorus, lime, potassium, soluble salts, soil texture, and more. However, a general test will not reveal insects, diseases, or chemical residues.
  • There are a few ways to get a soil test. First, you could buy an inexpensive soil test kit at your local garden store. Or, you could contact your local cooperative extension service office for a free (or low-fee) soil test. Or, see this gardening blog about a resource that provides soil types around the country.

Amending Your Soil pH

  • 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.

Learn more about NPK Ratio: What Do Those Numbers Mean?

Avoid applying excess chemical/synthetic fertilizer. It can damage roots and/or reduce the availability of other elements. Because organic fertilizers release their benefits more slowly, they are less likely to burn plants, although some, such as fresh manure, can do so. Once the nutrients are in the soil in an available form, however, plants can not distinguish between synthetic and organic fertilizers. Organic fertilizers, however, tend to also improve soil structure and encourage earthworms and microorganisms that improve soil health.

When is a good time to fertilize your vegetables? See our Growing Vegetables Guide.

Soil Fixes

  • If you have clay soil, add coarse sand (not beach sand), compost, and peat moss.
  • If you have sandy soil, add humus or aged manure, peat moss, or sawdust with some extra nitrogen. Heavy, clay-rich soil can also be added to improve the soil.
  • If you have silt soil, add coarse sand (not beach sand) or gravel and compost, or well-rotted horse manure mixed with fresh straw.

Soil Amendments

  • Bark, ground: made from various tree barks. Improves soil structure.
  • Compost: excellent conditioner.
  • Leaf mold: decomposed leaves that add nutrients and structure to soil.
  • Lime: raises the pH of acid soil and helps loosen clay soil.
  • Manure: best if composted. Good conditioner.
  • Peat moss: conditioner that helps soil retain water.
  • Sand: improves drainage in clay soil.
  • Topsoil: usually used with another amendment. Replaces existing soil.

If you are starting from scratch with a small field or large garden that is either over-spent or neglected, see our article about reclaiming your garden soil.