Building Soils for Better Crops, Third Edition

Organic Matter and Nutrient Availability

SARE Outreach
Fred Magdoff and Harold van Es | 2010 | 294 pages
PDF (6.8 MB)

This title is temporarily out of print. We expect to publish an updated edition in the spring/summer of 2021.

The best single overall strategy for nutrient management is to enhance the levels of organic matter in soils (figure 18.1). This is especially true of N and P. Soil organic matter, together with any freshly applied residues, are well-known sources of N for plants. Mineralization of P and sulfur from organic matter is also an important source of these nutrients. As discussed earlier, organic matter helps hold on to positively charged potassium (K+), calcium (Ca++), and magnesium (Mg++) ions. It also provides natural chelates that maintain micronutrients such as zinc, copper, and manganese in forms that plants can use. In addition, the improved soil tilth and the growth-promoting substances produced during organic matter decomposition help the plant develop a more extensive root system, allowing it to obtain nutrients from a larger volume of soil.


  • Build up and maintain high soil organic matter levels.
  • Test manures and credit their nutrient content before applying fertilizers or other amendments.
  • Incorporate manures into the soil quickly, if possible, to reduce N volatilization and potential loss of nutrients in runoff.
  • Test soils regularly to determine the nutrient status and whether or not manures, fertilizers, or lime is needed.
  • Balance nutrient inflows and removals to maintain optimal levels and allow a little “drawdown” if nutrient levels get too high.
  • Enhance soil structure and reduce field runoff by minimizing soil compaction damage.
  • Use forage legumes or legume cover crops to provide N to following crops and develop good soil tilth.
  • Use cover crops to tie up nutrients in the off season, enhance soil structure, reduce runoff and erosion, and provide microbes with fresh organic matter.
  • Maintain soil pH in the optimal range for the most sensitive crops in your rotation.
  • When P and K are very deficient, broadcast some of the fertilizer to increase the general soil fertility level, and band apply some as well.
  • To get the most efficient use of a fertilizer when P and K levels are in the medium range, consider band application at planting, especially in cool climates.