Building Soils for Better Crops, Third Edition

Organic Matter and Nutrient Availability


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.

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