Essential Elements for Plant Growth

Primary and Secondary Nutrients

All essential elements are by definition required for plant growth and completion of the plant life cycle from seed to seed. Some essential elements are needed in large quantities and others in much smaller quantities. However, from a practical standpoint, three of the six essential macronutrients are most often "managed" by the addition of fertilizers to soils, while the others are most often found in sufficient quantities in most soils and no soil amendments are required to supply adequate supplies.

From a management perspective only, the primary nutrients are N, P, and K, because they are most often limiting from a crop production standpoint. All of the other essential macronutrient elements are secondary nutrients because they are rarely limiting, and more rarely added to soils as fertilizers.

The ability of soils to supply secondary nutrients to plants indefinitely is is subject to the law of conservation of matter and is therefore dependent upon nutrient cycling. Continued crop removal of Ca, Mg, and S requires replentishment just as surely as primary nutrients, but most likely less frequently. Calcium and magnesium are often supplied by mineral weathering, either of natural soil materials or of aglime, ground limestone added to correct soil acidity. Sulfur is often added to soil as either atmospheric deposition (associated with air pollution) or as impurities in fertilizers, particularly common P fertilizers.

To demonstrate that this classification is more responsive to soil ability to supply nutrients than plant requirements, it should be noted that plant requirements for Ca, a secondary nutrient element, is greater than for P. Calcium is found as a principle exchangeable cation in most soils and an important soluble cation in the soil solution. Phosphorus, on the other hand, is only slightly soluble in most soils, and many soils (particularly acid soils and alkaline soils) have the potential for causing phosphorus deficiencies.

Whether a macronutrient or micronutrient, or whether a primary or secondary nutrient, the Law of the Minimum holds: the most growth-limiting nutrient will limit growth, no matter how favorable the nutrient supply of other elements. For example, a deficiency of Fe or Mn (most common in soils containing calcium carbonate) can severely limit plant growth in spite of adequate N, P, and K.

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This page was last modified by Phillip Barak, Univ. of Wisconsin, on 17 Oct 95. All rights reserved.