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When talking about soil and fertilizers, what are the primary nutrients in addition to NPK (Nitrogen, Phosphorus, Potassium) that play a major role in plant development?

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Primary macronutrients are NPK as you mention.

From this article on plant nutrients:

The secondary [macro]nutrients are calcium (Ca), magnesium (Mg), and sulfur (S).

and

Micronutrients are those elements essential for plant growth which are needed in only very small (micro) quantities. The micronutrients are boron (B), copper (Cu), iron (Fe), chloride (Cl), manganese (Mn), molybdenum (Mo) and zinc (Zn).

  • Just wondering: is there a reasoning behind the naming convention of macro-/secondary/micro-nutrient? – mfg Jun 8 '11 at 19:32
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    "macro" = big, need larger amounts. "micro" = small, need smaller amounts? I've edited the quote to make it clearer that the "secondary" refers to macronutrients that are usually present in sufficient quantities. – bstpierre Jun 8 '11 at 19:41
  • (Sorry, I didn't mean for you to have to define macro/micro, I mean are the macro's used whole while micro's are combined or broken down or something along those lines) – mfg Jun 8 '11 at 19:43
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    And copper is a well known plant poison. ie. They only need a very tiny amount in the right form. – winwaed Jun 8 '11 at 23:54
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    Indeed. Too much of any is not good. Excess potassium interferes with plants' ability to take up magnesium and/or calcium. See the table at the bottom of this page for the interactions that excesses cause: cartage.org.lb/en/themes/Sciences/BotanicalSciences/… – bstpierre Jun 9 '11 at 1:07

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