Available vs. unavailable forms of mineral nutrients

Aside from water and carbon dioxide, plants need about 14 mineral nutrients: NITROGEN, PHOSPHORUS, POTASSIUM, CALCIUM, MAGNESIUM, SULFUR, IRON, MANGANESE, COPPER, ZINC, BORON, MOLYBDENUM, SODIUM and CHLORINE (these last two are rarely deficient).

Each of these mineral nutrients occurs in both available and unavailable forms in the soil. For instance, only about 1-2% of a soil's potassium is actually available to roots; most of the other 98-99% is tied up as part of rock fragments or clay particles and is very slowly released over time.

Likewise, only about 1-2% of a soil's nitrogen is readily usable by plants. The rest is in the organic form as dead leaves, roots, and crop residues in various states of decomposition; organic nitrogen doesn't become usable until soil bacteria have converted it into ammonium or nitrate ions.

The same applies for each of the other nutrients to varying degrees, depending on soil conditions. As we'll see, soil pH can have a big effect on nutrient availability.