Agriculture Reference
In-Depth Information
T A B L E 10.3. Essential Elements for Plants and Humans
a
Essential for Plants
Essential for Humans
Nitrogen
Nitrogen
Potassium
Potassium
Calcium
Calcium
Magnesium
Magnesium
Phosphorus
Phosphorus
Sulfur
Sulfur
Manganese
Manganese
Iron
Iron
Chlorine
Chlorine
Zinc
Zinc
Copper
Copper
Molybdenum
Molybdenum
Boron
Sodium
Fluorine
Iodine
Selenium
Silicon
Chromium
Arsenic
Vanadium
Tin
a
The order of the elements is arranged by decreasing quantities needed.
function. It also is needed to control the opening and closing of pores in leaves
that regulate water movement in plants. Most of the potassium in maize plants is
concentrated in the leaves and stalks. Potassium fertilizers are mostly composed of
ground up potash rock. One of the largest potash rock mines in the world is in
Saskatchewan, Canada.
One hectare of maize producing 9500 kg of grain will remove about 45 kg of
potash (K
2
O) per hectare. This assumes that only the grain is removed from the
field, not the stalks and leaves. If the potassium is applied as pure potash rock (KCl
equal to 60 percent K
2
O), about 75 kg of potash rock must be applied per hectare.
The third most important element for plants is phosphorus. It is part of the ATP
molecule (adenosine triphosphate) used to move energy around in plants and
animals. In contrast to potassium, more of the phosphorus is concentrated in the
grain than in the stalk and leaves. Phosphorus fertilizers are most commonly formed
by treating the mineral, apatite, with acids to make the phosphorus more soluble.
The largest apatite mines are located in Morocco.
A farmer harvesting one hectare of maize grain producing 9500 kg of grain will
remove 60 kg of phosphorus. Phosphorus fertilizers are rated by the equivalent
amount of soluble P
2
O
5
. A common phosphorus fertilizer is triple superphosphate con-
taining about 45 percent P
2
O
5
. To replace the phosphorus removed in the grain, we will
need to add 133 kg of phosphate fertilizer per hectare.
