Chap. 10. The Geosphere, Soil, and Food Production 2672Ca 5 (PO 4 ) 3 F(s) + 7H 2 SO 4 + 3H 2 O →
2HF(g) + 3Ca(H 2 PO 4 ) 2 • H 2 O + 7CaSO 4 (10.8.5)Potassium as the potassium ion, K+, is one of the major fertilizer nutrients
required by plants to regulate water balance, activate some enzymes, and enable some
transformations of carbohydrates. Potassium makes up 2.6% of Earth’s crust, but much
of it is tied up in relatively unavailable forms in minerals such as potassium feldspar,
KAlSi 3 O 8. Potassium is a common cation held in an exchangeable form by clay minerals
in soil from which it is readily available.
Potassium is one of the most abundant elements in the Earth’s crust, of which
it makes up 2.6%. Despite its abundance in Earth’s crust, much of this potassium is
not easily available to plants. For example, some silicate minerals such as leucite,
K 2 O•Al 2 O 3 • 4SiO 2 , contain strongly bound potassium. Exchangeable potassium held by
clay minerals is relatively more available to plants. Potassium for fertilizer is simply
mined from the ground as salts, particularly KCl, or pumped from beneath the ground
as potassium-rich brines. Large potassium deposits occur in the Canadian province of
Saskatchewan.
Plants require several micronutrients, largely elements that occur only at trace
levels, for their growth. These include boron, chlorine, copper, iron, manganese,
molybdenum (for N-fixation), and zinc. Some of these are toxic at levels above those
required for optimum plant growth. Most of the micronutrients are required for adequate
function of essential enzymes. Photosynthetic processes use manganese, iron, chlorine,
and zinc. Since the micronutrients are required at such low levels, soil normally provides
sufficient amounts.
10.9. Pesticides and Agricultural Production
Pesticides and related substances such as growth regulators are widely used in
agriculture. The most common agricultural pesticides are insecticides and herbicides,
which are an integral part of modern agricultural production. The Federal Insecticide,
Fungicide, and Rodenticide (FIFRA) act regulates agricultural pesticides in the United
States. This act, first passed in 1947 and amended several time since then, requires
registration of agricultural pesticides, considering toxicity, degradability, and other
aspects of their use.
Recombinant DNA technology (see Section 10.12) is having some significant effects
upon pesticide use. One of the more interesting developments has been the splicing
of genetic material into cotton, corn, and other crops that cause them to produce an
insecticide that is generated by some kinds of bacteria. Another interesting aspect of
recombinant DNA applied to herbicides is the breeding of genetically modified plants
that are not affected by specific herbicides. Because they significantly reduce levels of
chemical production needed for pesticides as well as levels of pesticide residues on soil
and in water, these kinds of measures are very much in keeping with the practice of
green chemistry.