chemical vignettes ROBERT C. BRASTED University of Minneroto Minneapolis, 55455
Broiled Rocks With a Side Order of Baked Minerals-The Blue Plate Special Soils, Food Chemistry, Inorganic Chemistry, Geology The Editor i s indebted to Professor W . D. Keller' for the ideas used in this Vignette.
A chunk of mudstone, a choice cut of shale, or a prime rib of montmorillonite (medium rare) does not really grab us as a main course for the evening meal. Yet without these substances as raw materials we would not have the considerably more attractive: chunk of rib roast, cut of pork, or prime rib of beef. Few of us think of the clay minerals and rocks as the real bread basket of agricultural operations. So much time is devoted in our instrnction to the preparation and application of synthetic fertilizers that we underplay the importance of weathering of the rocks which provide most of the nutrients to the soil and which eventually end up in the "meat and potatoes" of our dinner. A good piece of beef will analyze about 12% by weight of inorganics such as phosphorus, iron, and calcium. Or putting it another way, two ounces of every pound of hard rock has to weather to a degree wherein these elements can be made available to the plant for eventual incorporation into the food system. While munching popcorn in your next double feature or skin flick (depending upon your taste in entertainment) think about its (that is, the popcorn's) 10% "rock content." Even a sea food fancier should keep in mind that the shrimp is tasty and nutritious because of the minerals that have been washed down to the sea during and after the weathering process. A very practical (andnow acceptable, because it is an ecological approach) interpretation is that we are pulling out these mineral species from our soil a t an alarming rate, producing animal feeds (as an example) at a rate that far exceeds nature's capacity to replenish by many orders of magnitude through the chemical or the bioweathering processes. The loss through assimilation by the plant may not compare to the loss through natural wind and water erosion, another bothersome subject. We can and do replenish some of the minerals, The purpose of "Chemical Vignettes" is to illustrate applicsr tiana of chemistry, especidly in engineering and other scientific areas. Readers are encouraged to send items appropriate to this column to the author. 'KELLER, W. I)., "Geochemical Weathering of Rocks: Sunrces of Raw Materials for Good Living," The Geochemical Education Committee, Journal of Geological Education, Vol. XIV, No. 1, 17-22 (1966).
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of course, in fertilization but probably not in the quantity and delicate balance provided by nature through the rock mineral composition. The havesting of one crop of alfalfa provides some interesting figures for our student of ecology and inorganic chemistry. I n producing four tons of this feed crop over two tons of "agstone" are needed to give the mineral part of the alfalfa Raw phosphate rock Ca, + P 148 lbs
-- + + -
Gypsum S Ca Mg Ca. Dolomite Limestone -+ Ca Granite K ~~~~
~
134 ibs 118 lbs 30 lbs 4000 1bs 4430 lbs of Agstone
These numbers gave your editor pause to wonder and to ponder on the question: Just exactly what do we mean by "surplus food" and a t what point must we slow down or stop production of food in light of what is removed from our net raw materials? Two major classifications of rock minerals are the kaolinites and the montmorrilonites. The former include lcaolinite itself, A12Si20s(OH)jwhich comes from feldspar, KAISisOs, through a very slow weathering process liberating the potassium ion to the soil which is finally either used by the plant or leeched and washed to the sea. Potassium can be accountedfor through simple bookkeeping (stoichiometry to the first year chemistry student). An aluminum ion, A13+, substitutes for one Si4+ in the Si-0 tetrahedra network. This substitution leaves a net of 1 e- deficit (a +3 for a $4). The total atom and charge balance is satisfied by the gain of one A13+and one I
