Agriculture Reference
In-Depth Information
calculate how many years a crop could be grown on that soil before the amount of
Calcium or other mineral nutrient had been completely taken up and shipped away.
We have found one chart showing a complete assay of soil minerals and an
estimate of how many years' supply the soil contains. The table was published in
Soil Chemistry
by Bohn, McNeal, and O'Connor in 1985. It is based on the work of
the scientist Vinogradov whom we assume was Russian. Here are a few examples
from that chart:
Total Soil Mineral Reserves to 1 Meter Depth
Mineral element
Plant uptake per year kg/hectare
Years of supply at average plant uptake
Calcium
50
260
Potassium
30
430
Magnesium
4
4,600
Iron
0.5
100,000
Sulfur
2
320
Selenium
0.0003
40
According to the notes on the original, these are the total reserves in the soil to a
depth of 1 meter. No indication is given of where this soil was from, but clearly it
was a fertile soil if it contained potential reserves of Potassium to feed crops for
430 years. The question of which crops is not addressed.
Looking at the Sulfur reserves, 320 years at 2 kg per year would give us 640 kg/ha
of Sulfur in this soil.
Perhaps some crops may only take away 2 kg/ha (1.8 lbs) of S per year from the
soil, but more commonly crops will uptake 20 or more kg/ha.Agood crop of sugar
beets will use as much as 50 kg/hectare per year of Sulfur. That 640 kg of Sulfur
would only be enough to grow sugar beets for a dozen years, even assuming the
plants could somehow access every bit of Sulfur in the top 100cm of soil (which of
course they couldn't).
We can see how a soil could be rapidly depleted.
An even more dramatic example is Selenium in the bottom row of the chart-- only
enough for 40 years of growing, and as our arithmetic above has shown, perhaps
for much less time than that.
We begin to see why soils are spoken of as “worn out”. This can be the
consequence of short-sighted exploitative agriculture where soluble NPK fertilizers
have been applied in large amounts in order to force maximum growth and yield.
Plants cannot live and reproduce solely on a diet of NPK; they must have the other
essential minerals too, so they draw them from the soil reserves, depleting those
reserves year after year.As the crops are harvested and sold away; the minerals
