Agriculture Reference
In-Depth Information
For the rest of this chapter, the amounts we calculate will be interchangeable
between kg/ha and lbs/acre.
The hash mark # indicates either lbs/acre or
kg/ha.
We will begin with the major cations Ca, Mg, and K.
Calcium Ca and
Magnesium
Mg
Calcium Ca++
ppm
Desired
Found
Deficit
1550
1250
-300
Add 300 ppm
(600 #)
Calcium Ca
Ca Base Saturation 60-70 %
55.0%
Magnesium Mg++
ppm
Desired
Found
Deficit
164
116
-48
Add 48 ppm
(96 #)
Magnesium Mg
Mg Base Saturation 10-20 %
8.5%
Below is the Ca and Mg section from our worksheet.
Magnesium
can be difficult to balance when writing a Certified Organic soil Rx as
there are no USDANOP approved sources that are high in Mg. Here are our
choices:
Dolomite lime: Ca 22%, Mg 13%
Epsom Salts (Magnesium sulfate): S 13%, Mg 10%
K-Mag (Sulfate of Potash Magnesia): 22% K
2
O, 22% S, 11% Mg.
Note that all three approved sources are running 10-13% Mg. That means we are
going to need 75-10 # of any of them to end up with 10# of Mg.As this soil
requires 96 # of Mg, we are going to need up to 960# depending on which source
we choose.
If we needed only Magnesium, no Ca, S, or K, the USDANOP rules would allow
the use of naturally occurring mined Magnesium carbonate MgCO
3
, magnesite,
which contains up to 20%Mg, but this is not commonly available. If government
organic certification is not an issue, Magnesium oxide MgO, at around 50%Mg, is
pure, safe, inexpensive, readily available, and is rapidly assimilated into the soil.
Here are some 2013 retail prices from the USA:
K-Mag 24 kg (50 lbs): $25
Epsom Salts 24 kg: $25
Dolomite Lime 24 kg: $8
At first glance it looks like Dolomite is our best choice if we are amending a large
field. If we are amending a 1000 ft
2
or 100 meter
2
garden, the price might not be a
major consideration. Other considerations enter into the decision. Dolomite lime is
about 22% Calcium, so for every 13# of Mg we are adding 22# of Ca. If our
worksheet told us that we didn't need any Ca, we would have to use one of the
other Mg sources. However, both of the other choices contain more Sulfur than
