Agriculture Reference
In-Depth Information
Note that Potassium and Magnesium are almost the same weight in parts per
million, despite the fact that Magnesium is filling 12% of the EC while Potassium
only fills 4% of the exchange sites. With few exceptions, exchangeable Potassium
and Magnesium should be approximately equal in weight in the soil.
********
Enough arithmetic for a bit. Let's review why we are doing what we are doing. The
primary cations Ca, Mg, K, and Na that we are balancing are all nutrients for plants
and animals. They are also chemical elements with their own properties. They
interact with each other, and compete for the available exchange sites. Too much
of one may mean not enough of another. If the whole CEC were saturated with
Calcium, where would the Magnesium be stored?Answer: It wouldn't be stored; it
would be either in the soil solution or heading for the water table on its way to the
ocean.
As we covered in Chapter 2: Cation Exchange Simplified, these nutrient minerals
are held on the clay or humus by a static electric charge. They are positively
charged
+
, the sites where they are attracted and held are negatively charged
-
.
Plant roots and microorganismscan donate a couple of H+ Hydrogen ions to fill the
two negative sites occupied by the Ca++ ion, thus freeing the Ca to be absorbed
as a nutrient. That's the exchange: 2H+ for 1Ca++, or 2H+ for 1Mg++.
Or 1H+ for 1K+
or Mg++ for Ca++
or 2K+ for 1Ca++
Acation nutrient that is held to an exchange site, say Ca++, may be exchanged for
2 H+ ions. Over time, as more and more Ca++ ions are exchanged for H+ ions
released from plant roots, from soil microorganisms, or simply from free H+ in
rainfall, more sites become filled with H+ and the soil becomes more sour or acid.
The pH gets lower.ApH of 4.9 would be 44% saturated with Hydrogen, far too
acidic for most plants.ApH of 7.0, neutral, would have no exchangeable H+
adsorbed on the colloids at all, and a pH of more than 7.0 would have more
+
minerals available in the soil than the exchange sites could hold.
In a garden, field, or orchard where crops are grown, harvested, and taken away,
the mineral nutrients are taken away along with the crop. The same is true for
pastures where animals are grazed for milk or meat. In order to continue to raise
high-quality nutrient dense food crops, we must replace what has been taken away;
we also want to continue to have the proper balance of mineral nutrients available.
Getting Started:
Below are the primary cation results from the soil report we will be using
throughout this topic. We will use them and the information discussed above to
determine the percent of base saturation of the cation nutrients, determine if we
need to add more, and how much we would need to add to achieve the Ideal Soil
