soil basics - how it works

 

3. Ions, Nutrition and all that “Scary” Chemistry

Nutrients that are held on soil particles yet are available to plants for use are called “exchangeable nutrients”, meaning they can be easily exchanged between the soil particle and the plant root. These nutrients are in the form of “ions”, atoms or molecules that have an electrical charge. Ions with a positive charge are called “cations”; those with a negative charge are “anions”. Common nutrients are shown here in the form in which they are available to plants:

Positive ions

Potassium
Calcium
Magnesium
Iron
Copper
Zinc
Manganese
Nickel

Negative ions

Nitrogen
Phosphorus
Sulfur
Boron
Molybdenum
Chlorine


The very small colloidal particles in soil, clay and humus have a large surface area covered by many negative electrical charges. Cations are attracted to and held by these negatively charged colloids (positive and negative charges attract – similar charges repel). Anions are mainly found free in the soil solution (the water between the soil particles). Clay particles generally have a flat, angular shape, while humus particles are irregular and lumpy. Humus particles have much more surface area and can hold about three times as many nutrients as clay particles. Scientists have a means of estimating a soil’s fertility by determining its cation exchange capacity (CEC). The CEC is only an estimate of a soil’s ability to hold major cations (calcium, magnesium and potassium), but it has proven to be a fairly accurate estimate of soil fertility. The CEC of various soil types and humus is shown in Table 1; note that the more clay and humus a soil has, the greater its ability to store nutrients. Thus, it is valuable to enrich clay soils with organic matter such as that found in the Bradfield natural fertilizer products to enhance their nutrient-holding ability.

Table 1. Cation Exchange Capacity of Soil Types

Soil Texture CEC

CEC

Sands

1 – 5

Sandy loams

5 – 10

Loams, silt loams

5 – 15

Clay loams

15 – 30

Peat

10 - 30

Clays

over 30

Humus

100 – 300

Now, remember that while a soil may be able to hold a lot of nutrients, those nutrients must be available to the plant to be useful. A soil may have a great capacity to store nutrients but may be depleted of available nutrients from many years of growing plants with little or no supplemental fertilization or organic matter recycling. Under these circumstances, the negatively charged sites on the soil colloids that normally bind nutrient cations such as calcium, magnesium and potassium, will become filled with positively charged hydrogen ions. This meets the need of the negative site to bind with a positively charged ion, but it does nothing for the plant. Indeed, as hydrogen ions pile up, the soil’s pH drops, becoming more acidic, which is a sign of nutrient-poor soil. “Liming” a soil to increase its pH is really a matter of replacing hydrogen ions with needed nutrients. The material used for liming should be designed to replace those nutrients specifically missing, so having soil thoroughly tested for elemental composition is important for proper restoration. Enhancing a soil with natural fertilizers such as the Bradfield products, which directly provide nutrients as well as organic matter that supports microbial growth and provides colloidal binding sites, can help to prevent excessive soil acidity in the first place. Proper fertilization with an organic matter- containing fertilizer will help to maintain the physical characteristics of soil that are so necessary for optimal soil fertility and superior plant health.


References

Zimmer, Gary. 2006. Soil Basics: How It Works. Acres U.S.A.
MicroSoil: Cation Exchange Capacity. www.microsoil.com/CEC.htm

Cation-Exchange Capacity. Tree Fruit Soil and Nutrition. Washington State University.
soils.tfrec.wsu.edu/webnutritiongood/soilprops/04CEC.htm

Soil Basics - How it Works

1. The Symbiotic Decay-Nutrition Cycle

2. Water Uptake

3. Ions, Nutrition and all that “Scary” Chemistry

4. Who are these Microbes, and what are they doing in my Soil?

Download Soil Basics - How it Works from Bradfield Organics® (Adobe Acrobat Required)

 

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