The soil is the stomach of the plant [Schriefer, 1984].

Plants take up soluble salts, but in a healthy soil, soil microbial digestion [similar to a cow's or sheep's rumen] occurs with microbes predigesting organic matter in order for the plants to recycle it. Plants can also cultivate different micro-organisms around their rhizosphere by regulating root exudates.

Plants feed in four "Official chemical" ways:

• Foliar [Feeding through the leaves]

• Mass flow [Suck it] Day time[?]
• Diffusion [Concentrate it, causing an "updraft"]
• Root interception [Swap it] Night time[?]

Root Diffusion = Movement of ions from a high to low concentration of nutrients
As a plant's root absorbs nutrients from the soil it creates a "sink" or vacuum or as Schriefer [1984] suggests an up draft which nutrients move into. Nutrients [ions] in areas of high concentration in the soil solution migrate to where the lower concentration of elements exist due to the plant root removing them. Most of a plant's phosphorous and potassium uptake moves to the root by means of diffusion.

Root interception = Contact exchange of ions
As new roots grow they come in contact with "new" soil and intercept or seize nutrients by exchanging plant exudates for ions on soil colloids i.e. humus and clay [both negatively charged]. Root interception is most active when plants are actively growing because as plants come in contact with, and are exposed to, a progressively larger soil surface area, the plant can swap or exchange ions / nutrients.

Ion exchange occurs when the root exudes for example hydrogen which leaves the root and is attached to a soil colloid. As the hydrogen separates from the root it leaves a negative charge on the root surface. Therefore, the positively charged cations like Calcium, Potassium, Zinc and Copper are electrically attracted to the root. Similarly, plants roots can also release negatively charged anions which leave vacant positive charges on the root's surface, which in turn attract anions like Boron and Phosphorous. As you can see plants to a point select what elements they require but this does not happen as efficiently when water soluble fertilisers are used as the plant by mass flow literally has to take them up as it "drinks" up water [and the soluble nutrients].

SOIL CHEMISTRY

The chemistry of the soil can be thought of as the service to the soil house and will dictate which tenants live there.

We will now go further and start to explore our soils by looking at soil chemistry, which can also greatly influence the physical and biological nature of the soil.

The subject of soil chemistry involves the investigation of identifying the different elements in a soil and how they influence that soil.

Your soil is "full" of elements which can be divided into positive and negative elements:
The elements that have a positive [+] charge are collectively called cations ["cat irons"].

Major Cations

Calcium	[Ca ++]	Potassium	[K +]	Hydrogen	[H +]
Magnesium	[Mg ++]	Sodium	[Na +]

Minor Cations

Iron	[Fe++]	Copper	[Cu++]
Zinc	[Zn++]	Manganese	[Mn++]

There are other elements that have a negative [-] charge and they are collectively called anions ["an irons"]. For example:

Phosphorous	[P-]	Sulphur	[S-]	Molybdenum	[Mo-]
Chlorine	[Cl-]	Carbon	[C-]	Boron	[B-]

Poor soils are full of the wrong balance of elements. They have an excess of some elements and a deficiency of other. For example in an acid soil there is an excess of Hydrogen. In an alkaline soil there is often an excess of two of the three dominant cations being Calcium [Calcareous soil], Sodium [Saline soils] and Magnesium [black or grey pug clay if high sodium].

Productive healthy soils are full of the correct balance of elements.

So to correct a poor soil and improve the health of the plants and animals we need to duplicate or copy the soil element balance found in productive healthy soils. We can do this by adding or changing the availability of the needed elements.

Dr William Albrecht together with other scientists found that a good soil should have over 95% of the available elements as cations on the Base Saturation Percentage! These cations were needed within certain ranges. The Albrecht cation balance model can act as an international benchmark to assess how close your soil is to a chemically balanced soil. So to correct a poor soil we need to duplicate or copy the required soil element balance that nature requires for each group of plant species that we wish to grow.

Our challenge is to work in harmony with nature to provide the correct chemical balance for the plants that we grow [without forgetting the biological and physical requirements of the soil].