Introduction

World population growth forecasts indicate that the world population in the year 2020 will be about 8 billion people. There is no way that soil reserves plus organic fertilizers will be able to produce the required food (drawing 1.01).

Only the use of fertilizers - primary linking hydrogen to air molecular nitrogen to produce ammonia - will enable the production of sufficient food.

The importance of the use of fertilizer, and primarily nitrogen, potassium and phosphorus fertilizers, is no longer argued.

CLAY MINERALS

The static negative electrical charges on clay minerals surface results in a concentration of cations sorbed to it. Due to the dipole character of the water molecules - water attracts to the cations creating hydration shell around them. Because the radius of the cations in their crystal state is specific to each cation, different cations with an identical electrical charge have a different "charge density" defined as: the number of charges on the cation divided by it's surface area. The attraction force of the water to the cation is therefore varies and specific to each cation. The charge density is inversely proportional to the radius of the cation in its crystal state. The charge density of the clay is very significant in determining the cation sorption to the surface. The strength of the bond between the clay and the cation, relative to its bond with water, will determine which cations will be sorbed to the clay, an during that process are loosing their hydration shell as compared with cations that hold the water so strongly that they do not lose their hydration shell near the clay surface. A cation might be replaced from their sorption site by another cation. The selectivity of cations on the clay surface is influenced by the hydration forces (drawing 1.02).

Magnesium is a small ion with a valence of 2. This small ion, is holding it's hydration shell very strongly while calcium, that is a larger divalent cation hold therefore less strongly the hydrated shell. As a result, the calcium looses it's hydration shell when it approach the clay surface and establish a stronger bond with the surface than magnesium whose close approach to the clay surface is prevented by the water molecules around it. This is the reason why magnesium is behaving similarly to sodium in soil conditions. 

In comparing Li, Na, K from the point of view of their concentration near the clay (drawing 1.3. Schematic ion distribution near a clay surface), we find that in the "Stern layer", the layer of "naked" cations attached to the clay without their water shell, the concentration of lithium with the smallest crystalline radius, that links water more strongly than sodium and potassium, is the lowest of the concentration of these ions, while the concentration of potassium, which has a weaker hydration energy - is the highest among them. A short distance away from the surface of the clay the picture changes with the concentration varying according to the laws of diffusion. This is the diffuse layer (Schematic ion distribution near a clay surface drawing 1.03)

This sorption mechanism is active and always applies so long as the specific charge of the clay surface is constant. With an identical concentration of ions in solution, the intensity of the water bond to the cation will determine the selectivity strength of the cations to the clay surface. If the charge strength on the surface is not constant, but is variable (such as in iron and aluminium oxide which change their charge density as a function of the pH) then the field strength of the clay surface will also influence the manner in which the ions are sorbed. When the electrical surface charge is sufficiently strong - even strongly hydrated ions will lose their water shell and their surface density. The selectivity order of their sorption will change according to their ionic radius and the surface charge density , which is expressed in Fig 1.04) as "r", the distance between adjacent charges on the surface (Eisenman). It is therefore not enough to know the characteristics of the ions, but the characteristics of the sorbing surface must also be considered. The charge density on surfaces of living and plant cell membranes is not constant. Changes in the ionic strength and the ionic composition in solution lead to changes of the structure, the charge and the charge density. Accordingly, in various solution conditions, the sorption characteristics and the membrane exchange vary with pH, total ionic strength and ionic composition that vary with time.

In such ways membrane selectivity of plant roots are created and or change with time.

Under equal conditions, with the same soil solution, one plant will flourish and another will die because of their differing ability to adapt themselves to the specific environment.

Cation adsorption- the model is relatively simple. It is assumed that the clay surface has a constant charge density per unit area as the case in uniform clay. A value for the cation exchange capacity is obtained, expressed in milli-equivalent of charge per unit weight of the sorbing material, which was expressed in the past by units of: meq/100g. The convention today is -mole of charge - and its symbol is Mc kg-1).

The quantity of charges that are sorbed on a unit surface differs from ion to ion. By convention, the quantity of charges that can be replaced by high concentration of ammonium ion is define as the "cation exchange capacity" in units of Mc kg-1.

Changing the replacing cations will give different result for cation exchange capacity.

Negative charges are rejected from the clay surface. Accordingly, an area "free" of anions will be developed near the clay surface. We term this phenomenon "negative sorption", in other words, the concentration of anions in this layer is lower than in the surrounding solution, further from the clay surface. Chloride and nitrate anions, for example, behave in this way.

Phosphate also bears a negative charge, however its behaviour is different from chloride and nitrate. This difference in anions behaviour can be explained by observing the clay mineral structure (drawing 1.05 drawing 1.06)

Kaolinite-type clay is constructed of a layer of aluminium atoms in octahedral organisation with oxygen and hydroxyl on the one side, and with only hydroxyl ions on the other. Silicon tetrahedrals are attached to the side bearing the oxygen. This is a mineral that is relatively poor in silicon (ratio of 1 between Al and Si) is common in acid soils. In nature there are layers, or stacks of such plates. Other minerals are created when there are two layers of silicone in tetrahedral arrangement on both sides of the aluminium octahedral layers. In such clays the ratio of silicon to aluminium is 2/1. These are the minerals found in relatively dry areas where the silicon remained and was not leached out from the minerals as silicic acid.

In very rainy areas, where the silicate is dissolved and largely washed away, a situation is also created where the soil is largely made up of aluminium or iron oxide minerals almost without silicate.

There is an isomorphic substitution in the clay's crystal unit, when about one out of six aluminum atoms (Al³⁺is exchanged by Mg²⁺ in the octahedral layer of montmorillonite type clay. As a result of this exchange, which does not change in the structure of the clay, (isomorphic exchange), sites are created where the positive electric charge is missing and in the balance of charges on the clay surface is a negative charge which is constant and not dependent o external pH.

With the illite clay, in contrast to montmorillonite the isomorphic substitution in fact takes place in the tetrahedral layer, with the silicon atom being exchanged by aluminium.

Here the positive charge is close to the surface of the clay and the potassium is balancing that charge and create s 12 coordinative bonds with the oxygen atoms in the 2 adjacent crystal units.This "fixed" potassium will be released only through the investment of a relative large amount of energy and time. About 6% of weight of the illite is potassium, this mean a huge amount of total K, even though with relatively low availability.

Anion sorption

The aluminium-oxygen-phosphorous bond produce a very similar type of electronic structure to that ofsilicon-oxygen (drawing 1.07)

If the silicon is removed from the alumo-silicate system there is a good chance that the "freed aluminium" from the silica bond will establish a bond with phosphate provide it is available in the same solution.

This in fact is the final stable situation of "maturing" an alumosilicate mineral exposed to an excess quantity of phosphorous.

When a very dilute suspension of sodium montmorillonite is shaken in a solution containing phosphate it is found that the phosphate concentration in the solution after the addition of the suspension is higher than in the clear phosphate solution. The explanation of this phenomenon is that the negative charge by the clay forced the negative anion away from the surface (this phenomenon is known as "negative sorption"). The concentration of the anion accordingly rose in the suspension far away from the surface, and is concentrated in the volume termed "the diffusion layer" (drawing 1.03 , drawing 1.08)

In conditions where high concentration of chloride is present with montmorillonite, positive sorption of phosphate is observed and no anionic competition between P and Cl is observed.

The phosphate, in contrast to chloride, can contribute a negative charge into the aluminium atoms present at the broken edges of the clay. When the phosphate ion approaches an aluminium site, a chemical bond is established between the aluminium and the phosphate. This bond is created by the contribution of OH- from the phosphate and the exit of H+ from the aluminium site so that the phosphate bond to the clay involve the exit of water molecules from the surface. In kaolinite clay, phosphate sorption takes place under conditions in which it does not on montmorillonite. Phosphate sorption on kaolinite is clearly dependent on the pH in the suspension (drawing 1.09) .

The montmorillonite in the ground, which is not distributed in suspension, absorbs phosphorous.

The iron hydroxides (like the aluminium) sorbed on clay surface also strongly sorbs phosphate from the solution.

Phosphates that come near the clay surface are linked to the aluminium through the oxygen and creates an hexagonal ring, in which it is linked to the structure of the clay (drawing 1.10), through 2 oxygens and is not exchangeable.

When aluminium is exposed to the edges of the fractured mineral - a chemical bond is created of the phosphate with the aluminium octahedral and a water molecule is formed and leaves the sorption site. 

If the pH increases greatly, i.e., OH- is added to the system - the phosphate will desorb from the aluminium and a water molecule will enter. In low pH - the phosphate sorption will increase (drawing 1.09, drawing 1.10) ,

In the case of phosphorous, therefore, there is no anion exchange but a kind of "titration" of acid surface by a weak base. Therefore, even though there are other anions in the environment - the phosphorous will be taken up and not a single other anion like chloride or nitrate. In other words - this is a "specific sorption" creating a P complex on the sorbing surface and it is accompanied by a change in the electrical charge of the clay or of the sorbing surface and an increase in the uptake capacity of the cations (drawing 1.08). The sorption of phosphate into lateritic soil (rich with aluminium and iron oxides) increases its cation exchange capacity. This does not occur in montmorillonitic soils

kaolinite has a variable electrical charge depending on the pH. Therefore with the increase in pH the absorption of phosphate decreases and the absorption of calcium to the kaolinite increases, dissolves with constant concentration of calcium and phosphorous. When the pH varies, the cations exchange capacity varies.

In mixing clay with weak solutions of phosphorous - phosphorous is absorbed from the solution. Increasing the P concentration increases it's uptake but when attempt is made desorb P from the surface very small amount of P can be released from the surface (drawing 1.11).

Re-uptake and rerinsing give even greater sorption values. The phosphate sorption to the clay is therefore influenced by the history of the process. An examination of the rate of isotopic exchange of phosphorous from the solution to the sorbed sites shows that only a small part of the sorbed phosphorous can undergo isotopic exchange (drawing 1.12).

These findings were used to develop the model of phosphate sorption as shown in Fig. 1.10