4.Soil Alkalinity & Sodic Soil Amendment.

Soil alkalinity, like acidity is divided into two groups (1) active and (2) potential alkalinity. Active alkalinity is that of soil solution and determined by the titration of soil extract (soil:water=1:5) with 0.05N H₂SO₄. Its carriers are carbonates and bicarbonates of sodium and other alkaline cations:

2Na₂CO₃ +H₂SO₄→2NaHCO₃+Na₂SO₄

^{(Phenolphthalein)}

2NaHCO₃+H₂SO₄→ Na₂SO₄⁺+2H₂O+2CO₂↑

^{(Methyl orange)}

The exchangeable sodium determines potential soil alkalinity. Soils containing 5-20% of CEC occupied by sodium and other alkaline cations (K⁺,NH₄⁺, etc) are sodicity – affected & need the application of gypsum to improve their properties. Sodic soils have been called alkali soils. Sodic soils are mostly non saline & have adsorbed or exchangeable sodium percentage (ESP) of 15 or more (of 20 or more in Ukraine). Defflocculation or dispersion of colloids occurs, together with a breakdown of the soil structure. This results in a massive or puddled soil with low water infiltration & very low hydraulic conductivity within the soil. The soil is difficult to till & soil crusting may inhibit seedling emergence. Sodic soils may develop when the leaching of a saline soil results in high exchangeable sodium & low exchangeable calcium & magnesium (K.K.Gedroiz).

The basis for treatment of sodic soil is the replacement of exchangeable sodium with calcium and the conversion of any sodium carbonate into sodium sulfate:

[SAC^2-]2Na⁺+CaSO₄ ↔[SAC^2-]Ca²⁺+Na₂SO₄

Na₂CO₃ + CaSO₄→CaCO₃ + Na₂SO₄

The amount of gypsum needed to replace the exchangeable sodium is the gypsum requirement. In this country, gypsum requirement (GR,mt/ha) is computed by the formula:

GR,mt/ha=0.086(E_xch.Na⁺-0.05CEC)hd_v,

where E_xch.Na⁺ is the amount of exchangeable sodium expressed in m.-eq. per 100g of soil; CEC is expressed in the same units; h is the depth (cm) of a soil layer subjected to amendment and d_v is the bulk density of the soil in this layer, g/cm³.

Sometimes it may be necessary to remove excessive sodium sulfate by leaching of the soil (washing it with an excess of water).

Gypsum application improves soil structure water &air permeability and other properties.

5. Soil Buffer Capacity & Significance of Soil pH.

The buffer capacity is the ability of ions associated with the solid phase to buffer changes in ion concentration in the solution phase. In acid soils, buffering refers to the ability of the Al³⁺,H⁺ and hydroxy-aluminum to maintain a certain concentration of H⁺ in solution. The amount of H⁺ in the soil solution of a soil with a pH of 6.0 is extremely small compared with the non dissociated H⁺ adsorbed & the amount of aluminum that can hydrolyze to produce H⁺. Neutralization of the active or solution H⁺ results in rapid replacement of H⁺ from the relatively large amount of H⁺ associated with the solid phase. Thus the soil exhibits great resistance to undergo a pH change.

Acids or alkalis appearing in soil solution interact with SAC which alleviates the change in soil reaction:

[SAC^2-]Ca²⁺ +2HCl =[SAC^2-]2H⁺ + CaCl₂

[SAC^2-]2H⁺ +Ca(OH)₂=[SAC^2-]Ca²⁺+2H₂O

Soil solution may contain weak acids and their salts with strong bases making up buffer mixtures.

Significance of soil pH displays itself in nutrient availability, effects on soil organisms, toxicities and pH preferences of plants.

The actual concentration of H⁺ or OH^- are not very important except under the extreme circumstances. The most important factor is the associated chemical or biological environment of a certain pH. Some soil organisms have a rather limited tolerance to variations in pH. Other organisms can tolerate a wide pH range. Nitrogen availability is maximum between pH 6 & 8, because this is the most favorable range for the microbes mineralizing the nitrogen in organic matter and those organisms that fix nitrogen symbiotically. In calcareous soil (pH 7.5-8.3) phosphorus availability is reduced by the presence of CaCO₃ that repress the dissolution of calcium phosphates. Maximum P availability is in the range 7.5 to 6.5.Below pH6.5, increasing acidity is associated with increasing Fe and Al in solution & the formation of relatively insoluble Fe and Al phosphates.

Potassium calcium & Mg are widely available in alkaline soil. As soil acidity increases, these nutrients show less availability as a result of the decreasing CEC & decreased amounts of exchangeable nutrient cations.

Fe & Mn availability increase with increasing acidity because of their increased solubility. These two nutrients are frequently deficient in plants growing in alkaline soils because of the insolubility of their compounds.

Boron, copper & Zn are leachable & can be deficient in leached, acid soils. Conversely, they can become insoluble (fixed) & unavailable in alkaline soils. In acid soils, Mo is commonly deficient owing to its reaction with Fe to form an insoluble compound. Good overall nutrient availability occurs near pH6.5.

For the students of plant protection it is important to know that the requirement of some disease organisms is used as a management practice to control disease.

Maintenance of acid soil controls potato scab, though potato varieties have now been developed that resist scab organisms in neutral & alkaline soils. Damping - off disease in nurseries is controlled by maintaining soil pH at 5.5 or less. Fungi, compared with bacteria, thrive in highly acid soils. Earthworms are inhibited by high soil acidity. And so are the moles for whom earthworms are the primary food. Manganese toxicity may occur when soil pH is about 4.5 or less. High levels of solution aluminum restrict root growth.

Intensively weathered acid soils have a low CEC saturated with Al. The main benefits from liming are: (1) reduced exchangeable Al & reduced Al in solutions (2) an increase in the amount of Ca & Mg, and (3) increased availability of Mo.

The addition of acid sphagnum peat to soil may have some acidifying effect. Significant & dependable increases in soil acidity are best achieved by the use of S which is slowly converted to H₂SO₄ by soil microbes.

The overall reaction representing the neutralization of Al-derived soil acidity is:

[SAC^6-]2Al³⁺+ 3CaCO₃ + 2H₂O=[SAC^6-]3Ca²⁺+2Al(OH)₃ + 3CO₂

Acid soils that have been limed are subjected to the natural acidifying effects of biological respiration, organic matter mineralization & precipitation. Soils become acid again & must be retimed about every 2 to 5 years (every 3 to 7 years in Ukrainian crop rotations).

LECTURE SIX.

Soil structure.

General physical and mechanical properties of soils.

Study outline:

1. The structure of mineral soils, its importance, genesis, and classification.

2. Managing soil structure.

3. Particle density and bulk density.

4. Soil porosity and aeration porosity.

5. Mechanical properties of mineral soils and their management.

1. The structure of mineral soils, its importance, genesis, and

classification.

To understand how the soil behaves, we must consider the manner in which the various particles are packed & held together. They form a continuous spatial network - the soil matrix, or fabric. Sand, silt, & clay particles are typically arranged into secondary particles called peds or aggregates. The soils which have no structure may be single grained or massive. Structured soils are granulated into aggregates. When soil particles are associated in quasi - stable small clods is generally the most desirable condition for plant growth, especially in the critical early stages of germination & seedling establishment.

Structure modifies the influence of texture with regard to water & air relationships & the ease of root penetration. The macroscopic size of most aggregates results in the existence of interaggregate spaces that are larger than those existing between adjacent sand, silt, & clay particles. The following types & subtypes of structure are found in mineral soils:

1. Spheroidal (granular, crumby, very porous), characteristic of the furrow slice;

2. Block-like (blocky, cube-like) & sub angular (nutty), more common in heavy sub soils;

3. Prism-like (prismatic, with level tops) & columnar (with rounded tops), both subtypes usually subsoil manifestations ;

4. Plate-like (platy, leafy or flaky) may occur in any part of profile, but more usual in elluvial horizons.

Soil Structure generally develops from material that is without structure. In Ukrainian Soil science, it is common to identify the following factors of structure formation:

1. physico-mechanical,

2. physico-chemical,

3. chemical,

4. biological.

Physico-mechanical factors include wetting & drying, freezing & thawing, physical activity of roots & soil animals and tillage at proper soil wetness (physical maturity).

Physico-chemical factors are determined by the exchange cations and Soil adsorbing complex consisting of enough organic and mineral colloids. Colloids flocculated by calcium favor soil granulation. Sodium - cation deflocculates soil colloids and disrupts soil structure. So do the other alkaline cations (K⁺ & NH₄⁺). AI³⁺ & Fe³⁺ also coagulate soil colloids but the peds they form are consolidated & non-porous. Clays are not much good for soil structure unless there is enough organic matter with its binding and granulating action, especially in clay soils.

Chemical factors include the formation of precipitates, such as CaCO₃, Fe(OH) _3, MgSiO₃ etc, which cement small aggregates or mechanical particles into larger ones. Excessive wetness may instigate the formation or soil peds by the action of ferrous compounds, which turn into ferric ones wish oxidation.

But the basic role in soil granulation belongs to the biological factors: plants and animals living in the soil. Perennial grasses developing powerful roots help to form - meadow soils with favorable structure. Earthworms leave their pellets which are water proof aggregates. Crop residues are a source of soil humus which in its turn helps to form soil structure.

In Ukraine there is a custom to speak well about "agronomically valuable” structure. This structure is made up of macro aggregates (10-0,25 mm in size) which are sufficiently porous (over 45% by volume) and contain over 55% of water-proof grains and clods. Aggregate stability is of great importance for soil resistance to erosion. Some granules readily succumb to the beating of rain and the rough of plowing. Others resist disintegration. As a general rule, the larger the aggregates, the lower their stability.