Analyse the relationship of soil moisture with structure & organic matter of soil.

soil moisture or soil water is the water present within the interstitial spaces in between soil particles which is held by matrix potential.Soil matrix is thought of as the hierarchy of capillaries whose precise characteristics depend on-

a)particle size of soil;

b)organic matter content;

c)nature of aggregates;

d)activities of soil fauna.

                                       the organic & inorganic minerals are present in colloidal state where soil moisture plays the role of dispersion medium.The hydration layer prevents the colloids from coming in contact with each other & settling down.

Soil aggregates which give structure to the soil is a cluster of particles brought together by physical forces.Aggregates comprise of clay rich soil at one end of the scale & organic faceal pellets of microarthopods & casts of annalids at other ends.The stability of aggregates depends on components like:-

1)capillarity of water:-Hydrogen bonding between water molecules & soil particles.

2)Divalent cations:-aggregates between clay particles are held by Ca bridges between adjacent platelets.

3)Colloidal oxides & hydroxides.

4)Humic colloids;

5)Exuded polysaccharides of plant/microbial origin;

6)Mucus of soil fauna;

7)Fibrous organic matter.

                                     Soil moisture contains & respectively.As an example it can be said that in case of silicate clays forms silicon-oxygen tetrahedra by combining with  ion &  ion to nutralise the cation charge giving rise to a stable soil structure.

Coagulation which is a phenomenon  of precipitation of colloidal particles due to aggregation by addition of an electrolyte results in aggregate formation & hence,promote different soil structures.Eg.-granular & crumb peds are formed when mineral aggregates are glued together by organic mater.This type of peds are found in grassland region where organic matter content is high.organic matter on the other hand by their presence & turnover influences water porosity which influences soil structure.

Platy structures caused by natural or anthropogenic compaction depends more on parent material than on clay or organic matter.Soil organic matter is an important examples of natural organic colloid.

Quantitatively humus is the most prominent fraction of soil organic matter & is negatively charged .The property of sorption is developed to a higher degree in organic colloids than in inorganic colloids.However,excess of sorbed Na & to a lesser degree K leads to dispersion & destruction of soil structure.Eg.-silicon-oxygen tetrahedron between   &  & is destroyed when sodium enters this stable system as it combines  with hydroxyl ions to form sols or gels by the process of peptization (which is the process of conversion of a precipitate into colloidal sol by adding an elecrolyte). Neutral & acidic polysaccharides are controlled by uronic acid & neutral polysaccharides (Dextrans) are produced by algae.

Tisdall & Oades has identifies 3 modes in which soil organic matter as a binding agent contributes to soil structure.These modes  are :-

1)Transient-;

2)Temporary;

3)Persistent.

Greenland,Rimmer & Payne (1975)have suggested that non-calcereous soils with organic matter content below 3.4% will suffer structural deterioration but those soils with more than 4.3% of organic matter are structurally stable.

The above figure itself shows how intricately is joined & is an important factor determining structural stability.This in turn shows the importance of soil moisture which is the source of  ions in the soil(water-hydrogen ion+Hydroxyl ion)dispersed clay tends to clog up the pore space in the soil causing water logging & poor aeration.Clay colloids holding large amounts of plant nutrition like- ,,,,etc. gets washed  away by run-off.Dispersed clog the interstitial spaces preventing the water from entering resulting in surface swamping.Since, ion is lost sodium,potassium & hydrogen ions are left behind to increase the soil acidity,the soil being deprived of bases.

                                         When saline ground water reaches soil surface in arid zone especially in depressions .high salt concentration leads to collapse of clay aggregates dependent on divalent cation bridges.this salt induced defloculation leads to poor drainage & poor aeration,due to depression of clay particles.However,development of anaerobic condition will depend on the presence of organic matter which in terms of soil function has  a distinct bulking-up effect,which may be thought of as a dilution of elements present in the earth's crust & soil.

                                       organic matter also increases soil porosity by aiding soil aggregate formation although porosity also depends on soil particle size,it's packing.

                                                                                                                                                                                                                                                                                                            

 

                                                                                                                                                                                                              

                                                                                                                                                                                    

                                                                                                                                                                                      

In conclusion to the above discussion it can be said that soil moisture plays the important role of dispersion medium for organic & inorganic colloids of different charges until they coagulate to form soil aggregates thus giving structure to the soil while organic matter glue together these aggregates.Organic matter also partly aids porosity which is important for the entry of the dispersion medium,water.