Soil water holding capacity is controlled primarily by the soil texture and the soil organic matter content. Soil texture is a reflection of the particle size distribution of a soil. An example is a silt loam soil that has 30% sand, 60% silt and 10% clay sized particles. In general, the higher the percentage of silt and clay sized particles, the higher the water holding capacity. The small particles (clay and silt) have a much larger surface area than the larger sand particles. This large surface area allows the soil to hold a greater quantity of water. The amount of organic material in a soil also influences the water holding capacity. As the level of organic matter increases in a soil, the water holding capacity also increases, due to the affinity of organic matter for water.

Hi

Agriculture sector is a major consumer of water resources in our country. To increase irrigation efficiency and optimal use of water resources Arid and semi-arid various measures such as the use of mulch, fertilizer and modify the physical properties of soil Various modifiers such as perlite, plus hydro and the like are made. To compare the absorbent material on the natural and artificial water Soil water retention, factorial experimental design completely randomized design with three treatments of three factors Jazbab) control ) Without water-absorbing material (rotted cow manure) 15 grams per kilogram of soil (and superabsorbent refreshing 2) A200 grams per kilogram of soil ((three Sandy loam and clay loam was repeated in two. Then the soil water content for each treatment suctions 5, 3, 1, 0/5, 0/3 0/1, 0, 15 Soil moisture measurement curves were plotted separately. The results showed that between soil texture and treatments  Different suctions and the interaction of these factors together, there are significant differences in the level of one percent. Generally, the amount of capacity Soil moisture retention in the tissues, using materials found Jazbab respectively. Super absorbent polymer in a sandy loam and Clay loam soil volumetric water content, respectively 3/5 and 1/3 times compared to control, while the increase for fertilizers Respectively 1/6 and 1/1 respectively. The use of polymers in the soil, especially in light textures with a higher water holding capacity. The success of irrigation projects in arid and semi-dry.

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&uact=8&ved=0ahUKEwjhuYGTofTPAhXH1iwKHTCNCe0QFggrMAI&url=http%3A%2F%2Fjise.scu.ac.ir%2Farticle_10796_51b6e7313613cf1ed756d7c647112b9c.pdf&usg=AFQjCNE3us9IgtPTN9IzhS-joOsadSqhjQ&bvm=bv.136593572,d.d24

It is definitely confusing.

Water holding capacity increase with structure development/modifications due to changes in pore size distribution, the fraction of micro-pores enhance and contributing to more water holding at field capacity, and thus increased available water. As far as total porosity is concerned, it may not be altered so saturated moisture content may not change. But practically, field capacity moisture content decides the available water under aerobic conditions

regards

Dear Kim,

yes sandy soil have less water-holding capacity than the others, however soil structure in sandy soil is mainly a biotic process due to soil flora (roots exudation) and fauna, like earthworms and others meso and micro fauna. So soil structure will develops in parallel to biologic activity which will incorporate organic matter in your sandy soil. So you start from the worse case and can only improve it.

Thank you all for comments :)

Dear Jerome Juilleret.

Thank you for your reply, Now I feel clearer about the sandy condition. However, I still have doubt.

What do processes mainly affect to develop soil structure in fine-textured soils? And why does water-holding capacity decrease as structure develops in fine-textured soils?

Hi Kim,

coarse-textured soil such as sandy soil does not have so micropores to retain water for a long time, the water easily infiltrates into subsoil . As the soil structure develops and gets better, the micropores enhance ,thus the soil has the ability to hold water among the the pore spaces for a longer time. 

Dear Kim,

in fine textured soil, development of macropores either biogenic (earthworms) or physicogenic (cracks in clayey soil, or even interpedal macropores)  enhanced drainage capacity of the soil.

Soil water holding capacity is controlled primarily by the soil texture and the soil organic matter content. Soil texture is a reflection of the particle size distribution of a soil. An example is a silt loam soil that has 30% sand, 60% silt and 10% clay sized particles. In general, the higher the percentage of silt and clay sized particles, the higher the water holding capacity. The small particles (clay and silt) have a much larger surface area than the larger sand particles. This large surface area allows the soil to hold a greater quantity of water. The amount of organic material in a soil also influences the water holding capacity. As the level of organic matter increases in a soil, the water holding capacity also increases, due to the affinity of organic matter for water.

Good discussion by colleagues on how structure influences water holding capacity of coarse- textured and fine-textured soils.One can take sandy/sandyloam soil and clay/clay loam soil as examples for the two textural types.Coarse- textured soils will have predominantely macropores and fine- textured soils the micropores.The total porosity is more in clay soil(because of very large number of micropores) compared to sandy soil.Without much aggregation and structure development, the microporosity is very less in coares-textured soils and so their waterholding capacity is very less compared a finetextured soils.In coarse- textuted soils the structure development increases the microporosity and consequently increases  water holding capacity(as also explained by Dr..Kalra).In fine textured soils the aggregation and structure development may decrease the microporosity to some extent  and possibly the water holding capacity may also come down to that extent.But the proportion of plant- available water will increase in clay soil with structure development because of increase in macroporosity.In addition to organic matter(crop residues,manures and composts),sesquioxides, fungal hypae,glomalin and synthetic polymers aid in structure build up.Farmers apply clay soil and tank silt to coare- textured soils to improve their waterholding capacity through increase in microporosity.They also add sand to clay soils to reduce the microporosity and increase the macroporosity, and consequently the water availabilty and aeration in soil.

Dear Kim,

(1) Coarser textured soils will generally have larger pore sizes and little soil structure, resulting in rapid water drainage. A poor or non-existent soil structure will have a reduced volume of soil pores.  Soil aggregates create pores which store water for plants to access. 

(2) Increasing soil organic matter content helps create and stabilise soil structure. Organic matter is considered integral in the capacity of a soil to maximise water storage through its effect on creating and stabilising soil pores and its absorption capacity. 

With the improvement in soil structure , supposedly aggregate sizes , proportion micropores to macropores increases , resultantly , there is an increase in water holding capacity of  the coarse textured soils , originally having reverse distribution of pore sizes , infact little micropores...

Once water enter a soil profile, one part of it is stored in rooting zone for plant use, some will evaporate and some will drain away from root zone. Plant available water is the difference between field capacity  and the wilting point . Beyond the wilting point there is still water in the soil profile, however it is contained in pores that are too small for plant roots to access. Soil texture, soil structure and plant rooting depth are the crucial factors in determining the amount of water available for plants to access.

To interpret and discuss water holding capacity we do need firstly to know about what is (was) potential that we are talking about. As water holding capacity change with the soil potential. Probably this book was trying to explain an enhance in water holding capacity at low potentials (near field capacity) that is dependent on the soil structure (large "pores") that may increase with development of structure in sandy soils.