Water in Soil

The basis of irrigation Soil Plant Evapotranspiration Plant requirements

Soil Composition Three main phases  g - gas  w - water  s – solids We want to know  What is the capacity of the soil to hold water  How much water is stored in the soil  How much water we can add to the soil

Soil porosity notations: g - gasV – volume/void w - waterW - mass s – solidst - total Void ratio– The ratio of pores volume to solid volume Porosity (n or ) – The ratio of pores volume to total volume Air filled porosity – The ratio of volume of air to total volume

Soil density  Soil density– The ratio of total mass to total volume Particle density– The ratio of total mass to total volume assume to be 2.65 gr cm -3 Bulk density– The ratio of solids mass to total volume typical values: clay soil gr cm -3 Sandy soils gr cm -3 Organic soils gr cm -3

What is the particle density of organic soils

Soil Texture Soil texture is a qualitative classification tool used in both the field and laboratory to determine classes for agricultural soils based on their physical texture. The classes are distinguished in the field by the 'textural feel' which can be further clarified by separating the relative proportions of sand, silt and clay using grading sieves: The Particle-size distribution (PSD). The class is then used to determine crop suitability and to approximate the soils responses to environmental and management conditions such as drought or calcium (lime) requirements. A qualitative rather than a quantitative tool it is a fast, simple and effective means to assess the soils physical characteristicssandsiltclayParticle-size distributioncalcium Equivalent diameter [mm]Particle type 2-0.2Coarse Sand Fine Sand silt <0.002Clay

Soil Texture

The water content in the soil Gravitational water Capillary water Hygroscopic water air dry Structure water oven dry 105 o C no water oven dry >>105 o C  s saturation  FC Field capacity  WP Wilting point

The water content   Water content (mass)– The ratio of mass of liquid content to the mass of the solids Volumetric water content – the fraction of the total volume of soil that is occupied by the water contained in the soil. {range 0-n} Degree of saturation – the fraction of the total volume of soil that is occupied by the water contained in the soil. {range 0-1}

Field Capacity is the amount of soil moisture or water content held in the soil after excess water has drained away and the rate of downward movement has decreased. Theoretical definition: amount of water held by soil against gravity. Wilting point is defined as the minimal point of soil moisture the plant requires not to wilt. If moisture decreases to this or any lower point a plant wilts and can no longer recover. The water content

Hygroscopic water - This water forms very thin films around soil particles and is not available to the plant. The water is held so tightly by the soil that it can not be taken up by roots. Structure water – water that are part of the particle chemical structure.

Forces on Soil Water Adhesion: The attraction of soil water to soil particles. Cohesion: The attraction of water molecules to other water molecules. Capillarity: A capilary is a very thin tube in which a liquid can move against the force of gravity as shown in fig 3 below. The narrower the tube the higher the liquid rises due to the forces of adhesion and cohesion.

Retention curve

Capillarity in soil The adhesion forces holds the water in the ground As the soil gets dryer only the thin capillaries are filled with water

Retention curve Moisture Condition Pressure [bar] Saturation0 Field capacity0.3 Wilting point-15 Air dry-31

 v Sand/clay Moisture Condition Pressure [bar] Saturation0 Field capacity 0.3 Wilting point -15 Air dry-31 What is the volumetric water content of the moisture conditions? How much water is available for the plants

Water depth The amount of water per an area unit. Water depth is described in units of length usually mm Water depth is also applicable for describing rain or evaporation If we know  and the root zone depth (h) we can calculate how much water we have in the soil column  100 m 10 m h=2 m

Example How much water are held in 100 cm 2 of soil with depth of 10 cm if  is 0.3. Home Work on-for-agrostudies/O51v1Rziw_4https://groups.google.com/forum/?hl=en#!topic/irrigati on-for-agrostudies/O51v1Rziw_4