BASIC SOIL PLANT WATER RELATIONS N.L Mufute mufutenl@msu.ac.zw

1. Soil Properties Texture Structure Definition: relative proportions of various sizes of individual soil particles USDA classifications Sand: 0.05 – 2.0 mm Silt: 0.002 - 0.05 mm Clay: <0.002 mm Textural triangle: USDA Textural Classes Coarse vs. Fine, Light vs. Heavy Affects water movement and storage Structure Definition: how soil particles are grouped or arranged Affects root penetration and water intake and movement

Bulk Density (b) Typical values: 1.1 - 1.6 g/cm3 b = soil bulk density, g/cm3 Ms = mass of dry soil, g Vb = volume of soil sample, cm3 Typical values: 1.1 - 1.6 g/cm3 Particle Density (p) P = soil particle density, g/cm3 Vs = volume of solids, cm3 Typical values: 2.6 - 2.7 g/cm3

Porosity () Typical values: 30 - 60%

2. SOIL WATER MEASUREMENT Soil water content Can be expressed as; Mass water content (m) - The mass of water associated with a given mass of soil. m = mass water content (fraction) Mw = mass of water evaporated, g (24 hours @ 105oC) Ms = mass of dry soil, g 2.Volumetric water content (v) – The volume of water associated with a given volume of dry soil. (Usually expressed as m3 of water / m3 of soil

Either approach is acceptable but use of V is more common V = volumetric water content (fraction) Vw = volume of water Vb = volume of soil sample V = As m As = apparent soil specific gravity = b/w (w = density of water = 1 g/cm3) Either approach is acceptable but use of V is more common Equivalent depth of water (d) or z d = volume of water per unit land area = (v A L) / A = v L d = equivalent depth of water in a soil layer L = depth (thickness) of the soil layer

Volumetric Water Content & Equivalent Depth (cm3) Equivalent Depth (cm3) (g) (g)

Volumetric Water Content & Equivalent Depth Typical Values for Agricultural Soils Soil Solids (Particles): 50% 0.50 in. 1 in. Very Large Pores: 15% (Gravitational Water) 0.15 in. Total Pore Space: 50% Medium-sized Pores: 20% (Plant Available Water) 0.20 in. Very Small Pores: 15% (Unavailable Water) 0.15 in. 1 inch = (approx ) 2.54 cm

Soil Water Measurement : Measuring soil water content and water potential. A. Direct Methods Gravimetric Measures mass water content (m) Take field samples  weigh  oven dry  weigh Advantages: accurate; Multiple locations Disadvantages: labor; Time delay Feel and appearance Take field samples and feel them by hand Advantages: low cost; Multiple locations Disadvantages: experience required; Not highly accurate

Soil Water Measurement: Indirect Methods Neutron scattering (attenuation) Measures volumetric water content (v) Attenuation of high-energy neutrons by hydrogen nucleus Advantages: samples a relatively large soil sphere repeatedly sample same site and several depths accurate Disadvantages: high cost instrument radioactive licensing and safety not reliable for shallow measurements near the soil surface Dielectric constant A soil’s dielectric constant is dependent on soil moisture Time domain reflectometry (TDR) Frequency domain reflectometry (FDR) Primarily used for research purposes at this time

Soil Water Measurement Neutron Attenuation

Soil Water Measurement Tensiometers Measure soil water potential (tension) Practical operating range is about 0 to 0.75 bar of tension (this can be a limitation on medium- and fine-textured soils) Electrical resistance blocks Tend to work better at higher tensions (lower water contents) Thermal dissipation blocks Require individual calibration

Tensiometer for Measuring Soil Water Potential Water Reservoir Variable Tube Length (12 in- 48 in) Based on Root Zone Depth Porous Ceramic Tip Vacuum Gauge (0-100 centibar)

Electrical Resistance Blocks & Meters

Water and Plant Growth:SOIL-WATER-PLANT RELATIONSHIPS. Field Capacity (FC or fc) Soil water content where gravity drainage becomes negligible Soil is not saturated but still a very wet condition Traditionally defined as the water content corresponding to a soil water potential of -1/10 to -1/3 bar Permanent Wilting Point (WP or wp) Soil water content beyond which plants cannot recover from water stress (dead) Still some water in the soil but not enough to be of use to plants Traditionally defined as the water content corresponding to -15 bars of SWP

Available Water Definition Available Water Capacity (AWC) Water held in the soil between field capacity and permanent wilting point “Available” for plant use Available Water Capacity (AWC) AWC = fc - wp Units: depth of available water per unit depth of soil, “unitless” (in/in, or mm/mm)

Soil Hydraulic Properties and Soil Texture

Fraction available water depleted (fd) (fc - v) = soil water deficit (SWD) v = current soil volumetric water content Fraction available water remaining (fr) (v - wp) = soil water balance (SWB)

Total Available Water (TAW or TAM) TAW = (AWC) (RZD) TAW = total available water capacity within the plant root zone, (mm) AWC = available water capacity of the soil, (mm of H2O/mm of soil) RZD = Root zone depth, (mm) If different soil layers have different AWC’s, need to sum up the layer-by-layer TAW’s TAW = (AWC1) (L1) + (AWC2) (L2) + . . . (AWCN) (LN) - L = thickness of soil layer, ( mm) - 1, 2, N: subscripts represent each successive soil layer RZD =L1+L2+…+LN

Management Allowable Depletion (MAD) The growth and yield of plants or crops get severely hampered when the AWC is depleted below a certain level, i.e. as water content approaches wilting point. Readily Available Moisture (RAM) refers to that part of AWC which the plant can easily extract from the root zone. This varies with the root zone depth (RZD) of the plant. So as someone involved in water management in Irrigation need to set an allowable or acceptable depletion of the AWC and then come in with an irrigation to replenish the lost water.

Management Allowable Depletion (Cont.) This depletion level is what is called Management Allowable Depletion (MAD). It actually means at that level of moisture depletion, crop growth and yield are not affected. For most crops such as maize and wheat the MAD is 50% of the AWC. For crops like cotton and sugarcane the MAD can go as far as 60% of the AWC and for vegetables and related crops, the MAD is limited to around 40% of the AWC.

Water-Holding Capacity of Soil Effect of Soil Texture Coarse Sand Silty Clay Loam Dry Soil Gravitational Water Water Holding Capacity Available Water Unavailable Water

Depth of Soil Water Cont. The concept of depth of soil water is very important in irrigation agronomy. If the root zone depth of the soil is RZD or Rz, then the volume of soil per unit area of field surface is given by (Rz*1 ). If d or z is the depth of the soil water available for plant use (assuming that all the soil water is brought together at one place), then the weight of water per unit area (Ww) is given by:   Ww = (z*1)ρw  Where ρw is the bulk density of water (standard at 1000kg/m3 or 1g/cm3).

Depth of Soil Water Cont. The percentage water content, RAM (or Pw ) readily available for plant use i.e. that fraction of AWC that can safely be depleted without affecting plant performance is given by:   Pw = (z* ρ w/ ρ sRz)*100 Where ρs is the bulk density of the soil. Alternatively , the depth of soil water (z) can be expressed as z= Pw *Rz*(ρs/ρw)/100 Note that z will have the same units as Rz.

Worked example A soil has a FC of 22%, PWP of 10% and a bulk density of 1400kg/m3. The Rz depth is 100cm and the MAD is 60% of the AWC. Given the above information, calculate or determine the AWC, percentage water content readily available for plant use (Pw) and the depth of soil water which can be safely depleted (z).

Worked example :Solution AWC= FC-PW = (22-10)% = 12%,   Pw = 60% of AWC = 60/100*12% = 7.2% z= Pw/100*(s/ w)* Rz = 7.2/100*1400/1000*1000mm= 100.8mm.

An Alternative Approach In this approach (Which gives an approximate answer), the density of soil is not taken into consideration and hence; z = MAD x AWC x Rz or Pw xRz   For the above example therefore z= 60%x12%x1000mm =72mm Or z = 7.2% x 1000mm = 72mm