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*Saturation is the moisture content when lack of oxygen will adversely affect plant growth and may induce denitrification. Note that saturation (on a weight basis) occurs well below 100% and even below 50% on most soils. Plants cannot extract all the available water between field capacity and permanent wilting point with equal ease. Soil water is more readily available to plants when soils are near field capacity and less so as soil moisture content approaches the permanent wilting point. Table 3.2 Determining available soil moisture by feel or appearance
Soil moisture definitions for other purposes
Table 3.3 Moisture contents (by weight) for selected soil types and depths
Reporting soil moisture
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![]() Figure 3.2 Relationship between hydraulic conductivity and soil texture |
Hydraulic conductivity is the rate at which water
can pass through a soil material, usually measured under
saturated conditions (i.e. when a small volume of soil has
been sufficiently saturated) to ensure water is moving
through the soil via gravity and positive head pressure.
Saturated hydraulic conductivity (Ksat)
provides the simplest and most consistent means of measuring
the rate of water movement through soils. The rate of water movement through a given soil is largely determined by the texture. Large soil particles (sands) create large pore spaces between the particles, allowing water to move through these pores relatively quickly and with little adhesion to soil particles. Small soil particles (clays) pack together more tightly, producing numerous small pore spaces that represent a larger volume than the pore volume of sandy soils, but allow the transmission of water at a much slower rate. Movement of water through clay soils is restricted by the small pore size and the significant adhesive forces between water and soil particles. Other factors affecting water movement through soil are the internal drainage (i.e. depth to water table), soil structure, amount of organic matter present and the presence of soluble salts (salinity). |
Several concepts need to be discussed to understand how to calculate the depth of water infiltration.
Soil porosity is the percentage of a given volume of soil that is made up of pore spaces. Soils are oven-dried to measure bulk density, so porosity is a measure of air-filled pore space.
Bulk density is the apparent density of a soil, measured by determining the oven-dry mass of soil per unit volume. The volume of soil is determined using sampling cores and is measured before soil is oven-dried to avoid any changes in volume due to drying. Bulk density is usually expressed in g/cm3 or Mg/m3.
Table 3.4 Typical bulk densities for various soil series
Soil Series | Bulk Density (g/cm3) 0-6" depth (0-15 cm) |
Stockton fine sand | 1.34 |
Newdale clay loam | 1.26 |
Red River heavy clay | 1.07 |
Most rocks | 2.65 |
Compacted soil | 1.80 |
Particle density is the grain density, or the mass per unit volume of the soil particles. Pore spaces found in bulk soil samples are excluded. Particle density is usually expressed in g/cm3 or Mg/m3, and the particle density for most agricultural soils is 2.65 g/cm3.
These three factors are used to calculate the depth an inch of precipitation moves into a given soil.
The distance an inch (25 millimetres) of water (precipitation) moves into the soil depends on several factors including initial soil moisture content, amount of water lost as runoff, texture, structure, organic matter content and porosity. A general estimate can be calculated for dry soil using the following formulae:
% Porosity = [1-(bulk density ÷ particle density)] x 100
where particle density = 2.65 g/cm3
Depth of water infiltration for dry soil
~ [depth of water ÷ (% porosity/100)]
E.g. 1) A sandy soil with a bulk density of 1.2 g/cm3:
% Porosity = [1 – (1.2 ÷ 2.65)] x 100
= 55%
Depth of water infiltration
~ [1 inch ÷ (55/100)] = 1 inch ÷ 0.55 = 1.8 inches
Therefore, an inch of precipitation will move 1.8 inches (4.5 centimetres) in a dry sandy soil.
E.g. 2) A clay soil with a bulk density of 0.9:
% Porosity = [1 – (0.9 ÷ 2.65)] x 100
= 77%
Depth of water infiltration
~ [1 inch ÷ (77/100)] = 1 inch ÷ 0.77 = 1.3 inches
Therefore, an inch of precipitation will move 1.3 inches (3.25 centimetres) in a dry clay soil.
Table 3.5 Relative crop suitability on various soil types
Texture | Coarse
(sand) |
Medium (loam, clay loam) |
Fine (clay) |
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Drainage | Well | Imp. | Poor | Well | Imp. | Poor | Well | Imp. | Poor |
Crops: | |||||||||
Cereals | P | P | W | P | P | W | P | W | W |
Flax, canola | M | M | W | P | P | W | P | P | W |
Peas, lentils | M | P | W | P | P | W | W | W | W |
Field beans | P | P | W | P | W | W | W | W | W |
Sunflowers | P | P | W | P | P | W | P | P | W |
Soybeans | M | M | W | P | P | P | P | P | W |
Faba beans | M | M | W | P | P | W | P | P | W |
Corn | P | P | W | P | P | W | P | W | W |
Buckwheat | P | P | P | P | P | W | P | W | W |
Canary seed | M | P | W | P | P | W | P | W | W |
Potatoes | P | P | W | P | P | W | H | H | W |
Hybrid popular | M | P | W | P | P | W | P | W | W |
Forages: | |||||||||
Alfalfa | M | P | W | P | P | P | P | W | W |
Drought tolerant grasses* | P | P | P | P | P | W | P | W | W |
Flood tolerant grasses** | M | M | P | P | P | P | P | P | P |
Orchardgrass | M | M | P | P | P | P | P | W | W |
* = tame species of wheatgrasses, wild rye, etc.
** = reed canarygrass, meadow foxtail, fescues, etc.
P= suitable most years
M = moisture challenges in normal-dry years; suitable in wet
years
W = wetness challenges in normal-wet years; suitable in dry years
H = harvesting challenges (i.e. potatoes on clay)
1 inch (25 millimetres) of precipitation = 22,500 gallons/acre (252,675 litres/hectare) of H20
actively growing plants transpire approx. 1/3 inch (8.3 millimetres) of water per day (which is 7500 gallons/acre/day or 84,225 litres/hectare/day).
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Figures 3.3 and 3.4 Examples of soils with drought limitations |
B. Wet soils (soils with an agriculture capability modifier “W”) require moisture removal, which includes the following practices:
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Figures 3.5 and 3.6 Examples of soils with excess water limitations |
Yield losses are greatest on clay soils during periods of excess water in July, regardless of crop (Rigaux & Singh, 1977).
Table 3.6 Indicator weeds of soil moisture problems
Dry Soils ("M" limitation) | Poorly Drained Soils ("W" limitations) |
Tumble mustard Stinking mayweed Thyme-leaved sandwort Stork's bill Purslane Prostrate pigweed |
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Table 3.7 Cropping and management strategies
Droughty soils and drier weather conditions | Wet soils and wetter weather conditions | Soils with both moisture (M) and wetness (W) limitations |
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| Understanding the Soil Landscapes in Manitoba | Using Soil Survey Information | Water Use and Moisture Management | Nutrient Management | Soil Salinity | Drainage | Soil Erosion | Tillage, Organic Matter and Crop Residue Management | Soil Compaction | Soils Information for Planning Purposes | Other Applications of Soils Information | Summary | References | Appendices|
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