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Determining slope and soil texture

Estimating slope

Slope can be expressed in a number of ways. One is a percentage which can also be converted to the amount of fall over a 100 metre distance. The table below shows the conversion of percent slope to fall over 100 metres. A visual assessment can be made using the amount of fall over 100 metres and converting back to a percentage.

Table 1:Conversion of percent slope to fall over 100 metres

Precent slope

Amount of fall over a 100 metre distance

0%

Flat with no fall

1%

1 metre

3%

3 metres

8%

8 metres

25%

25 metres

A clinometer can be used for a more accurate measurement of slope. Clinometer apps are available for smart phones.

A dumpy level can also be used to accurately determine the fall over 100 metres or more (or less).

Slope may also be determined using a GPS or topographic map. To determine the slope using a topographic map you will need the rise (the difference in elevation between two points) and the run (the distance between two points calculated using the map scale). Slope can then be determined with the following calculation: rise / run x 100 = % slope

Assessing Soil Texture

Soil texture refers to how coarse or fine the soil is: that is, how much sand, silt and clay it contains. Texture has a major influence on how much water a soil can hold. Generally, the smaller and finer the soil particles (the more silt and clay), the more water a soil can hold, and the less susceptible it is to wind erosion with adequate rainfall.

Soil texture can be estimated by hand using the ribboning technique, noting that it takes practice to produce a consistent result.

Carry out this ribbon test on a sample of soil from the area to be cleared using the Code. If soil differs across the area to be cleared, assess each area separately.

Do this several times for confirmation and compare the average ribbon length with those in Table 4 below. Each soil texture is classified within a ribbon length range (for example, sandy clay loam ribbon length is 25 to 40 mm long).

Once a consistent ribbon length is being produced, you can be reasonably confident that the correct soil texture has been identified.

Table 2: Soils textures using the ribboning technique

Broad Groups

Texture Grade

Behaviour of the
soil ball

Ribbon (mm)

Sands

Sand

Ball will not form

0

Loamy sand

Ball just holds together

5

Clayey sand

Ball forms, sticky-clay stains fingers

5-15

Sandy Loams

Sandy loam

Ball forms, feels sandy, but spongy

15-25

Silty loam

Ball forms, feels smooth and silky

25

Loams

Loam

Ball forms, feels smooth and spongy

25

Sandy clay loam

Ball is firm, feels sandy and plastic

25-40

Clay Loams

Silty clay loam

Ball is firm, smooth, silky, plastic

40-50

Clay loam

Ball firm, feels smooth and plastic

40-50

Clays

Light clay

Ball very strong, feels plastic

50-75

Medium clay

Ball very strong, feels like plasticine

75+

Heavy clay

Ball very strong, stiff plasticine

75+

Assessing Soil Texture using the ribbon test

  • Take a small handful of soil.  
  • Add enough water to make a ball. If you can't make a ball, the soil is very sandy
  • Feel the ball with your fingers to find out if it is gritty (sand), silky (silt) or plastic/sticky (clay).
  • Reroll the ball and with your thumb gently press it out over your forefinger to make hanging ribbon.  
  • If you can make a short ribbon, your soil texture is loamy, a mixture of sand and clay.
  • The longer the ribbon, the more clay is in your soil.
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