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The Soil Triangle

The soil triangle
The soil triangle

Part of performing a garden soil type test is to classify your garden soil as sandy, loamy, or clay. You have already calculated the percentages of each layer of sediment of your garden soil as describe in our page on garden soil testing. Now you can easily determine what type of garden soil you have by transferring the results onto the soil triangle.

This is quite easy to do. Say you have 12cm of soil in your jar or bottle, with the bottom layer of sediment being 6,8cm, the middle layer of sediment being 3,5cm and the top layer of sediment being 1,7cm.

The bottom layer of sediment is sand, which is 6,8cm high. We take 6,8 and divided by the height of the soil sample, which in this example is 12cm, and then multiply it by 100 to convert it to a percentage. This works out to approximately 57% (6.8 ÷ 12 × 100 = 56.667). Plot 57 on the bottom axis of the soil triangle (A) and draw a line parallel to the axis that plots the percentage of silt.

The middle layer of sediment is silt and is 3,5cm high in this example. We take 3,5 and divided by the height of the soil sample, and then multiply it by 100. This works out to approximately 29% (3.5 ÷ 12 × 100 = 29.167). Now we plot 29 on the left axis of the soil triangle (B) and draw a line parallel to the axis that plots the percentage of clay.

The top layer of sediment is clay and is 1,7cm high in this example. So we take 1,7 and divided by the height of the soil sample, and then multiply it by 100. This works out to approximately 14% (1.7 ÷ 12 × 100 = 14.167). Now we plot 14 on the right axis of the soil triangle (C) and draw a horizontal line across to the axis that plots the percentage of silt.

Now determining what type of garden soil we have is simply a matter of checking where the three lines intersect on the soil triangle.

In this particular example the three lines intersect in the sandy loam area of the soil triangle. We can now proceed to make the appropriate soil amendments to make our garden soil more loamy. In this particular example this can be achieved by adding some organic matter such as compost.