Different soil types have different water carrying capacities. Therefore, knowing your soil type will help you to optimise irrigation for your plants’ health.

The key to categorising your soil type is to determine its relative percentages of clay, silt and sand. This experiment demonstrates the simple way you can do this to benefit your home or work garden.

The Science Behind the Experiment

About Soil Moisture 

Soil moisture is an essential factor for the growth and development of healthy plants.

If plants and crops are overwatered – even waterlogged – roots will die, and your plants suffer. They will not absorb enough oxygen and may develop root rot. On the other hand, underwatering plants will lead to excessively dry soil, plant wilting, leaf loss, slow growth, and eventually plant death.

The ideal soil moisture varies from plant-to-plant. To get it right, you need to know what level of wetness your plant variety requires - AND your soil type.

Soil Types

As we mentioned in the introduction, you categorise your soil by its relative percentages of clay, silt and sand. Figure 1 (below) elegantly summarises these classifications.

Figure 1: A soil texture triangle showing soil textures based on the percentage of sand, silt, and clay. Reference.

Different soil types have different physical properties, including differences in water infiltration, retention, and availability. Listed below are the four fundamental soil compositions. You break down these further to describe texture subtypes, which are essentially varying combinations of each main group. (See a practical application of soil texture subtypes in Figure 2 below).

Once you know your soil composition, there are two critical values you need to know. These are its:

1. Wilting Point: Where the moisture level falls below a critical point, and your plants will begin to wilt and may die.
2. Field Capacity: The maximum amount of water your type of soil can hold after excess water has drained away. When the moisture level remains over its field capacity, your soil is said to be waterlogged.

Figure 2 This graph shows the field capacity and wilting points of various texture classes of soil. 

The Experiment

So let us show you how you can discover what soil type you have in your garden.

Materials Used

1. Two clear containers
2. Water
3. Stirring stick
4. Ruler
5. Shovel

Method

1. Use a shovel to collect a soil sample. Fill the clear container to about halfway. For this experiment, we have taken one sample from a home garden and one from the work garden.
2. Fill the jar with water to just over ¾ full. Don’t be too precise, so long as the soil is well covered.
3. Stir the slurry thoroughly.
4. Set your sample aside and leave it to settle for a few hours.
5. After a few hours, the soil will have separated. Measure each of the three clear levels (sand, clay, and silt) with a ruler, then convert your measurements into percentages. Finally, use the soil texture triangle (Figure 1) to determine your soil type.

Results and Discussion

Figure 3 This image shows the home and work soil jars having separated into their sand, silt, and clay components.
Jar 1 has clearly defined lines of soil components. Jar 2 is a bit more difficult to determine.

Figure 4 This image shows the home (2)  and work (1) soil jars having separated into their sand, silt, and clay components with measurements

As you can see in Figure 4, the sand particles are the largest and heaviest, so they have settled at the jars' lowest level. Silt particles are slightly smaller and weigh less than sand. They occupy the layer above the sand. As clay particles are the smallest, you’ll find them at the top of the three soil particle layers - unless your soil is very thick with clay – in which case you might find some large lumps sitting on the bottom of your jar(s).

Layered immediately above the soil components, you can observe water and rotted organic material. At the very top of the mix are floating organic particles yet to rot. Ignore the layers above the sand, silt and clay; they are not relevant for classification purposes.

Now, we can determine our soil types:

Jar 1 Work Soil Sample: Sand 54% Silt 32%, Clay 14%. Referring back to figure 1, we can classify this work garden soil sample as Sandy Loam.

Jar 2 Home Soil Sample: Sand 86% Silt 9%, Clay 5%. Again, referring back to figure 1, we can deduce that the home garden's soil must be Loamy Sand.

The home garden soil had a higher sand content than the work garden. By referring to Figure 2 (above), we can predict that the home garden will retain the least amount of water, but you can easily replenish its moisture content. The Sandy Loam soil of the work garden will retain water slightly better than our home garden.

Conclusion

If you want healthy plant growth, you’ll need to optimise your garden's irrigation; but first, you must determine the soil structure and deduce your soil type.

For more information on this experiment and soil moisture meters, speak with an Instrument Choice Scientist: Call 1300 737 871 or email [email protected].