Soil texture

Texture is how something feels. You can often judge texture by the look of a material. Some materials have a coarse texture, while others are fine to the touch.

Consider the clothes you are wearing right now. Do they all have the same feel? It’s most likely some will have a smooth feel while others will be much rougher or coarser to the touch.

Soil texture is much the same. There are soils with coarse or rough texture and there are those with fine, smooth texture.

Soil texture is due to the size of the particles that make it up. There are three main types of soil particle:

• sand, with coarser or larger particles
• silt, with medium sized particles
• clay, with the finest or smallest particles.

The table below shows you the actual size of some of these particles.

       

Soil particle sizes

Soil types are often named after the proportions of these particles they contain. For instance, sandy soils are made up mainly of sand particles.

Silty soils are made up mainly of silt particles. Clay soils are made up mainly of clay particles.

The following diagrams show the proportions of particles in a sandy soil and a clay soil.

Soil type characteristics

Particle size is very important because it affects:

• the proportion of air in a soil
• the amount of water a soil will hold
• the rate at which a soil drains
• how easily a soil can be cultivated.

Here are the characteristics of clay and sandy soils, which are important in horticulture. Silty soils are midway between these two in their characteristics and have not been included in the table below.

Silty soils are generally fine-grained soils formed from the deposits carried by rivers and streams. Silt particles clump together. They form soils that can be dry and lacking in nutrients due to the large pore spaces but they are well drained and aerated.

                                               

Soil type	Positive features	Negative features	How they look
Clay soils	They are very fertile    because they hold onto their original nutrients.           They are easily improved    by cultivation and adding lime and organic matter.       They have a high    water-holding capacity.       Their total pore space is    large.       Soil temperature is slow    to cool down in autumn.       Clay particles bind other    soil particles together.	Air can’t get between the    particles because they are very small and tightly packed together so they are    poorly aerated.           Water can’t drain away    easily.       They get very sticky when    wet, making cultivation difficult.       When they dry out they    get hard and are difficult to cultivate, especially as they stick together in    lumps.       Plant roots find it    difficult to grow through clay.       Clay soils warm up slowly    in spring because they hold a lot of water.
Sandy soils	Have good aeration    because air gets in easily.           Have good drainage    because water moves easily through them.       They are easy to    cultivate.       These soils warm up    quickly in spring.	Have poor water-holding    capacity.           They dry out quickly    because the pore spaces are large and they drain fast and too much air is    left around the plant roots.       They are not very fertile    because nutrients are washed out quickly.       These soils are hard to    improve.       These soils have no    structure.       Total pore space is    small.       Need lots of organic    matter to grow plants well.       These soils cool down    quickly in autumn.       They may be blown or    washed away easily.

Loams

Loams have a mixture of particles sizes. They are usually the ideal soils to grow plants in because they have the good qualities of both sand and clay soils. They are usually a mixture of clay, silt and sand particles and organic matter.

Loams drain well and contain plenty of air. They hold enough water for plant growth, but have good drainage and aeration. They contain lots of nutrients. Loams are the most fertile soils.

Loams are classified by which particle type has the highest proportion.

• A sandy loam would have more sand particles than silt or clay particles.
• A silty loam has more silt particles than sand or clay particles.
• A clay loam has more clay particles than sand or silt particles.

A soil’s texture depends on the proportion of sand, silt or clay it contains. For example, a soil described as a silt loam contains mainly silt but also will contain some sand and clay in smaller proportions.

Working out soil texture

By feeling the soil you can tell a lot about the types of particles it is made up of. This will give it different characteristics. Sand is gritty because it has rough sharp edges. Silt particles feel floury when dry and smooth and soapy when wet. Clay is sticky when wet.

Soil texture is one of the most important soil properties to know about because it indicates:

• how much water and nutrients can be stored in the soil
• how fast the water will move through the soil
• how much air space the soil has
• what kind of organisms live in the soil
• what type of plants will grow.

Overseas students: Do NOT send soil into New Zealand.

The soil texture triangle

Texture by feel gives a broad idea of the soil type. Working out the percentage of soil particles gives a more accurate measurement of the proportion of particles. A texture triangle is another way to work out the soil type if you know the percentage of sand, silt and clay present.

Using the triangle

A soil is made up of 50% clay and 20% silt. To work out the soil’s texture, follow steps 1-4 and use the diagram.

Step one

Find the 50% clay mark on the texture diagram. Start at 0 on the clay line. Move your pencil along until it comes to 50%. Now draw a line from the 50% mark horizontally across and parallel to the base of the triangle.

Step two

Look at the horizontal line with the silt numbers. Find where 0 is on this line. Move your pen along until it reaches the 20% silt mark. Now draw an upward line parallel to the left hand side of the triangle.

Step three

The two lines will meet in an area with the name of the soil you have. The soil is clay.

Step four

You don’t need to know the percentage of the third kind of particles in this soil. If you wanted to find out how much sand is in the soil this is what you would do.

100% – 20% silt – 50% clay = 30% sand

Try the following 5 multiple choice questions.

What's next?

Go to: 4 Soil structure.