Soil texture is mostly based on 'Feel'

Soil texture relates to the 'feel' of the soil - sandy, loamy, heavy or light. It results mainly from the range of particle sizes present , along with the percentage of organic matter and a few other things.

Before getting into all that though, there are some terms used in relation to soil texture that need a little explanation. The first two of these are quite commonly used and so easily misapplied. The terms ‘Light’ and ‘Heavy’ come from the pre-soil-testing era when soils were judged by how hard they were to cultivate. A heavy soil was simply one that was harder to draw a plough through than a Light soil.

Today, these terms are mostly used to distinguish sub-grades of a given texture and they tend to reflect the relative amount of clay in the soil. Since clay tends to have the effect of binding the soil together, light and heavy sub-grades are usually distinguished by their relative cohesiveness.

Loam is also a term that can cause confusion. This generally indicates a somewhat 'spongy' feel, relative to what you would normally expect. For instance, a clay loam will feel slightly spongy relative to an equivalent clay. Soil organic matter percentage generally has the most impact on whether or not a soil feels 'loamy'.

Factors influencing Soil Texture

In addition to the proportions of various particle sizes, the following properties also affect the soil texture:

1. Clay minerals.
   This influences the tractability of the bolus. Montmorillonite clays suggest a heavier, more clay-like texture than the per cent clay content would indicate. By contrast, kaolinite clays make the texture appear less clay-like than the per cent content would indicate.
2. Silt content.
   This confers a silky smoothness to the soil texture as it fills in the particle size range between sand and clay.
3. Organic matter.
   This gives cohesion to sandy soils and ‘greasiness’ to some soils that contain about 40 – 50 % clay. With sufficient organic matter present (over 5%) these soils behave as clay loams medium or heavy clays, as the clay percentage would suggest. Also, large amounts of organic matter in dry soil may resist wetting.
4. Oxides (chiefly those of iron and aluminium).
   When these are present in significant amounts, the addition of extra water may be required in order to reach the same level of moisture content as would otherwise be the case, thus indicating a less clayey texture than the actual clay content would suggest.
5. Calcium-Magnesium carbonates.
   In the fine earth fraction (particles less than 2mm in diameter) carbonates may impart a peculiar, porridge-like consistence to the soil. They tend to make sandy and loamy soil textures seem to be more clayey. Amounts of 10 - 30 % CaCO₃ can cause an increase in soil texture of about one grade above that obtained when the carbonates are not present.
   On the other hand,carbonates may also make clay textures appear less clayey than would otherwise be the case.
6. Exchangeable cations.
   The balance of exchangeable cations can have a significant effect on soil texture. In general, calcium saturated clays accept water readily and are easy to cultivate. Sodium and magnesium saturated clays on the other hand are often difficult to wet and work, producing a slimy, tough feel that often appears to have a more clay-like texture than would be indicated by the actual clay content.
7. Strong, fine structure.
   This will tend to make texture feel less clayey than it is.

How to do a Soil Texture assessment

This method relies on the ‘feel’ of the soil and so requires practice to perform reliably. In the case of sandy soils, sufficient water is used to thoroughly wet a small amount of soil in the palm of the hand without it being possible to squeeze water out of the soil by pressure.

In the case of the heavier soils containing more clay, the soil is thoroughly mixed with the water until it becomes ‘plastic’, but does not stick to the fingers when pressed. The water is best added drop-wise. The amount of water needed to wet dry soil to the required degree of wetness itself can give some idea of its soil texture.

Sandy soils require less water that heavier soils and the amount of water needed increases as the clay and organic matter content of the soil increase.

NB. If too much water is added, mix in a small amount of dry soil to adjust the wetness.

After wetting the soil to the proper degree it is pressed in the hand with the fingers to form a sausage shaped ‘Bolus’. In the case of sands and loamy sands, this bolus will not stand even gentle handling without breaking up. Thus, it cannot be picked up without breaking, nor can it be tossed from hand to hand without losing its shape.

As the soil texture becomes heavier, the bolus becomes more able to withstand these treatments. Sand particles are not sticky and because of their size they give the soil a gritty feel when pressed between the thumb and the fingers. Because of this lack of stickiness or cohesion, they easily break up when this is done. Coarse sand even produces a sandy sound when rolled between the fingers.

Clay particles on the other hand impart stickiness to a soil and cause it to be more coherent when wet. As the amount of clay in a soil increases it becomes stickier and more plastic.

Sandy loams and Loamy sands will break up when pressed through the fingers by the thumb, but clay loams hold together when subjected to this treatment, although lacking the ‘smooth’ feel of clays. They exhibit some degree of surface cracking when pressed out and often have a somewhat ‘spongy’ feel when pressed. The more clay there is in the soil, the thinner that a ribbon or cylinder can be made without breaking.

In addition, as the amount of clay and silt increases and the amount of sand correspondingly decreases, the soil has a less gritty feel, and becomes smoother and more slippery when pressed in the hand.