Soil texture, the physical feel of the material, is determined by the relative proportions of mineral particles—sand, silt, and clay—that make up its structure. Loam is the textural class recognized as having a balanced mix of these three particle sizes, often considered the most desirable soil type for agriculture and gardening. This balance gives loam its unique and beneficial properties.
The Specific Composition of Loam
Soil scientists classify mineral particles into three distinct groups based on their diameter. Sand particles are the largest, measuring greater than 63 micrometers, which gives them a gritty feel. Silt particles are medium-sized, falling between 2 micrometers and 63 micrometers, and feel smooth or floury. Clay particles are the smallest, measuring less than 2 micrometers, and are associated with a sticky or plastic feel when wet.
Loam is characterized by a composition that typically hovers around 40% sand, 40% silt, and 20% clay, though percentages can vary slightly within the official textural classification. This balanced ratio creates a mixture of large and small pore spaces within the soil structure. The larger pores, contributed by the sand component, allow for efficient drainage and aeration, which is important for preventing waterlogging.
The smaller clay and silt particles are responsible for water retention and nutrient holding capacity. Clay’s microscopic size and high surface area allow it to hold water as a film, while silt retains water but drains at an average pace. The resulting texture allows loam to hold moisture long enough for plant roots to absorb it, yet still permits excess water to drain away freely.
Sensory Characteristics of Loam Texture
The texture of loam is often described as mellow and crumbly, making it easy to work with. When dry, loam should break apart readily in the hand, lacking the hard clods that are characteristic of clay-heavy soils. The presence of sand gives it a slight grittiness, while the silt imparts a velvety, smooth, or silky quality to the overall feel.
When moistened, loam becomes cohesive and will form a ball when squeezed. This moist ball of soil should feel slightly sticky and plastic due to the clay content, but the influence of the sand and silt prevents it from feeling overly slick or heavy. The balanced texture ensures that it is not excessively gritty like a sandy soil nor extremely hard and dense like a clay soil.
A handful of moist loam should feel pliable, like soft modeling clay or cookie dough, but must not be overly sticky, which would indicate a higher clay percentage. The ability to form a shape while maintaining a soft and friable quality is the defining sensory characteristic of loam. This texture allows plant roots to spread quickly and easily through the soil to find water and nutrients.
Simple Methods for Identifying Loam
A practical way to check if a soil sample is loam is by using a field test that assesses its physical behavior when wet. The initial “squeeze test” involves moistening a small amount of soil until it is workable, like putty, but not dripping wet. When this sample is formed into a ball and then gently poked, true loam will hold its shape initially but crumble easily under slight pressure.
The “ribbon test” is a technique that helps estimate the clay content, which indicates loam texture. After moistening the soil, a small portion is pressed between the thumb and forefinger to create a flat, thin ribbon. The length of the ribbon before it breaks under its own weight correlates with the amount of clay present.
A soil classified as loam will typically form a short, weak ribbon, generally less than 1 inch long. If the ribbon is much longer, it suggests a higher clay content, and if the soil fails to form a ribbon at all, it points to a predominantly sandy texture. When manipulating the ribbon, loam should feel neither extremely gritty nor excessively smooth, reflecting the near-equal influence of all three particle sizes.