The variability of wood is vast, ranging from species dense enough to sink in water to others lighter than cork. This difference in mass and physical resistance results from how trees form their internal cellular structures. Determining the softest wood requires employing scientific measurement standards.
Quantifying Wood Softness and Hardness
Scientists and industry professionals rely on precise testing methods to determine a wood’s relative softness or hardness. The most widely accepted measurement for this physical property is the Janka Hardness Test.
The Janka test uses an 11.28-millimeter (0.444-inch) diameter steel ball, determining the pounds of force (lbf) necessary to press this ball halfway into the wood sample. A lower Janka rating indicates a softer wood, as less force is needed to cause the indentation. The test is standardized to be performed on wood dried to a 12% moisture content, ensuring comparable results across different samples and species.
Another fundamental metric is Specific Gravity, which measures a wood’s density relative to the density of water. Any wood with a specific gravity value below 1.0 will float, while a value greater than 1.0 means the wood will sink. Specific gravity is a proxy for the total amount of wood fiber present in a given volume, directly correlating with both hardness and weight.
The World’s Softest Wood Species
The undisputed champion of lightness and softness among commercially available timbers is Balsa, scientifically known as Ochroma pyramidale. Balsa wood exhibits an extremely low Janka hardness rating, often falling in the range of 67 to 100 pounds-force (lbf). For comparison, common woods like Douglas Fir and Red Oak have ratings of approximately 660 lbf and 1,290 lbf, respectively.
The specific gravity of Balsa is exceptionally low, typically ranging from 0.10 to 0.15, meaning it is only 10% to 15% as dense as water. While some rare, non-commercial species, such as Aeschynomene, may have lower densities, Balsa remains the practical answer. This tropical hardwood is botanically classified as a hardwood because it comes from a flowering tree and is primarily sourced from Ecuador, where its rapid growth is commercially cultivated.
Cellular Structure and Commercial Applications
The extreme lightness and softness of Balsa are directly linked to its unique cellular anatomy. The wood is composed of a high percentage of large, thin-walled fiber cells, which are the main structural components.
These fibers contain vast empty spaces, known as cell lumens, which are significantly larger than those found in denser woods.
The cell walls themselves are thin, measuring only about 1 to 2.5 micrometers in thickness. These thin walls and large lumens mean that a cubic foot of Balsa wood contains very little actual solid material, accounting for its low density and high buoyancy. When kiln-dried, the wood is mostly air trapped within a lightweight scaffolding of cellulose.
This unusual combination of lightness and relative rigidity makes Balsa an ideal material for specialized uses. Its primary applications include use as a core material in structural sandwich panels, such as those found in wind turbine blades and racing yachts, providing thickness and stiffness without substantial weight. Balsa is also a favorite for model aircraft and architectural models, and its high insulating properties make it useful for thermal insulation and flotation devices.