silty or clayey soils. This is because finer grained particles have a larger amount of pore space than coarser grained particles. The table below displays the deal bulk densities that both allow and restrict root growth for the three main texture classifications. The porosity of a soil is an important factor that determines the amount of water a soil can hold, how much air it can hold, and subsequently how well plant roots can grow within the soil. Soil porosity is complex. Traditional models regard porosity as continuous. This fails to account for anomalous features and produces only approximate results. Furthermore it cannot help model the influence of environmental factors which affect pore geometry. A number of more complex models have been proposed, including fractals, bubble theory, cracking theory, Boolean grain process, packed sphere, and numerous other models. Soil texture Soil texture is the analysis and classification of the varying amounts of particles present in the soil. Soil texture triangles are used to determine the measure of clay, sand and silt present. The volume of these particles determines the appearance, feel and chemical properties of the soil. Methods of determining texture The measure of texture by hand is a simple way of estimating the sand, silt and clay amounts and ultimately estimating the soil type. This is typically done in the field when precise measurement tools are unavailable. Soil texture estimation in the field To estimate by hand, scientists in the field will take a handful of sifted soil and moisten it with water until it holds together. The soil is then rolled into a ball nearing 1-2 inches in diameter, called a bolus, until it is no longer sticky. With the bolus in hand, the thumb is then used to attempt to press out the soil into what is called a ribbon. A ribbon is successfully achieved when the soil is 2 mm in thickness, and 1–2 cm wide. Characteristics such as the length of the ribbon, color of the moistened soil and the level of crumble are taken into account and used to estimate the soil type. Soil texture estimation in the lab An experienced soil scientist can determine soil texture in the field with decent accuracy, as described above. However, not all soils lend themselves to accurate field determinations of soil texture due to the presence of other particles that interfere with measuring the concentration of sand,