Soil is composed of sand, silt, clay and organic matter. The minute particles of sand and silt are bound by clay and organic matter into aggregates. These are the crumbs or lumps soil breaks into when you dig it. The technical term for them is peds.
The arrangement of aggregates gives soil its structure. Good soil structure has adequate spaces (pores) between aggregates to allow water and air to enter the soil and drain easily, while holding enough moisture to maintain plant growth. Poor soil structure has few aggregates and few pores between soil particles.
Digging a small hole 20–30cm deep will give you some idea of the structure and strength of your topsoil, although dry soils are often hard and not necessarily poorly structured. Dig the hole, take a slice off the side and carefully lift it out so it stays intact. Lay the slice on its side to examine it. A crumbly soil is usually softer and more encouraging to root and shoot development than a massive soil where soil particles are all the same size (such as fine silt), or cloddy soil which breaks apart into large clods and is difficult to break down further, or platey soil which breaks into flat plate-like layers (such as soil crusts).
Examine the spadeful of soil closely for channels created by earthworms, ants, old plant roots and crack lines. These are very important features of a well-structured soil as they allow easy root penetration and water and air flow.
This exercise illustrates the proportions of gravel, sand, silt and clay in your soil.
The combination of sand, silt and clay gives soil its texture. There are 19 soil textures, ranging from sand through loam to heavy clay. The following test helps you assess the texture of your soil.
It is important to test for dispersibility and slaking if you plan to clear land for cultivation or build earthworks. In dispersible soils, moist or wet clay breaks up into individual clay particles due to a chemical reaction between water and sodium in the clay. These particles block the soil pores and seal the soil surface. This is good for dam building, but not for pastures or crops, because water and roots will have difficulty penetrating the soil.
Slaking occurs in poorly structured soils when dry soil is wet rapidly. Water moves into the pores and forces air out. The force of the escaping air causes the aggregate to burst and break down into smaller particles. These wash into the soil and block soil pores, and form a crust on the soil surface.
This test helps you decide if your soil is too wet for vehicle/animal traffic or cultivation.
Rapidly squeeze a small lump of soil into a ball and attempt to roll it into a 3-mm diameter rod. If you can make a cohesive rod easily the soil is too wet and should not be worked with machinery. If you cannot make a rod at all, then it is only suitable to cultivate if it is a clay soil; if it is a loam, this indicates the soil is too dry to cultivate. If you can just make a crumbly rod, the water content should be right for cultivating all soil types.
From the Soil Sense leaflet 9/92. Agdex 504 produced by Rebecca Lines-Kelly, formerly soils media officer, Wollongbar Agricultural Institute, for CaLM and NSW Agriculture, north coast region, under the National Landcare Program, October 1992.