The graph below shows how aggregates created by tillage are affected by the soil moisture content. The nearer to the lower plastic limit that cultivations take place, the minimum amount of large and the maximum amount of smaller aggregates are created (Soil structures, Keller et al., 2007). Consequently, it is useful to be able to identify this critical point when planning cultivations. Furthermore, if conditions are far from this point (for example, too wet), then it is often better to avoid moving the soil at all, as any cultivation or trafficking will cause damage and make conditions worse.
A simple "field" method of identifying the soil condition relative to the lower plastic limit is to roll out a small ball of soil worked in the hand, as shown in the pictures below.
Ideal soil conditions for tillage:
Soil cannot be rolled, but crumbles and breaks into hard crumbs: It is below the plastic limit and compaction by normal farm traffic is unlikely to occur. Care should be taken in severely dry situations to ensure that cultivation (for example, seedbed making) is cost effective, as clods can be formed that will not break until soil moisture increases.
Soil can be rolled, but is just “on the edge” of crumbling: It is near the plastic limit (top right). Soil in this condition can be called “friable.” Cultivation will normally be effective, however ground pressures of above 15psi (100kPa) can compact the soil to some extent.
Avoid tillage in these conditions:
Soil can be easily rolled into long thread (bottom right): Soil is wetter than the plastic limit and compaction will result from traffic by many vehicles, even if exerting pressures as low as 6 to 12psi (40 to 80kPa). Clay-based soils will be most prone to damage if cultivated in this state.
Soil cannot be rolled but smears easily: It is much wetter than the plastic limit; trafficking and cultivations will generally be detrimental in all situations.
Soil is light and forms powder: Conditions are too dry to be effective and pulverization of the ground will be harmful and potentially will risk erosion by wind or water.