The optimum water content for tillage (OPT) is defined as the

moisture Moisture is the presence of a liquid, especially water, often in trace amounts. Small amounts of water may be found, for example, in the air (humidity), in foods, and in some commercial products. Moisture also refers to the amount of water vapo ...

content of

soil Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former te ...

at which

tillage Tillage is the agricultural preparation of soil by mechanical agitation of various types, such as digging, stirring, and overturning. Examples of human-powered tilling methods using hand tools include shoveling, picking, mattock work, hoein ...

produces the largest number of small aggregates.

Overview

The Optimum Water Content of soil is the water content at which a maximum dry unit weight can be achieved after a given compaction effort. A max dry unit weight would have no voids in the soil. If you were trying to compact a hard dry soil to make it more dense, you might want to get it wet. The OPT is the water content of the soil in which you could compact it the most. If there is too much water you would have too much pore water pressure during compression to compact any further. If there is too little water the soil would naturally resist compaction via shear strength/friction/effective stress. The determination of the OPT is important because if tillage is carried out on fields that are wetter or drier than the OPT many problems can be caused, including soil structural damage, through the production of large clods, and an increase in the content of readily dispersible

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4). Clays develop plasticity when wet, due to a molecular film of water surrounding the clay par ...

which is indicative of the soil stability. The OPT can be determined in relation to the

volumetric Volume is a measure of occupied three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch). The defi ...

water content at the lower

Plastic Limit The Atterberg limits are a basic measure of the critical water contents of a fine-grained soil: its shrinkage limit, plastic limit, and liquid limit. Depending on its water content, soil may appear in one of four states: solid, semi-solid, plastic ...

of the soil (PL). Some examples of suggested OPT: *On a lateritic sandy loam : 0.77 PL *On a sandy loam : 0.9 PL For several soils the OPT has been found to equal 0.9 PL, although there are a number of limitations with the use of the lower plastic limit in determining the optimal moisture content. Firstly it is a property of a moulded soil and not an undisturbed soil in the field and secondly, many

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class of s ...

y soils are not plastic and do not have a lower plastic limit.

Relationships between water content at field capacity (FC) and Plastic Limit(PL)

* When FC < PL Soil will drain to a water content at which no excessive structural damage will occur on tillage. * When FC > PL Soil will never drain to a water content ideal for tillage Many

soils drain very slowly, and as a result they are usually wetter than PL unless they are dried by water extraction by plant uptake.

Soil friability

Soil friability is the tendency of a mass of soil to

crumble A crumble is a dish that can be made in a sweet or savoury version. Crumbles became popular in Britain during World War II, when the topping was an economical alternative to pies due to shortages of pastry ingredients as the result of rationi ...

under the action of an applied force. This has been found to be maximum at the PL of a soil.

Overview

Relationships between water content at field capacity (FC) and Plastic Limit(PL)

Soil friability

See also