Using motties to measure soil strength

The Adams and Stewart soil binding test (ASSB) or Motty test as it is commonly known is still a very good method for measuring soil strength. Particularly in that it can be simply done at home without the need to use specialist equipment.

Soil strength is very important when selecting clay loam dressings for cricket pitches. The potential success of the wicket is directly related to the percentage of clay content found in the soil profile of the cricket pitch.

There are a number of clay loam suppliers in the UK who specialise in blending and mixing loam soils for use in the turf grass industry. The clay content in these products can vary immensely ranging anything between 22-32%. The larger the percentage of clay content in your soil, the stronger the binding strength will be, however, having the highest clay content is not always the criteria to base your requirements on.

You need to look at the age of the users and the standard of cricket you are catering for. Using higher clay content soils also require a greater input of resources. High clay content loams tend to shrink and swell more readily, thus you need to control the rate of drying by the use of covers and sheets. Not all clubs have these resources available, therefore, it may be more desirable to use lower clay content loams which are easier to manage.

The performance of your wicket is dependant on a number of factors:

• The choice of clay loam being used
• Type of square construction
• Preparation and maintenance procedures
• Machinery, equipment and resources available
• Knowledge of the Groundsman

The combination of these factors will dictate the condition of your square and the performance of your pitches, however, the end result will be influenced entirely by the type of clay loam you manage and the percentage of clay material there is in the soil profile.

As a groundsman it is important to accurately identify your soil classification. Ideally, you should take a number of soil samples from your square at a depth between 20-100mm.

These samples should be labeled with the date and location taken and sent to a laboratory for classification, measuring for Particle Size Distribution (PSD). This will give you a breakdown of the clay, silt and sand percentages in your soil. Once identified you will be in a better position to manage your square.

Once you know your clay content percentage, you can then decide on what clay loams are desirable for you. This is where the ABBS /Motty test becomes an aid to choosing a compatible loam.

The concept behind the Motty test is to determine the strength of the sample taken on your square and compare it with other loam samples, thus finding a compatible soil for use on your facility.

The process begins with the taking of some soil extracted from your square, obtaining enough material to produce several Motties, remember you need enough soil to create a ball approximately 20mm (3/4") in diameter for this test.

Procedure:

(a) Crumble the loam down into a workable size (less than 2 mm). Remove all fibrous matter from the sample (i.e. living roots, grass plant leaves, dead and decaying matter and thatch, any small stones). Transfer half a cupful to a clean plate.

(b) Moisten the sample with distilled water if available (otherwise with tap water) until it can be moulded by hand. Water must be added slowly to ensure that the soil does not become too wet to mould.

(c) Mould vigorously by hand over a period of 5 minutes to destroy all aggregation. Add more water if necessary.

(d) Roll the soil into a ball and place it on a clean plate. Cover the soil with a wet cloth (wrung out by hand) and leave standing for 2 hours.

(e) Re-mould the soil by hand, making sure that it is plastic but not sloppy. It should have the consistency of putty and have little tendency to stick to the hands. It should also readily mould into a ball without forming cracks.

(f) Roll into a sausage shape approximately 12.5 mm (0.5 inch) in diameter and cut into sections 25 mm (1 inch) long.

(g) Carefully mould each section into a ball and then roll between the palms of the hands.

(h) Place the Motty balls onto clean plates and allow to air dry in a cupboard for 7-10 days.

(i) Place each dried ball in turn on a bathroom scale and slowly exert pressure (with a piece of wood) until the ball breaks. Make a note of the reading when the ball breaks.

(j) Discount the lowest value in each set and obtain the mean value of the rest.

(k) Readings between 45 and 90kg (100 to 200lbs) have been shown to be optimum for British soils.

• Soils which disintegrate at a pressure below 45 kg are not suitable for use on a cricket pitch.
• Soils which disintegrate between 45kg and 70kg are suitable for club pitch use.
• Soils which disintegrate between 70kg and 90kg are suitable for county and international pitch use.
• Soils which disintegrate at pressures greater than 90kg tend to be too strong for cricket pitch use

Apart from local materials, there are several recognised loam suppliers in the UK. Use of Marl is not encouraged nowadays as it can cause layering. A simple modification to the motty test was proposed and endorsed by Professor W A Adams who recognised the potential to assess loam compatibility, particularly when topdressing.

The modification involves joining two half motties of different loams together along the flattened face of each hemisphere. It is important not to trap any air along the joining surfaces, as this may affect results.

If there is excessive differential shrinkage between the two halves on drying, the Motty will duly separate, thus the soils are not compatible. If both loam types remain strongly bonded and only break after pressure, these two loams are likely to be compatible.

The Motty test is a simple reliable field test that can play an important part in determining the selection of clay loams for cricket. Groundsmen should be made aware of this technique and readily try it out to assess their soil strengths.

Having a better understanding of soils is paramount in the management of natural grass pitch facilities.

---

Article written by David Goodjohn, Green Infrastructure (Europe) Ltd