Simple tests for the determination of soil texture and topsoil quality

A.1
Soil texture assessed by handling properties

1. Moisten 10–20 g (i.e. a dessert-spoonful) of soil and knead into a firm dough—sand will feel gritty, silt will feel soft and silky, clay will be stiff and tenaciously sticky.
2. By trying to form shapes in the sequence 1–5 in Table A. 1 the shape achieved furthest down the list will indicate the soil texture class.

A.2
Clay content assessed by the Adams and Stewart soil-binding test (ASSB or ‘motty test’)

A.2.1
Procedures

Because of the risk of errors in preparation it is advisable always to include a standard soil with each batch of determinations.

1.Place on a saucer a golf-ball-size sample of soil roughly 50 g (2 oz) for a mineral soil. Then add 15–20 ml of water, the lesser quantity for sands.

2.Rub down by firm thumb pressure, smearing on the surface of the saucer to break down all aggregation.

3.(a) Make into a ball and thump down on to absorbent paper to test for, and eliminate, excess moisture. Samples that are too wet will smear and stick to the paper; too dry samples will shatter and crack.

(b) If too wet to thump drier, leave to stand for a while in contact with absorbent paper. If too dry, add 1 ml water, knead to incorporate and try (a) again, adding further 1-ml increments of water if necessary.

4.Place clod on dish in one drop of water, and cover with a small glass or cup. Leave to stand for several hours, e.g. overnight. This will allow time for the water to become uniformly dispersed.

5.Check on the moisture state of the clod after equilibration, then:

TABLE A.1 Handling properties and soil texture class

	Shape	Texture
1.	Cone only	Sand
2.	Cone and ball only	Loamy sand
3.	Cone, ball and worm formed, worm cracks when bent; ball inclined to be soft-centred	Silty loam
4.	Cone, ball and worm formed, but worm cracks when bent; ball not noticably soft-centred	Remainder of loams
5.	Cone, ball and worm formed, but worm not cracked when bent into a ring around finger. Ball typically stiff to work and difficult to round off into a perfect sphere when just moist enough to mould without cracking. The surface will readily take on a polish.	Clay

6.When the clod is satisfactorily wet, rework in the hand and then roll into a cylinder 25 mm (1 in) in diameter. Avoid incorporating folds and cut off any involuted ends. Cut the cylinder into three or four, 20 mm (3/4 in) lengths, discarding any residue. Ensure that all sections used are of equal size.

7.Mould cylinder sections into round balls, avoiding folds. Add one drop of water and thoroughly re-work if there is a tendency to crack. Round off by progressively rolling more firmly between the palms of both hands for at least 45 seconds.

With strongly coherent soils such as loams and clays, maximize consolidation by repeatedly throwing the ball down on to a smooth, solid surface covered with clean absorbent paper. Roll to restore shape after each throw. Continue until the reduced earthy staining of the paper on impact indicates that all excess water has been squeezed out, then round off for the last time, aiming to form a perfect sphere with a smooth, polished surface.

With silts and sands there may well be insufficient cohesion to enable a moist ball to hold together on impact. Therefore, consolidation and removal of excess water will have to be achieved at the ball-forming stage by very careful squeezing and blotting during the rounding-off process. If the ball is left to stand periodically on absorbent paper, this can help, but great care will be required when handling during the rounding-off process as any motty, with little or no clay, will become brittle as it dries out.

In every case, aim to form a perfectly smooth, well-consolidated sphere, with no cracks.

8.Leave the moulded soil balls uncovered overnight to begin drying. If they have been rolled too wet they will be moist and flattened at the base next morning and may stick to the dish. They should be left to dry out completely over a period of 5 days at normal room temperature.

9.On the sixth day, crack the dry motty between two, smooth, hard surfaces laid one over the other on the platform of a bathroom scales. Apply foot pressure from above and record weight required to shatter the dry motty. Zero the scales, plus any base plate used, before applying pressure

TABLE A.2 Classification of clay content and breaking strength (ASSB) values

ASSB value		Equivalent clay content for well-made motties	Strength category
113kg	(250 lb) and over	over 55%	Exceptionally strong
91–113 kg	(200–250 lb)	44–55%	Very strong
68–91 kg	(150–200 lb)	33–14%	Strong
45–68 kg	(100–150 lb)	22–33%	Moderately strong
23–15 kg	(50–100 lb)	11–22%	Weak
9–23 kg	(20–50 lb)	4–11%	Very weak
under 9 kg	(20 lb)	below 4%	Non-binding

from above. If the motty is well made the shatter point will be well defined and sudden.

10.Because of the risk of errors in preparation affecting individual motties, eliminate the lowest breaking value in every three determinations before averaging the remaining values to give the rating for the sample.

A.2.2
Interpretation of breaking strength values

Table A.2 classifies ASSB breaking strengths for well-made, properly air-dried motties of normal topsoil. Every 2 kg (414 lb) of breaking strength corresponds to approximately 1% of clay but, for those inexperienced in motty making, assume 1.6–1.8 kg (31/2–4 lb) of breaking strength corresponds to 1% of clay.

If you regard efficient motty making as a skill worth acquiring, then practice with materials of known clay content.

Clay type and degree of dispersion can affect the ASSB value. Small, potentially very active clays, and marked dispersion typical of structurally degraded and salt-affected soil, both increase the binding strength of a given quantity of clay. However, these effects will also be expressed in the field so that, if regarded as a measure of the degree to which a soil will display the characteristics typical of its clay content, clay type and state of dispersion, then ASSB rankings will not mislead.

A.3
Modification of the motty test to assess topdressing compatibility with the topsoil already in place

As reported in Parks and Sports Grounds, May, 1987, Dr W.A.Adams has now introduced a modification to the motty test which is of particular value for checking on the compatibility of different clay soil top-dressings used to promote pace on cricket pitches. The modification involves joining two half-motties together along the flattened face of each hemisphere. To help achieve a firm join, avoid trapping air along the junction by ponding water over the upturned face of one hemisphere, then, after wetting the face of the other hemisphere, bring the two together by inverting the second hemisphere over the ponded surface of the first. Place the composite motty on absorbent paper for a while to dry off excess water, and when no longer too tacky to handle, remould and consolidate into a ball, dry and determine breaking strength, as recommended for loams and clays in paragraphs 7–9 of section A 2.1. If there is excessive differential shrinkage between the two halves on drying then the motty will readily separate along the boundary. Compatible soils will hold together until the whole motty shatters at random. This difference cannot be confidently predicted from differences in clay content alone, other factors such as clay type and state of dispersion also being involved.

Needless to say, only materials that cohere well in such a test with the existing surface soil should be used for topdressing. This also suggests that when trying to effect change by upgrading the binding strength of the soil used for topdressing, progress may have to be gradual.

A.4
Sand content assessed by decantation of the silt and clay

When all the mineral particles in a soil have been freed to act as individuals they settle out in water at very different rates according to size. From a totally dispersed sample, 180 mm (7 in) deep, left to sediment out in still water, all the sand will have settled to the bottom in one minute. A decantation of the supernatant will then contain only silt and clay. Repeated dispersions, followed by decantation, will eventually remove all the silt and clay, leaving only the sand.

For practical reasons it is probably best to carry out this determination starting with a cup-sized sample of soil left to dry for a week at room temperature and passed through a 2 mm sieve. From this weigh out 110 g (4 oz will do) for typical topsoil (5–10% organic matter), or 103 g for subsoil, and then puddle, using no more than a cupful of water. Work in a 9-litre (2-gallon) bucket, and rub the soil against the wall of the bucket with a pliable spatula. To assist with clay dispersion, add to the water a pinch of table salt and a pinch, or a few drops of a non-frothing detergent (e.g. Calgon). The aim should be to free the individual soil particles by rubbing down any granular aggregations between the fingers. Once the soil has been converted to a smooth paste, make up with water to a height of 200 mm (8 in) from the base and stir vigorously to try and form a uniform suspension of the soil in the water. Quickly counter any residual swirling motion in the water and note the time when sedimentation is allowed to begin.

Exactly one minute later carefully tip out most of the supernatant liquid, in a continuous, steady stream, leaving only the equivalent of 20–30 mm (1 in) of water to remain in the bucket. Refill with water to the 200 mm (8 in) height and repeat for a total of four decantations. By the fourth decantation the supernatant liquid will scarcely be cloudy and it will be possible to pour off virtually all the free water, including any sludge or organic debris half suspended in the last dregs of the water. The total sand fraction should be dried, e.g. at 100°C in an oven or near a room heater, until it flows easily without sticking, then weigh. The weight in grammes will represent the percentage sand in the mineral skeleton of the original sample. A value in excess of 70 g (21/2 oz) will indicate a soil with the workability and free-draining characteristics of a sand, i.e. a soil particularly prone to be hungry and thirsty unless maintained well-supplemented with organic matter. A value falling short of 70 g (i.e. less than 70% sand) will indicate a loam or a clay, requiring further investigation by means of its reaction to the ‘worm’ test, as described in section A.1, or by the breaking strength of its motty being higher or lower than 80 kg (180 lb). However, from the evidence of sand content alone, the probability zone on the triangle of texture (Figure A. 1) indicates that the soil is very unlikely to be a clay unless the sand value is less than 40 g (40%).

A.5
Designation of soil texture classes

A.5.1
Considering the mineral, fine earth only

From systematic sampling of soil data, it would appear that 90% of the soils in England and

FIGURE A.1 Triangles of texture defining 90% probability zone for topsoils in England and Wales. (a) Plot of systematically sampled data for topsoils in England and Wales. (b) 90% probability zone superimposed on triangle of texture defining commonly used BS particle-size classes.

Wales lie within the probability zone defined in Figures A.1 and A.2 and most of these lie close to the line X-Y (Figure A.2).

For practical purposes, when dealing with normal soils which are neither markedly organic nor excessively stony, there is much to be said for limiting the objective to the recognition of just the three texture categories shown in Figure A.2. These can fairly readily be recognized by their handling properties but, as corroborative evidence, the ASSB value or the sand content will provide quantitative data indicating lines for clay or sand content along which the sample must lie. Plotting both for sand and clay will give an intersection defining the texture exactly. However, if only one of these—sand or clay—is known, a projection into the 90% probability area will give useful general guidance which the handling properties will refine sufficiently to enable a choice to be made between the three simple categories on offer.

Alternatively, as the data for England and Wales indicate that most soils contain silt and clay in a ratio close to 6:4, an estimate of any one fraction, sand, silt or clay, will also allow an estimate to be made of all three. For example, if clay content is 20%, silt is likely to be of the order of 20×1/4=30% and sand 100 –(20+30)=50%.

A.5.2
Taking account of stones and organic matter

Stones contribute very little other than weight to a soil. They are essentially inert filler, occupying space without contributing to water or nutrient supply. They dilute the influence of the active soil.

Organic matter is itself a significant source of

FIGURE A.2 Triangle of texture showing particle-size classes reduced to just three major categories: sand, clay and loam.

a wide range of nutrients beneficial to plants and soil organisms, and it may supplement the available water reserve. However, organic matter is light in weight and, when dominant, as in peat, may adversely affect anchorage.

Both components should be commented upon in a full texture description as they can significantly modify predictions based solely on the particle-size composition of the mineral, fine earth.

Classifications of stone and organic matter contents are subjective, like the particle-size classes used to define the texture of the mineral, fine earth. They tend to be ignored, and no one system of classification seems yet to have found general acceptance, but for precise specification it is essential that categories are defined.

Stones

Table A.3 defines categories based on size and quantity. These are used by soil scientists for general descriptive purposes. Note, this system dispenses with the idea of distinguishing the finer fractions described in Tables 7.1 and 13.1 as fine and coarse gravel particles (2–4 mm and 4–8 mm in nominal diameter). Based on these categories, a loam with 50% stones by weight (35% by volume) would be more correctly described as a very stony loam and, if some of the stones were boulders, these would have to be specifically mentioned also.

To assess stone content accurately requires very large samples, and correspondingly robust equipment, but it can be assessed adequately enough in the field by eye using the volume categories as guidance.

On sports fields, boulders are never welcome within drain trenching depth, and if less than 2–3 m apart, they will severely impede tillage and subsoiling. Stones over 6–8 cm will interfere with efficient slit trenching and spiking. Ideally a topsoil for sports turf should not contain stones much larger than 2 cm, especially if sharply angular and liable to cause injury.

A topsoil classified in Table A.3 as more than slightly stony should be considered undesirable, especially if it is to be imported. By the time it has been spread, harrowed, levelled and seeded, the mechanical agitation will have brought the stones to the surface, concentrating them where they are least welcome. Any process involving soil agitation, such as cultivation or frost heaving, will cause stones to rise because the fine soil will more easily slump back into place. The only natural processes that will tend to bury stones are the surface casting of earthworms, and in the absence of earthworms, the accumulation of a surface, organic mat. Under sports turf the former will at best take years to build up; the latter is undesirable. Better to avoid the problem by choosing a soil that is no more than slightly stony in the first place.

Organic matter

Table A.4 indicates categories of organic matter content that should be used to qualify a texture classification. For example, a natural loam soil

TABLE A.3 Ranking of stone and boulder categories—sizes and quantities

Longest diameter (mm)	Size category	Percentage by weight	By volume (approx %)	Classification
over 600	boulder	under 1		stoneless
600–200	V.L. stone	1–5	under 3	V.S. stony
200–60	L. stone	6–15	3–8	S. stony
60–20	M. stone	16–35	9–21	M. stony
20–6	S. stone	36–70	22–53	V. stony
6–2	V.S. stone	71–80	54–66	Ex. stony
under 2	Fine earth	over 80	over 66	Insufficient fine earth to fill cavities between stones

TABLE A.4 Rough guide to the significance of organic matter content in British soils

Percentage by weight (oven dry)	Percentage by volume (oven dry)	Practical significance
under 3	under 16	Mineral soil with organic matter in short supply; typical of subsoils and degraded topsoils
3–10	16–38	Mineral soil, adequately organic enriched; typical of worm-worked topsoil
10–25	38–65	Humose—organic matter content beginning to dominate soil properties; organic decay probably restricted seasonally by low temperature and/or excessive wetness
25–35	65–75	Peaty—soil essentially organic because site and/or climate has caused organic decay to be impeded by persistent waterlogging
over 35	over 75	Peat—an accumulation of organic residues where decay has been arrested by site and climate combining to cause permanent waterlogging

containing 12% organic matter by dry weight (about 43% by volume) should be described as humose; a loam containing 6% organic matter by dry weight (about 25% by volume), loam with normal topsoil organic matter content. (Figure A.3 gives weight/volume conversions.)

The organic matter categories defined in Table A.4 have practical significance. Below 3% organic matter by dry weight (about 16% by volume) there begins to be insufficient organic matter to benefit structure. Organic matter contents of around 6–8% by dry weight (25–31% by volume) are typical of natural topsoils in Britain sustained in good heart by earthworms. Organic matter contents in excess of 10–12% by dry weight (37– 43% by volume) indicate soils in which there has been at least some seasonal restriction in organic matter decay, probably low temperature and/or excess moisture, slowing down biological activity. At

FIGURE A.3 Rough guide to relationship between percentage organic matter by weight and percentage organic matter by volume. In each zone the upper margin relates to sand/peat mixtures; the lower margin to mixtures in which the mineral component is loam in particle-size composition. A strongly granulated soil would lie outwith the lower margin.

over about 25% organic matter by dry weight (65% by volume) the material is peat in origin, a product of prolonged waterlogging arresting organic decay.

However, as organic matter contents can be enhanced artificially, organic matter content alone should not be used as definitive evidence of topsoil quality. For loams and clays in particular, there needs also to be satisfactory evidence on water-stable structure.

A.6
Practical significance of the three primary soil texture classes: sands, clays and loams

A.6.1
Strong clay soils

Winter games

Soils with a binding strength of over 82 kg (180 lb), equivalent to clay content of 40% are likely to be strong cracking clays, i.e. given the opportunity to dry out thoroughly, as in a long dry summer, they will form a system of deep cracks. If these are filled from the surface with fine, dry sand they will continue to assist with surface drainage throughout the succeeding winter.

To ensure a good packing of sand within the cracks, the sand-filling operation should be carried out with a uniform fine sand, when both sand and surface are dry, and the cracks wide open. Six millimetres (1/4 in) of sand, equivalent to 100 tonnes/hectare (40 tons/acre), spread over the surface and brushed into the cracks, would not be an over estimate of the amount of sand required in the initial application. Topping up will be necessary in succeeding years when conditions are dry enough to re-open the cracks again.

Because of the tendency for strong clay soils to structure themselves by cracking, and then to maintain this form by virtue of their binding strength, they are the soils most likely to respond favourably to mole ploughing (Chapter 2, page 35). For this reason, true clay soils are a much better proposition for drainage then the silty soils with which they are so frequently confused. It is inadvisable therefore, to assume a texture based solely on superficial evidence without checking by means of a motty test. The difference in binding strength between silt and clay is so obvious that it leaves no margin for doubt.

Cricket tables

Pace is an essential feature of the performance of a cricket pitch and for this the clay content of the soil is critical (Table 12.4, page 185). Appropriate ASSB values are as follows:

1. school and club standard, where durable pace is secondary to trueness and ease of preparation, 36–64 kg (80–140 lb);
2. first class County standard, where skilled time-consuming preparation is essential, 64–82 kg (140–180 lb);
3. breaking strength at which pitch preparation is likely to be difficult, and an inadequately prepared pitch dangerous, 82 kg (180 lb) and over.

A.6.2
Weak, very sandy soils

Soils with a binding strength of under 18 kg (40 lb), equivalent to a clay content of 9%, are likely to be sands, loamy sands or low clay versions of silt loams. To further distinguish between these three alternatives, use handling properties or visual examination (section A.l).

If revealed to be a loamy sand, such a soil can be expected to retain a useful amount of rapid drainage, even when compact, providing the infiltered water can be adequately discharged below ground.

If revealed to be a sandy silt loam or a silt loam, (i.e. less than 70% sand and less than 10% clay) such a soil should be avoided where, as under sports turf, it cannot be repeatedly cultivated. It will be inclined to slump and retain moisture. Water may not lie for long on the surface but the problem will be to clear water from the uniform, fine pore system of the soil itself.

Even amongst soils more than 70% sand, problems of drainage can arise.

1. If the sand component is not of a uniform, particle-size composition, e.g. if it does not have of the order of 80% in the medium-to-fine (0.125–0.500 mm) particle-size range. Too wide a range of particle sizes allows for a substantial amount of interpacking within the sand itself, reducing the amount of silt and clay required to block the remaining pore space. Contrast the cement required to make up a cement mortar with that required for an equal volume of concrete. Where there is a wide range of particle sizes present in a sand, e.g. no two adjacent categories together exceeding 80%, then it may not be until the total sand content exceeds 90% that there will be insufficient silt and clay to fill the available pore space when the soil is compact.
2. If the silt and clay present is allowed to segregate into layers. This is particularly liable to happen if the clay is freely dispersible in water, i.e. not associated with organic matter in water-stable clusters. Such a dispersed state is typical of soil that has been stockpiled (refer to section A.7).

A.6.3
Loamy soils

The sands and strong clays together constitute only one quarter of the soils of England and Wales. The loams, that is soils with no more than 70% sand or 40% clay, are by far our commonest soils. For them particle aggregation is essential to provide the proper balance of water retentive (small) and free-draining (large) pores required to ensure an acceptable air/ water balance. As already discussed in section 1.4, with a fairly balanced mixture of particle sizes we do not automatically get the nice mixture of large and small pores that one might expect because the different particle-size grades tend to interpack, leaving the finer grades in control of the pore space. Therefore, to provide a free-draining, macro-pore system in a loamtextured soil some form of aggregation into water-stable granules is absolutely essential.

Soil granulation in the loams is primarily a by-product of the processing of soil by earthworms, as they burrow and process fresh organic residues into the mineral soil. Typically it occurs under grass but only when earthworms are present.

In sports turf the twin problems of soil compaction and thatch appear when earthworms are absent. Associated features are: surface rooting, sensitivity to drought, and problems with those pests and diseases that are encouraged by an organic-rich surface and the absence of earthworm browsing.

However, water-stable, soil granulation, though fairly stable to the patter of raindrops, cannot survive the sort of foot traffic that a football pitch receives in the winter. When present in such circumstances earthworms will continually re-open their burrows and thereby contribute to drainage, but the loss of soil granulation between the worm burrows ensures that the bulk of the soil is compact and water holding. When earthworm activity is absent, as on many made-up sites, or discouraged by the acidity promoted by sulphate-rich fertilizers, or failure to lime, a loam can rapidly become surface waterlogged, even when perched over a free-draining gravel bed that is no more than 150 mm (6 in) under the surface.

Where a loam soil is to be subject to intensive winter use, beyond the capacity of an active earthworm population to continually rehabilitate, spiking and surface sanding can at best provide only temporary relief. In fact, spiking, if overdone, can lead to new problems of subsurface compaction. The only long-term solution for sports use is to install an intensive system of surface-opening, vertical slits, stabilized by appropriate permeable fill, and linked for discharge below into a pipe, or soakaway system.

A.7
Assessment of topsoil condition in loamy soils

A loam in good condition is likely to be associated with visual evidence of the presence of earthworms, including their casts and burrows. There will be no evidence of a surface, organic mat. Instead, the organic matter content (5–10%) and the roots will be evenly distributed throughout a surface layer of soil, typically 225 mm (9 in)—300 mm (12 in) deep, well aggregated and water stable. The pH will probably lie between 5.5 and 7.5, limits ideal both for earthworms and grass.

As the water-stable aggregation is both the consequence of all the conditions being right to encourage earthworm activity, and the cause of the fertility which benefits grass growth, a test to assess the degree to which a loam soil is water-stably aggregated can be used as a general criterion of quality.

To carry out the simplest form of this test, air dry the soil, then sieve off any loose particles that will pass a domestic, culinary or flour sieve. Place a dessert spoonful of the material retained on the sieve into a glass containing approximately 100 mm (4 in) depth of cold water. Leave to soak for five minutes and then swirl ten times. If the solution rapidly clears and the granules of the soil remain more-or-less coherent, the structure is water stable and the soil could be expected to be of a kind that a farmer would describe as topsoil in ‘good heart’. Check if the soil units that appear to be stable are indeed single, solid particles, or aggregates of particles that will crush down further to smaller components merely under finger pressure.

This test is not appropriate for sands, loamy sands and loams which have insufficient clay (less than 10%) to hold a structure. They will probably break down completely at the dry sieving stage anyway. However, of these, the sands and loamy sands have more than 70% sand and can be readily amended for use as free-draining topsoils, merely by the addition of an organic compost. Loams with less than 10% clay are uncommon and are potentially quite difficult to manage, even for agriculture, unless the cropping system and the climate allow for periodic, mechanical disturbance to relieve compaction.

To improve the quality of the water-stability test there are several modifications that should be considered:

1.The whole sample can be used—set aside until just brittle-dry, then ease clods apart, teasing out roots so as to create a tilth that will all pass a 10-mm (1/4–3/8-in) riddle. Avoid working the soil when it is inclined to smear or dust.

2.Sub-samples of the total soil sample prepared as above should be compared with:

3. Allow about one hour for the soil to soak, then swirl and allow five minutes for the material to settle. Note the relative cloudiness of the supernatant liquid. Looking up through the base of the glass beaker, against the light, decant off the supernatant if particularly cloudy, and then observe the extent to which the aggregation has remained water stable. The less cloudy the supernatant and the greater the amount of water-stable aggregation the better is the condition of the soil.

4.A loam or clay in good heart will settle out in water to leave a clear, or translucent, supernatant solution with light showing through the gaps between granules. This is best seen when the solution is gently agitated and viewed from below, against the light. A loam, or clay in poor condition will leave a cloudy effect in the supernatant solution, varying a little in proportion to the silt and clay content of the soil; any aggregation will collapse to form an opaque sludge at the base of the beaker. A chalk or limestone soil, because of its high lime content, may cause the supernatant solution to clear by flocculating any suspended clay. However, the true state of affairs will be evident in the curdiness of the flocculated deposit compared with the discrete, granular character of water-stable aggregates.