The Groundsman

September 2014

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TECHNICAL UPDATE 23 the Groundsman September 2014 Visit www.iog.org for more information and digital editions he definition of consistency is the soil's material consistency – its material state (not to be confused with consistency of a pitch in terms of ball bounce or pace) and there are three states: solid –the soil breaks by fracture; plastic - the soil fails by flowing or moulding like plasticine; and liquid - the soil has no yield stress and will flow, such as in mudflows. As water is added to a dry, compacted clay soil, the soil will move from a solid state to a plastic consistency. The addition of more water would move it from a plastic to a liquid consistency. The liquid consistency state should be avoided at all times but the transformation between the solid and plastic consistency is commonly used in rolling and pitch preparation. The pitch is wetted to make it plastic so that when it is rolled it is easily deformed and smoothed. It is then allowed to dry and move to a solid consistency where it becomes hard and ball bounce and player traction (grip) is improved. Tine penetration is easier when the soil is plastic but the soil flows around the tine. When the soil is friable (not so hard it is impenetrable, but it still crumbles rather than flows), the tine will help to break the soil around it, creating T Aeration guidelines – Part 3 We continue our aeration guidelines series by looking at soil consistency, profiles and the causes of root breaks By: Dr Iain James fissures and new pores which extend beyond the region of the tine hole. Unfortunately in cricket soils, the friable region has a very narrow range of water content so there is a rapid transition between very hard and plastic, so it can be difficult to get the soil at the right water content. If the pitch profile is too dry, only shallow penetration can be achieved and this will damage the profile as it will fail upwards – it will lift as the tine is pulled out, damaging the profile and pitch surface. If the pitch is too wet, the tine will simply push the soil into the walls of the hole and the effect will be limited. Harm can also be done when the soil is too wet that the aeration equipment damages the soil as it travels over the pitch. The ideal consistency to be at on cricket pitches is just on the wet side of friable. This might reduce the effectiveness of the operation but it significantly reduces the risk of harm to the pitch from shallow penetration and vertical uplift. For solid tine aeration of outfields, a little drier is OK because the effect on the outfield is less. One tip for getting the right soil conditions is to keep trying as the pitch gets wetter to get good penetration. How do profiles affect grass growth? Healthy grass plants with deep roots are less susceptible to drought, are able to access more nutrient resources within the soil and can help dry the pitch to a greater depth. Shallow rooting in cricket pitches occurs for two reasons: • As the grass is cut shorter the plant responds by reducing rooting depth. • As the soil is compacted it becomes harder for the roots to grow through the soil. The first cause cannot be solved by aeration, it is just a case of allowing grasses to be longer when pitches are not in preparation for play and even longer over winter to encourage deeper rooting. The second cause is related to rolling and compaction. The total mass of roots in the soil is actually similar, but it is the depth of rooting that decreases with increasing soil density. Grass roots will grow and establish in a high density (1.90 g/cm 3 ) soil, it is just that they are all in the top 75 mm of the profile. Near-surface roots dominate at lower densities but the quantity of roots at depth increases as the soil bulk density decreases. In high density soils, the plant cannot physically break through to lower depths so it increases near-surface rooting to compensate. This has an interesting effect on density: at 1.20 g/cm 3 the original packing density is not affected; at 1.55 g/cm 3 original density there is some reduction of density near surface; and at 1.9 g/cm 3 there is a significant reduction in density near the surface where the roots can exploit any soil porosity and reduce soil density. This creates a 50 mm layer of lower density soil over a higher density base (>100 mm). Shallow rooting is easily identified in a core. If you look at the bottom of the core and can see live roots (live roots are usually light in colour and are flexible), then you have roots to that depth. Showing how the consistency of a soil will change from solid to plastic to liquid as the water content is increased. At the plastic limit, the soil will change from failing elastically to failing plastically – it stops crumbling and shattering, and starts to mould and flow. At the liquid limit, the soil has no yield stress – it will flow under its own weight, like liquid water. The actual values for the plastic limit and liquid limit presented here are for Ongar Loam, other loams will have similar but slightly different limit values. The ideal zone represents the range of water contents at which the soil is in an ideal consistency for solid tine aeration in cricket pitches, ie it is not too solid and not too plastic. This range of water content is very small in cricket loams Water content (by weight) 0% 18% 40% Plastic limit Liquid limit t

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