On Weds 6/Feb Alex Vickers gave us a talk summarising research done at Cranfield University on the subject of Optomising Rolling in Pitch Preparation. The research is funded by the ECB. The project is continuing with latest results due to be published this spring.
Alex described how a number of possible techniques for pre-season rolling were tried and measured using the test facilities at Cranfield. The experimental results and the theories to support them will come as a surprise to groundsmen brought up with the belief that "all pre-season rolling is good - and the more hours the better".
Hopefully this is a reasonable summary -
(1) The aim of the game is to compact the top layer of the clay based pitch. The degree of compaction is measured by the density of the soil (loam).
(2) The researcher found that the greatest increase in density was achieved by short bursts of rolling (max 5 passes over any one part of the square per burst) - with the bursts separated by a fair amount of time (day or so?)
(3) Using this technique a 20% increase in the density of the pitch surface could be achieved - but the interesting thing is that this co mpaction actually happened in the periods between the bursts of rolling not during the rolling.
(4) Other tests included "no rolling" and "single lengthy period of rolling" - none produced significant increases in density.
The explanation for this was -
Clay contracts as it dries.
Drying is achieved by one or all of:- gravity pulling the water away, evaporation from the surface or transpiration from the grass. The amount of water accessible to these three mechanisms is directly related to the size of the spaces between the particles of soil, the smaller the space the harder it is to get the water out.
So the theory to explain the result of the rolling experiment is that rolling does not itself significantly change the density of the medium, but it does have the effect of rearranging the particles and embedded water and air to make more water available for the drying mechanisms. This means the soil dries more efficiently, thereby shrinking and increasing in density and strength as it does so.
So the practical conclusion to all this is, I guess, it's best to roll when drying conditions are optimum (sun, wind), don't spend too long on the roller (no more than 4-5 passes over the same bit of soil at one time) but repeat the exercise a few times over a week or so. Presumably, too, anything we can do to increase the length and density of grass roots will also help getting moisture up from deeper in the soil.DT 10/Feb/2008