La técnica de los ocultadores e instrumento de análisis 61

To increase the productivity of modern cropping systems and reduce production costs has required agricultural machinery to become more powerful and therefore heavier. Soil degradation due to compaction from this machinery is a major problem for agriculture and affects an area of 68 million ha globally of which 33 million ha is in Europe. When a stress is applied to a soil in excess of the soil's strength, compaction occurs. This increases the bulk density of the soil, as a result of a reduction in soil porosity, reducing the proportion of large to small pores. This change in pore structure affects soil strength and thermal

conductivity and reduces soil aeration, which decreases crop root and shoot growth, leading to reduced nutrient use efficiency that affects growth and reduces yield. The degree of compaction of a soil from agricultural machinery depends upon the stress applied by the vehicle tyre, the strength of the soil and the soil moisture. Wet soils are less able to resist compaction and increases in soil moisture lead to increased soil compaction. Uncontrolled agricultural field traffic (Random Traffic Farming (RTF)) is a common feature of modern farming which can lead to in excess of 85% of a field being trafficked annually by agricultural vehicles. Controlled Traffic Farming (CTF) is a field traffic system that restricts agricultural machinery to sacrificial traffic lanes whilst leaving separate crop zones free from traffic. Well designed CTF systems require all machinery to have matched working widths and rely on the use of navigation and auto-steer technology but they can restrict the area of a field that is trafficked by vehicle tyres to 15%. The lack of adoption of controlled traffic management systems in the UK is thought to be due to non compatibility of machinery working widths but there can be beneficial increases in crop yield using existing farm equipment by using a controlled traffic system. Where field traffic is

necessary, the use of low ground pressure systems (low inflation pressure tyres or tracks) can reduce the risk of soil compaction by increasing the ground contact pressure for a given load. Researchers have found that low inflation pressure (LTP) tyres can reduce soil compaction and increase crop yields compared to using standard inflation pressure tyres. Stresses under LTP tyres have been found to reduce soil stresses in the top 100 mm of the soil but they do not influence stresses in the subsoil below 300 mm.

Tillage is an important component of modern cropping systems that is used to remove biological, chemical and physical limitations within the soil to provide better conditions for crop establishment and growth. To produce the maximum crop yield tillage must be tailored to the local environmental and climatic conditions. Tillage is used to incorporate crop residues and nutrients into the soil, destroy weeds and improve soil aeration and water infiltration. Long term use of inversion tillage can lead to soil degradation by erosion.

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This can be by wind or water runoff, the latter can also lead to pollution of water bodies by pesticides and nutrients. Repeated cultivations at the same depth can lead to plough pan, a region of high bulk density below the cultivation depth, that forms a barrier to crop roots and limits water infiltration. Plough pan removal requires deep loosening of the soil which reduces the bearing capacity of the soil and leaves the soil at risk of further compaction from agricultural traffic. Non-inversion tillage (reduced tillage) can be used as an

alternative to mouldboard ploughing as the use of broad-spectrum herbicides has

removed the need to bury weeds. Minimising tillage, maintaining year round soil cover and employing diverse crop rotations (the three principles of Conservation Agriculture)

promotes biological processes in the soil that allows sustainable high crop yields. In the UK reduced tillage is used on 40% of arable land but only 5% of arable land is under no- till (direct drilling) compared to 20% in the USA. The susceptibility of soil to erosion is reduced under no-till systems, as runoff is reduced and infiltration is increased, due to increased macro pore stability and connectivity. However crop yields in climates with cold springs or on heavy or poorly drained soils can be reduced for no-till systems. The main areas associated with cereal growing in the east of the UK are most suited to no-till cereal farming.

This literature review highlighted the importance of good soil structure to provide suitable conditions for high crop yields and that the most important feature of soil structure is soil porosity. Soil porosity is usually calculated from bulk density and soil pore size distribution by means of water desorption or mercury intrusion. Although these methods have given a good understanding of the nature of soil porosity they are indirect methods that describe porosity. The use of image analysis on thin soil sections can provide information on pore size, shape, orientation and connectivity but is not only difficult but also time consuming. Although there is a quantity of research on soil structure connecting good structure to crop yields or soil water transport, there seems to be little information on what is a good

structure from its actual properties. The development of X-ray CT in the use of soil analysis has allowed measurement of soil pore size and distribution as well as pore shape, orientation and connectivity to be relatively quick. Much of the previous X-ray CT research has concentrated on developing the technique in soil analysis especially around the technical problems associated with segmenting the images. Studies on soil

compaction and tillage has mainly been focussed on 'two field' studies. It is believed that this research is the first time that X-ray CT has been used to measure the properties of soil from a long-term traffic and tillage trial.

Previous research by Smith, E. (2016) on the traffic and tillage plots on Large Marsh at Harper Adams University found no significant differences in establishment of winter wheat and winter barley in the traffic or tillage treatments (P>0.05). Although not significant,

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yields were increased in the low inflation pressure tyres treatments compared to standard inflation pressure tyre treatments for winter wheat (4%) but decreased for winter barley (2%). Zero tillage significantly (P<0.001) reduced the combinable yield of winter wheat compared to deep and shallow tillage treatments but was not significantly different for winter barley. Soil physical properties measured were soil bulk density, penetration resistance and hydraulic conductivity. Vehicular traffic significantly increased soil bulk density (P=0.001) but there was no significant differences in soil bulk density or

penetration resistance between the low inflation pressure and standard inflation pressure treatments. Hydraulic conductivity was significantly (P<0.001) higher in the untrafficked soil in the CTF plots compared to the wheelways.