This is a complicated question to answer. On a like for like basis, where you have two tractors exactly the same, the one with more surface area contact to the soil will have a lower foot-pressure ratio, so a larger tire or tracks would, theoretically, be better. However, the ability to drive the tracks (or a larger tire) requires an exponential increase in the power of the engine unit and more substantial drive train, increasing the weight of the vehicle. So in order to achieve the same outcomes as a 'normal' tractor, using a tracked vehicle requires a larger vehicle with substantially more weight and subsequently greater soil compaction. 

While this is a limitation on the employment of tracked vehicles, it is not the full answer on why they are not more widely used- two other factors need to be considered. First is the turning circle and the resultant impacts. A tracked vehicle can theoretically turn within its own radius, often less, but the churn in the soil and the risk of throwing a track increases substantially. A less dramatic turn results in a turn outside its diameter (If you drive the vehicle in a circle, the resultant distance across that circle is described as its diameter, half that diameter is its radius). While this is substantially true for most vehicles, especially large ones, the difference when using tracked vehicles of a similar size is substantial. This not only requires greater amounts of POL but also increases wear and tear. 

Which brings us to the other problem with tracked vehicles- the wear and tear on drive trains and engines is substantially greater, requiring both detailed maintenance which cannot be readily provided in a farm environment and regular replacement if the power to weight ratio is to be maintained. The power to weight ratio is important in tracked vehicles as even a minimal drop-off can result in drive failure, where the tracks cannot be made to complete a full revolution in order to propel the vehicle. Moreover, throwing a track requires far more time, skill and equipment to repair than is required to replace flat tire on a tractor. Rubber tracks have been used in recent times to try and overcome this problem but this requires more maintenance and results in shorter replacement times for the drive wheels and return sprockets. 

Thank you for your reply, Prof. Carruthers. Are there papers on that matter I could reference in my writings?

Apart from a comprehensive detail as given above, the tired tractors have greater operational speed, maneuverability and are also free from track maintenance jobs that is cumbersome and expensive. However, in sloping and watery ground conditions with presence of angular stones/boulders, track mounted tractors are better choice. 

Dear Thiago

the question is not technical but economic

as there is a system that has the advantages

for all soil types and user needs, so if there are machines

using both systems but have not spread for economic reasons.

There are some patents for converting wheeled tractor crawler tractor

and are  patent U.S. 5343960 and U.S. 4865141

There also exists half-tracks system , it is combining the advantages of both

as it is shown in Patent U.S. 2 706663

I am the author of a book where I approach these issues

only this I'm in Spanish translated into English when ready you'll let you know

Dear Thiago,

To your question, the previous remarks contain worthfull information. In addition on it :

Considering similar  wheights, tracked vehicles (generaly higher contact area) cause less compaction.

Premise: homogeneous distribution of the weight across the interface soi/track by pulling on the field (the distancebetween thesoil surface and the poing at which the attachment is hitched must be adjusted)

Additional remarks: i) by pulling traked vehicles make sense specially on high cohesive wet soils (clay soil). Soil cohesion c high -  internal friction angle or angle of soil shear resistance phi low. ii) by pulling on the field, slip occurs. Under similar conditons, topsoil failure occurs under tracks by lower slip level then under tyres by higher slip level. The traction force at the soil failure point remains higher under the tracks.

I wish full success on your research!

Etienne

Both systems will work similarly in most situations. If a farm is width-restricted or needs to operate in wet soil conditions, a track system may be the best fit. If an operation wants to minimize upfront costs and is able to match axle load to inflation pressures, the wheeled system may be the best fit. The longer footprint of the tracks also improves mobility in wet conditions by allowing the track to move over soft spots with little to no sinking, further protecting the soil. The tracks can help reduce the amount of surface area compacted, as well. The length and width of tracks cover a larger area than wheels do. This significantly reduces load transfer and creates much lighter PSI sharing than wheels can ever accomplish. Tracks actually work to float the machine over the ground rather than cut into it. Tracks tend to be most advantageous on uneven ground or muddy surfaces. With regard to traction, the reason tracks have the edge over tires is because they provide a more even weight distribution. Rubber tracks have a noticeable traction advantage over steel tracks on most surfaces. Rubber has a higher intrinsic friction and melds to uneven surfaces, increasing the overall surface area. On any relatively smooth, dry surface, rubber tracks will have the advantage.