Over the years slurry tankers have increased in size, which hasn’t been all to the benefit of the soil. These larger tankers can damage soil when spreading early or late in the season. Correct tyre choice and the use of different equipment configurations such as umbilical spreading can reduce the load. Whether you are spreading slurry with your own equipment or using a contractor, you need to be aware of these challenges and what to do about them.

The weight increase in tankers is dramatic (see Table 1).

Today, 2,500-3,000 gallon recessed wheel tankers working behind 200hp tractors are not uncommon. The loaded tanker weight has more than doubled, with axle load increasing from 6t to 15t. The pursuit of scale does not stop there, however. Tankers on triple axles with capacities of over 30t of slurry and an empty weight of 12t are available. But what does this mean for the soil? And can tyre features such as size, type and diameter help reduce this damage.

A 3,000g tandem axle tanker on standard tyres (600/75 R38) is almost equivalent to a 3,000g single axle tanker on 900/60 R38 tyres. If the tandem axle is fitted with wider 750/60R26.5 at 0.65 bar pressure a soil stress of a much lower 91 kPa can be achieved.

Ground damage

The damage caused by heavy machinery, particularly in soft or high moisture conditions, can be divided into three categories:

1. Soil and sward damage: the grass or stubble surface is destroyed, often leaving a rut as well as smearing the soil and dragging the sod. This affects grass growth. Reducing tyre ground pressure by fitting larger tyres and running at low pressures will help, as will flexible sidewalls and a traction type tread pattern. Large diameter can have a big impact in soft conditions, allowing the tyre to stay turning rather than dragging.

2. Near-surface compaction: the soil structure in the top 200mm is compacted, affecting water drainage, root growth and nutrient availability to plants. This is reduced by having low ground pressure by either reducing weight or increasing tyre size. Damage caused may take years to recover.

3. Deeper compaction: the soil is damaged below 200mm and can be very slow to repair. Very large axle loads will tend to cause this damage even at ground pressures which would prevent such damage with lower axle loads. The absence of ruts is not a good guide to safe working with large axle loads. The best way to reduce the risk of subsoil compaction is to limit individual axle loads to less than 6t – but as Table 1 shows, that is challenging. Bigger axle loads need to be operated at extremely low ground pressures to avoid subsoil damage.

An umbilical system even on a tractor with 650/65 R38 tyres at 0.9 bar pressure can achieve a soil stress of 91kPa. This is down to the reduced overall weight.

Slurry spreading configurations

There are a number of different machine configurations for high-capacity slurry spreading. Trailing shoe units can be used with each and would add about 600-1,000kg to the tankers.

Recessed wheel tankers: while this concept was originally designed to improve the field performance of a 1,600 gallon tanker, it has now been extended to tankers of up to double that size. The large-diameter tyres allow progress through very wet spots but their on-road stability can be poor. Axle loads can be very high-risk factors for subsoil damage.

Tandem axle tankers: this is the more common approach to getting reduced ground pressure in other countries. It is generally better on the road than a single-axle tanker with very large wheel/tyres. If small-diameter tyres are used in wetter conditions, dragging and scrubbing makes them less able to travel than very large singles.

An umbilical system even on a tractor with 650/65 R38 tyres at 0.9 bar pressure can achieve a soil stress of 91kPa. This is down to the reduced overall weight.

Umbilical spreading system: this system where the slurry is directly pumped from the storage tank, or nurse tank, to a tractor-mounted applicator reduces the soil loading as there is no tanker hauling loads of slurry to and on the fields. This leaves just the spreading tractor to be sorted.

Comparison of recessed wheel, tandem and umbilical systems

So which system is best? We have compared three high-capacity systems in Table 2.

  • A large 3,000 gallon recessed-wheel tanker with a 180-200hp tractor.
  • A 3,000 gallon tandem-axle tanker with a 180-200hp tractor.
  • An umbilical system with a tractor-mounted trailing shoe applicator and front-mounted reeler on a 140hp tractor.
  • The axle loads of each were calculated, tyre options were selected and the inflation pressure for the specific axle load at 40kmh was calculated. Finally, a soil stress model was used to calculate soil stresses in moist/wet conditions with the selected loads, tyres and pressures. The target in moist soil is to have a soil stress value of less than 100kPa to avoid soil structure damage. Key findings from this analysis were:

  • A 3,000g (13,600l) recessed wheel tanker has an axle load that makes it very difficult to prevent structure damage in moist or wet soil. Even the very large tyre option still poses a threat despite the mobility of these systems. Unless conditions are dry, this size and type of tanker should be restricted.
  • A tandem axle tanker of similar size can achieve low ground pressure and low soil stresses, but only if very large tyres are used, which will add considerably to the cost of the tanker.
  • An umbilical system imposes much less load on the soil and is far easier to sort form a tyre selection perspective.
  • The calculation here assumed that the tanker options were operated at 40km/h on the road transporting the slurry. If a central inflation pressure system was used allowing pressures to be dropped on entering the field, or the unit was just used to spread in the field and not on the road (via nurse tank or similar), then lower inflation pressures could be used reducing the soil stress.

    An umbilical system imposes much less load on the soil and is far easier to sort from a tyre-selection perspective.

    In summary

    Modern slurry tankers pose a serious threat to soil structure damage in anything but dry conditions. Single-axle tankers should be limited to 2,000 gallon capacity if they are to be used in these conditions. Tandems can work better but only if fitted with very large tyres. Umbilical systems have an inherent advantage in terms of axle load.