Tight margins are driving prairie grain producers to use larger and larger air and fertilizer carts to supply their seeding tools. Direct seeders see the efficiencies in this trend but some are also concerned about what these heavy loads are doing to their soils.
One of the fortunate side effects of our cold winters is freeze thaw cycles. These cycles seem to eliminate the effects of soil compaction, which our counterparts in warmer climates have to deal with. Although not all experts agree that compaction is totally eliminated by these cycles, there is no evidence that shows otherwise. Vern Hoffman with North Dakota State University adds, that on expansive clay soils even rainfall can eliminate compaction. If you are concerned about tire loads causing soil compaction the best method for evaluation to date is to look for reduced crop establishment, growth, or yield. In other words if you can't see tire tracks in the crop there are probably no detrimental effects. Often, especially under dryer conditions, the crop comes up best or first in the cart tracks.
The soils with the most potential for problems with heavy axle loads are clay soils. The lower the organic matter, the more sensitive these soils will be. Also, the wetter the soils the more potential there will be for problems. Besides compaction, crusting or glazing can also occur which could really effect crop emergence. The good news for direct seeders is that direct seeded soil tends to have less of these problems. For one thing there is a build up of organic matter in direct seeded soil. This organic matter and straw residue helps to increase the weight the soil can hold up without rutting and also reduce crusting problems. The other thing is that direct seeded soils tend to be firmer and the soil in between the seed rows tends to carry more of the weight of the cart than in the actual seed row itself.
To counter these effects on problem soils Saskatchewan producers can reduce the air pressure in the tires on the seeding rigs they use. The effective soil pressure on bias ply tires is equal to tire pressure plus three lb/in2 for the stiffness or pressure of the tire. What this means in essence is that tires will squash out to cover enough area to balance ground pressure to the actual air pressure in the tire. With radial tires you only need to add one lb/in2 for tire stiffness. One of the detriments of low pressure tires is that the horse power needed to pull that cart will increase.
With this principle in mind decide what pressure you want to run the tires on your seeding rig at. Use the tire manufacturer's load charts to determine minimum inflation pressure at the maximum load that cart axle will be carrying. It is important to note here that these charts are developed based on whether the tire is a traction tire or not, whether the load is continuous or cyclical like an airseeder cart which is never run full for very long, and at what speed the tire will be run. Find a tire to match the kind of load you will be carrying with the tire pressure you want to run at. It is also helpful to note that inflation pressures listed in the air cart operators manual will be based on the heaviest per volume material you will likely ever carry. Potash fertilizer weighs 66 lb/ft3 and wheat weighs about 47 lb/ft3.
An example is the seeding rig Meier Brothers of Ridgedale set up on their farm to begin direct seeding. They farm a lot of Arborfield soils which are gray wooded heavy clay soils and can be very prone to crusting and baking if traveled on when wet. Garry Meier says they found that they could get the tire pressure of their 4-wheel drive down to 6 - 8 lb/in2 with 18.4 - 46 triples. They wanted the rest of their seeding rig to follow suit so they matched the 1200 gal anhydrous tank with the appropriate tires to run at this pressure. Their Bourgault 300 bu air cart has optional over sized tires to get down to 16 lb/in2. Mark Cresswell, one of Bourgault's engineers, comments that at today prices this option would cost an extra $2400.
Cresswell also says that most of their air seeders equipped with standard tires will be in the 20 lb/in2 range. Some models come standard in the 15 lb/in2 range and most of the rest can be equipped with larger optional tires that will reduce the pressure down to the 15 lb/in2 range and some as low as 7 to 10 lb/in2. Bob Purton with Morris Industries says if their customers desire low pressure tires they are referred to the Trelleborg tire. This European tire manufacture suggests 7 lb/in2 for no soil compaction on European soils says Purton.
Although there are no research projects to prove this summary on soil compaction, it seems that if you farm loamy or lighter land in Saskatchewan following the major Saskatchewan manufacturers' standard cart setup will virtually eliminate any concerns over soil compaction or soil structure problems. Lawrence Papworth, with Alberta Farm Machinery Research Center, says, they have not seen any problems in their tests under these conditions. If you farm clays or low organic matter clays reducing effective ground pressure on air or fertilizer carts to about 15 lb/in2 will likely be satisfactory. If you're running two carts, offsetting axles and or keeping one in front of the seeding tool may also help to reduce potential soil structure problems.