Harrowing may be required to cover the seed with soil but there may still be considerable variation in seedling emergence. Normally, the openers following the tractor tracks need more pressure applied to them than the other openers because the soil is compacted by the tires. Double Disc Press Drills Double disc press drills are similar to the other double disc drills except for the addition of press wheels, one following each furrow opener, which pack the soil over the seed. The fully enclosed convex press wheels are the best except in moist clay soils, where the semi-pneumatic are preferred because they pick up less moist soil. The press wheels help cover the seed with soil and prevent premature drying of the seedbed, as well as good soil-seed contact for fast germination and therefore good stand establishment.
Hoe drills are similar to double disc press drills with better penetration characteristics.
Narrow openers take less power, clear crop residues better, penetrate the soil easier and create fewer problems in covering the seed with soil than the wide openers. They however, do plug more frequently on moist clay soils. Hoe drills give better penetration than double disc drills in firm soils. Hoe drills can be used to seed without pre-seeding tillage because it can place the seed through more surface straw than double disc drills. Shallow seeding is difficult, especially in loose soils.