Improper seeding depth is the No. 1 cause of establishment failures in forages, according to Penn State University extension forage specialist Marvin Hall. “If seeding depth isn’t correct, you might as well not bother to plant,” he says.
Forage seeds have a very small supply of stored energy to support seedlings until they emerge and begin making their own energy, Hall notes. Seeds placed too deep are not likely to emerge.
Optimum seeding depth varies with soil type, soil moisture, time of seeding and seedbed firmness. Generally though, it’s not more than 3/8” deep. “A rule of thumb is that 5-10% of the forages seeds planted should be on the surface after seeding,” Hall says.
A firm seedbed will help ensure seeds are placed at proper depth. “It is extremely difficult to accurately regulate seeding depth if the soil is soft and fluffy,” says Hall. “On properly firmed soil, an adult’s footprint should not be deeper than ½”. Seeds should be covered with enough soil to provide moist conditions for germination but not so deep that the shoot cannot reach the surface.”
Dribbling a basketball on the field is another way to determine whether an alfalfa or grass seedbed is firm enough for planting, says Bruce Anderson, University of Nebraska extension forage specialist. “The ball will bounce easily on a firm seedbed,” he says. “If it doesn’t bounce, the bed isn’t ready. Firm the seedbed with a flat harrow, a roller or maybe even irrigate.”
Anderson notes that loose seedbeds can have up to 50% dead air space in the seeding zone. “The first roots that emerge into that dead air space often do not live, and your stand will suffer,” he says. “A firm seedbed reduces the dead air space, which helps you get thicker stands that develop more rapidly.”