By Genevieve Slocum and Dave Wilson
When we are threatened with a dry year, and especially a dry spring in the Northeast, one of our best defenses (and arguably a cheaper one than buying irrigation) is packing after seeding. This concern arose with alfalfa planting season, since 2012 was a record year for alfalfa replant requests at King’s.
What we did observe in many fields in 2012 was that where the horse or mule prints were we had better packing and the alfalfas grew well in the hoof-prints. We also saw that in the compressed soil in the tractor tire tracks, alfalfa germination and emergence was greatly improved.
This is quite pertinent to alfalfa, but applies to almost all seeds, since seeds and seedlings need good contact with the soil for germination and emergence. Seed germination and very small seedling stage are plants’ most vulnerable life stage and the point at which the crop needs a fairly specific set of growing conditions. For alfalfa, the critical period is before three-leaf stage. After that, roots get deeper and more efficient at reaching for moisture.
Alfalfa seeds need to absorb at least their own weight in water before they can germinate. After germination, better seed-to-soil contact helps roots access soil moisture and prevent seedling dessication. Both seeds and seedlings absorb water from the surrounding soil, so good seed-to-soil contact is critical, which means proper seedbed preparation. If soil is too cloddy or loose at the time of planting, seed-to-soil contact will be compromised. Well-packed soil not only improves consistent contact, but also helps retain moisture, since airflow between soil particles is reduced. If soil is properly firmed, an adult footprint will be about ¼ inch deep.
Conventionally tilled soil should be disked finely and cultipacked prior to planting and then packed again after planting. Press wheels on a grain drill can also work. For alfalfa, packing ensures both proper seeding depth and seed-to-soil contact.
A firm seedbed is important both for seed-to-soil contact and seed placement. Seed should be placed deep enough to reach moisture, but not so deep that the shoot can’t reach the surface. This is a fine balance to achieve that varies for every species and soil type. With coarser, sandy soils, more pore space between particles means less even seed-to-soil contact and less moisture retention near the surface, so be sure to increase the seeding rate in these conditions.
The amount of tillage also influences seed contact with the soil. Not enough tillage will leave soil surface cloddy and trashy, impeding consistent seed-to-soil contact, but too much tillage can cause soil drying, erosion, or crusting.
Be careful using cultipacker seeders on heavy soils that have been finely tilled, since this increases the potential for soil crusting. In no-till situations and where there is residue on the soil surface, a drill with good presswheels works better and additional packing after planting isn’t needed.