The majority of our direct seeding drills in Alberta are shank type machines matched to a variety of openers. On row packing is considered a must and fairly heavy pressures are used depending on soil conditions at seeding.
Lyle Carter, Ag Engineer USDA-ARS published an article on “Proper Seedbed Preparation”
way back in 1990 that looks at the key factors necessary for germination In the article he drew on H.D. Bowen
(1966) who had identified four soil environment factors that are necessary and sufficient for cotton germination:
1. air permeability for oxygen diffusion and respiration
2. moisture for metabolic processes
3. temperature for metabolic processes
4. soil strength or density lower than seedling capacity
For the most part these conditions are the same for the cereal, oilseed and pulse crops we grow.
Carter says that: “germination and emergence obviously require moisture transfer from the soil to the seed and root. Both cases require moisture movement across a membrane as a liquid rather than a gas. For germination the planting system must assure an intimate seedcoat to soil contact. This has been traditionally accomplished by pressing (packing) either the seed or soil to assure contact. Unfortunately there is a delicate balance between time to germinate and loss of soil moisture through evaporation or transpiration. Once the radical has found a suitable downward path the root generally can extend about 6 times faster than soil drying with depth. Bowen determined for one soil that the root extended 1.5 inches per day and the permanent wilting point interface proceeded at 0.25 inches per day. However if insufficient moisture is transferred across the seed coat to complete germination during the first wetting cycle, the seed may die.”
Carter contends that “the soil to seed contact should be controlled with a seed firming device such as a seed firming wheel rather than by firming the covering soil over and around the seed. Packed soil, a continuous media above the seed results in increased soil water evaporation. However “if the planter does not disturb the soil below the seed, if the seed is pressed into that soil and if the covering soil has less moisture and lower density or greater pore space, a soil textural discontinuity exists at midline or slightly above. This discontinuity retards water loss from below, while the loose soil above dries quickly, allowing higher temperatures and increased air permeability.”
gives four fundamentals of seed placement:
1) Cut residue & soil to create the furrow of proper depth,
2) place seeds consistently into the bottom of the furrow,
3) firm the seeds into moist soil at the bottom of the furrow by applying the right amount of pressure exactly where it is needed and
4) close the furrow by chopping the sidewall, to prevent drying and allow good root exploration.
We both close the furrow and pack from the surface down in our systems. Could it be that we are sometimes creating poor conditions for germination and emergence? If Carter is right we are losing moisture. However, On the whole we get pretty good germination and emergence. Canola might be the big exception with emergence around 50-55% in research plots at Lacombe.
While my thoughts are speculative I do think of Robert Ruwoldt's comment from Direct Seeding Advantage 2008, “If you don’t challenge your systems and yourself, who will?”
 Carter, L.M. 1990. Proper seedbed preparation. The Cotton gin and Oil Mill Press. 14-16.
 Bowen, H. D. 1966. Measurement of edaphic factors for determining planter specifications. Trans. ASAE, 9(5): 725-735.