Jack Moes P.Ag.

Manitoba Agriculture, Brandon, MB

Aside from the universally recognized need to place seeds into moist soil to enable germination, the particulars of seed placement remain somewhat under-rated in importance. Recommendations often give crop seeding depth as a range. For example, Manitoba recommendations suggest sowing wheat from 1.5 – 3" in depth. Unfortunately, recommendations stated like this may leave the false impression that sowing at 3" will have the same result as sowing at 1.5" – this simply is not the case.

To explore the importance of seed placement, it is helpful for us to restate the obvious: the goal of seeding is to quickly establish a vigorous crop, in turn toward the goal of achieving the highest economic yield. The central claim of this paper is that attention to seed placement, in particular to seeding depth, is an important (and free) step in attaining the highest economic yield.

Any basic plant physiology textbook will inform us that seeds require three conditions to germinate: adequate moisture, adequate oxygen, and adequate temperature (some species, such as lettuce, also require light).

Lack of oxygen is only a problem on waterlogged soils – presumably we won’t be seeding under such conditions. Waterlogging after sowing might be a problem, but not one we can readily do something about in the first place. Thus for practical purposes, this factor has no implications for management.

For adequate (or non-restrictive) temperature the primary management consideration is seeding date. Most of the crops we are concerned with will germinate reasonably well in cool soil (say 3-5 °C), so early seeding is the norm. All crops will germinate more quickly in warmer soil – for some, such as field beans, a delay in seeding until after mid-May is called for (this is also to reduce the risk of frost damage after emergence). Seed placement also enters as a factor in adequate temperature – early in the season especially, each additional increment in sowing depth brings the seed into what is on average a progressively cooler environment (albeit a more uniform temperature environment). Thus relatively shallow sowing favours the seed with the warmest possible environment for germination.

Adequate moisture then becomes the major germination factor over which we have control in terms of seed placement. We all know that seeds must be placed into moisture to germinate – in dry soil, germination just doesn’t take place; in soil with sub-optimal moisture, germination and emergence are slow and uneven.

But then, how deep into moisture is deep enough? And what happens if the seed is sown too deep? The answer to the first question is good and scientific: "It depends!" The answer to the second follows with a look at a number of crops.

Wheat. As sowing depth increases, emergence is progressively slower and less complete. Larger seeds (within the variety) are affected to a lesser degree. The duration of emergence (from first to last plants emerged) also increases with slowing depth. Late-emerging plants growing from deeper in the soil produce fewer tillers and lower yield per plant. Not only was tillering reduced for deep-sown plants, but crown root development was also greatly reduced (Hadjichristodoulou, 1977). Deeper sowing increased seedling to seedling variability in wheat emergence, and the effect was greater at lower temperatures (deJong and Best, 1979). In other work at the University of Manitoba, wheat sown at 2" and 3" and 6% less than wheat sown at 1", under conditions where 1" was adequate (Ngoma, 1990).

Winter Wheat. Deep seeding of winter wheat resulted in poor establishment, reduced winter survival, reduced competitiveness, late maturity and low yields (Hultgren and Fowler, 1987). Even a small increase in seeding depth from 1 to 1.5" resulted in a 20% drop in emergence.

Barley. Seeding barley beyond 4cm (1.5") in depth resulted in reduced emergence and reduced yield, and also in increased symptoms of common root rot. The effect of soil packing after seeding – time to emergence is reduced, and percent establishment (of seeds sown) is increased, due to enhanced seed/soil contact and reduced depth of soil above the seed.

Canola/Mustard. Both Polish and Argentine canola varieties emerged faster and more completely when sown at 1.5cm(0.6") than when sown at 3cm(1.2")(Nuttall, 1982). Sowing deeper than 1" leads to progressively later and poorer emergence, poorer seedling vigour, increased incidence of seedling disease, and reduced yield. For mustard, increased depth of sowing beyond 1" led to progressively later emergence, later maturity, and lower yield (Brandt, 1992).

Flax. Increased depth of sowing results in delayed, reduced, and increasingly non-uniform emergence of flax. Plant stand was reduced when sowing depth increased beyond 1", while yield was reduced when depth increased beyond 2" (unpublished work by G.G. Rowland and K.M. Panchuk, cited by Stoskopf, 1981).

Type of Emergence. Emergence from depth for crops with hypogeous emergence (cotyledons come above ground – eg. Dry beans, lupins) is more difficult than for crops with epigeous emergence (cotyledons remain below ground – eg. Peas, fababeans, lentils, cereals)(White and Robson, 1989; Stoskopf, 1981). Although this means that the latter group may emerge more successfully from deeper in the soil, it does not mean there is no cost associated with deep sowing.

To summarize, the following problems may manifest themselves if sowing is deeper than absolutely necessary:

-delayed and reduced emergence, poor non-uniform stand

-reduced tillering of cereals

-reduced competitiveness of crop

-increased disease risk

-delayed, non-uniform maturity

-reduced yield.

It is clear – seeding any deeper than necessary under the circumstances carries a high risk of yield loss. Given that the only cost is a few minutes to check the depth and adjust the seeder, grains made by adjusting seeding depth are essentially free. The following "rules-of-thumb" sum up the message of this paper:

1. The goal is to get seedlings up and running as quickly, uniformly, and completely as possible.
2. Seeding should always be as shallow as possible under the circumstances – just deep enough to be in moisture adequate for germination and early growth.
3. Large-seeded crops will need to be sown deeper into moisture than small-seeded crops.
4. Packing after seeding helps put moist soil in contact with the seed, and to maintain soil moisture in the vicinity of the seed.
5. Preseeding tillage dries the soil and makes deeper seeding necessary – zerotillers will be able to seed more shallowly than their conventional-tilling neighbours.

Sometimes deep seeding may be deemed necessary in a dry seedbed, and rightly so- in this case, the disadvantages of deep seeding are expected to be outweighed by the advantage of sowing into moisture. Even so, some of the disadvantages will remain, and germination, emergence, and yield are likely to be poorer than if shallow seeding in a moist seedbed could have taken place.

Too shallow – poor germination and (perhaps no) emergence; too deep – progressively poorer emergence leading to reduced yield. For all this, it is still not possible to give an absolute recommendation for seeding depth for any crop – hence the need for sound judgement which can only be based on the experience of actually getting off the tractor from time to time to see what’s happening to the seed.

Brandt, S.A. 1992. Depths, rates and dates of seeding and yield of yellow mustard (Sinapis alba L.) in west-central Saskatchewan. Can.J.Plant Sci. 72:351-359.