Agronomy Library > Soil Conservation

Seeding Depths and Seeding Rates in Direct Seeding Systems
Author: Nick Underwood, Reduced Tillage LINKAGES
Date Created: April 02, 2002
Last Reviewed: February 01, 2007

 
Here are two statements that are true:

Direct seeding conserves moisture.
Direct seeded fields are cooler than cultivated fields.

There are some people who assume from this information that their land will not allow reduction or elimination of tillage. Most of the area in Western Canada that is now direct seeded or zero tilled, had people with that belief in the recent past. I will call them the ‘non-believers’.
 
Neither of the above statements tells the whole story about moisture and soil temperature in direct seeding, even together they don’t. We can however add to the information in those two simple statements and use them to our advantage when deciding about seeding rates and seeding depth. The same thought process helps to encourage the ‘non-believer’ to look again at reduced tillage.
 
Statement #1 says that we conserve moisture in direct seeding, and there are three main reasons for that. First stubble traps snow, second residue cover reduces evapouration losses, and third the stubble and residue together reduce surface run off and evaporation. BUT, the ‘non-believers’ will say, “my heavy clay ground is flat and I have to cultivate so that it is dry enough to seed and warm enough for germination.” OK - so if you do that cultivation and presumably harrow, what is your seeding depth for wheat or canola?   It is probably 1.5” or more for wheat, and 1” for canola. The direct-seeder however will probably seed wheat at 1” and canola at 0.5”. These depths may be slightly less in moist clay soils.
Remember the seeding depth is defined as the distance from the seed to the soil surface immediately above it, after the last implement passes over the planted field.
 
Statement #2 is based on measurements that have been taken. The only measurements that I have seen were at 2”. At that depth direct seeded fields range from being 0 to 3 degrees C. cooler than conventional fields. In the top 1” the difference is less, of course. Cultivated land is warmer than residue covered land at the soil surface on sunny days, and the diurnal range is greater. The point I am trying to make is that the difference in temperature is not great at the depth to which the seed is placed in the different systems. Direct seeding at a shallower depth can offset much of the heat advantage that tilled land has at 2”.
 
Whatever your seeding system, the job of your seeding implement and its ground openers, is to place the seed into moist soil where it is surrounded by soil particles small enough to reduce the open spaces in the seedbed. There should be good seed to soil contact, and that is one of the jobs of the packer. The on row packer reduces the seeding depth while it is firming the soil.
 
Narrow single shoot openers provide the best seed placement. By that I mean that it is easier to consistently achieve the desired seeding depth. However, you are limited to the amount of fertilizer that can be placed with the seed.
 
Sometimes people ask if seeding rates should change when you switch to direct seeding.
The short answer is no. But there are two reasons why you might want to increase the seeding rates:
 
1 - The seeder you use to direct seed may have much wider row spacing than your old conventional seed drill. This will result in a higher number of seeds per row at a given seeding rate. In drier areas the increased moisture conservation of direct seeding will increase the production potential compared to a tillage system. This potential may result in a longer growing period while the crop responds to the additional soil moisture. Increasing seeding rates by ten to twenty percent can also capture this potential. Maturity should not be delayed. The effectiveness of this will depend on what your present seeding rates are, and the moisture conditions in your area.
 
2 – Weed control in direct seeding is not done by tillage. We depend on timely application of herbicides. To prevent the herbicide costs from increasing, you can adopt some other tactics to control weeds such as rotation, competitive varieties, and increased seeding rates. Drs. Neil Harker and John O’Donovan have done very useful work in this area.
 
Facts about seeding depth:
  • Seeding depth is defined as the distance from the seed to the soil surface immediately above it, after the last implement passes over the planted field.
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  • The on row packer reduces the seeding depth while it is firming the soil.
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  • The direct-seeder will probably seed wheat at +/- 1” and canola at <0.5”. These depths may be slightly less in moist clay soils.
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  • At a depth of 2 inches direct seeded fields range from being 0 to 3 degrees C. cooler than conventional fields.
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  • Direct seeding at a shallower depth can offset much of the heat advantage that tilled land has at 2”.
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  • The job of your seeding implement and its ground openers is to place the seed into moist soil where it is surrounded by soil particles small enough to reduce the open spaces in the seedbed.
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  • Narrow single shoot openers most consistently achieve the desired seeding depth.
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  • Increasing seeding rates by ten to twenty percent can capture the potential from increased moisture conservation of direct seeding in drier areas.
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  • Increasing seeding rates is one of several tactics to control weeds without using additional herbicides.