Agriculture Reference
In-Depth Information
(r
2
= 0.94 or even higher). However, this alone does not mean that ultrasonic control
of seeding depths is ready for applications.
Precise sensing of the sowing depth with this technique implies an unimpeded
access of the ultrasonic waves to a resting soil surface. Therefore, the waves should
hit the soil well in advance of the opener front in order to prevent inaccuracies
arising from soil in motion due to mechanical stirring. Bulky crop residues must be
removed from the row area (see Sect.
8.4.1
). Interference by clods must and can
be taken care of by averaging of signals and by adjusting the response time. And
finally, under very dry conditions, dust may influence the results.
But there are important advantages too. Control adjustments are possible on-the-
go, which is a prerequisite for site-specific operations. And since the control devices
need less space in the lateral direction than gauge wheels, realizing of seeding in
narrow rows is facilitated.
The tests at the Alberta Agricultural Research Centre (
1994
) and by Kiani et al.
(
2010
) were about the precise adjustment of sowing depth to preset
constant
numerical values for the field. But when it comes to site-specific control, the question
is, on which criterion a
varying
seeding depth within a field should be based. Such
a control could be oriented
e.g.
at the temperature, the texture, the organic matter or
the water content of a soil. Sensing techniques for site-specific recording and mapping
of these soil properties are available. However, what is reasonable?
It is well known that a specific temperature range is needed in the soil for every plant
species for germination as well as for emergence. However, except for selecting the
proper season of the year, predicting the soil temperature around the seeds for several
days is hardly possible. Therefore, sensing the site-specific temperatures at the tip of an
opener does not make sense. This would provide for the situation just at the time of
seeding, not more. Varying weather can change the temperature within a short time.
Contrary to this, the
texture
as well as the
organic matter content
of a soil are
constant on a time basis. In most cases, these factors are interrelated. Organic matter
contents of soils tend to increase with the clay content. This means that a site-speciic
control oriented on the clay content or texture of a soil simultaneously includes differ-
ences in the organic matter content, at least to some extent. All site- specific recom-
mendations that are based on clay content or texture probably intrinsically will do this.
Generally, it is recommended to increase the crop-specific seeding depth with
decreasing clay- and hence with rising sand content of a soil. This holds especially
for regions in continental climate. Yet the widely used control systems that keep the
positions of the openers constant to the soil surface prevent a site-specific adapta-
tion of the seeding depth in fields with varying soil texture. Ultrasonic distance
sensors would allow to adjust the seeding depth on-the-go to the respective texture.
The logical procedure for realizing such a control system would be starting with
maps about the site-specific texture that was estimated from sensing electrical
conductivities or perhaps infrared reflectances. These maps could then be used for
many years as a base for a site- as well as a crop-specific seeding depth control via
ultrasonic distance sensing.
However, the most important factor for an adequate control of the seeding depth
probably is soil moisture. This is dealt with in the next section.
