This thesis is dealing with soil quality in relation to soil compaction
and soil loosening. Chapter 2 presents definitions of sustainability and soil
quality as well as presents three research teams view on how to model the
concept. Since the models have problems dealing with real life data they can be
considered conceptual.
Larson and Pierce (1991) add a suggestion of which tests to use to
measure the quality in the field. The number of tests makes it realistic to
measure the quality at field-scale but unrealistic at regional, national and
international scale.
At the international level a global index of human induced soil
degradation, GLASOD, has been developed. The index is the first overview of the
problems with decreasing soil quality around the world. The index is used as
background material of the strategy of soil protection recently presented by
the EU Commission as well as in the proposal for a UN soil convention, also
presented in chapter 2.
Naturally the results of GLASOD is limited for a number of reasons. The
research is only done in small-scale, and therefore not appropriate for a
national level. The results also only show the bad news, which is inappropriate
to show the true condition of the soil. Thus, it is needed to add further
investigations to create concrete politics.
Due to the complexity of the topic it has been decided to limit the
research to four main areas: water and wind erosion as well as physical and
chemical degradation. The results are also containing some visual
overestimation since every polygon, which is not 100% stabil, shows a
degradation colour, even if only 1-5% is affected.
The results are based on expert evaluations and are therefore
qualitative and thereby potentially subjective. That is also the case for the
definitions and models presented in the chapter.
Chapter 3 presents soil compaction which is one of the most serious
reasons of soil degradation in modern agriculture.
Soil compaction occurs when heavy machinery is pressing the soil pores
together and the problem is increasing with water and clay content in the soil
as well as with the number of passes, wheel type, axle load and tire pressure
affects the degree of compaction.
The largest tractors are having axle loads around 20 tonnes and
combiners often exceeds 50 tonnes, as well as slurry equipment is creating
serious compaction. Tractors with axle loads as low as 5 tonnes has been proven
to create compaction as deep as 35 cm. The compaction is relative proportional
with the axle load but the soil reaches a form of maximum compaction which
again increases with passes of heavier machinery.
Larger machinery also creates a dilemma since the make it possible to
cultivate the soil at times where it is most vulnerable to compaction, namely
in the spring and autumn month.
The soil characteristics are traditionally divided into physical,
chemical and biological parameters. Compaction is primarily affecting the
physical parameters since the number of macropores are decreasing which affects
the bulk density, penetration resistance and soil strength and thereby the infiltration
rate, water holding capacity and drainage of the soil.
Also the root growth resistance is increased while the aeration and the
uptake of nutrients by the plants are decreased which results in bad plant
growth and low yields.
Many organisms are living in the soil and affects its quality by their
life processes. Their activities are decreased by compaction, mainly due to the
changes in aeration and water content in the soil. They are crucial to the turn
over of organic material, the release of nutrients and to the N-fixation and
therefore especially organic agriculture is affected by compaction.
Since soil compaction is seriously degrading the quality of the soil it
seems reasonable to investigate nature’s ability to recover from the stress.
Many studies have shown that the structure damages are persisting more than 5-7
years and it is therefore necessary to investigate the effects of different mechanical
loosing tools on the soil and yields.
Chapter 4 presents different research results where subsoilers, double
diggers and paraplows have been used to break up plow pans and loosening the subsoil. All tools are
common in mechanised agriculture.
The chapter also presents a trial with the Dutzi system under Danish
conditions in an organic rotation and a trial with soil loosening to 90 cm. The
last trial has shown improved yields of different vegetables, potatoes, and
beans, while trials with digging machines shows several irregularities.
As a part of the thesis a field experiment with soil loosening was
carried out at the KVL research farm Højbakkegård. The investigated mechanical
looseners used were a one-tined subsoiler and deep digging to 90 cm with a
excavator. Ploughing as reference.
The effects were studied for two crops: barley and oil radish, which
later was replaced by oil seed rape.
The deep digging gave, as expected, the best structure measured as the
penetration resistance, and it followed the literature. Subsoiling gave a lower
resistance in D30 and D60 in spring and in D30 in the autumn. The measures of
the penetration resistance in spring showed that the subsoiler mainly loosened
the soil in the furrow, while the effect of the tools was erased in autumn.
Despite the fact that digging gave the lowest resistance the method had
problems with creating the an appropriate seedbed, and especially the oil
radish had a lower emergence in the loosened parcels. For barley there was a
tendency that digging gave a reduced emergence, while subsoiling had the best
emergence rate.
The emergence rated affected the remote sensing (fig. 5.6.a.) in the
beginning of the growing season, where subsoiling followed by ploughing had the
highest reflectance. But after 54 days the dug parcels exceeded the subsoiled,
which at the end of the measure period (81 days) was reached by the ploughed parcels.
Despite the uneven emergence all parcels had the same yields.
De ploughed parcels showed the highest seed weight while there was no
variation between the loosened parcels.
To get the same yields the loosed parcel must have had more plants/m2
(as for the subsoiled) or the plants had more stems. This seems as the case for
the dug parcels which despite a low emergence rate reached the same level of
remote sensing after 54 days.
There was a correlation between remote sensing and yields of barley, so
that none of them showed differences between the soil management.
Based on the level of remote sensing of rape seed the soil loosening had
a positive influence on the growth. A relation between the penetration
resistance and the remote sensing was also found. The relation was that an
increase of 1 MPa in the penetration resistance gave a reduction in the remote
sensing of 1,26.
There was no differences in the penetration resistance between spring
and autumn samples and it is therefore concluded that neither barley nor rape
had any soil loosening effects during the growing season.