T. VIDRIHBiotechnical Faculty, AGRONOMY DEPARTMENT Jamnikarjeva 101, 61000 Ljubljana. Slovenia
Introduction
Materials and methods
Results
Discussion
References
Many lowland pastures in Slovenia are located on heavy soils. On dairy farms, with such less favourable soils for grazing, soil compaction has become increasingly frequent and severe, because of humid climate and high stocking rate. Soil compaction may affects the overall possibilities for root penetration of the sward plants. To partly overcome this problem continuous system of grazing is in use because denser sward could be maintained and animals graze at lower stocking density. Slow establishment and poor persistence of white clover in heavy soils or cold wet soils could be improved if any mechanical measure on soil disturbance can be implement for such compacted soils.
The objective of this work was to measure the level of soil compaction on lowland pasture., to look at the quantity and distribution of the roots in such soil, and to investigate how long the effect of shallow subtillage on improved physical properties of the soil will last.
In open sward white clover was sod seeded (Vredo seeder) in early spring 1991, at the seeding rate 7 kg ha-1. Seedlings growth and herbage mass was controlled at monthly intervals. At beginning of autumn detailed botanical composition was made and penetrometer resistance was measured. Next spring (1992) in March pasture was fertilized with 400 kg ha-1 (8: 26: 26). Dry cows grazed experimental site continuously during spring and late summer/autumn period. Summer regrowth was cut for conserved feed. Protective cages were used for to monitor sward growth and total herbage mass was measured with Pasture Probe at 2-weekly intervals. In beginning of June sward botanical composition was estimated again and root sample cores were obtained by hand driving a steel tube of an internal diameter of 72 mm to a depth of 30 cm. For each measurement 8 cores were taken. Four soil cores were used to wash roots out. No attempt was made to distinguish between grass and clover roots, or to between live and dead one. Root lenght was determined by Delta-T Area Meter (Root lenght measurement procedure). Roots were than dried and weighed. From data obtained this way root lenght density (cm cm-3), root area index (cm cm-2) and specific root lenght (m mg-1) were calculated. Other 4 soil cores were used to determine the number of roots at specified depths at lower and upper core face by core break method. The lenght of white clover stolons in the top layer of soil cores were determined too. Equal core sampling was repeated in September and October and penetrometer resistance was measured at each core sampling.
In mid June half of the experimental site (paddock) was subtilled to a depth of 15 cm with the rows of 60 cm apart. The other half of paddock was subtilled at the end of autumn.
Good establish of white clover sod seeded in spring was obtained due to proper grazing management. In mid summer there was 25 % of white clover in the sward and later in the same year even 62 %. Penetrometer resistance values were 4.0 MPa for top 15 cm soil layer and 3.3 MPa for soil under below. Little higher values on penetrometer resistance were obtained in spring sampling on soil with bulk density 1.1 Mg m-3, porosity 57.6 % and water content 28.2 %. In autumn because of wetter soil penetrometer resistance values were much lower, but the similar pattern could be found as in the spring sampling. This data agree with the topsoil compressibility results of Mulholland and Fullen (1991), who found that most structural damage occurred at a depth of about 10 cm. After shallow subtillage in June, values recorded on penetrometer resistance were less than half of normal ones. This effect has been noticed 15 cm aside from the line of subtillage and persisted up to the end of grazing season.
Root lenght density in average over rooted zone was found to be low compared to the data found in literature for grass swards (De Willigen and Van Noordwijk, 1987).For white clover high number of nodes must to be establish in first part of growing season and than in moist late summer strong adventitious roots could be developed. Similar pattern of root lenght density and root area index can be seen from data in table 1.
High number of roots per square cm of core break face was found for top layer of soil. At depth greater than 15 cm less than 1 root cm-2 was found. These were white clover roots expanding even under depth of samplig (30 cm). These roots were growing in cracks and fissures. On clay soils, soil compaction mainly affects the soil structure, the size and the shape of soil aggregates. This way the uptake of nutrients by plants could be seriously hindered because roots cannot penetrate large aggregates when rhizosphere is depleted. In practice this problem is often overcome by applying fertilizers as a source of nutrients.
Table 1: Data on root quantity measurements and penetrometer resistance values sampled at three occasions.
|
Date of sampling |
June |
September |
October | |
|
Root lenght density cm cm-3 |
0.44 |
0.61 |
0.39 | |
|
Root area index cm cm-2 |
13.2 |
18.3 |
11.6 | |
|
Specific root lenght m g |
17.3 |
20.8 |
11,8 | |
|
Penetrometer resistance MPa |
|
|
| |
|
Soil depth |
Top layer |
4.18 |
3.96 |
1.40 |
|
" |
5 cm |
4.40 |
4.12 |
2.10 |
|
" |
10 cm |
4. 17 |
3.82 |
1.88 |
|
" |
15 cm |
3.90 |
3.24 |
1.88 |
|
" |
20 cm |
3.55 |
3.35 |
1.64 |
|
" |
25 cm |
3.55 |
3.92 |
1.84 |
The soil used for grass production and grazing must have sufficient mechanical strength to prevent the collapse of soil water and air pathways by animal trampling. But to high compaction may limit root growth because of large mechanical resistance and restricts the rate of oxygen supply to roots (Oussible, 1988).
Good root development and homogeneous distribution pattern allows maximum production at lower current fertility levels, or stabilize sward growth during less favourable conditions. Because the rate of nutrient losses to the environment depends on current fertility levels, larger root system may contribute to an Increased nutrient use efficiency.
Coarsely aggregated soils, or soils which can only be penetrated by roots in cracks and fissures, result in an inhomogeneous root distribution. Reduction in root elongation of such compacted soil Is because of lower aeration capacities in soil (Asady and Smucker, 1989). The CO2 production by roots and associated rhizospheres could exacerbate the effects of mechanical resistance to root penetration in deeper soil layers.
The soil compaction produced by cattle trampling tends to be shallow but can lead to ponding, thus rendering the soil more susceptible to further poaching. This compacted top layer of soil has a pronounced effect on oxygen diffusion rates of soil below it. Asady and Smucker (1989) found that even when root lenght density approach high value and occupies more than 5 % of the air-filled porosity, O2 consumption rates appear to exceed the supply rate and CO2 concentration Increase. It is not hard to understand that high pore plugging values can soon be obtained with shallow rooted grass on compacted soils with high density of white clover stolons and high potential to develop adventitious roots in moist late summer. More work is needed to understand better and be able to explain the reasons for the sometimes sudden changes in white clover content in grazed sward especially In heavy soils.
Asady, G.H. and Smucker, A.J.M. 1989. Compaction and root modifications of soil aeration. Soil Sci. Soc. A.J. 53: 251-254.
De Willigen, P. and Van Noordwijk, M. 1987. Roots, plant production and nutrient use efficiency. PhD thesis, Agricultural University Wageningen, 282 p.
Oussible, M. 1988. Effect of subsoiling a compacted clay loam soil on the nitrogen uptake of wheat. Procc. of the 11th Conf. of the Int. Soil Tillage Res. Org. 2. 797-802.
Mulholland. B. and Fullen, M.A. 1991. Cattle trampling and soil compaction on loamy sands. Soil Use and Management 4: 189-192.
