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Experiences with deep tillage in the Netherlands
Soil & Tillage Research, 7 (1986) 273--283 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
273
Short Communication EXPERIENCES
WITH DEEP TILLAGE
IN THE NETHERLANDS
C. VAN OUWERKERK and P.A.C. RAATS Institute for Soil Fertility, Department o f Soil Physics and Soil Tillage, P,O. Box 30003, 9 750 R A Haren Gn (The Netherlands) (Accepted for publication 20 February 1986)
ABSTRACT
Van Ouwerkerk, C. and Raats, P.A.C., 1986. Experiences with deep tillage in The Netherlands. Soil Tillage Res., 7: 273--283. A review is given of a national symposium on deep tillage, which was held in December 1984 at Wageningen, The Netherlands. By nature, in sandy loam soils (8--17% clay) with a low organic matter content, and in sand soils without organic matter, the subsoil may be so dense that root development is unsatisfactory, crops suffer from drought and yields are reduced. Ploughpans are c o m m o n in all Dutch soil types. Light soils (< 17% clay) do not crack on drying and here, ploughpans may hamper root growth and transport of water and gases. This negatively affects emergence, crop development and the number of work days in autumn and in spring. Deep tillage by subsoiling, rotadigging or rotamixing has a pronounced effect on total porosity and macroporosity, but usually recompaction starts very quickly and after 2--3 years, throughout the loosened layer, bulk density may be higher and macroporosity lower than before deep tillage. To conserve the loosening effect of deep tillage, field traffic and tillage systems should be modified and, preferably, deep rooting crops with a long growing period (alfalfa, seed grass) should be grown. Deep tillage should never be performed to a greater depth than the b o t t o m of the compact layer. In view of the high costs of deep tillage, it is advisable to consider alternatives, such as tile drainage to improve emergence and to increase the number of work days, and sprinkler irrigation to improve crop development and yield.
INTRODUCTION On 13 December 1984, the Working Group "Soil Tillage, Technical Aspects" organized a well-attended national symposium on the timely s u b j e c t " D e e p T i l l a g e in T h e N e t h e r l a n d s " . L e c t u r e s w e r e g i v e n b y D r . M.J. Kooistra and Mr. A. Jager (Soil Survey Institute, Stiboka, Wageningen), D r . Ir. A . J . K o o l e n ( W a g e n i n g e n A g r i c u l t u r a l U n i v e r s i t y , T i l l a g e L a b o r a t o r y ) , Ir. C . F . P o e l m a ( E i n d h o v e n T e c h n i c a l U n i v e r s i t y , T H E ) , I n g . M . C . S p r o n g ( I n s t i t u t e o f A g r i c u l t u r a l E n g i n e e r i n g , I M A G , W a g e n i n g e n ) , D r . Ir. G . P . Wind and Mr. R. Wiebing (Institute for Land and Water Management Res e a r c h , I C W , W a g e n i n g e n ) a n d I n g . J. A l b l a s ( R e s e a r c h S t a t i o n f o r A r a b l e
0167-1987/86/$03.50
© 1986 Elsevier Science Publishers B.V.
274 Farming and Field Production of Vegetables, PAGV, Lelystad). The main points made in these lectures and subsequent discussions are reported below. SOIL COMPACTION IN THE NETHERLANDS Most Dutch soils have developed in unconsolidated sediment, deposited by water or wind. By nature, only a small proportion of these soils, characterized by a low clay and/or organic matter content and a specific granular composition, is so dense that they present problems in arable farming and field production of vegetables. Dense subsoils may occur underneath thin clay topsoils in the coastal areas or below the arable layer of organic soils and wind deposited sands (Wind and Pot, 1976). These subsoils may have a porosity as low as 40% (v/v). Consequently, penetration resistance may be too high and oxygen content too low for satisfactory root development (Goedewaagen et al., 1955). As a result, especially in extended dry periods, crops suffer from drought and yields are low. Irrespective of the natural bulk density, compacted layers just below the arable layer, caused by field traffic and ploughing (ploughpans) are c o m m o n in all Dutch soil types. On light soils (< 17% clay), which do not crack on drying, ploughpans may hamper root growth or vertical transport of water and gases. This may have a negative effect on soil temperature in spring, seedling emergence and crop development, and on the number of work days. Extended dry periods may result in too high a penetration resistance (on sand > 2.5 MPa). As a result, roots may be ranged, which especially for sugar beet is disastrous (low yield, high soil tare). OBJECTIVES OF DEEP TILLAGE Generally, the main objective of deep tillage is to loosen compact layers in order to promote root development throughout the soil profile, thus improving the supply of water from increased storage and capillary rise. Secondary goals may be the improvement of internal drainage and, thus, improvement of workability in spring and in autumn, and getting rid of excess water in the growing period of the crops. Deep tillage operations may include mixing subsoil material through the arable layer to improve its granular composition. At the basis of all this of course is the expectation that deep tillage will lead to a substantial increase in net returns over the duration of the crop rotation. However, relevant economic data are scanty. IMPLEMENTS FOR DEEP TILLAGE, PROCESSES IN THE SOIL AND ENERGY REQUIREMENTS Deep tillage may be performed with drawn or with powered implements. The group of drawn implements consists of (1) chisel plough, (2) blade-type
275
subsoiler, (3) plough-mounted subsoiler, (4) deep plough and (5) mixing plough subsoiler {Fig. la). These implements are simple but heavy and, thus, they require a heavy tractor. However, due to wheel slip and rolling resistance, energetic efficiency is only about 50%. ~ l
.
.
j
.
.
.
.
.
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.
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.
.
.
(a)
L i)..
~
(b)
FS i (c)
_
•~. __
_
.., / ~ _ - - _ ' ~/'1 ~ . J
Fig. 1. Implements and machinery for deep tillage. (a) mixing plough subsoiler; (b) mixing rotor; (c) cyclomixer. Source: C.F. Poelma, Eindhoven Technical University.
276
The group of powered implements includes (1) pendulum subsoiler, (2) luffing-blade subsoiler, (3) rotadigger and (4) mixing rotor (Fig. l b ) . Although the energetic efficiency is higher, the total energy requirement (kJ m -3) of powered implements is higher than that of drawn implements (Table I), because of the more intensive crumbling and mixing of the soil. At the Eindhoven Technical University, a p r o t o t y p e "cyclomixer" has been developed (Fig. lc), which has a working depth of 0.35 m and a low specific energy requirement (150 kJ m-3; Table I). TABLE 1 Characteristics of implements and machinery for deep tillage a Implement/machine
Working Working Forward width depth speed (m) (m) (m s ')
Power requirement
Capacity Specific (ha h -1) energy (kJ m -3)
(kW) Fixed-blade subsoiler Luffing-blade subsoiler Pendulum subsoiler Mixing plough subsoiler Mixing rotor Rota-digger Cyclo mixer (calculated)
1.0 0.65 2.5 0.7 1.5 3.0 5.0
1.0 1.0 0.7 1.4 1.2 0.5 1.0
1.0 0.4 0.4 0.85 0.2 0.5 0.3
200 75 150 220 150 140 200
0.35 0.10 0.35 0.20 0.10 0.50 0.50
200 290 220 260 420 190 150
aSource: C.F. Poelma, Eindhoven Technical University (THE), Eindhoven.
The most probable type of process when using a c o m m o n blade-type subsoiler, is illustrated in Fig. 2 (Koolen and Kuipers, 1983). In the inclined hatched zone in front of the blade, the soil is impelled obliquely upwards. In light soils, this causes a series of more or less parallel shear fractures; in firm, cohesive soil, S-type fractures may occur. The soil slice moves upwards over the inclined blade and expands sideways. Consequently, vertical tension fractures occur in the top part of the slice and shear fractures along its lower sides. During the process, much bending occurs, which gives rise to zones with tensile fractures and zones with shear fractures. The horizontal pressure of the vertical shank causes shear strain and extra tensile fractures. As a result, the soil is loosened, i.e. the volume of the soil slice is increased, resulting in some upheaval. In dense sand, maximum loosening of the soil occurs when the soil is lifted over a height of 0.10--0.15 m, the o p t i m u m vertical blade angle being between 15 and 25 ° (Sprong, 1982). Usually a layer of about 0.3 m above the blade is loosened by the blade action; the effective working width practically coincides with the width of the blade. Thus, the cross section of the loosened soil is not an inverted trapezium (as in cohesive soils, Fig. 2), b u t a rectangle.
277
cross section A - A
direction of travel
i
/
/
/
A longitudinal section
Fig. 2. Soil behaviour due to subsoiling with a blade-type subsoiler (Koolen and Kuipers, 1983)• Since the vertical effect is limited to 0.3 m, a blade-type subsoiler is o n l y effective w h e n working at a d e p t h less than 0 . 7 m . For greater working depths, say 1.0 m, it may be advisable to use t w o subsoilers in o n e frame, the o n e in front working at a shallower depth than the o n e in the rear. This arrangement results in a substantially lower specific resistance. EFFECTS OF DEEP TILLAGE Soil s t r u c t u r e
On light loam soils, subsoiling has a p r o n o u n c e d effect on b o t h total p o r o s i t y and m a c r o p o r o s i t y (pores > 30 p m diameter), but the effect is
278 usually relatively short-lived (Table II). After 1 year, at 0.32--0.36 m depth, total porosity is still appreciably higher than in the original (primary) ploughpan, but macroporosity is already smaller. After 3 years, total porosity has decreased to the same low value (40%, v/v) as in the primary ploughpan, while the macroporosity (0.8%, v/v) is less than half the value in the primary ploughpan (1.9%, v/v). In the course of time, the loosened subsoil is also recompacted. After 3 years, macroporosity (15.8%, v/v) is still appreciably higher than in the original, undisturbed subsoil (3.4%, v/v), but total porosity has already decreased to a lower value (43%, v/v). It may be expected that recompaction throughout the subsoiled layer will continue and that the ultimate bulk density will be higher than in the original situation. Consequently, deep tillage should never be performed to a greater depth than the bottom of a seriously compacted layer. Working under too wet conditions should be avoided as it may result in a smeared layer with an extremely low macroporosity (0.5%, v/v) directly below the slit (Table II, 1 year after subsoiling, 0.51--0.52 m depth).
Water retention and hydraulic conductivity In the primary ploughpan, continuous pores are scarce, but in the secondary (recompacted) ploughpan they are virtuany lacking (Kooistra et al., 1984, 1985). Here, macroporosity consists mainly of isolated packing voids, continuous channels made by plant roots and soil fauna being practically absent. However, at saturation, water content is strikingly higher in the secondary than in the primary ploughpan (Fig. 3). This difference must be due to a larger volume of pores < 30 pm diameter, possibly due to puddling during deep tillage (Kooistra et al., 1984) or to the smearing action of the furrow wheel at subsequent ploughings. Between saturation and a pressure head of -0.1 m, hydraulic conductivity is significantly lower in the secondary ploughpan which, again, indicates the lack of continuous pores. The low values at saturation imply a higher probability for the occurrence of ponding and perched water tables during extended wet periods.
Crop response Effects of deep tillage are most pronounced in dry years as, for example in 1964 at Borgercompagnie on an organic soil, underlain by dense sand (Wind and Pot, 1976). Deep ploughing carried out in 1961 resulted in a 40-cm increase in rooting depth and in a 37-% higher yield of spring wheat. However, these effects could also be obtained by sprinkler irrigation (Table III). In this case, on deep ploughed soil, irrigation did not have any additional effect. In several long-term experiments on sand, crop response fluctuated
27 9
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280 Hydraulic conductivity K (cm d-l) 103
\
........
pasture land
-
arable land, with primary ploughpan
- -
102 ~\~\. .\ "!\.~"x-
---
-
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deeply loosened arable land, with secondary ploughpan
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(cm3 cm-3} 0.55
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-100 -50 pressure head h (cm)
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6
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Fig. 3. Hydraulic conductivity (top) and water retention curves (bottom) of the 0.30-0.35-m layer of pasture land (A), and of the primary (B) and secondary (C) ploughpan in a Typic Haplaquent. Separate curves represent different locations within the same treatment (Kooistra et al., 1984, 1985).
strongly and, on average, yield increase was only 5--6% (Alblas, 1984). Only when controlled traffic is applied is the effect of deep tillage sufficiently long-lasting to ensure large yield increases. When, moreover, expensive vegetables such as asparagus are grown, deep tillage may be profitable. For clay-on-sand profiles and sandy loam soils, deep tillage, especially
28:1 TABLE III Grain yield (t ha l) of spring wheat at Borgercompagnie in the dry year 1964 a Treatment in 1961
Nil Deep ploughing
Rooting depth (cm) 25 65
Sprinkler irrigation No
Yes
3.8 5.2
5.3 5.1
aSouree: Wind and Pot, 1976. when performed with rotating (mixing) implements, usually resulted in a strong decrease in clay and organic matter content of the arable layer (Havinga, 1978). This may improve workability in spring, but it may also delay early growth and induce slaking of the topsoil during winter. Yield increases were only observed during the first one or two years. RECOMPACTION To conserve the loosening effect of deep tillage, field traffic and tillage should not be performed when the soil is too wet and tractors and other vehicles used should be as light as possible. Also, properly adapted running gear (low inflation pressure) and, especially for field production of vegetables and flower bulbs, controlled traffic should be recommended. The compacting and smearing action of the furrow wheel is avoided if ploughing is replaced by rota-digging. On sandy soils, new methods of deep tillage promise longer lasting results. For example, subsoiling may now be performed such that at the same time, clay or peat from deeper layers are mixed through the sand, which has a stabilizing effect. In the field experiment at Borgercompagnie, where peat was mixed through the sandy subsoil, recompaction had not occurred even after 20 years and repetition of deep tillage did not have significant effects on crop yield (Table IV). Also, complete removal of a dense, sandy layer in 0.08-m wide slits at 0.4-m distance, and backfilling with stable, black peat material may be a solution (R. Wiebing, Institute for Land and Water Management Research, personal communication 1984). At present, probably the best way to stabilize the loosening effect of deep tillage is to grow deep rooting crops, such as alfalfa, or crops with a long growing period, such as seed grass. They may also have favourable effects on the soil fauna which, in turn, may be beneficial for soil structure. Even when recompaction does not occur, as in organic soils with dense peat layers underneath, subsoiling may give unsatisfactory results, due to the low pH (< 3.5) of the peat layers, which prevents root penetration (Wind and Pot, 1976). Subsoiling with a double subsoiler (vertical distance between the blades 0.1 m) and application of sugar factory lime sludge in
282 TABLE IV 1982--1984 average grain, tuber and root yields (t ha ~) at Borgercompagnie a Crop
Spring wheat Starch potatoes Sugar beet
Treatment in 1961
Nil Deep ploughing Nil Deep ploughing Nil Deep ploughing
Treatment in 1982 Nil
Deep subsoiling
5.5 6.2 61.2 64.6 54.3 59.8
6.3 6.4 66.7 63.8 56.2 60.1
aSource: G.P. Wind, Institute for Land and Water Management (ICW), Wageningen.
one pass, may increase the pH to about 4.0 (Sprong, 1980. Boer and Tick, 1983). This pH is high enough for satisfactory root development and low enough to prevent oxidation of the peat. CONCLUSIONS
(1) On sandy loam soils (8--17% clay), which are liable to compaction, the ploughpan does not crack on drying and, thus, it can only be loosened by subsoiling. Deeper seated compacted layers may be loosened by rota-digging or by using a wide-blade type subsoiler. (2) Restricted root development is not always caused by compacted layers, but may be caused by a low pH in the subsoil. In such cases, deep tillage should be supplemented by liming. (3) After deep tillage, recompaction over the full depth of the loosened layer usually starts very quickly and the ultimate bulk density may be higher than it was prior to deep tillage. Therefore, the working depth of deep tillage implements should never exceed the depth of the compact layer. (4) To conserve the loosening effect of deep tillage, field traffic and tillage treatments should be modified such that the soil is not unduly compacted. Deep rooting crops such as alfalfa, and crops with a long growing period such as seed grass, may have a beneficial effect. (5) Deep tillage is expensive and the effect is usually only short-lived. Therefore, sprinkler irrigation or improvement of tile drainage may be economically attractive alternatives in some cases. (6) When considering deep tillage, firstly the soil profile and the root development should be properly examined to be able to judge the seriousness of the compaction.
283 REFERENCES Alblas, J., 1984. Results of deep loosening of loam soils in the southwestern part of The Netherlands; 1978--1982. Proefstn. Akkerbouw Groenteteelt Vollegrond (PAGV), Lelystad, Rep. No. 22,22 pp. (in Dutch). Boer, J. and Tick, J.J., 1983. Results of subsoil liming 1980--1982. Bedrijfsontwikkeling, 1 4 : 3 2 1 - - 3 2 4 (in Dutch). Goedewaagen, M.A.J., Van den Berg, C., Van den Bosch, D., Butijn, J., Jonker, J.J., Van der Schaaf, D. and Schuurman, J.J., 1955. Root development in soils consisting of a top layer of clay and a sandy subsoil. Versl. Landbouwkd. Onderz., 61.7., 137 pp. (in Dutch with English summary). Havinga, L., 1978. Effects and crop reaction as a result of deep tillage of clay on sand soils. Cultuurtechn. Tijdschr., 1 8 : 9 7 - - 1 0 2 (in Dutch). Kooistra, M.J., Bouma, J., Boersma, O.H. and Jager, A., 1984. Physical and morphologica] characterization of undisturbed and disturbed ploughpans in a sandy loam soil. Soil Tillage Res., 4: 405--417. Kooistra, M.J., Bouma, J., Boersma, O.H. and Jager, A., 1985. Soil structure differences and associated physical properties of some loamy Typic Fluvaquents in The Netherlands. Geoderma, 36: 215--228. Koolen, A.J. and Kuipers, H., 1983. Agricultural Soil Mechanics (Advanced Series in Agricultural Sciences, Volume 13). Springer-Verlag, Berlin, 250 pp. Sprong, M.C., 1980. Implement for injection of liquid sugar factory lime sludge. Landbouwmechanisatie, 31 : 799--800 (in Dutch). Sprong, M.C., 1982. Deep tillage of sand soils. Landbouwmechanisatie, 3 3 : 6 3 5 - - 6 3 7 (in Dutch). Wind, G.P. and Pot, R.A., 1976. Bodenverbesserung in den holl~ndischen Veenkolonien. Z. Kulturtech. Flurbereinig., 17: 193--206.
273
Short Communication EXPERIENCES
WITH DEEP TILLAGE
IN THE NETHERLANDS
C. VAN OUWERKERK and P.A.C. RAATS Institute for Soil Fertility, Department o f Soil Physics and Soil Tillage, P,O. Box 30003, 9 750 R A Haren Gn (The Netherlands) (Accepted for publication 20 February 1986)
ABSTRACT
Van Ouwerkerk, C. and Raats, P.A.C., 1986. Experiences with deep tillage in The Netherlands. Soil Tillage Res., 7: 273--283. A review is given of a national symposium on deep tillage, which was held in December 1984 at Wageningen, The Netherlands. By nature, in sandy loam soils (8--17% clay) with a low organic matter content, and in sand soils without organic matter, the subsoil may be so dense that root development is unsatisfactory, crops suffer from drought and yields are reduced. Ploughpans are c o m m o n in all Dutch soil types. Light soils (< 17% clay) do not crack on drying and here, ploughpans may hamper root growth and transport of water and gases. This negatively affects emergence, crop development and the number of work days in autumn and in spring. Deep tillage by subsoiling, rotadigging or rotamixing has a pronounced effect on total porosity and macroporosity, but usually recompaction starts very quickly and after 2--3 years, throughout the loosened layer, bulk density may be higher and macroporosity lower than before deep tillage. To conserve the loosening effect of deep tillage, field traffic and tillage systems should be modified and, preferably, deep rooting crops with a long growing period (alfalfa, seed grass) should be grown. Deep tillage should never be performed to a greater depth than the b o t t o m of the compact layer. In view of the high costs of deep tillage, it is advisable to consider alternatives, such as tile drainage to improve emergence and to increase the number of work days, and sprinkler irrigation to improve crop development and yield.
INTRODUCTION On 13 December 1984, the Working Group "Soil Tillage, Technical Aspects" organized a well-attended national symposium on the timely s u b j e c t " D e e p T i l l a g e in T h e N e t h e r l a n d s " . L e c t u r e s w e r e g i v e n b y D r . M.J. Kooistra and Mr. A. Jager (Soil Survey Institute, Stiboka, Wageningen), D r . Ir. A . J . K o o l e n ( W a g e n i n g e n A g r i c u l t u r a l U n i v e r s i t y , T i l l a g e L a b o r a t o r y ) , Ir. C . F . P o e l m a ( E i n d h o v e n T e c h n i c a l U n i v e r s i t y , T H E ) , I n g . M . C . S p r o n g ( I n s t i t u t e o f A g r i c u l t u r a l E n g i n e e r i n g , I M A G , W a g e n i n g e n ) , D r . Ir. G . P . Wind and Mr. R. Wiebing (Institute for Land and Water Management Res e a r c h , I C W , W a g e n i n g e n ) a n d I n g . J. A l b l a s ( R e s e a r c h S t a t i o n f o r A r a b l e
0167-1987/86/$03.50
© 1986 Elsevier Science Publishers B.V.
274 Farming and Field Production of Vegetables, PAGV, Lelystad). The main points made in these lectures and subsequent discussions are reported below. SOIL COMPACTION IN THE NETHERLANDS Most Dutch soils have developed in unconsolidated sediment, deposited by water or wind. By nature, only a small proportion of these soils, characterized by a low clay and/or organic matter content and a specific granular composition, is so dense that they present problems in arable farming and field production of vegetables. Dense subsoils may occur underneath thin clay topsoils in the coastal areas or below the arable layer of organic soils and wind deposited sands (Wind and Pot, 1976). These subsoils may have a porosity as low as 40% (v/v). Consequently, penetration resistance may be too high and oxygen content too low for satisfactory root development (Goedewaagen et al., 1955). As a result, especially in extended dry periods, crops suffer from drought and yields are low. Irrespective of the natural bulk density, compacted layers just below the arable layer, caused by field traffic and ploughing (ploughpans) are c o m m o n in all Dutch soil types. On light soils (< 17% clay), which do not crack on drying, ploughpans may hamper root growth or vertical transport of water and gases. This may have a negative effect on soil temperature in spring, seedling emergence and crop development, and on the number of work days. Extended dry periods may result in too high a penetration resistance (on sand > 2.5 MPa). As a result, roots may be ranged, which especially for sugar beet is disastrous (low yield, high soil tare). OBJECTIVES OF DEEP TILLAGE Generally, the main objective of deep tillage is to loosen compact layers in order to promote root development throughout the soil profile, thus improving the supply of water from increased storage and capillary rise. Secondary goals may be the improvement of internal drainage and, thus, improvement of workability in spring and in autumn, and getting rid of excess water in the growing period of the crops. Deep tillage operations may include mixing subsoil material through the arable layer to improve its granular composition. At the basis of all this of course is the expectation that deep tillage will lead to a substantial increase in net returns over the duration of the crop rotation. However, relevant economic data are scanty. IMPLEMENTS FOR DEEP TILLAGE, PROCESSES IN THE SOIL AND ENERGY REQUIREMENTS Deep tillage may be performed with drawn or with powered implements. The group of drawn implements consists of (1) chisel plough, (2) blade-type
275
subsoiler, (3) plough-mounted subsoiler, (4) deep plough and (5) mixing plough subsoiler {Fig. la). These implements are simple but heavy and, thus, they require a heavy tractor. However, due to wheel slip and rolling resistance, energetic efficiency is only about 50%. ~ l
.
.
j
.
.
.
.
.
~
.
.
.
.
.
(a)
L i)..
~
(b)
FS i (c)
_
•~. __
_
.., / ~ _ - - _ ' ~/'1 ~ . J
Fig. 1. Implements and machinery for deep tillage. (a) mixing plough subsoiler; (b) mixing rotor; (c) cyclomixer. Source: C.F. Poelma, Eindhoven Technical University.
276
The group of powered implements includes (1) pendulum subsoiler, (2) luffing-blade subsoiler, (3) rotadigger and (4) mixing rotor (Fig. l b ) . Although the energetic efficiency is higher, the total energy requirement (kJ m -3) of powered implements is higher than that of drawn implements (Table I), because of the more intensive crumbling and mixing of the soil. At the Eindhoven Technical University, a p r o t o t y p e "cyclomixer" has been developed (Fig. lc), which has a working depth of 0.35 m and a low specific energy requirement (150 kJ m-3; Table I). TABLE 1 Characteristics of implements and machinery for deep tillage a Implement/machine
Working Working Forward width depth speed (m) (m) (m s ')
Power requirement
Capacity Specific (ha h -1) energy (kJ m -3)
(kW) Fixed-blade subsoiler Luffing-blade subsoiler Pendulum subsoiler Mixing plough subsoiler Mixing rotor Rota-digger Cyclo mixer (calculated)
1.0 0.65 2.5 0.7 1.5 3.0 5.0
1.0 1.0 0.7 1.4 1.2 0.5 1.0
1.0 0.4 0.4 0.85 0.2 0.5 0.3
200 75 150 220 150 140 200
0.35 0.10 0.35 0.20 0.10 0.50 0.50
200 290 220 260 420 190 150
aSource: C.F. Poelma, Eindhoven Technical University (THE), Eindhoven.
The most probable type of process when using a c o m m o n blade-type subsoiler, is illustrated in Fig. 2 (Koolen and Kuipers, 1983). In the inclined hatched zone in front of the blade, the soil is impelled obliquely upwards. In light soils, this causes a series of more or less parallel shear fractures; in firm, cohesive soil, S-type fractures may occur. The soil slice moves upwards over the inclined blade and expands sideways. Consequently, vertical tension fractures occur in the top part of the slice and shear fractures along its lower sides. During the process, much bending occurs, which gives rise to zones with tensile fractures and zones with shear fractures. The horizontal pressure of the vertical shank causes shear strain and extra tensile fractures. As a result, the soil is loosened, i.e. the volume of the soil slice is increased, resulting in some upheaval. In dense sand, maximum loosening of the soil occurs when the soil is lifted over a height of 0.10--0.15 m, the o p t i m u m vertical blade angle being between 15 and 25 ° (Sprong, 1982). Usually a layer of about 0.3 m above the blade is loosened by the blade action; the effective working width practically coincides with the width of the blade. Thus, the cross section of the loosened soil is not an inverted trapezium (as in cohesive soils, Fig. 2), b u t a rectangle.
277
cross section A - A
direction of travel
i
/
/
/
A longitudinal section
Fig. 2. Soil behaviour due to subsoiling with a blade-type subsoiler (Koolen and Kuipers, 1983)• Since the vertical effect is limited to 0.3 m, a blade-type subsoiler is o n l y effective w h e n working at a d e p t h less than 0 . 7 m . For greater working depths, say 1.0 m, it may be advisable to use t w o subsoilers in o n e frame, the o n e in front working at a shallower depth than the o n e in the rear. This arrangement results in a substantially lower specific resistance. EFFECTS OF DEEP TILLAGE Soil s t r u c t u r e
On light loam soils, subsoiling has a p r o n o u n c e d effect on b o t h total p o r o s i t y and m a c r o p o r o s i t y (pores > 30 p m diameter), but the effect is
278 usually relatively short-lived (Table II). After 1 year, at 0.32--0.36 m depth, total porosity is still appreciably higher than in the original (primary) ploughpan, but macroporosity is already smaller. After 3 years, total porosity has decreased to the same low value (40%, v/v) as in the primary ploughpan, while the macroporosity (0.8%, v/v) is less than half the value in the primary ploughpan (1.9%, v/v). In the course of time, the loosened subsoil is also recompacted. After 3 years, macroporosity (15.8%, v/v) is still appreciably higher than in the original, undisturbed subsoil (3.4%, v/v), but total porosity has already decreased to a lower value (43%, v/v). It may be expected that recompaction throughout the subsoiled layer will continue and that the ultimate bulk density will be higher than in the original situation. Consequently, deep tillage should never be performed to a greater depth than the bottom of a seriously compacted layer. Working under too wet conditions should be avoided as it may result in a smeared layer with an extremely low macroporosity (0.5%, v/v) directly below the slit (Table II, 1 year after subsoiling, 0.51--0.52 m depth).
Water retention and hydraulic conductivity In the primary ploughpan, continuous pores are scarce, but in the secondary (recompacted) ploughpan they are virtuany lacking (Kooistra et al., 1984, 1985). Here, macroporosity consists mainly of isolated packing voids, continuous channels made by plant roots and soil fauna being practically absent. However, at saturation, water content is strikingly higher in the secondary than in the primary ploughpan (Fig. 3). This difference must be due to a larger volume of pores < 30 pm diameter, possibly due to puddling during deep tillage (Kooistra et al., 1984) or to the smearing action of the furrow wheel at subsequent ploughings. Between saturation and a pressure head of -0.1 m, hydraulic conductivity is significantly lower in the secondary ploughpan which, again, indicates the lack of continuous pores. The low values at saturation imply a higher probability for the occurrence of ponding and perched water tables during extended wet periods.
Crop response Effects of deep tillage are most pronounced in dry years as, for example in 1964 at Borgercompagnie on an organic soil, underlain by dense sand (Wind and Pot, 1976). Deep ploughing carried out in 1961 resulted in a 40-cm increase in rooting depth and in a 37-% higher yield of spring wheat. However, these effects could also be obtained by sprinkler irrigation (Table III). In this case, on deep ploughed soil, irrigation did not have any additional effect. In several long-term experiments on sand, crop response fluctuated
27 9
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280 Hydraulic conductivity K (cm d-l) 103
\
........
pasture land
-
arable land, with primary ploughpan
- -
102 ~\~\. .\ "!\.~"x-
---
-
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deeply loosened arable land, with secondary ploughpan
..... • ~ . _ .
01
o
~. . . . . ' :---"'~"-~.~..::.... . . . . . .
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pressure head h (cm}
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(cm3 cm-3} 0.55
i
050
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0.45
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O40 O.35 )30
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.
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-100 -50 pressure head h (cm)
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6
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.
.
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Fig. 3. Hydraulic conductivity (top) and water retention curves (bottom) of the 0.30-0.35-m layer of pasture land (A), and of the primary (B) and secondary (C) ploughpan in a Typic Haplaquent. Separate curves represent different locations within the same treatment (Kooistra et al., 1984, 1985).
strongly and, on average, yield increase was only 5--6% (Alblas, 1984). Only when controlled traffic is applied is the effect of deep tillage sufficiently long-lasting to ensure large yield increases. When, moreover, expensive vegetables such as asparagus are grown, deep tillage may be profitable. For clay-on-sand profiles and sandy loam soils, deep tillage, especially
28:1 TABLE III Grain yield (t ha l) of spring wheat at Borgercompagnie in the dry year 1964 a Treatment in 1961
Nil Deep ploughing
Rooting depth (cm) 25 65
Sprinkler irrigation No
Yes
3.8 5.2
5.3 5.1
aSouree: Wind and Pot, 1976. when performed with rotating (mixing) implements, usually resulted in a strong decrease in clay and organic matter content of the arable layer (Havinga, 1978). This may improve workability in spring, but it may also delay early growth and induce slaking of the topsoil during winter. Yield increases were only observed during the first one or two years. RECOMPACTION To conserve the loosening effect of deep tillage, field traffic and tillage should not be performed when the soil is too wet and tractors and other vehicles used should be as light as possible. Also, properly adapted running gear (low inflation pressure) and, especially for field production of vegetables and flower bulbs, controlled traffic should be recommended. The compacting and smearing action of the furrow wheel is avoided if ploughing is replaced by rota-digging. On sandy soils, new methods of deep tillage promise longer lasting results. For example, subsoiling may now be performed such that at the same time, clay or peat from deeper layers are mixed through the sand, which has a stabilizing effect. In the field experiment at Borgercompagnie, where peat was mixed through the sandy subsoil, recompaction had not occurred even after 20 years and repetition of deep tillage did not have significant effects on crop yield (Table IV). Also, complete removal of a dense, sandy layer in 0.08-m wide slits at 0.4-m distance, and backfilling with stable, black peat material may be a solution (R. Wiebing, Institute for Land and Water Management Research, personal communication 1984). At present, probably the best way to stabilize the loosening effect of deep tillage is to grow deep rooting crops, such as alfalfa, or crops with a long growing period, such as seed grass. They may also have favourable effects on the soil fauna which, in turn, may be beneficial for soil structure. Even when recompaction does not occur, as in organic soils with dense peat layers underneath, subsoiling may give unsatisfactory results, due to the low pH (< 3.5) of the peat layers, which prevents root penetration (Wind and Pot, 1976). Subsoiling with a double subsoiler (vertical distance between the blades 0.1 m) and application of sugar factory lime sludge in
282 TABLE IV 1982--1984 average grain, tuber and root yields (t ha ~) at Borgercompagnie a Crop
Spring wheat Starch potatoes Sugar beet
Treatment in 1961
Nil Deep ploughing Nil Deep ploughing Nil Deep ploughing
Treatment in 1982 Nil
Deep subsoiling
5.5 6.2 61.2 64.6 54.3 59.8
6.3 6.4 66.7 63.8 56.2 60.1
aSource: G.P. Wind, Institute for Land and Water Management (ICW), Wageningen.
one pass, may increase the pH to about 4.0 (Sprong, 1980. Boer and Tick, 1983). This pH is high enough for satisfactory root development and low enough to prevent oxidation of the peat. CONCLUSIONS
(1) On sandy loam soils (8--17% clay), which are liable to compaction, the ploughpan does not crack on drying and, thus, it can only be loosened by subsoiling. Deeper seated compacted layers may be loosened by rota-digging or by using a wide-blade type subsoiler. (2) Restricted root development is not always caused by compacted layers, but may be caused by a low pH in the subsoil. In such cases, deep tillage should be supplemented by liming. (3) After deep tillage, recompaction over the full depth of the loosened layer usually starts very quickly and the ultimate bulk density may be higher than it was prior to deep tillage. Therefore, the working depth of deep tillage implements should never exceed the depth of the compact layer. (4) To conserve the loosening effect of deep tillage, field traffic and tillage treatments should be modified such that the soil is not unduly compacted. Deep rooting crops such as alfalfa, and crops with a long growing period such as seed grass, may have a beneficial effect. (5) Deep tillage is expensive and the effect is usually only short-lived. Therefore, sprinkler irrigation or improvement of tile drainage may be economically attractive alternatives in some cases. (6) When considering deep tillage, firstly the soil profile and the root development should be properly examined to be able to judge the seriousness of the compaction.
283 REFERENCES Alblas, J., 1984. Results of deep loosening of loam soils in the southwestern part of The Netherlands; 1978--1982. Proefstn. Akkerbouw Groenteteelt Vollegrond (PAGV), Lelystad, Rep. No. 22,22 pp. (in Dutch). Boer, J. and Tick, J.J., 1983. Results of subsoil liming 1980--1982. Bedrijfsontwikkeling, 1 4 : 3 2 1 - - 3 2 4 (in Dutch). Goedewaagen, M.A.J., Van den Berg, C., Van den Bosch, D., Butijn, J., Jonker, J.J., Van der Schaaf, D. and Schuurman, J.J., 1955. Root development in soils consisting of a top layer of clay and a sandy subsoil. Versl. Landbouwkd. Onderz., 61.7., 137 pp. (in Dutch with English summary). Havinga, L., 1978. Effects and crop reaction as a result of deep tillage of clay on sand soils. Cultuurtechn. Tijdschr., 1 8 : 9 7 - - 1 0 2 (in Dutch). Kooistra, M.J., Bouma, J., Boersma, O.H. and Jager, A., 1984. Physical and morphologica] characterization of undisturbed and disturbed ploughpans in a sandy loam soil. Soil Tillage Res., 4: 405--417. Kooistra, M.J., Bouma, J., Boersma, O.H. and Jager, A., 1985. Soil structure differences and associated physical properties of some loamy Typic Fluvaquents in The Netherlands. Geoderma, 36: 215--228. Koolen, A.J. and Kuipers, H., 1983. Agricultural Soil Mechanics (Advanced Series in Agricultural Sciences, Volume 13). Springer-Verlag, Berlin, 250 pp. Sprong, M.C., 1980. Implement for injection of liquid sugar factory lime sludge. Landbouwmechanisatie, 31 : 799--800 (in Dutch). Sprong, M.C., 1982. Deep tillage of sand soils. Landbouwmechanisatie, 3 3 : 6 3 5 - - 6 3 7 (in Dutch). Wind, G.P. and Pot, R.A., 1976. Bodenverbesserung in den holl~ndischen Veenkolonien. Z. Kulturtech. Flurbereinig., 17: 193--206.