Ann. app/. B i d . (1990), 117, 653 -666 Printed in Grear Britain 653

Effect of soil compaction on growth, yield and light interception of selected crops

By A. M. ASSAEED, M. McGOWAN, P. D. HEBBLETHWAITE and J. C. BRERETON

Department of Agriculture and Horticulture, University of Nottingham, School of Agriculture, Sutton Bonington,

Loughborough, Leics LEI2 5RD, UK

(Accepted 20 August 1990)

Summary The response of spring barley (Hordeum vulgare, cvs Carnival and Atem), faba

beans (Vicia faba, cv. Maris Bead), sugar beet (Beta vulgaris, cv. Monoire), forage maize (Zea mays, cv. Leader), forage peas (Pisum sativum, cv. Poneka) and white turnip (Brassica campestris, cv. Barkant) to topsoil compaction was investigated in a three year trial. Soil compaction was induced by tractor wheeling after crop sowing.

Compaction reduced leaf area and dry matter accumulation in all crops in every season. Yield of barley was reduced by 290711, 27% and 40% in 1984, 1986 and 1987 respectively. Yield of maize, peas and turnip decreased by 33'3'0, 14% and 13% in 1986 and 25%, 16% and 19% in 1987. Yields of beans and sugar beet were decreased by 34% and 35% respectively in 1984.

Light interception was decreased in all crops in all three years of study but, with the exception of maize in 1987, the efficiency of conversion of radiant energy to dry matter was not significantly affected by soil compaction. It is concluded that reduced dry matter production and yield due to soil compaction was more a consequence of reduced light interception because of restricted leaf area development rather than as a result of an impaired ability of crops to utilise intercepted radiant energy.

Key words: Soil compaction, light interception, light conversion coefficients, water use efficiency

Introduction Soil compaction has frequently been reported to reduce the yield of many crop species but

the reasons for reduced crop yields are not entirely clear. There is a general belief that yield losses arise mainly because of impaired nutrient and water acquisition by restricted root systems (Phillips & Kirkham, 1962; Juang & Uehara, 1971; Davies, 1975) but evidence of specific effects of soil compaction on plant morphology, e.g. deformed and distorted roots, shorter internodes, increased stem diameter etc. has led to suggestions that plant responses to soil compaction may be influenced by mechanisms other than impaired nutrient and water capture (Dawkins, Hebblethwaite & McGowan, 1984; Brereton, 1986) and some workers have speculated on the possible role of hormonal 'signals' between root and shoot systems (Brereton, McGowan & Dawkins, 1986; Masle & Passioura, 1987). 0 1990 Association of Applied Biologists

654 A. M. ASSAEED, M. McGOWAN, P. D. HEBBLETHWAITE AND J . C. BRERETON

The most commonly observed effect of soil compaction is the production of smaller, stunted plants which, together with reduced plant populations, leads to a smaller canopy cover (e.g. Smittle, Threadgill & Seigler, 1981; Dawkins, 1982). One important question raised by such observations is to what extent poor plant performance in overcompacted soils can be attributed to reduced light interception - or does soil compaction also affect the efficiency of light conversion to dry matter?

The analysis of dry matter production in terms of the amount of radiation intercepted by a crop canopy and its coefficient of conversion to dry matter (Gallagher & Biscoe, 1978; Monteith & Elston, 1983) has been one of the more significant developments in crop growth analysis in recent years. Radiation interception depends on the extent, structure and duration of leaf canopy and is known to be affected by many stress factors; conversely the coefficient of conversion is thought t o be more conservative (Monteith, 1981), although there appears to be no information about the effects of soil compaction.

This paper reports the results of three years’ field trials to examine the effects of soil compaction on the growth and development of a series of crops grown at the School of Agriculture, University of Nottingham. The principal aim of these experiments was to assess the impact of soil compaction on crop dry matter production and yield in terms of light interception and conversion coefficients.

Materials and Methods The trials were conducted at Sutton Bonington (UK) in 1984, 1986 and 1987 in different

fields on soils of the Arrow series, an imperfectly drained, coarse textured sandy loam (Hebblethwaite & McGowan, 1977).

In 1984, comparison was made of the effect of soil compaction on three arable crops, namely spring barley (cv. Carnival), faba beans (cv. Maris Bead) and sugar beet (cv. Monoire). The 1986 and 1987 trials compared three forage crops, namely maize (cv. Leader), peas (cv. Poneka) and white turnip (cv. Barkant), also spring barley (cv. Atem) grown for whole crop forage. In 1984, barley and beans were sown on 20 March and sugar beet was sown on 6 April. The other crops were sown on 28 May 1986 and on 28 April 1987. In each year the soil was chisel ploughed in the autumn and tine cultivated in the spring. All plots were rolled after sowing. The compaction treatment was applied immediately after crop sowing, when the soil was moist, across the plot with either one pass (1984) or two passes (1986, 87) of a John Deere 2140 tractor (wheel base weight 3500 kg). Recorded soil bulk densities of the top 10 cm increased from 1.03, 1.49 and 1.35 g/cm3 for the control treatment to 1.51, 1.75 and 1.58 g/cm3 in the compacted treatment in 1984, 1986 and 1987 respectively. Penetrometer resistance was significantly increased to a depth of 20 cm from 0.9 MPa to 2 MPa in the compacted treatment in 1984. Soil shear vane strength of the upper 5 cm increased from 10 and 3 kPa in the control treatment to 50 and 19 kPa in 1984 and 1986 respectively; no measurements were taken in 1987. Further experimental and management details are given in Table 1. The maize and turnip crops were oversown and once emergence was complete the compacted and non-compacted treatments were thinned to the same plant density. Each trial was arranged in a randomised complete block design with four replicates.

Dry matter accumulation was assessed from destructive growth analysis every 4 to 14 days. Leaf area was determined using a photo-electronic planimeter (Licor. Inc., Model 3100).

Tube solarimeters, calibrated against a standard Kipp pyranometer at the beginning of the season, were used to measure the fraction of light intercepted by the crop canopies. One tube solarimeter was mounted above the crops to measure incident radiation at the site. One tube

Effects of soil compaction in crops 655

656 A. M . ASSAEED, M. McCOWAN, P . D. HEBBLETHWAITE AND J . C. BRERETON

solarimeter was also placed in each replicate plot across the inner rows just above ground level, oriented in an east-west direction. These solarimeters were used to record the radiation transmitted through the crop canopy, hence estimating the fraction of light intercepted (fI) by the crops. Tube solarimeters were connected to millivolt integrators developed by Campbell (1974). Radiation measurements over a period of 3 t o 5 h around noon GMT were integrated every 5 to 6 days.

During early growth in 1984, the fraction of canopy cover of barley and beans was used to estimate light interception using the photographic technique of Steven, Biscoe, Jaggard & Paruntu (1986), while due to severely reduced plant population in the compacted treatment, the fraction of light interception (fI) of the 1984 sugar beet crop was estimated from leaf area index (LAI), i.e.

fI = 1 - exp (-KL) . ......... (Monteith, 1981)

where L is LA1 and K, the light extinction coefficient for sugar beet, was assumed constant (0.52) through the season.

The amount of light intercepted during a given period was determined as the product of average fI for that period and the total incident solar radiation measured at a standard meteorological station located less than 1 km from the experimental sites at Sutton Bonington. Cumulative light intercepted during the growing season was obtained by summing values for consecutive periods.

Conversion coefficients were estimated by plotting cumulative above-ground biomass produced (g/m’) against the corresponding cumulative light intercepted throughout the season (MJ/m2). The slope of the linear regression, forced through the origin, was taken as equal to the mean seasonal conversion coefficient. In the case of barley 1984 and 1986 and beans 1984 where the relationship between dry matter production and light interception were evidently curvilinear (curves drawn by hand), two conversion coefficients were calculated. A mean seasonal conversion coefficient, given by the ratio of dry matter production at the end of light measurements divided by the corresponding cumulative light interception, and a maximum conversion coefficient during the vegetative growth period taken as the slope of a line up to anthesis.

In 1984 and 1986 soil moisture was monitored with a Wallingford neutron probe (Bell, 1969, 1973) at 10 cm depth intervals to 100 cm depth in barley (cv. Carnival) and beans, 130 cm in sugar beet and 140 cm for all crops in 1986. Soil water deficit within the ‘effective’ extraction depth of water by roots was calculated by the method described by McGowan (1974). Total water use by crops was assumed to be equal to the sum of soil water deficit and rainfall received and was used to calculate water use efficiency, defined here as the ratio of dry matter produced to total amount of water used, i.e. crop transpiration plus soil evaporation.

Results

Crop development and yield Soil compaction reduced plant populations in 1984 by 50% (P < 0.01) in barley, 41 Vo (NS)

in beans and 64% (P < 0.001) in sugar beet. The soil compaction treatment in 1986 and 1987 had only slight effects on plant stands possibly because of the rain after sowing which helps alleviate the influence of soil compaction on seedling emergence (Dawkins, 1982). Because maize and turnip crops were sown at high rates, both the compacted and control treatments were thinned to the same population once plant emergence was complete (see Table 1) .

Effects o f soil compaction in crops 65 7

Soil compaction decreased leaf area production of all the crops in all three years (data not shown; see Assaeed, 1989). Soil compaction delayed leaf area expansion during the crop establishment phase especially in 1984 and 1986. Maximum leaf area index (LAI) achieved for all crops in all three years was also reduced. Leaf senescence was sometimes delayed by soil compaction particularly for barley and peas in 1986 (see Assaeed, 1989).

Soil compaction also reduced crop dry matter production over time and yield of all crops in all three years (Brereton, 1986; Assaeed, 1989). Maximum dry matter production was reduced on average by 30%, ranging from 13Vo in peas in 1987 to 46% in sugar beet in 1984. Yield reductions averaged 26% and varied from 13% in turnip in 1986 to as high as 40% in barley in 1987 (Table 2).

Table 2. Yields of spring barley (grain), field beans (grain), forage maize (total DM), forage peas (total DM), sugar beet (roots) and white turnip (total DM) ( t ha-/)

Control Compact S E D Sig. ( 3 D I )

Spring barley 84 86 87

Field beans 87

Forage maire 86 87

Forage pea5 86 87

Sugar beet 84

White iurnip 86 87

7.4 7. I 7.9

5.2 5.2 4.8

0.23 P < 0.01 0.35 P < 0.05 0.88 P < 0.05

P < 0.001 3.8 2.5 0.03

10.4 11.3

6.7 11.6

7.2 8.4

5.8 9.8

P < 0.05 0.97 0.70 P < 0.05

0.45 NS 0.36 P < 0.05

65.0 42.6 5.5 P < 0.05

7.0 8.4

6. I 6.8

0.5 I 0.65

NS NS

NS indicates P 2 0.05

Water use efficiency Soil compaction generally decreased the water use efficiencies (4) of crops (Table 3) . These

decreases were statistically significant for beans in 1984 and for barley, peas and turnip in 1986. The most likely explanation was a greater soil evaporation from the compacted plots during the early stages of plant growth when canopy cover was incomplete but this cannot be quantified since no measurements were taken of soil evaporation. The one exception to the general trend of lower q in the compacted plots was for sugar beet. The trend to higher q of the 1984 compacted sugar beet crop probably resulted from the reduced plant population which meant that the widely scattered plants in the compacted treatment were able to continue transpiration throughout the season, whereas the control plants ran out of soil water reserves.

658 A. M. ASSAEED, M . McGOWAN, P. D. HEBBLETHWAlTE AND J . C. BRERETON

Table 3. Water use efficiency g/kg H 2 0 (4) and the ratio of water use efficiency to conversion coefficient (q/e) in 1984, 86

Control Compact S.E.D. (D.F.) Sig.

5.6 2.7

4.7 2.7

0.55 (14) -

NS

7.9 4.4

5. I 4.1

1.15 (12) -

P < 0.05

5.9 2.6

6.2 3.0

1.18 (8) -

S.E.D. (3 D.F.)

NS

6.7 4.6

4.4 3.2

0.65 0.53

P < 0.05 NS

8.8 4.2

6.3 3.5

1.01 0.70

NS NS

8.4 4.8

5.8 3.6

0.42 0.37

P < 0.05 NS

7.5 3.8

5.3 3 .O

0.67 0.75

P < 0.05 NS

NS indicates P 2 0.05

Light interception Apart from maize in 1987, soil compaction generally decreased the fraction of light

interception by all crops (Fig. 1) including maximum recorded fractional light interception. Duration of light interception however was unaffected apart from beans in 1984 when fractional radiation interception declined in the control treatment towards the end of the season while that of the compacted treatment continued to increase. Differences between the two treatments generally narrowed as the season progressed, probably because of the delayed senescence of plants in the compacted treatment.

Values of extinction coefficient (K), estimated by plotting the natural logarithm of radiation transmitted against LA1 during the vegetative phase of plant growth, were not consistently affected by soil compaction. In both 1984 and 1986 K values for barley increased from 0.29 to 0 . 3 5 but in 1987 decreased from 0.21 to 0.18 as a result of the compaction treatment. The K value for beans in 1984 decreased from 0.67 to 0.49 while that for sugar beet in 1984 increased from 0.46 to 0.52. K values for maize and peas in 1986 were unaffected by the compaction treatment while that for turnip increased from 0 . 3 3 to 0.48. The corresponding changes in K values for the three crops in 1987 were an increase from 0.21 to 0.29 in maize, a decrease from 0.22 to 0.19 in peas and a decrease from 0.40 to 0.26 in turnip. Taken together, these results give no indication of a consistent effect of soil compaction on leaf inclination, i.e. no evidence that compaction encouraged the production of plants with either more erect or

Effects of soil compaction in crops 659

, - . . . . I

x a 'i #

I 3

\ - *

r.

0 0 0 0 0 ~ 0 0 0 0 0 0 0 0 0 0 0 0 0 , 0 0 0 0 0 - 0 9 ' 9 6 " - 0 9 \ 9 6 N - 0 9 Y i b . N - 0 9 \ 9 b . p !

660 A. M. ASSAEED, M . McGOWAN, P. D. HEBBLETHWAITE AND J. C. BRERETON

more lax leaves and thus no support for the suggestion that soil compaction may affect light interception also via altered crop canopy structure rather than simply via altered extent of canopy development.

Tables 4 and 5 summarise information on the maximum biomass production, cumulative seasonal light intercepted and the mean seasonal conversion coefficients (calculated from the

Table 4a. Effect of soil compaction on dry matter production at the end of light measurements, cumulative radiation intercepted and conversion coefficients in 1984

Barley Control Compact

Beans Control Compact

Sugar beet Control Compact

Maximum Cumulative Cumulative biomass radiation inter-

(g/m') cepted (MJ/m')

1680 1113

1444 1105

1766 956

82 1 726

932 848

649 406

Conversion coefficient

W M J )

2.11 1.71

S.E.D. = 0.332 (14 D.F.)

1.78 1.25

SE.D. = 0.467 (12 D.F.)

2.31 2.10

S.E I). = 0.557 (8 D.F.)

Table 4b. Effect of soil compaction on dry matter production at the end of light measurements, cumulative radiation intercepted and conversion coefficients in 1986, 1987

Maximum cumulative biomass (g/m2)

1986 1987

Barley Control 976 1500 Compact 643 94 I

S.E.D. (3 D.F.) 81.2 105.1 P < 0.05

Maize Control 1463 1425 Compact 1041 1200 S.E.D. (3 D.F.) 368.7 180.8

NS NS

Peas Control 1182 1147 Compact 904 996 S E.D. (3 D . F . ) 136.1 58.0

NS N S

Turnip Control 1284 530 Compact 903 368 S.E 11. (3 D.S ) 274.9 61.1

NS NS

Cumulative radiation intercepted (MJ/m2) 1986

790 596 25.1

P < 0.005

823 689

51.8 NS

702 533

38.2 P < 0.05

649 503 46.7 NS

1987

697 469

39.0 P < 0.01

440 428 32. 5 NS

843 744 21.7

P < 0.05

317 225

39.4 NS

Conversion coefficient (g/MJ)

1986 1987

1.44 2.47 1.38 2.33 0.136 0.121 NS NS

2.17 3.30 1.83 2.44 0.301 0.258 NS P < 0.05

I .75 1.38 1.64 1.35 0.122 0.065

NS NS

1.97 1.89 1.76 1.86 0.390 0.552

NS NS

NS indicates P 0.05

Effects of soil compaction in crops 66 1

Table 5. Radiation intercepted, conversion coefficients and maximum dry matter production of the compact treatment expressed as a ratio of the control

Radiation Conversion Dry matter intercepted coefficient production

Barley 84 86 87

Beans

Maize 84

86 87

Peas 86 87

Sugar beet 84

Turnip 86 87

0.88 0.75 0.67

0. 91

0.84 0.97

0.76 0.88

0.82 0.96 0.94

0.70

0.84 0.74

0.94 0.98

0.66 0.66 0.63

0.77

0.71 0.68

0.76 0.87

0.63 0.92 0.54

0.78 0.71

0.89 0.98

0.70 0.69

linear regressions between dry matter production and cumulative light intercepted or for barley 84, 86 and beans 84 as the ratio of dry matter production at the end of light measurements to the corresponding cumulative light intercepted). Generally, soil compaction had greater effect on the amount of light intercepted than on the mean seasonal conversion coefficients. For example, in seven out of eleven cases, soil compaction reduced the amount of light intercepted more than the efficiency of conversion (Table 5). O n average, light interception was reduced by 25% (range 3 to 37%) whereas the conversion coefficient was reduced by only 12% (range 2 t o 30%). Indeed, apart from maize in 1987, soil compaction had no significant effect on any of the conversion coefficients.

The relationships between dry matter accumulation and light interception are also given in Fig. 2. For barley in 1984 and 1986 and beans in 1984 it appears that the conversion coefficient decreased during grain and pod filling. Similar seasonal changes in conversion coefficients have been reported for millet (Marshall & Willey, 1983) and wheat (Green, 1987). The maximum conversion coefficient (1.e. during the vegetative phase of growth) for barley decreased from 2.34 +- 0.14 to 2.31 * 0.09 g /MJ in 1984 and from 1.63 -+ 0.08 to 1.46 k

0.07 g i M J in 1986 as a result of the soil compaction treatment. The corresponding figures for beans in 1984 were 2.40 2 0.22 and 1.32 2 0.11 g /MJ in the control and compacted treatments respectively.

Radiant energy intercepted by crop canopies is mainly partitioned into latent and sensible heat losses. In the absence of any sensible heat loss, the ratio of q (gikg H,O) to conversion coefficient e (g iMJ) would be equal to the latent energy of water evaporation, i.e. q/e =

2.45 MJ/kg. The q / e ratios in Table 3, indicate significant sensible heat losses occurred for all crops for both treatments, i.e. q/e > 2.45, considering especially that the q values are biased by inclusion of soil evaporation (i.e. the values for transpiration efficiency would be even greater than those for 9). The generally lower q /e values for compacted crops probably arise

662 A. M. ASSAEED, M. McGOWAN, P. D. HEBBLETHWAITE AND J. C. BRERETON

Effects of soil compaction in crops 663

1987

1800.

1000.

800.

-5 0 200 400 600 800 k L

;; 2 1800

1600

0 1400

1200.

1000 .

8 0 0 . 600 . 400 . 200.

0' 0 200 400 600 800

0 200 400 600 800

Turnips

Cumulative radiation intercepted MJ/m2 Fig. 2 continued

from the greater soil evaporation losses in the more open crop canopies. The higher q/e value for the compacted sugar beet reflects the observation that the control crop depleted its soil water reserves sooner and ran into drought conditions, i.e. was disadvantaged by continued interception of radiation while transpiring at a reduced rate, whereas the compacted sugar beet crop, consisting of a few scattered plants, was better able to continue transpiration through the season.

Discussion Previous workers have shown that reduced plant population and reduced performance of

plants are both important factors whereby soil compaction can affect growth and yield of several crops (Hebblethwaite & McGowan, 1980; Dawkins et al., 1984). The principal aim of this study was to examine to what extent restricted growth and yield of crops grown in compacted soil could be related to impaired light interception or to impaired conversion coefficients.

664 A. M . ASSAEED, M . McCOWAN, P. D. HEBBLETHWAITE AND J . C. BRERETON

Results of this study showed that, apart from maize, conversion coefficients were not significantly affected by the compaction treatment. Values for conversion coefficients were broadly comparable with other values reported in the literature. For example, the conversion coefficients for beans (1.25 to 1.78 g/MJ) were similar to values of 1.35-1.53 reported by Thompson (1983). Likewise, the values for peas (1.35 to 1.75 g/MJ) were in line with the values of 1.46 and 1.25 g/MJ reported by Heath & Hebblethwaite (1985) and Pyke & Hedley (1985) respectively, while the conversion coefficients for the two root crops, sugar beet and turnip, were similar to a value of 2.6 g/MJ reported for sugar beet by Kumar & Monteith (1981).

Expressed in terms of photosynthetically active radiation (PAR) absorbed (following the method of Marshall & Willey (1983) and allowing for 10% reflection of radiation from foliage surface), conversion coefficients for barley (3 .O-3.4 g/MJ PAR) were higher than those reported by Gallagher & Biscoe (1978) (2.2-2.6 g/MJ PAR). Conversion coefficients for beans (1.9-3.7 g/MJ PAR) were similar to the 2.3 g/MJ PAR reported for an irrigated bean by Green, Hebblethwaite & Ison (1985) but lower than the 4.1 g/MJ PAR given by Fasheun & Dennett (1982). Values for maize (3.4-5.9 g/MJ PAR) were comparable to 3.8 g/MJ PAR reported by Sivakumar & Virmani (1980) while values for sugar beet (4.5-4.6 g/MJ PAR) and turnip (3.3-4.0 g/MJ PAR) were similar to the value of 3.5 g/MJ PAR for sugar beet found by Biscoe & Gallagher (1977).

Year to year variation on conversion coefficients were also observed. In the case of barley, maize and turnip, the lower values for 1986 were probably due to greater crop water stress. The cumulative soil water deficit estimated from the Penman equation for the period from crop emergence to anthesis of barley was 108 mm in 1986 compared to only 56 and 39 mm in 1984 and 1987 respectively. Water stress has been shown to reduce the conversion coefficients of beans (Green et al., 1985) and of wheat (Gallagher & Biscoe, 1978). The lower conversion coefficients of peas in 1987 relate to the premature leaf senescence of that crop,

For most of the crops, soil compaction had greater, often significant, influence on the amount of light intercepted compared to the non-significant effects on the conversion coefficients. A distinct effect of soil compaction on light interception was expected from the observed sparse crop canopy covers. The implication of these results is that soil compaction leads to reduced dry matter production primarily through a reduction in the amount of light intercepted rather than through a reduction in the efficiency of converting intercepted light to dry matter. For maize the significantly reduced conversion coefficient in 1987 implies that this crop was unable to cope with the stresses imposed by soil compaction solely through reduction in leaf canopy. Muchow & Davis (1988) also reported that inadequate nitrogen reduces both the amount of light intercepted and the conversion coefficients of maize and sorghum.

The main conclusions to be drawn from this study are that (1) the reduced performance of crops in compacted soils may be largely attributed to the reduced amount of light intercepted rather than the impaired ability of plants to convert intercepted light into dry matter, and (2) continued slower dry matter production in compacted soils appears to be a compound effect whereby the delayed initiation of leaf cover on emergence leads to lower interception of radiant energy, hence less photosynthates for leaf expansion so that compacted plants are unable to ‘catch up’ with the control plants.

The important questions posed by these observations are: what causes delayed initiation of leaf cover, what physiological mechanisms are involved? Is delayed initiation of leaf cover simply a response to impaired water or nutrient status as a result of restricted root volume in compacted soil or does a restricted root volume per se have specific effects on top growth, possibly through root or shoot hormonal ‘signals’? In a subsequent paper we examine for the possibility of specific effects of restricted root volume on the growth of plants.

Effects of soil cornpaction in crops 665

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54, 29-34.

(Received 27 November 1989)