K. A. McVay, D. E. Radcliffe,* and W. L. Hargrove. ABSTRACT. Winter legume cover crops have been shown to provide significant amounts of N to subsequent ...
Published November, 1989
Winter Legume Effects on Soil Properties and Nitrogen Fertilizer Requirements K. A. McVay, D. E. Radcliffe,* and W. L. Hargrove fungal hyphae, and their development was a function of soil management. In the laboratory, Yaacob and Blair (1981) saw a 26% increase in water-stable aggregates >2 mm in the 0- to 0.15-m depth of soil following 3 yr of continuous legumes. A winter legume cover crop can influence soil fertility status differently than a grass cover crop or fallow. Wilson et al. (1982) found soil pH, organic C, total N, and exchangeable cations generally higher, and Bray P lower under legumes than grasses in the 0- to 0.15-m depth of soil. Data from Touchton et al. (1982) suggest that P mineralized from crimson clover may be a significant source of plant-available P. In Georgia, Hargrove (1986) found that winter legumes lowered soil pH and extractable P in the 0- to 0.075m depth and redistributed K to the surface. Research in Kentucky (Utomo et al., 1987) showed that a cover crop of hairy vetch increased soil organic matter over that of fallow and big flower vetch. Objectives of this study were to determine the equivalent fertilizer-N supplying capacity of several winter annual legumes to subsequent nonleguminous grain crops, and the effect of the legumes on soil physical and chemical properties under no-tillage management.
ABSTRACT Winter legume cover crops have been shown to provide significant amounts of N to subsequent nonleguminous crops, but benefits beyond those directly attributed to N are rarely cited. A 3 yr field study at two Georgia locations utilizing a randomized, completeblock, split-plot design with four replications was begun in 1985 to measure the equivalent fertilizer N supplied by winter annual legumes and to monitor changes in soil physical and chemical properties. Corn, Zea mays L. was grown on a Rome gravelly clay loam soil (fine-loamy, mixed, thermic Typic Hapludult) at the Limestone Valley location, and grain sorghum [Sorghum bicolor (L) Moench] on a Greenville sandy clay loam soil (clayey, kaolinitic, thermic Rhodic Paleudult) in the Coastal Plain. Main plots were cover crops of hairy vetch (Vicia villosa Roth.), crimson clover (Trifolium incarnatum L), berseem clover (Trifolium alexandrinum L), winter pea [Pisum sativum subsp. arvense (L) poir], wheat (Triticum aestivum L), and fallow. Subplots were broadcast NH4NO3 fertilizer of varying rates. Hairy vetch and crimson clover replaced the greatest amount of fertilizer N averaging 123 and 99 kg N ha ', respectively. More water-stable aggregates were found following cover crops than fallow in the 0- to 0.025-m depth at the Coastal Plain location. Higher infiltration rates were found following cover crops than fallow at both locations and infiltration rates were greater following hairy vetch than following wheat at the Coastal Plain site. An adapted winter legume cover crop can replace all of the fertilizer N necessary for optimum rain-fed grain sorghum and up to two-thirds of that required for corn production, and improve soil physical properties.
INTER LEGUME COVER CROPS can be a signifiW cant source of N for subsequent nonleguminous crops, replacing as much as 120 kg N ha~' (Ebel-
har et al., 1984; Hargrove, 1986). Documentation of benefits beyond those directly attributable to N are rarely found in the literature. During the 1st yr of their study, Mitchell and Teel (1977) noted that moisture stress in midsummer depressed grain N yields of corn where there was no cover crop, compared with cover crops of rye, spring oat plus rye, or rye plus vetch. Wilson et al. (1982), using a double-ring infiltrometer showed that there was improved infiltration under cover crops compared with fallow, and that cover crops generally improved soil structure and porosity. Touchton et al. (1984) found an increased infiltration rate, using a 0.6m ring infiltrometer, when either hairy vetch or crimson clover was the cover crop, compared with fallow, for no-tillage cotton. Greater infiltration following cover crops could be attributed to a mulch effect that physically protects the soil, or it could also be a function of aggregate stability. Tisdall and Oades (1982) showed that macroaggregates (>250 /im) were stabilized by roots and K.A. McVay, Dep. of Agronomy, Univ. of Missouri, Columbia, MO 65211; D.E. Radcliffe, Dep. of Agronomy, Univ. of Ga., Athens, GA 30602; and W.L. Hargrove, Dep. of Agronomy, Georgia Agric. Exp. Stn., Griffin, GA 30222. Received 1 Mar. 1989. "Corresponding author. Published in Soil Sci. Soc. Am. J. 53:1856-1862 (1989).
1856
MATERIALS AND METHODS Two field experiments were conducted for 3 yr (19851987), in the Limestone Valley and Coastal Plain regions of Georgia. A randomized, complete block, split plot design with four replicates was used. Cover crops were the whole plots and fertilizer-N rates were subplots. At the Limestone Valley location, the soil series was a Rome gravelly clay loam representative of the Southern Appalachian Ridge and Valley soil province, and previously managed as conventionally tilled corn. At the Coastal Plain location, the soil series was a Greenville sandy clay loam representative of the red soils of the upper Coastal Plain of Georgia, and previously managed as conventionally tilled soybeans. At the beginning of the experiments, the Limestone Valley location was limed with 4.5 Mg CaCO3 equivalent ha-', and fertilized with 34.4 kg P ha-' and 133 kg K ha-', and the Coastal Plain location was limed with 4.5 Mg CaCO3 equivalent ha-', and 1 then fertilized with 28 kg N ha-', 24 kg P ha' , and 70 kg K ha"1 in accordance with University of Georgia Extension Services recommendations. Fertilizer and lime were tilled into the soil by one operation each of a chisel plow, disk, and moldboard plow, and then rototilled. Legumes were inoculated with appropriate commercial rhizobial inoculant and all cover crops established with a no-tillage planter (Table 1). Cover crops at the Limestone Valley location included 'Stacy' wheat seeded at 100 kg ha-', Tibbee' crimson clover at 22.5 kg ha-', hairy vetch at 33.5 kg ha-', 'Bigbee' berseem clover at 26 kg ha-', and fallow. Cover crops at the Coastal Plain location were the same except that Austrian winter pea at 67 kg ha-'was substituted for berseem clover. In the spring, the cover crops were dessicated with an application of 0.56 kg ha-'paraquat (l.r-dimethyl-4.4'-bipyridinium ion) plus a nonionic surfactant in 750 L water ha"'. Pioneer 3165 corn was planted at the Limestone Valley location in 0.75-m rows with a fluted-coulter no-tillage planter at a seeding rate of 60 500 seeds ha-'. At the Coastal Plain
MCVAY ET AL.: WINTER LEGUME EFFECTS
Soil Analysis At the conclusion of these studies, the soil was sampled at depths of 0 to 0.05, 0.05 to 0.10, 0.10 to 0.20, and 0.20 to 0.30 m on all plots corresponding to fallow, wheat, hairy vetch,1 and crimson clover at N rates of 0, 112, and 224 k N ha- at the Limestone Valley site and 0, 90, and 180 kg N ha-' at the Coastal Plain location. After air drying and grinding to pass a 2-mm sieve, the samples were analyzed for pH, organic C, and total N. The samples were also extracted with Mehlich I solution for P, Ca, and Mg, and with 0.01 M. CaCl2 for Mn and Al (Johnson et al., 1984). The amount of P was determined colorimetrically, (Murphy and Riley, 1962) and the amount of Ca, Mg, and Mn were determined by atomic absorption spectroscopy. Aluminum was determined colorimetrically (Wilson, 1984). Soil pH was determined on a 1:1 soil/liquid suspension using either water or 0.01 M CaCl2. Soil organic C was determined by a dry-combustion procedure (Nelson and Sommers, 1982),and total N was determined by a micro-Kjeldahl analysis (Bremner and Mulvaney, 1982). Percent infiltration was measured at the end of these studies using a sprinkler infiltrometer similar to that described by Miller (1987). Electrical conductivity of the water used was 12.53 and 5.18 S nr1 for the Limestone Valley and Coastal Plain locations, respectively. Measurements were taken after harvest in late August and early September of 1987. Leaves and stalks from the 1987 summer crop were removed by hand. This resulted in a ground cover of approximately 10 to 20% in the winter fallow treatment and approximately 100% in the winter cover treatments. The hairy vetch and fallow treatments were compared at the Limestone Valley site and the hairy vetch, wheat, and fallow treatments were compared at the Coastal Plain site. The sprinkler infiltrometer was operated for 1 hr at each plot. Water that collected at the downslope end of a 0.76-m square metal enclosure was siphoned off and measured as runoff in alternate minutes. Aggregate stability was determined on soil samples obtained from the 0- to 0.025-m depth using a wet-sieving method (Kemper and Rosenau, 1986) modified for a single sieve of 0.25mm. RESULTS AND DISCUSSION Dry Matter Production and Nitrogen Content of Cover Crops
Table 1. Dates of planting, harvest, and chemical application for legume/corn and legume/grain sorghum studies. Grain crop
Cover crops Year
Plant
1984-1985 1985-1986 1986-1987
18 Sept. 30 Sept. 24 Sept.
1984-1985 1985-1986 1986-1987
17 Oct. 15 Oct. 17 Oct.
Kill
Plant
Limestone Valley 8 Apr. 12 Apr. 12 Apr. 14 Apr. 10 Apr. 20 Apr. Coastal Plain 19 Apr. 29 Apr. 17 Apr. 24 Apr. 21 Apr. 9 June}
Fertilize
Harvest
14 Mayf 16 Apr. 20 Apr.
10 Sept. 1 1 Sept. 26 Aug.
20 May 28 Apr. 6 May
5 Sept. 9 Sept. 15 Sept.
1857
t Date of the first of two equal applications. I Replanted due to poor stand following incomplete chemical kill of berseem clover.
location, the crop was Pioneer 8333 grain sorghum planted in 0.75-m rows at approximately 192600 sees ha"1. Ammonium nitrate fertilizer treatments were 0, 28, 56, 112, or 224 kg N ha-' at Limestone Valley, and 0, 22.5, 45, 90, or 180 kg N ha-' at the Coastal Plain location. At the Limestone Valley location, one-half of the fertilizer N was broadcast within 1 wk of planting, and the rest applied when corn was at the eight-leaf stage. At the Coastal Plain location, N treatments were broadcast in a single application within 3 wk of planting. Preemergence weed control was obtained by applying alachlor (2-chloro-4-ethylamino-6-isopropylamino-Striazine), and paraquat at planting. Herbage production by cover crops was measured each year by hand harvesting 1 m2 from each whole plot. Grain yields were determined by harvesting two 6-m lengths of row. After oven drying at 700 ° C, subsamples of cover crops and grain were ground to pass a 425-mm screen and N concentrations were determined by a micro-Kjeldahl procedure (Bremner, 1965). Regression equations were obtained for each cover condition using yield or N content of the grain as a function of fertilizer-N rate. The yield or grain N of the legume at zero fertilizer N was used as the intercept of the regression equation. To estimate the equivalent amount of fertilizer N replaced by the legumes, the equations for wheat and fallow treatments were used to determine what fertilizer-N rate would produce a yield or grain N equivalent to that produced at zero fertilizer N in the legume. In those cases where a significant regression model for a legume cover crop could not be developed, the mean value over all rates was used.
Hairy vetch and crimson clover produced the greatest amount of herbage at each location (Table 2).
Table 2. Herbage, N concentration, and N content by year of four cover crops grown in the Coastal Plain and Limestone Valley regions, and means across locations and years. Herbage Cover Crop
1985
1986
1987
1985
1987
1985
4822a* 4499a 1398c 1869b
3305b 5173a 1605c 822c
4239a 3040b 4846a 1901c
Hairy vetch Crimson clover Winter pea Wheat
3513a 3756a 2459b 2302b
2144b 2591a 727d 1680b
2518a 2208a 687c 1680b
Coastal Plain 44.9a 32.3a 23.0b 22.0bc 30.4b 30.4ab 18.9c 21.5b Limestone Valley 57.4a 39.1a 42.0c 32.8b 51.8b 41.3a 23.9d 17.9c Meant 39.3a 32.2b 17.7c
* Means with the same letter are not significantly different at P = 0.05. f Mean of Coastal Plain and Limestone Valley location for 3 yr.
1986
1987
- kg ha-' -
&
Hairy vetch Crimson clover Berseem clover Wheat
3433a 3544a 1775b
1986 1 • PK kg' O
- kg ha-' -
Hairy vetch Crimson clover Wheat
N Content
N Concentration
149a
23.0b 42. la 22.9b 9.8c
155a 99b
118a
42c
35c
49b 17c
39.7a 31.5b 40.5a 23.9d
139a 123ab lOlb 41c
123a 109b 37c 40c
128a 108b 32c
98b 128a HOab 19c 102a 70b 30c 40c
1858
SOIL SCI. SOC. AM. J., VOL. 53, NOVEMBER-DECEMBER 1989 12000
6000-
Hotly Vetch Y - 8335 + 25.61N - 0.097N2
Crimson Clow Y - 5053 + 1.68N -0.0593N2 R2 - 0.61
T- 9000
o 0> 6000 TJ < 3000
whcot Y - 3312 + 64.08N - 0.0164N2 fP-OM
128
64
—I— 192
3
Y - 2342 + 46.57N - 0.2417N2
2 2000
R2 - 0.69
N_X
256
36 80
ISO
Hairy V«tch Y - 87 + 0.48N - 0.0014N2 R2-0.45
108
144
180
CrinwonClovw Y - 70 -I- 0.09N - 0.001N2 R2 - 0.59
I
o
I
O 01
Wheat
•z.
c
8 o
72
C "g O
0.0007N2 2
R - 0.94
64
128
192
296
1
Fertilizer N (kg ha" )
Y - 16 + 0.72N - 0.0031 N2 R2 - 0.71
204
36
72
144
108
180
1
Fertilizer N (kg ha~ )
Fig. 1. Corn grain yield (top) and grain N content (bottom) in 1986 as a function of fertilizer N for a crop grown under two different winter cover conditions at the Limestone Valley location.
Fig. 2. Grain sorghum yield (top) and grain N content (bottom) as a function of fertilizer N for a crop grown under two different winter cover conditions at the Coastal Plain location.
Hairy vetch and Austrian winter pea had the greatest N concentrations and wheat had the lowest N concentrations. This resulted in hairy vetch producing a greater N content than the other cover crops. Only
a function of fertilizer-N rate. Where the cover crop was a legume, the equations had large intercepts and small linear and quadratic coefficients, indicating little response to fertilizer N. Often, equations for legume
crimson clover and hairy vetch consistently produced
cover crops were not significant, indicating no re-
more than 100 kg N ha"1. Production of herbage by cover crops in the Coastal Plain was much greater than in the Limestone Valley. This was probably due to an earlier harvest date at the Limestone Valley location to facilitate early corn planting. The less-mature cover crops at the Limestone Valley location also had a greater N concentration than those at the Coastal Plain location. With the data combined over locations, the order for greatest to least N content was: hairy vetch, crimson clover, and wheat. It is important to note that, although roots were not harvested, results from Mitchell and Teel (1977) indicate that less than one-third of the total N content of winter annual cover crops is contained in the roots. Hairy vetch and crimson clover appeared to be well adapted to Ultisols and the predominant climatic conditions of the southeastern USA. These results agree with those found by Hargrove (1986), Ebelhar et al. (1984), and Decker et al. (1987) in that hairy vetch is the most consistent producer of organic N. Grain Yields and Total Nitrogen Regression equations for both grain yield and grain N as a function of fertilizer N applied were developed for each crop, cover crop, and year combination (data not shown). Wheat and fallow treatments produced significant quadratic equations for yield at total N as
sponse to fertilizer N. The regression lines in Fig. 1 are from 1986 data at the Limestone Valley location, and are typical of most comparisons between an adapted winter legume cover crop and wheat. When wheat was the cover crop, corn grain yield would increase with increasing amounts of fertilizer N in quadratic fashion approaching some plateau. When the cover crop was hairy vetch, yield at the zero fertilizerN rate was much greater, the slope of the regression line was less, and the plateau was reached at a lesser amount of fertilizer N. When grain N was compared for the same two cover crops, the plateau for each regression line was reached at a greater amount of fertilizer N. As noted by Hargrove (1986), grain N seemed to be a more sensitive parameter to measure available N than grain yield. At the Coastal Plain location, grain sorghum yields were depressed at the highest fertilizer-N rate for all cover crops. When wheat or fallow was the cover crop, a positive response to fertilizer N was obtained up to 100 kg N ha'1. When a legume such as crimson clover was the cover crop, the highest yields of grain or grain N were obtained at the zero fertilizer-N rate (Fig. 2). The depressed yield of grain sorghum at higher fertilizer-N rates resulted from greater weed competition. Soil samples taken in midsummer of the final year showed surface soil pH values between 4.5 and 5.0. Nitrification resulting from the accumulation of
MCVAY ET AL.: WINTER LEGUME EFFECTS Table 3. Nitrogen fertilizer replaced by three legume cover crops based on yield and grain-N regression equations for wheat and fallow at the Limestone Valley location.
1859
Table 4. Nitrogen fertilizer replaced by three legume cover crops based on yield and grain-N regression equations for wheat and fallow at the Coastal Plain location. N fertilizer replaced
N fertilizer replaced Cover crop Year
Based on grain N of
Based on yield of Wheat
Fallow
Wheat
Fallow
Cover crop
1
87
Fallow
Fallow
Wheat
—————— kg ha-'
—————— kg ha" Winter pea 1985 1986 1987 Mean Hairy vetch 1985 1986 1987 Mean Crimson clover 1985 1986 1987 Mean Year 1985 1986 1987
Based on grain N of
Based on yield of Wheat
Year
92
113 73
78 108 50
109 85
91
79
96
119 103 130 117
110 96 124 110
180 137 212 176
202 105 205 171
94 115 124 110
85 110 115 103
156 125 139 140
202 91 110 134
0.91 0.88 0.70
0.91 0.56 0.61
0.86 0.94 0.84
0.78 0.69 0.72
90 73 57 73
Berseem clover 1985 1986 1987 Mean Hairy vetch 1985 1986 1987 Mean Crimson clover 1985 1986 1987 Mean Year 1985 1986 1987
53 15 83 50
t 13 59 36
116 146 119
110
97 88
106 69 62 79
127
113
57 53 97 69
21 53 80 51
46 84
t
118 83
73 122 98
0.82 0.92 0.69
0.28 0.85 0.81
0.71 0.93 0.71
t 0.88 0.89
47 15 64 27
16 97
53 92 74
T
116
t Equation for fallow was not significant at the 0.05 probability level in 1985.
NH4NO3 fertilizer plus decomposing legume residue at the soil surface created an acidic condition that probably inhibited herbicidal activity and allowed greater weed pressure. Visual inspection of these plots showed a greater incidence of weeds and delayed maturity of the grain crop. The amount of fertilizer N replaced by legumes at each location (Table 3 and 4) was determined by comparing the regression equations for wheat and fallow production of yield and grain N with the legume production at zero fertilizer N. Estimates for fertilizer N replaced were greater where corn was the summer crop. This was probably due to the greater yield potential and higher N requirement of corn than of grain sorghum at these locations. Comparing values among cover crops within locations, hairy vetch consistently replaced the greatest amount of fertilizer N, followed by crimson clover. The other legumes replaced lesser amounts of fertilizer N. The two legume cover crops common to each location, hairy vetch and crimson clover, replaced1 an average value of 123 and 99 kg fertilizer N ha" , respectively, averaged over locations. These values are within the range of values found by other researchers such as Mitchell and Teel (1977), Ebelhar et al. (1984), Hargrove (1986), and Hons et al. (1987). Based on the University of Georgia Extension Service recommendations, these adapted winter legumes are capable of providing all the N needed for production of grain sorghum, and up to two-thirds of the N needed for corn production at these locations. Soil Chemical Properties Broadcast NH4NO3 fertilizer combined with no-tillage management, lowered soil pH in the 0- to 0.05m depth increment of soil at the Limestone Valley location and the 0- to 0.10-m depth increment at the Coastal Plain location compared to the zero N treat-
ment (Fig. 3). At greater depths there were no differences in soil pH due to fertilizer N. Blevins et al. (1977) in Kentucky also saw a reduction in soil pH as a result of fertilizer-N application. At the Limestone Valley location, no differences in soil pH or ion concentration across cover crops were found (data not shown). At the Coastal Plain location, soil pH following crimson clover was lower than pH following other cover crops or fallow in the 0.05- to 0.30-m depth increment (Table 5). Soil pH following hairy vetch was lower than that following fallow at the 0.05- to 0.10-m depth increment. The changes in soil pH explain most of the differences observed in exchangeable ions. Concentrations of Al and Mn following crimson clover in the 0.05- to 0.10-m depth increment were greater than those after any other pH (CaCi2) 4
pH (CaCI2) 6 4
5
5
u.uu •
A
1
^N°\
0.10£ £
-
0.20
a.
/
Q
0.30 •
0 kg ha~1 o— o 112 kg ha~1 • ——•
224 kg ha~1
A ——A
A 0 kg ha~1 O——O 90 kg ha~1 • ——• 180 kg ha~1 A——A
O Af\ .
Limestone Valley
Coastal Plain
Fig. 3. Soil pH in 0.01 A/CaCl2 as a function of soil depth for three fertilizer-N rates (averaged over winter cover conditions) at the Limestone Valley location (left) and Coastal Plain location (right). LSD(O.OS) is shown for each depth.
I860
SOIL SCI. SOC. AM. J., VOL. 53, NOVEMBER-DECEMBER 1989
Table 5. Soil chemical analysis from the Coastal Plain location by cover crop for each depth after 3 yr. Cover crop
Ca
Mg
P
Al
Mn
pHf
Table 6. Soil organic C, total N, and C:N ratios for each depth by cover crop at the Limestone Valley and Coastal Plain locations after 3 yr. Cover crops
0-0.5 m
Fallow Wheat Crimson clover Hairy vetch
286 270
89.9 87.7
285 283
Fallow Wheat Crimson clover Hairy vetch Fallow Wheat Crimson clover Hairy vetch
415a 405a
132 127
359b 397a
107 121
0.05 0.00
21.0a* 20.3a
15.6
82.2 0.78 85.4 1.07 0.05-0. 10m
14.5b 18.8a
17.4
379 350
HOa 102a
0.03b 0.1 4b
18.0a 14.7b
6.2c 7.5b
281 341
71. Ib 0.54a 95.6a 0.09b 0.1 0-0.20 m
11. 5c
12.1a
12.4c
8.1b
0.00
16.0a
2.2
0.00
14.4a
2.7
10.5b 10.8b
4.3
1.07 0.88
Limestone Valley
Coastal Plain
_,
13.8
18.9
3.2
5.6a 5.5ab 5.0c 5.3b 6.0a 5.9a 5.5b 5.9a
* Depth means in each column with the same letter are not significantly different at the 0.05 probability level using Fisher's LSD. Absence of letters indicates lack of any treatment effect. t 0.01 M CaClj.
cover crops. Concentration of Mg was lower following crimson clover at the 0.05- to 0.10-m depth increment and Ca was lower at the 0.10- to 0.20-m depth increment. Zinc concentrations were not affected by treatments (data now shown). Soil P was also lower following legumes, compared with fallow or wheat, to depths as great as 0.10 to 0.20 m. Lower soil pH following legumes would reduce the availability of P due to greater competition from Al and Mn for P (Tisdale and Nelson, 1975). The lesser amount of P under legumes could also be a reflection of the greater average grain yield and greater removal of P from the soil profile following legume cover crops. Results by Touchton et al. (1982) in Georgia showed P mineralized from clover tissue to be a significant source of plant-available P. At both locations, soil organic C in the 0- to 0.05m depth increment was greater when legume cover crops were grown than following fallow, and at the Coastal Plain location total N was greater following legumes compared with either wheat or fallow (Table 6). Carbon-tb-nitrogen ratios were not different, except at the Limestone Valley in the 0.05- to 0.10-m depth increment, where they were lower following hairy vetch. These results are consistent with those found by Utomo et al. (1987) and Wilson et al. (1982), who found greater amounts of organic C following winter cover crops than following fallow. Soil Physical Properties Aggregate stability varied by location, with a greater percentage of aggregates found at the Limestone Valley location (Table 7). This was probably due to the lesser degree of weathering of the Limestone Valley soil compared with the Coastal Plain soil (Hapludult and Paleiidult, respectively) and the higher organic-C content overall of the Limestone Valley soil (Table 6). Comparisons between cover crops at the Limestone
N
C:N
——— gkg~ i
5.0a 5.0a
4.9ab 4.8b
C
ke& Rke~' a
R
C ——— gk
N g-L
——————
C:N
kg kg-'
0-0.5 m
Fallow Wheat Crimson clover Hairy vetch
lO.lb 11. 8a 12.8a
1.3
8.1
1.4 1.5
8.9 8.4
1.3a 7.8 0.05-0. 10m 0.9 7.9 1.0 7.7
11. 8a
1.5
7.8
1.0
1.0
10.3a
1.2
8.6a 8.2ab 8.3ab
9.3b
1.2
7.7b
7.9b 9.3a lO.la
1.1 1.1 1.2
7.5 8.5 8.3
7.9b
1.1
7.1
0.7c 0.8b 0.9a
6.5
0.7c
6.3
8.5c* 8.9c 10.6a
I.Ob l.lb 1.3a
10.2b
8.5 8.4 7.8
Fallow Wheat Crimson clover Hairy vetch
7.2 7.3 7.7 7.4
1.0
Fallow Wheat Crimson clover Hairy vetch
6.8 6.6 7.1
0.9 0.9 0.9
6.7
0.9
Fallow Wheat Crimson clover Hairy vetch
4.0 3.7 4.1
0.5 0.5
7.6 7.2
0.6
7.4
4.3b 5.7a 5.7a
3.7
0.6
6.7
4.4b
7.7
7.5 0.1 0-0.20 m 7.9 7.6 7.6 7.6 0.20-0.30 m
8.7b 9.5ab
1.2
7.0
6.4
' Means within a depth column followed by the same letter are not significantly different at the 0.05 probability level using Fisher's LSD. Absence of letters indicates lack of any treatment effect.
Valley location showed no differences in percent stable aggregates. At the Coastal Plain location when a cover crop was grown, the soil had a significantly
greater percentage of aggregates compared with fallow. Further, if the cover crop was a legume, the percentage of stable aggregates tended to be greater than when the cover crop was wheat. This difference in aggregate stability was probably due to a greater amount of covercrop herbage (Table 2) and residue returned to the soil under this management system and a more favorable C/N ratio (Table 6). Cover crops had a significant effect on infiltration in the treatments examined (Fig. 4 and 5). At both locations, infiltration rates were lower in the fallow treatments, compared with the hairy vetch treatments, after about 10 min. Not only was there a difference between cover crops and fallow, but, at the Coastal Plain location where the wheat plots were included in the infiltration tests, the legume cover crop had a higher infiltration rate than the nonlegume cover crop. Using time as a covariate and comparing at the mean time (30 min), infiltration was greater where hairy vetch was the cover crop (Table 8). Fallow had the lowest average infiltration rate and, when wheat was the cover crop, infiltration was intermediate. The sprinkler infiltrometer simulates actual rainfall by allowing raindrops to impact the soil or residue surface at a measurable rate. Wilson et al. (1982) and Touchton et al. (1984) found cover crops to increase infiltration over fallow using ring methods, but these results cannot be directly compared because the en-
MCVAY ET AL: WINTER LEGUME EFFECTS 100-r
Table 7. Percentage of water-stable aggregates >250 nm in the 0to 0.025-m depth at the Limestone Valley and Coastal Plain locations.
£
Stable aggregates Cover crop Crimson clover Hairy vetch Wheat Fallow
Hairy vetch Wheat Fallow
38.8*
58.4a 42.3b 23.4b 37.8c * Means with the same letter within a location are not significantly different at the 0.05 probability level using Fisher's LSD.
ergy imparted to the soil by raindrop impact can be a significant added effect. In this experiment, when the soil surface was covered by even a minimum of residue, the amount of infiltration was significantly greater than that of the fallow or mostly bare soil, in addition, the greater aggregate stability found under cover crops could have reduced crusting of the surface soil, allowing greater infiltration. Depending on how long a surface crust persisted, differences in infiltration could have continued beyond canopy closure. The higher infiltration rate in hairy vetch compared with wheat (Fig. 5, Table 8) may have been due to a difference in cover or aggregate stability. The orientation of the wheat residue remains somewhat vertical even after growth of the grain crop, while the legume residue tends, to lie flat on the soil surface soon after dessication. The wheat cover allows a certain percentage of soil to be exposed to raindrop impact, which could account for a lower percent infiltration compared with the legume cover crops. Higher percent infiltration at the Coastal Plain site following legumes could also be due to the greater herbage and lower C:N ratio, which produced greater aggregate stability under legume cover crops compared with wheat. CONCLUSIONS Results from this study showed that an adapted legume cover crop provided the equivalent of the total fertilizer-N needs for the production of rain-fed grain sorghum, and could replace from 99 to 123 kg ha'1 of the fertilizer-N requirement of corn. This constituted a significant energy savings in no-tillage corn and grain sorghum production. With energy prices predicted to rise, the savings in energy costs alone should make winter legume cover crops an economical alternative to fertilizer N.
'•&
40
£
22 mm h" 20 - • Limestone Valley location
10
20
30
40
50
60
Time (min) Fig. 4. Infiltration rate as a percent of the sprinkling rate (40 mm rr1) under two winter cover conditions at the Limestone Valley location. Infiltration rate after 60 min is given at the right. 100T
Coastal Plain
- mm h~'
60
O
Mean infiltration rate Limestone Valley
V c
o
37.9a* 36.7a 32.6ab 28.9b 56.3 * Means within a column with the same letter are not significantly different at the 0.05 probability level using Fisher's LSD. Absence of letters indicates lack of any treatment effect. 55.0 58.2 65.1
Cover crop
80--
Q) -4J
Coastal Plain Limestone Valley - % by weight -
Table 8. Mean percent infiltration rate measured at the mean of time using a sprinkling infiltrometer at the Limestone Valley (sprinkling rate = 40 mm rr') and Coastal Plain (sprinkling rate = 68 mm h ') locations.
1861
K
80-
Q)
-M
o
o
40 mm h 1
60-
40--
20 • •
Coastal Plain location 28 mm h"
10
20
30
40
—i— 50
60
Time (min) Fig. 5. Infiltration rate as a percent of the sprinkling rate (68 mm h~') under three winter cover conditions at the Coastal Plain location. Infiltration rate after 60 min is given at the right.
Most of the significant differences in soil fertility status were found in the top 0- to 0.10-m depth, where decomposing crop residue and broadcast fertilizer N lowered soil pH. Because NH4NO3 was the fertilizerN source and the legume cover crops contributed large amounts of NHJ, soil pH in the 0.- to 0.10-m depth increment was lowered compared with deeper depths, especially at the Coastal Plain location. Reduced availability of P following legumes was largely due to a lower soil pH, but removal by the grain crop contributed. At the Coastal Plain location, concentrations of Al and Mn increased in the 0.05- to 0.10-m depth increment. The use of hairy vetch as a cover crop improved soil water infiltration late in the season, compared with fallow or wheat. Greater aggregate stability at the Coastal Plain location following legume cover crops, compared with fallow or wheat, probably influenced infiltration rate, and was another measure of the improved physical condition of the soil when managed under winter legume cover crops.
1862
SOIL SCI. SOC. AM. J., VOL. 53: NOVEMBER-DECEMBER 1989
