www.elsevier.com/locate/still

Soil & Tillage Research 87 (2006) 80–88

Effects of soil management practices on soil fertility

measurements on Agave tequilana plantations

in Western Central Mexico

Alayne Gobeille a,*, Joseph Yavitt a, Philip Stalcup b, Ana Valenzuela c

aDepartment of Natural Resources, Fernow Hall, Cornell University, Ithaca, NY 14850, USAbDepartment of Earth and Atmospheric Sciences, Snee Hall, Cornell University, Ithaca, NY 14850, USA

cUniversidad de Guadalajara, Centro Universitario de Ciencias Biologicas y Agropecuarias, Zapopan, Jalisco, Mexico

Received 8 June 2004; received in revised form 22 February 2005; accepted 28 February 2005

Abstract

Agaves have historically been an important crop in theWestern Central Highlands of Mexico, used for food, livestock fodder

and beverage production. The best known of these products, and one of the most financially significant, is the distilled spirit,

Tequila. The success of agaves in this region has been attributed, in part, to unique environmental adaptations. Agaves used for

production of the distilled spirit Tequila (Blue agave) have also been considered to have a minimal impact on soil fertility

because they are harvested only every 6–10 years. In the past 20 years, with the rising global consumption of Tequila, cultivation

of Blue agaves has grown more intense, prompting questions regarding the impact of commercial practices on soil quality.

Researchers on related agave species have shown the potential for severe nutrient depletion and subsequent declines in yield in

the absence of proper soil fertility management. This study examined the key physical and chemical indicators of soil fertility

and how they were affected by tillage, the amendment of soils with distillery effluent, and the grazing of livestock. It was found

that tillage decreased soil carbon levels along with nutrients associated with organic matter; mean levels of 2.44% C, 0.20% N

and 10.99 mg P/kg soil were found on untilled sites compared with 1.29% C, 0.10% N and 3.53 mg P/kg soil for tilled sites. The

addition of distillery effluent was found to increase soil cation levels (mean soil K, Mg, and Ca were 436.50, 364.40 and

1416.2 mg/kg on amended sites versus 299.50, 228.36 and 1013.5 mg/kg on non-amended sites) while livestock grazing had

only a small effect on nutrient levels. The effects of tillage and livestock grazing need to be addressed by proper soil fertility

management strategies need to be addressed to insure the long term health of soils in the region.

# 2005 Elsevier B.V. All rights reserved.

Keywords: Agave tequilana; Tequila; Soil fertility; Distillery effluent

* Corresponding author. Present address: Mount Sinai School of

Medicine, Department of Community and Preventive Medicine,

One Gustave L. Levy Place, Box #1043, New York, NY 10029,

USA. Tel.: +1 212 241 7865; fax: +1 212 996 0407.

E-mail address: alayne.gobeille@mssm.edu (A. Gobeille).

0167-1987/$ – see front matter # 2005 Elsevier B.V. All rights reserved

doi:10.1016/j.still.2005.02.033

1. Introduction

Agaves have been cultivated in the Western Central

Highlands of Mexico for at least 400 years (Parsons

and Parsons, 1990; Valenzuela, 2003). With their

.

A. Gobeille et al. / Soil & Tillage Research 87 (2006) 80–88 81

shallow rooting system and succulent morphology,

agaves can prosper in regions where traditional annual

crops cannot. Crassulacean acid metabolism (CAM)

allows them to conserve water, and many agave

plantations are not irrigated (Nobel, 1988). These

factors contribute to the success of the cultivation of

Agave tequilana Weber Blue variety, ‘‘Blue agave’’,

used in the production of Tequila. It has been assumed

that because of these unique environmental adapta-

tions, along with an infrequent harvesting schedule

occurring every 6–10 years, that Blue agave cultiva-

tion does not lend itself to the rapid depletion of soil

nutrient levels associated with commercial agricul-

ture. Consequently, little work has been done to

understand the impacts of commercial Blue agave

production on soil fertility.

However, the decline of the sisal agave (Agave

sisalana) industry in eastern Africa between the 1960s

and 1980s has been largely attributed to poor soil

fertility management (Hartemink et al., 1996; Harte-

mink, 1997a,b). These findings have prompted interest

in other agricultural systems based on agave cultiva-

tion. Agronomists investigating the growing practices

of Blue agaves used by the Mexican Tequila industry

have commented on poor soil fertility management

practices especially during times of excess supply

when agave prices drop (Valenzuela-Zapata and

Nabahn, 2003).

The work of Nobel (1989) indicates that the

productivity of Blue agave is limited by levels of soil

nitrogen, phosphorus, potassium and boron. Over

short time periods, Blue agave productivity has been

shown to increase in response to application with

inorganic N–P–K (Uriarte, 1987). However anecdotal

reports indicate that often no fertilizer is used at all due

to their high cost (Valenzuela-Zapata and Nabahn,

2003). Another common practice is ‘‘ganaderia’’

where livestock are allowed to graze on mature fields,

a supposed benefit by controlling weeds and deposit-

ing manure (Valenzuela-Zapata and Nabahn, 2003).

In recent years the disposal of distillery effluent,

‘‘vinazas’’, off-site has become difficult and expensive

due to government regulation. As a result, some Tequila

companies are using this material as an organic

fertilizer. In 1990 a study evaluating the use of low

levels of vinazas as a fertilizer on the Herradura

plantation reported that application of vinasas resulted

in elevated soil calcium and foliar phosphorus and

magnesium (Reyes, 1999). Subsequently, much higher

levels of effluent have been used on the plantation. High

salt levels have been reported in other types of distillery

effluents and have been shown to increase ion

concentration in soils elsewhere (Pathak et al., 1999).

In this paper we examine common agricultural

practices involved in Blue agave cultivation and their

effects on soil fertility. The effects of nutrient addition,

tillage practices and livestock grazing are considered.

2. Materials and methods

2.1. Study location characteristics

Research was conducted on Blue agave plantations

of Herradura in the Valle de Amatitan, southeast of the

town of Tequila. The entire area is considered part of the

‘‘Zona Centro’’ or ‘‘Tequila Region’’ in the western

central highlands of the State of Jalisco inMexico. Zona

Centro is home to the three largest Tequila producers in

Mexico: Jose Cuervo, Sauza and Herradura (INEGI,

1997). The region has a ‘‘warm subtropical’’ climate

(Pimienta-Barrios and Robles-Murguia, 2001) where

annual precipitation is between 800 and 1000 mm of

rainfall annually (Valenzuela, 2003). There are distinct

wet and dry seasons, with rainfall concentrated between

the months of June and September (Nobel and

Valenzuela, 1987). Mean annual temperature ranges

from 22 to 26 8C (INEGI, 2003). Soils of the region are

classified as an association of Cambisols, Luvisols and

Lithosols and much of the region has igneous bedrock

(Cetenal, 1974; FAO, 1975).

Thirteen sites were selected within the study area.

Sites were located approximately 1200 m above sea

level in a valley floor largely free of relief. The Valle

de Amatitan lies between the Sierra Madre Occidental

and the Jalisco Block in the Plan de Barrancas-Santa

Rosa graben and is characterized by volcanic activity

of Quaternary age (Ferrari and Rosas-Elguera, 1999;

Rossotti et al., 2002).

Tequila production has a long history in the study

area. The company managing the study sites, Tequila

Herradura, was founded in 1870 by Don Feliciano

Romo at the San Jose del Refugio Site in Amatitan,

Jalisco, Mexico (Zamora, 1991). It is reasonable to

believe that the study sites have been used for agave

production for 100 years or longer.

A. Gobeille et al. / Soil & Tillage Research 87 (2006) 80–8882

2.2. Characteristics of Blue agave cultivation

Blue agaves are cultivated from cloned specimens

that are grownwith minimal interference for 7–10 years

until they reach sexual maturity. The ‘‘Pina’’ or heart of

the plant is harvested and surrounding leaves are left in

the field. This practice leaves behind roughly half of the

above-ground biomass of the plant which is eventually

reincorporated into the soil. Fallow periods are not

frequently employed (Valenzuela, 2003).

2.3. Site characteristics

Study sites were characterized by berms with a row

spacing of about 3 and 1 m in between plants. Most of

the sites were tilled to a depth of approximately 30 cm,

but some exceptionally rocky areas termed ‘‘cebor-

uco’’, were left untilled and tended by hand. Cattle and

horses were allowed to graze freely at all sites,

excepting four. The use of herbicides is ubiquitous in

the tequila industry and was observed at all sites

studied. No fertilizer was used at the study sites

although application of distillery effluent (vinazas)

was observed in two locations.

2.4. Soil sampling

Sites were selected from a pool of sites that had

been studied by Nobel (1989) for further study based

on variability in tillage, distillery effluent application,

Table 1

Site characteristics

Site# Name Tilled or

untilled

Livestock

present

1 Pozo Santo Tilled Yes

2 Coranilla Tilled Yes

3 Los Arcos Tilled Yes

4 Venaderos #1 Tilled Yes

5 Venaderos #2 Yes

6 Chiqueno (+vinazasb) Tilled Yes

7 Portrero Grande #1 Untilled Yes

8 Portrero Grande #2 Tilled Yes

9 Portrero Grande #3 Untilled Yes

10 Zapote Sur #1 Untilled No

11 Zapote Sur #2 Tilled No

12 Zapote Sur #2 (+vinazas) Tilled No

13 Bajio Tilled No

a Standard error.b Vinazas = distillery effluent.

and livestock grazing (Table 1). Only sites with plants

at least 4 years old were used as young plants take up

nutrients at an accelerated rate compared to mature

plants. In addition, a land area previously used for

Blue agave cultivation, but abandoned for the past 7

years, was used as the location for two uncultivated

control sites. Sites were sampled during a 2-week

period in June 2002.

Within each of the 13 sites, plots were selected

away from the edge of the planting and in places of

minimal slope where possible. Grids of plants 30 rows

by 30 plants were identified within the larger fields,

and 24 plants per site were randomly selected within

each grid. Because agaves have radial growth, three

soil samples per plant were taken from underneath the

canopy, half the distance of one leaf. These three

samples were pooled into one composite sample per

plant. Samples were taken from square holes of sizes

averaging 12 cm2 and dug out to a depth of roughly

20 cm, to correspond to the center of the active plant

rooting zone (Nobel, 1988) and soil was taken by

extending the hole by 1–2 cm on three of the square’s

four sides.

Volumetric samples for bulk density were obtained

with a stainless steel cylinder, which was pounded into

the ground with a rubber mallet. Four samples were

taken per site. This instrument was also used laterally

to sample from soil profiles. Bulk density samples

were dried to 105 8C the same day of collection. All

other samples were air-dried.

Plant age (years) Plant diameter

(cm) + S.E.aProfile Herbicides

4 107.2 � 11.8 Yes Yes

6 167.7 � 26 No Yes

5 132.2 � 22.3 Yes Yes

Non-cultivated NA No No

Non-cultivated NA Yes No

7 205.3 � 31.4 Yes No

5 137.8 � 14.5 Yes Yes

5 183.1 � 24.5 Yes

5 164.3 � 21.9 No

5 188.9 � 17.9 No Yes

5 193.0 � 28.5 No

5 193.8 � 30.8 Yes

4 127.8 � 18 No No

A. Gobeille et al. / Soil & Tillage Research 87 (2006) 80–88 83

Cubic meter soil profiles were dug on seven sites by

Herradura field workers which were photographed and

measured and their horizons evaluated. Samples for

nutrient analysis were taken from each of the four

sides and pooled into a composite. Composites were

taken at depths of 20 cm to represent the uppermost

horizon and an average depth of 50 cm to represent a

lower horizon. Because pits dug on untilled sites were

not deep enough to observe horizonation, data is only

shown for tilled sites.

All samples for nutrient analysis were shipped to

Ithaca, NY.

Bulk density determination (Blake and Hartage,

1986) and sample drying were conducted at the

Instituto de Tecnologico y Agropecuario (ITA) in

Tlajolmulco, Jalisco, Mexico.

Soil samples were sifted and the <2 mm non-

gravel fraction was retained for analysis. Particle

size was determined by the hydrometer method (Gee

and Bauder, 1986). Cation exchange capacity was

determined using an unbuffered salt extraction

method followed by ammonium analysis on an OI

autoanalyzer (Sumner and Miller, 1996). A 1:1 soil

to water ratio was employed for measuring pH while

soil K, P, Na, Mg and Ca were extracted with

Mehlich-3 solution (Kuo, 1996). Boron was

extracted by a pressurized hot water method (Webb

et al., 2002). ICP–AES analysis of extraction

solutions was performed at the Cornell Nutrient

Analysis Laboratory (CNAL) in Ithaca, NY. Carbon

and nitrogen were determined by dry combustion on

a LECO C/N analyzer at the CNAL. Subsamples

were treated with 0.05 M HCl to determine if

carbonates were present.

Mineralogical analysis was preformed on a small

subset of the soils (from the surface of Site 1, and from

the surface and subsurface of Sites 7 and 8) at the

X-ray diffraction (XRD) facility associated with the

Cornell Department of Earth and Atmospheric

Sciences in Ithaca, NY.

Multivariate analysis (Tukey method), unpaired

t-tests and linear regression were used to investigate

the variability of soil physical and chemical

properties between sites (Dawson and Trapp,

2001). Soil properties were treated as variables

dependent upon either site location or pooled groups

associated with previously identified agricultural

practices.

3. Results

Soils in the study area were characterized by low to

moderate values for pH, bulk density and percentage

sand content (Table 2). Concentrations of carbon and

nitrogen range from 1.02 to 2.89% for carbon, and

from 0.07 to 0.24% for nitrogen. Soil CEC varied four-

fold among the 13 sites. Carbonates were not detected

in any of the samples while quartz (SiO2) and

halloysites (aluminum silicate) were positively iden-

tified in all samples. Feldspar (potassium sodium

aluminum silicate) was identified in both the surface

and subsurface samples from Site 8. Low bulk density

was observed in Site 10 (0.68 g/cm3).

Concentrations of exchangeable ions distinguish

sites from one another (Table 3). Mean CEC values for

non-tilled sites and vinazas-amended sites were

significantly higher (11.88 and 11.06 cmolc/kg) than

the tilled sites (8.34 cmolc/kg) and abandoned sites

(6.62 cmolc/kg). Non-tilled sites had also higher mean

concentrations of carbon and nitrogen (2.44 and

0.20%) as well as elevated pH (pH = 6.07) compared

to tilled sites (1.29% C; 0.10% N; pH = 4.84). In

addition to high levels observed for CEC, vinazas-

amended tilled sites had a high mean pH (5.76), mean

phosphorus concentration (23.85 mg/kg soil) and

mean exchangeable cations potassium (436.5 mg/

kg), sodium (23.04 mg/kg), and calcium (1416.2 mg/

kg) compared to mean values on untreated sites

(3.53 mg P/kg soil; 299.5 mg K/kg soil; 10.16 mg Na/

kg soil; and 1013.5 mg Ca/kg soil) (Table 3). Live-

stock grazing effects were investigated on tilled sites

which had not been amended with vinazas. Sites with

freely grazing livestock had significantly lower mean

concentrations of the nutrients potassium (270.15 mg/

kg soil), boron (0.732 mg/kg soil), and calcium

(769.2 mg/kg soil) compared to tilled sites with no

livestock grazing (355.75 mg K/kg soil; 1.62 mg B/

kg soil; 1480.7 mg Ca/kg soil). Lower mean CEC

(7.29 cmolc/kg) and pH levels (4.8) were also

observed compared to ungrazed sites (10.06 cmolc/

kg and pH = 5.89) (Table 5). Analysis of subsurface

samples from tilled sites revealed decreases in soil

carbon (0.7%) and nitrogen (0.023%) in samples

collected at 50 cm depth compared to 20 cm samples

(1.3% C and 0.11% N), though differences were not

statistically significant. Decreases in mean levels of

potassium, phosphorus, magnesium, and calciumwere

A. Gobeille et al. / Soil & Tillage Research 87 (2006) 80–8884

Table 2

Soil physical properties (means presented � 1 S.E.)

CECsoil (cmolc/kg) % Clay % Sand % C % N pH Bulk density (g/cm3)

Site 1 Mean 5.29 70.16 16.07 1.12 0.11 4.58 1.10

S.E. 0.26 3.53 2.94 0.09 0.01 0.10

Site 2 Mean 9.40 66.81 12.72 1.37 0.10 5.86 1.05

S.E. 0.16 2.01 1.70 0.70 0.01 0.08

Site 3 Mean 5.97 63.80 26.30 1.02 0.07 4.51 1.20

S.E. 0.08 1.20 2.06 0.04 0.01 0.08

Site 4 Mean 4.87 44.16 40.53 1.10 0.06 5.20 1.54

S.E. 0.19 2.42 2.71 0.21 0.01 0.06

Site 5 Mean 8.89 55.57 28.36 1.19 0.10 5.28 1.46

S.E. 0.49 3.76 2.58 0.10 0.02 0.04

Site 6 Mean 11.88 56.42 15.85 1.47 0.12 5.55 1.02

S.E. 0.30 0.96 0.50 0.07 0.00 0.02

Site 7 Mean 10.53 58.72 17.38 1.98 0.15 5.88 1.04

S.E. 0.34 1.53 1.68 0.04 0.00 0.04

Site 8 Mean 8.33 51.61 28.81 1.59 0.11 5.41 1.05

S.E. 0.18 4.13 2.58 0.04 0.00 0.01

Site 9 Mean 13.32 37.94 31.31 2.89 0.24 6.43 0.85

S.E. 0.38 2.90 0.95 0.56 0.06 0.11

Site 10 Mean 11.32 40.30 31.80 2.46 0.20 6.03 0.68

S.E. 0.31 2.67 2.71 0.13 0.02 0.03

Site 11 Mean 10.35 53.31 22.80 1.31 0.11 6.30 0.89

S.E. 0.37 0.60 1.09 0.05 0.01 0.06

Site 12 Mean 10.24 52.67 27.59 1.42 0.13 6.26 0.95

S.E. 0.13 1.72 5.20 0.04 0.01 0.04

Site 13 Mean 9.72 51.87 20.40 1.37 0.10 5.68 1.04

S.E. 0.84 1.13 3.06 0.05 0.01 0.05

also observed in the subsurface samples while sodium

levels and physical properties remained unchanged

(Table 4).

4. Discussion

Soils on the study sites were previously classified as

an association of finely textured chromic and vertic

luvisols (Cetenal, 1974). In this study on sites that

were tilled and unamended, estimated CECclay values

were all less than 24 cmolc/kg (data not shown), no

argic horizon or evidence of clay skins was observed

and clay content did not increase with soil depth

(Table 4). Dark B horizons looked cambic in nature at

Site 1 and mollic on Site 8 at a higher elevation. The

third site observed had no such horizon. This suggests

either a considerable change in soil physical properties

since the soils were classified approximately 30 years

ago or an incorrect initial soil classification. We

suggest that the soils are chromic and eutric cambisols

but acknowledge that further work is necessary to

adequately classify this area.

Levels of soil phosphorus and nitrogen were lower

in this study than those reported by Nobel (1989) as

optimal for plant productivity. The ammonium acetate

extraction employed by Nobel is less effective than the

Mehlich-3 extraction used here; therefore it gives the

appearance of lower concentrations of these soil

nutrients. To address this discrepancy a subset of

samples was extracted with ammonium acetate.

Dramatically higher concentrations of soil potassium

and phosphorus were found with the Mehlich-3

extraction (averaging 117 mg potassium and

A. Gobeille et al. / Soil & Tillage Research 87 (2006) 80–88 85

Table 3

Tillage and vinazas effects on soil physical and chemical propertiesa

Tilled (1, 2, 3, 8, 11, 13) Not tilled (7, 9, 10) Abandoned (4, 5) Vinazas-amended (6, 12)

Mean S.E.b Mean S.E. Mean S.E. Mean S.E.

% Clay 61.21 b 1.94 46.66 a 3.02 50.68 ab 3.19 54.75 ab 1.09

pH 4.84 b 6.07 a 5.24 ab 5.76 a

CEC (cmolc/kg) 8.34 b 0.19 11.88 c 0.27 6.62 a 0.48 11.06 c 0.12

% C 1.29 a 0.04 2.44 b 0.203 1.14 a 0.12 1.44 a 0.02

% N 0.10 a 0.04 0.20 b 0.02 0.07 a 0.09 0.12 a 0.05

Bulk density

(g/cm3)

1.05 b 0.03 0.90 a 0.06 1.50 c 0.04 0.99 ab 0.27

K (mg/kg soil) 299.50 a 12.2 346.50 a 15.3 303.00 a 30.3 436.50 b 21.6

P (mg/kg soil) 3.53 a 0.82 10.99 a 2.13 3.08 a 0.61 23.8 b 9.30

Na (mg/kg soil) 10.16 a 0.54 10.60 ab 0.48 15.68 b 1.70 23.04 2.62

B (mg/kg soil) 1.06 ab 0.07 1.51 bc 0.07 0.74 a 0.07 1.37 bc 0.08

Mg (mg/kg soil) 228.36 a 8.76 410.5 b 12.61 221.19 a 117.69 364.40 b 14.17

Ca (mg/kg soil) 1013.5 a 61.4 2156.9 c 91.2 786.3 a 134.4 1416.2 b 41.9

a Values followed by the same letter in a row did not differ significantly per the Tukey t-test (p < 0.05).b Standard error.

11.8 mg P/g soil). This suggests that that the phos-

phorus deficiency is more severe than originally

predicted with the Mehlich-3 data alone.

Since nitrogen, carbon and phosphorus levels do

not vary between cultivated and abandoned sites

suggests that unmanaged fallow periods do not restore

nutrients exported by the harvest of plant biomass.

This is consistent with similar findings with regard to

A. sisalana on more weathered soils, where an 18-year

fallow period had little effect on soil organic matter or

nutrient levels (Hartemink et al., 1996).

Tillage results in a significant decrease in soil

organic matter and associated nutrient levels. An

examination of tilled sites reveals that carbon and

nitrogen levels are lower than the untilled sites (1.29%

C and 0.10% N on tilled sites; 2.44% C and 0.44% N

on non-tilled sites). Nitrogen levels do not increase in

the subsurface samples (Table 5); therefore leaching

Table 4

Soil profile characteristics from tilled, unamended sites (N = 3)

K

(mg/kg soil)

P

(mg/kg soil)

Na

(mg/kg soil)

B

(mg/kg soil)

Mg

(mg

Tilled surface (20 cm)a

Mean 297.72 2.33 11.32 0.85 223

S.E. 52.05 1.04 2.48 0.22 50

Tilled subsurface (50 cm)

Mean 52.48 0.07 12.04 0.88 175

S.E. 73.88 0.07 3.12 0.18 37

a Differences observed between surface and subsurface samples were n

is unlikely to be a significant mechanism of loss. That

the untilled sites also had greater CEC levels than

tilled sites (11.88 cmolc/kg versus 8.34 cmolc/kg) is

most likely related to their increased concentration of

soil organic matter. This observation is supported by

Hickman (2002) aswell by a strong linear relationship

between observed between CEC and carbon levels

(R2 = 0.546) in these sites. The elevated CEC levels

observed are likely an important factor in the higher

concentrations of magnesium and calcium also

observed in the untilled sites (R2 = 0.556 and

0.467, respectively). Bulk density values on untilled

sites may be deceptively low due to the presence of

large rocks which were removed from the sample

prior to analysis.

Sites amended with distillery effluent had high

levels of mineral salts and elevated CEC levels. These

results are consistent with those reported by Reyes

/kg soil)

Ca

(mg/kg soil)

%

Clay

pH CEC

(cmolc/kg)

% C % N

.29 723.06 55.59 4.65 8.01 1.33 0.11

.25 196.24 5.33 0.85 0.19 0.02

.64 531.55 60.37 4.8 7.95 0.70 0.06

.70 143.79 5.15 0.87 0.07 0.02

ot statistically significant.

A. Gobeille et al. / Soil & Tillage Research 87 (2006) 80–8886

Table 5

Physical and chemical characteristics of vinazas

Property Value

pHa 3.35

Total nitrogena (mg/l) 243

Coppera (mg/l) 0.364

Zinca (mg/l) 0.399

Total phosphorusa (mg/l) 20.6

Suspended solidsa (mg/l) 3400

Potassiumb (% solid weight) 11

Magnesiumb (% solid weight) 0.08

Calciumb (% solid weight) 0.21

Potassiumb (meq/l) 2.80

Sodiumb (meq/l) 5.4

a Tequila Herradura, unpublished data, 2004.b Reyes (1999).

(1999) and other researching the use of rum distillery

effluent in India (Jabeeen et al., 1987; Rajannan

et al., 1998) and with the high levels of potassium and

sodium in the vinazas themselves (Table 5). While

sodium levels were higher on sites treated with

vinazas than on any other sites (Table 3), these levels

were still well below the estimate threshold of plant

toxicity estimated in 1989 at 150 mg Na/g soil

(Nobel, 1989).

Livestock grazing lowered pH, CEC values and

concentrations of potassium, boron and calcium, when

compared to non-grazed sites (Table 6). To accom-

modate the export of potassium from 1 ha to a depth of

20 cm, approximately 300 kg/year of agave tissue

would have been consumed, using published estimates

of foliar potassium levels. When the same calculation

is applied to calcium, it is found that 2200 kg of tissue

export would be needed to accommodate the calcium

loss in grazed sites (Nobel, 1988). Differences

between these two figures are most likely the result

Table 6

Soil physical and chemical properties on sites with and without livestock

Ka

(mg/kg soil)

P

(mg/kg soil)

Na

(mg/kg soil)

Ba

(mg/kg soil)

Mg

(mg/kg so

Tilled, livestock present (N = 4)

Mean 270.15 1.91 9.88 0.73 209.94

S.E. 10.44 0.2 0.67 0.06 10.67

Tilled, no livestock present (N = 2)

Mean 355.75 6.63 10.55 1.62 60.47

S.E. 26.50 2.24 0.90 0.09 12.61

a Mean values differed significantly per unpaired Student’s t-test (p � 0

of other factors that effect soil nutrient levels in situ.

Both estimates are reasonable export values, based on

livestock daily food intake estimates, of approxi-

mately 20 kg/day (Makhijani, 1990). Using these

figures, it would take 10 cows, deriving 10% of their

daily calories from agaves only 110 days to consume

the 2200 kg tissue required to accommodate the

predicted calcium loss, assuming manure left behind

was negligible.

Blue agave cultivation has resulted in the depletion

of important soil nutrients and has altered soil physical

properties. Hartemink (1997a,b) has shown that the

effects of intensively cropping agaves are mitigated by

the resilience of volcanic soil. The ‘‘sustainability’’ of

these systems is also helped by infrequent harvesting

and a return of much of the above-ground biomass.

Yet, despite these buffers, Blue agave plantations are

not benign. Cultivation has resulted in a net loss of soil

nitrogen, phosphorus and organic matter and also

altered soil physical properties such, lowering CEC

and pH.

5. Conclusions

A significant driver of soil fertility decline is tillage,

which dramatically affects the soil organicmatter stocks

along with associated nutrients. In this study, sites that

were tilled had significantlyhighermean levels of soilC,

N and CEC. Additionally, sites in which cattle were

grazed had significantly lower mean pH and CEC levels

aswell as lower levels of importantmicronutrientsK,B,

and Ca. The application of distillery effluent increased

mean levels of soil K, P, Mg, Ca and B, as well as CEC

on those sites, but soil carbon levels were similar to

present

il)

Caa

(mg/kg soil)

%

ClayapHa CECa

(cmolc/kg)

% C % N Bulk density

(mg/cm3)

769.20 64.45 4.80 7.29 1.26 0.10 1.10

57.35 52.87 0.20 0.07 0.01 0.04

1480.70 52.87 5.89 10.06 1.34 0.11 0.97

74.36 0.66 0.22 0.04 0.01 0.04

.05).

A. Gobeille et al. / Soil & Tillage Research 87 (2006) 80–88 87

untreated sites. The region studied has been productive

agriculturally since the mid-19th century and will most

likely be productive well into the future. But, this

success may not be indefinite if trends observed

continue. A key question generated by this research

is whether a lower availability of soil nutrients is

associated with reduced productivity. Plant size, a

seasonally dependent staticmeasurement, did not reveal

differences between sites or agricultural practices.More

intensive and dynamic measurements, such leaf

unfurling counts, may reveal more subtle differences

in plant performance.

Acknowledgements

This study was funded, in part, by an IGERT in

Biogeochemistry and Biocomplexity as well as a

small grant by the Andrew Mellon Foundation. The

authors would also like to acknowledge the generous

assistance of Tequila Herradura, especially Dr. Aidee

Orozco Hernandez, PhD, Luis Segura, Joel Zaragoza

and Martin Ortega. Manuel Alvarez and Ruben

Ravelero, of ITA Jalisco, also provided invaluable

assistance with field sampling methods and ideas.

Meghan Manion and Gretchen Guzek provided

laboratory support at Cornell University.

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