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Soil factors that influence the fruiting of Tuber melanosporum (black truffle).

Introduction

Tuber melanosporum (black truffle of Perigord) is a hypogeous ascomycete, ectomycorrhizal fungus of the Pezizales order, with great culinary and commercial value (more than 1000 [euro]/kg) due to its intense aroma. Different studies have examined which soil properties have the greatest impact on truffieres; these include comparisons of the soil requirements of different species of truffles (Delmas et al. 1981; Raglione et al. 1992; Mamoun and Olivier 1993a, 1993b; Zanini et al. 1995; Bencivenga et al. 1998; Raglione et al. 1999), analyses of soil variability and its relationship with climate (Bencivenga et al. 1988, 1992), and studies relating soil parameters to production and development of T. melanosporum (Delmas et al. 1982; Letacon et al. 1982; Verlhac et al. 1989; Bencivenga et al. 1992; Callot and Jaillard 1996; Bragato 1997; Bragato et al. 1999; Callot 1999; Callot et al. 1999; Lulli et al. 1999; Souzart 1999; Castrignano et al. 2000; Garcia-Montero 2000; Ricard 2003).

These authors indicate that Tuber melanosporum is strictly calcicolous and grows in calcareous soils with a C/N relationship of close to 10, in which the soil texture is balanced and tends to be simple and well constructed. However, Souzart (1999) indicates that the soil parameters that T. melanosporum tolerates are highly variable. Callot et al. (1999), Lulli et al. (1999), Castrignano et al. (2000), and Ricard (2003) report that the development of Tuber melanosporum increases in deep, well-drained, aerated soils with a reservoir of available water during dry periods. Total organic carbon content must be low in order not to modify their pH, and they require considerable porosity, originated by biological activity (above all, by worms and ants).

Nevertheless, Bencivenga (1999) and Chevalier (1999) point out that the ecology of T. melanosporum has not been thoroughly explained and that its study is of great importance to improve harvesting, cultivation, and natural protection. Callot et al. (1999), Ricard (2003), and Granetti et al. (2005) highlight the lack of knowledge as to how the physicochemical properties of fine soil influence truffle development. Moreover, many authors fail to supply quantitative data on the relationship between soil properties and truffle production.

The objective of the present work is therefore to explain more fully the impact of soil on the productivity of Tuber melanosporum carpophores through a quantitative statistical estimation of the influence of conventional surface horizon properties (granulometric texture, pH, calcareous fractions, organic carbon, total nitrogen, exchangeable cations). A study was done for this purpose in the Alto Tajo Nature Reserve in the centre of Spain, a location with several areas of natural populations of T. melanosporum (Garcia-Montero et al. 2005).

Materials and methods

Case study area

Twenty plots were selected close to the village of Peralejos de las Truchas (Guadalajara) (40[degrees]35'49.65"N; 1[degrees]54'12.78"W), but their exact geographic coordinates are not given to maintain the secrecy of the Tuber melanosporum locations. The study was conducted in a mountainous area at an altitude of > 1000 m in a supra-Mediterranean bio-climatic belt with a sub-humid, shadowy climate. Average annual precipitation is 797 mm, with low yearly average temperatures (9.7[degrees]C). Its lithology comprises predominantly Jurassic and Cretaceous limestone and dolomites, and soils are frequently lithic and rendzic leptosols with considerable surface stoniness and high peaks. Its vegetation is made up of forests of Quercus faginea Lam. (Cephalanthero longifoliae-Querceto Jagineae S. geobotanical classification type), river forests (Astrantio-Coryleto avellanae S.), and oak forests (dunipero thuriferae-Querceto rotundifoliae S.) (Garcia-Montero et al. 2005).

Soil analysis

Twenty soils were selected according to their production of Tuber melanosporum carpophores: 8 samples were taken from the interior of the brules (the name given to productive sites, recognised by their typical bare appearance), with high carpophore productivity (2000-6000 g/year); 6 samples from brules with low productivity (100-600 g/year), and 6 reference samples without production, situated at least 50 m from the brules. The term brules is used to describe spots where Z melanosporum grows and refers to its phytotoxic capacity and ability to create clearings in the vegetation where its carpophores bear fruits (Montacchini et al. 1972; Papa 1992). Only the first 30 cm of each soil profile was studied because T. melanosporum usually bears fruit in this range (Verlhac et al. 1990). These samples include soil surface horizons that have not been differentiated, as T. melanosporum natural brules contain a constant mixture of horizons, due to the continuous digging that the harvesters, their dogs, and wildlife engage in to extract the carpophores.

Sampling was done according to the FAO (1990). The gradient of each plot was measured with a clinometer. Surface stoniness and litter were estimated as percentages. The following soil determinations were made: pH, total organic carbon (TOC), total calcium carbonate (equivalent calcium carbonate), granulometric analysis, cationic exchangeable capacity (CEC), sum total of the exchangeable cations (S), and the exchangeable cations saturation percentage (% V), following the methods of the ISRIC (1995); textures are classified according to the International Society of Soil Science (ISSS); total nitrogen was analysed with the variant of Bouat and Crouzet (1965); and active limestone (calcium carbonate extractable with ammonium oxalate) was determined according to AFNOR (1982). The determination of the exchangeable cations of [Ca.sup.2+] and [Mg.sup.2+] was done using AAS (Philips UP9100x), and [K.sup.+] and [Na.sup.+] with a flame photometer (Sherwood 410).

Quantitative analysis

The maximum annual production of carpophores in Tuber melanosporum brules was evaluated as an expression of the greatest quantity (in g) of fresh carpophores picked that year. The harvesting data from 7 local truffle collectors were used, and brules in the field were tracked from 1994 to 2000 (Garcia-Montero 2000; Garcia-Montero et al. 2005). Statistical treatment was performed with the Statistica Program v.6 (StatSoft, Inc., Tulsa, OK). Before proceeding to the analysis, the distributions of the variables were adjusted to comply with the prerequisites of the statistical analysis. These transformations were selected using the Box and Cox (1964) Test. Normality was checked using the Shapiro-Wilks test and homogeneity of variances was verified by the Levene Test.

Results

Tables 1 and 2 show the characteristics of the surface horizons studied. Many of the soils have a texture that tends towards sandy clay loam soil. The majority present a moderately basic pH and a low percentage of total carbonate but have an elevated concentration of active carbonate. Levels of organic carbon are moderate and the C/N ratio is close to 10. Their exchangeable cation complex has high values. They display good structure with a granular tendency and a wealth of pores. These soil properties are suitable for proper development of T. melanosporum.

Simple correlations were obtained between the quantitative variables of the surface soil horizons and the productivity of Tuber melanosporum. The results show that active limestone had a positive and very significant impact on the production of carpophores in T. melanosporum brules. Thus, the percentage of active limestone explains 40% of the variance in production (r=0.635; P=0.003; n = 20). The correlation coefficients obtained between active limestone and truffle production indicate a moderately strong relationship between these variables (Table 3).

A principal components analysis (PCA) was done with the quantitative variables of the surface horizons studied, in order to synthesise the variation patterns in those elements that are potentially associated with the production of Tuber melanosporum carpophores and to quantify the overall impact of the surface horizons on carpophore production in this fungus. Table 4 shows that the first 3 factors ([PC.sub.1], [PC.sub.2], and [PC.sub.3]) accounted for 71.27% of the variance contained in the original matrix. Factors [PC.sub.1], [PC.sub.2], and [PC.sub.3] were selected and studied on the basis of the criteria of a visual inspection of scree plots, shown in Fig. 1. Table 4 provides the correlations between the original variables and the 3 factors generated by the PCA, as well as the factor loadings that permitted these PCs to be interpreted.

[FIGURE 1 OMITTED]

[PC.sub.1] accounted for 34% of the variance, which shows the differences between soils with a greater capability of cationic interchange and a greater amount of exchangeable [Ca.sup.2+] , silt, organic carbon, potassium, and soils rich in fine and coarse sand (Table 4). The second factor ([PC.sub.2]) represented 23% of the variance and highlights the differences between soils rich in active limestone and interior and surface stoniness, and soils with a greater quantity of exchangeable [Mg.sup.2+], organic carbon, clay and exchangeable [K.sup.+]. The third factor ([PC.sub.3]) accounted for 14% of the variance and underlines differences between soils rich in fine sand and [Mg.sup.2+], and soils with total calcium carbonate and coarse sand.

Finally, a positive significant correlation was noted between the situation of the soils in Axis 2 ([PC.sub.2]) and the production of Tuber melanosporum (r = -0.521; P = 0.018; n = 20) (Fig. 2). Thus, Factor 2 ([PC.sub.2]) explained 27% of the variance in productivity of T. melanosporum while [PC.sub.1] (r=-0.260; P=0.268; n=20) and [PC.sub.3] (r=0.262; P=0.265; n=20) showed no significant correlation. In summary, the more productive soils tended to have a higher proportion of active limestone and surface and below-ground stoniness, and lower percentages of exchangeable [Mg.sup.2+], organic matter, clay, and exchangeable [K.sup.+], in the conditions of the study area.

[FIGURE 2 OMITTED]

Discussion

The PCA showed that Tuber melanosporum productivity is influenced by the overall action of active limestone, stoniness, organic carbon, clay content, and exchangeable cations present in the soil surface horizons. Nevertheless, the collective influence of these conventional soil features is low, as [PC.sub.2] explains only 27% of the variance in T. melanosporum productivity. In this respect, the proposals of Callot (1999) and Ricard (2003) indicate that the conventional physicochemical properties of soil surface horizons have a limited impact on truffle culture. We have finally confirmed and quantified these proposals.

These authors and other studies have emphasised the importance of carrying out a thorough analysis of the properties accounting for porosity, permeability and aeration (small aggregates, macroporosity, continuity of pores, etc.), water balance, soil flora and fauna, and recarbonation throughout the entire profile, as these factors determine the growth and production of Tuber melanosporum (Bragato et al. 1999; Callot 1999; Callot et al. 1999; Ricard 2003).

Regarding the study of limestone, a simple, positive, and very significant correlation was found between the percentage of active limestone in the surface horizons and the production of Tuber melanosporum carpophores. Active limestone explained 40% of this variance. Active limestone is a finely divided fraction of calcareous rock measuring <50 [micro]m, susceptible to rapid mobilisation and very chemically active. Active limestone indicates the extent and reactivity of the limestone surfaces and constitutes an important reserve of exchangeable [Ca.sup.2+]. Continuous formation of active limestone preserves high levels of [Ca.sup.2+] in the soil solution and maintains the exchange complex, as it counterbalances losses from leaching and other processes. Exchangeable [Ca.sup.2+] comes mostly from calcareous rocks, whose rate of dissolution depends on the particles' hardness and size, given that at constant hardness, the dissolution speed increases as the size decreases (Lopez and Lopez 1990; Maranes et al. 1994; Breemen and Buurman 1998).

Ricard (2003) indicated that it is difficult to judge the impact of this type of limestone on truffle culture because several factors must be considered. Ourzik (1999) points out that the main problem stems from the current lack of knowledge of T. melanosporum, which is insufficient to establish a direct relationship between this calcareous fraction and the biology of T. melanosporum. Nevertheless, Ricard (2003) suggests that the lack of studies on active limestone in T. melanosporum culture is an oversight. The presence of active limestone and exchangeable [Ca.sup.2+] is very important to Tuber melanosporum due to the action of several factors. These include its capability to regulate the soil pH and its participation in the flocculation of the colloidal fraction (clays and humus), which contributes to the organisation and maintenance of soil structure and plays a part in the truffle's nutrition (Delmas et al. 1982; Poitou et al. 1983; Poitou 1988; Callot 1999; Ourzik 1999; Ravazzi 2003; Ricard 2003; Granetti et al. 2005).

The surface horizons of the Tuber melanosporum sites studied have an elevated concentration of active limestone (Table 2). The availability of active limestone in the soil depends on complex pedologic, climatic, and biological factors. The dissolution, transport, and accumulation of the soil's limestone depend on the relationship between water dissolution and gaseous C[O.sub.2], soluble [H.sub.2]C[O.sub.3] and HC[O.sub.3.sup.-], and solid CaC[O.sub.3] phases. The solubilisation and leaching of limestone increases with pluviosity, the respiration and metabolism of the roots and mycelia, and other factors that increase the partial pressure of the C[O.sub.2] (pC[O.sub.2]) in the soil. Bicarbonate precipitates in the form of calcium carbonate when it is concentrated in capillary water (evaporated or absorbed by the roots). This also occurs when the pC[O.sub.2] diminishes due to the aeration generated by macrofauna and to the disappearance of biological respiration at depth, among other factors (Lopez and Lopez 1990; Wild 1992; Maranes et al. 1994; Breemen and Buurman 1998).

In summary, conventional soil properties (granulometric texture, pH, calcareous fractions, organic carbon, total nitrogen, exchangeable cations) have a limited impact on the production of Tuber melanosporum carpophores, as they account for only 27% of production variance. However, an elevated percentage of active limestone explains 40% of the variance in T. melanosporum production. For this reason, we propose a study of calcareous amendments with fine limestone to maintain the concentration of active limestone in practical truffle culture. However, these conclusions should be interpreted with caution. In fact many, other biotic and abiotic factors play an important role in truffle productivity in truffieres and natural brules.

Ricard (2003) also suggests calcareous amendments of fine limestone in practical truffle culture, although he recommends that they should be used with care and in moderation. New studies are required into the effects of active limestone on the biology of T. melanosporum and into the role played by soil and biological processes in maintaining elevated concentrations of active limestone in T. melanosporum truffieres and natural brules.

Acknowledgments

The research in this article forms part of an agreement signed between the I.T.D.A. and the Instituto Madrileno de Investigaciones Agrarias (I.M.I.A.) entitled 'Estudio ecologico y edafico preliminar de habitats ibericos de la trufa negra en relacion con el cultivo del I.T.D.A.' and the Project FP-01-41 of I.M.I.A. research project, granted in 2001. We thank Domingo and J. Diaz for their teachings, and Margarita, Luis, J. L. Manjon, P. Diaz, I. Valverde, S. Martin-Fernandez, and Valentina Urbano for their support and collaboration. The authors wish especially to thank Domingo hijo, Domingo padre, Tono, Emilio, Justo, Abel, Pepe, Jose Maria, Gonzalo, and the other truffle collectors, and all the people and Council of Peralejos de las Truchas for their invaluable contribution to this study. We also thank the Alto Tajo Nature Reserve Institution. In the IV International Workshop on Edible Mycorrhizal Mushrooms (University of Murcia. Nov. 2005) we presented a partial communication entitled: Influence of active limestone and soil features on Tuber melanosporum productivity.

Manuscript received 13 April 2006, accepted 3 October 2006

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L. G. Garcia-Montero (A,C), M. A. Casermeiro (B), J. Hernando (B), and I. Hernando (B)

(A) Dept of Forestry Engineering, E.T.S.I. Montes, Universidad Politecnica de Madrid, Ciudad Universitaria, Madrid, E-28040 Spain.

(B) Dept of Soil Science, Universidad Complutense de Madrid, Facultad de Farmacia, Ciudad Universitaria, Madrid E-28040, Spain.

(C) Corresponding author. Email: luisgonzaga.garcia@upm.es
Table 1. Description of the location, vegetation, stoniness,
and topography of the soils studied

No. Max. g Vegetation Alt.
 truffles
 per year

1 6000 Open pine forest with 1250
 Quercus faginea
2 5000 Open forest of Quercus faginea 1200
3 4000 Thicket with Quercus faginea 1185
4 3500 Thicket with Quercus faginea 1000
5 3500 Open forest of Quercus faginea 1090
6 3000 Thicket with Quercus ilex 1290
 and Q. faginea
7 2000 Open forest of Quercus ilex 1140
8 2000 Thicket with Quercus ilex 1235
9 600 Open forest of Tilia platyphyllos 1080
 with Quercus faginea
10 400 Open forest of Quercus faginea 1215
11 300 Mixed forest of Corylus avellana 1000
 and Quercus faginea
12 200 Open forest of Quercus faginea 1095
13 200 Thicket with Quercus faginea 1155
14 100 Forest of Corylus avellana, 1200
 Tilia platyphyllos
 and Quercus faginea
15 0 Thicket with Quercus faginea 1185
16 0 Open forest of Quercus faginea 1200
17 0 Pine forest with Quercus faginea 1150
18 0 Pine forest with Quercus faginea 1100
19 0 Forest of Corylus avellana 1150
20 0 Forest of Corylus avellana 1150

No. Vegetation Orientation

1 Open pine forest with E-NE
 Quercus faginea
2 Open forest of Quercus faginea S
3 Thicket with Quercus faginea S-SW
4 Thicket with Quercus faginea S-SW
5 Open forest of Quercus faginea W
6 Thicket with Quercus ilex E-SE
 and Q. faginea
7 Open forest of Quercus ilex S-SW
8 Thicket with Quercus ilex S
9 Open forest of Tilia platyphyllos W-NW
 with Quercus faginea
10 Open forest of Quercus faginea E-NE
11 Mixed forest of Corylus avellana --
 and Quercus faginea
12 Open forest of Quercus faginea SW
13 Thicket with Quercus faginea E-SE
14 Forest of Corylus avellana, S
 Tilia platyphyllos
 and Quercus faginea
15 Thicket with Quercus faginea N
16 Open forest of Quercus faginea N-NE
17 Pine forest with Quercus faginea NE
18 Pine forest with Quercus faginea NE
19 Forest of Corylus avellana N
20 Forest of Corylus avellana NE

No. Vegetation Topography

1 Open pine forest with Flat on slope
 Quercus faginea
2 Open forest of Quercus faginea Slope
3 Thicket with Quercus faginea Hilly
4 Thicket with Quercus faginea Hilly
5 Open forest of Quercus faginea Slope
6 Thicket with Quercus ilex Slope
 and Q. faginea
7 Open forest of Quercus ilex Slope
8 Thicket with Quercus ilex Almost flat
9 Open forest of Tilia platyphyllos Slope
 with Quercus faginea
10 Open forest of Quercus faginea Slope
11 Mixed forest of Corylus avellana Almost flat
 and Quercus faginea
12 Open forest of Quercus faginea Slope
13 Thicket with Quercus faginea Low undulate hills
14 Forest of Corylus avellana, Steep craggy slope
 Tilia platyphyllos
 and Quercus faginea
15 Thicket with Quercus faginea Low undulate hills
16 Open forest of Quercus faginea Hilly
17 Pine forest with Quercus faginea Low undulating hills
18 Pine forest with Quercus faginea Moderate slope
19 Forest of Corylus avellana Moderate sloped hills
20 Forest of Corylus avellana Slightly hilly

No. Vegetation Slope % Surface
 (%) stoniness

1 Open pine forest with 30 60
 Quercus faginea
2 Open forest of Quercus faginea 15 60
3 Thicket with Quercus faginea 15 100
4 Thicket with Quercus faginea 10 20
5 Open forest of Quercus faginea 20 90
6 Thicket with Quercus ilex 70 20
 and Q. faginea
7 Open forest of Quercus ilex 20 100
8 Thicket with Quercus ilex 5 25
9 Open forest of Tilia platyphyllos 40 70
 with Quercus faginea
10 Open forest of Quercus faginea 30 50
11 Mixed forest of Corylus avellana 0 30
 and Quercus faginea
12 Open forest of Quercus faginea 40 90
13 Thicket with Quercus faginea 5 20
14 Forest of Corylus avellana, 70 85
 Tilia platyphyllos
 and Quercus faginea
15 Thicket with Quercus faginea 5 5
16 Open forest of Quercus faginea 5 20
17 Pine forest with Quercus faginea 15 10
18 Pine forest with Quercus faginea 30 50
19 Forest of Corylus avellana 5 40
20 Forest of Corylus avellana 3 5

No. Vegetation % Below-
 ground
 stoniness

1 Open pine forest with 20
 Quercus faginea
2 Open forest of Quercus faginea 50
3 Thicket with Quercus faginea 60
4 Thicket with Quercus faginea 20
5 Open forest of Quercus faginea 70
6 Thicket with Quercus ilex 15
 and Q. faginea
7 Open forest of Quercus ilex 80
8 Thicket with Quercus ilex 25
9 Open forest of Tilia platyphyllos 40
 with Quercus faginea
10 Open forest of Quercus faginea 30
11 Mixed forest of Corylus avellana 0
 and Quercus faginea
12 Open forest of Quercus faginea 75
13 Thicket with Quercus faginea 10
14 Forest of Corylus avellana, 40
 Tilia platyphyllos
 and Quercus faginea
15 Thicket with Quercus faginea 10
16 Open forest of Quercus faginea 0
17 Pine forest with Quercus faginea 10
18 Pine forest with Quercus faginea 50
19 Forest of Corylus avellana 40
20 Forest of Corylus avellana 0

Table 2. Analytical results of soils with Tuber melanosporum
production and reference soils
TOC and N expressed in g/kg; CEC, and exchangeable cations
expressed in [mol.sub.c]/kg

Max. Structure Sand Silt Clay
g/year (%)

6000 Moderate granular 44 19 37
5000 Moderate granular 66 22 12
4000 Coarse granular 41 14 45
3500 Moderate granular 82 4 14
3500 Not aggregated 28 59 13
3000 Moderate granular 64 13 23
2000 Not aggregated 45 32 23
2000 Not aggregated 55 10 35
600 Weak granular 82 8 10
400 Weak granular 76 9 15
300 Weak granular 59 7 34
200 Moderate granular 47 19 34
200 Weak granular 68 13 19
100 Particular 37 38 25
0 Well-structured granular 64 12 24
0 Moderate-weak granular 76 7 17
0 Medium polyhedral 57 9 34
0 Medium polyhedral 14 17 69
0 Not aggregated 62 17 21
0 Granular 64 10 26

 CaC[O.sub.3]
 (%)

Max. Structure [MATHEMATICAL Total Active
g/year EXPRESSION NOT
 REPRODUCIBLE
 IN ASCII]

6000 Moderate granular 7.90 3.12 2.25
5000 Moderate granular 7.88 7.88 7.01
4000 Coarse granular 7.50 12.79 1.63
3500 Moderate granular 7.90 9.18 0.38
3500 Not aggregated 7.80 7.19 2.88
3000 Moderate granular 8.15 4.18 2.63
2000 Not aggregated 7.90 11.50 8.14
2000 Not aggregated 8.15 4.53 2.38
600 Weak granular 7.90 10.21 3.38
400 Weak granular 8.07 6.28 0.38
300 Weak granular 7.92 4.38 1.57
200 Moderate granular 7.95 7.81 2.88
200 Weak granular 7.10 14.56 1.50
100 Particular 8.17 13.38 0.21
0 Well-structured granular 8.17 9.66 1.50
0 Moderate-weak granular 8.06 15.63 0.15
0 Medium polyhedral 7.52 17.92 0.00
0 Medium polyhedral 7.92 16.35 0.18
0 Not aggregated 7.85 0.21 0.00
0 Granular 7.00 18.44 0.00

Max. Structure TOC N [Ca.sup.2+]
g/year

6000 Moderate granular 34.03 2.54 13.19
5000 Moderate granular 35.62 2.73 21.14
4000 Coarse granular 23.02 2.50 20.17
3500 Moderate granular 31.52 2.90 5.23
3500 Not aggregated 75.24 5.60 46.16
3000 Moderate granular 11.58 1.77 11.20
2000 Not aggregated 36.07 2.30 18.86
2000 Not aggregated 10.80 0.87 15.49
600 Weak granular 25.51 2.73 27.49
400 Weak granular 5.25 -- 5.16
300 Weak granular 19.58 1.80 12.71
200 Moderate granular 31.10 2.87 12.04
200 Weak granular 1.00 -- 9.65
100 Particular 28.60 3.80 18.43
0 Well-structured granular 7.39 0.58 10.14
0 Moderate-weak granular 13.30 3.30 11.87
0 Medium polyhedral 49.73 3.60 20.14
0 Medium polyhedral 53.08 3.60 24.90
0 Not aggregated 91.70 7.40 23.49
0 Granular 43.22 3.80 16.02

Max. Structure [Mg.sup.2+] [K.sup.+] CEC
g/year

6000 Moderate granular 7.92 1.03 24.27
5000 Moderate granular 0.74 1.10 23.44
4000 Coarse granular 0.60 1.47 22.32
3500 Moderate granular 4.98 0.37 10.66
3500 Not aggregated 3.15 0.73 50.04
3000 Moderate granular 4.89 0.29 16.46
2000 Not aggregated 0.41 0.22 19.49
2000 Not aggregated 1.85 0.51 18.12
600 Weak granular 1.48 0.29 29.26
400 Weak granular 6.34 0.22 11.80
300 Weak granular 2.13 0.07 14.99
200 Moderate granular 2.63 0.66 15.33
200 Weak granular 1.79 0.22 11.66
100 Particular 0.78 1.03 20.33
0 Well-structured granular 3.62 0.29 14.13
0 Moderate-weak granular 1.89 0.07 13.83
0 Medium polyhedral 4.39 0.96 25.49
0 Medium polyhedral 8.35 1.99 35.32
0 Not aggregated 6.50 0.66 30.65
0 Granular 1.54 1.10 18.66

Table 3. Simple correlations between the production of Tuber
melanosporum and soil variables

 Correlation with square root
 of production of
 Tuber melanosporum

Variables r P n

% CaC[O.sub.3] total -0.433 0.056 20
Ln (x + 1) % CaC[O.sub.3] active 0.635 0.003 ** 20
% TOC -0.045 0.851 20
Ln (x + 1) % N -0.086 0.718 20
Course sand -0.183 0.440 20
Fine sand 0.083 0.727 20
Total sand -0.081 0.733 20
Ln (silt) 0.187 0.430 20
Ln (clay) -0.170 0.475 20
Ln (CEC) 0.126 0.597 20
[Ca.sup.2+] exchangeable 0.128 0.592 20
[Mg.sup.2+] exchangeable -0.054 0.820 20
Ln ([K.sup.+] exchangeable) 0.184 0.438 20
SQRT (% surface stoniness) 0.423 0.063 20
% Interior stoniness 0.417 0.067 20

** P < 0.01.

Table 4. Factor loadings (unrotated); extraction: principal
components analysis

Variables Factor 1 Factor 2
 ([PC.sub.1]) ([PC.sub.2])

% CaC[O.sub.3] total 0.066 -0.288
Log (x + 1) % CaC[O.sub.3] active -0.282 0.885
% TOC -0.723 -0.485
Log (x + 1) % N -0.656 -0.484
% Coarse sand 0.580 0.086
% Fine sand 0.500 0.145
Log (% silt) -0.813 0.219
Log (% clay) -0.160 -0.390
Log (CEC) -0.886 -0.226
[Ca.sup.2+] exchangeable -0.859 -0.047
[Mg.sup.2+] exchangeable -0.003 -0.566
Log ([K.sup.+] exchangeable) -0.622 -0.370
Root (% surface stones) -0.697 0.456
% Interior stoniness -0.545 0.646
Expl. Var (A) 5.057 3.509
Prp. Totl (B) 0.337 0.234

Variables Factor 3
 ([PC.sub.3])

% CaC[O.sub.3] total -0.883
Log (x + 1) % CaC[O.sub.3] active 0.129
% TOC 0.154
Log (x + 1) % N 0.017
% Coarse sand -0.569
% Fine sand 0.711
Log (% silt) -0.081
Log (% clay) 0.061
Log (CEC) 0.095
[Ca.sup.2+] exchangeable -0.054
[Mg.sup.2+] exchangeable 0.573
Log ([K.sup.+] exchangeable) -0.132
Root (% surface stones) 0.018
% Interior stoniness -0.270
Expl. Var (A) 2.125
Prp. Totl (B) 0.142

(A) The own value or eigenvalue, that is, the number of original
variables that equal the new factor in informational content.

(B) The percentage of initial variance (matrix of n cases by v
variables) retained by the factors.
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Article Details
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Author:Garcia-Montero, L.G.; Casermeiro, M.A.; Hernando, J.; Hernando, I.
Publication:Australian Journal of Soil Research
Geographic Code:4E
Date:Dec 1, 2006
Words:5697
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