Study area

This research study was performed in San Blas municipality, Nayarit, Mexico; it has a surface of 849.78 km², the type of climate is warm-subhumid (25.2 °C) with an average annual precipitation of 1,316 mm (^{Cossio et al., 2008}); limits with Pacific Mexican Ocean, where the physiographical provinces of Neovolcanic Axis and Pacific Coastal Plain converge, corresponding to geostructures according to ^{Zinck (2012)}, in which six topoform systems converge, corresponding to geomorphological landscapes according to ^{Zinck (2012)} (Coastal Plain, Delta Coastal Plain, Saline Coastal Plain with Coastal Lakes, Littoral ridges Plain, Volcanic Belt with steep hillsides and Volcanic Isolated Elevations) (Figure 1). The northwestern part of the Neovolcanic Axis has been included, a Plio-Quaternary volcanic structure that is East-West superimposed to the most ancient structures, in the region Graben Chapala-Tepic; the principal eruptive center is San Juan volcano, to which secondary eruptive centers such as La Yerba and La Cebadilla are associated, with diverse small volcanic cones with lava flows partially buried by recent alluvium, among them Chacalilla, La Contaduría and Ceboruco-Las Islitas (^{Demant et al., 1976}). As well, the South part of the Pacific Coastal Plain formed by alluvium of Santiago river during late Pleistocene Epoch, followed by a transgressive phase in the first millennia of the Holocene Epoch where the coastal line progressed and migrated inside under the primitive surface of the Delta; posteriorly, the transgressive movement stabilized with a slow ascent of the sea level, from 5,600 to 6,800 years ago, age from which a regressive behavior of the coast occurred with the formation of littoral ridges, phenomenon that has persisted since the last thousands of years (^{Curray et al., 1969}).

Figure 1 Physiographic location of the area under study. 

The mountainous region of the study area corresponds to Nayarit Volcanic Belts sub-province or geomorphological environments according to ^{Zinck (2012)} are constituted by basalts and andesite rocks, with dominant vegetation of tropical semi-deciduous forest and fruit trees plantations; while the coastal part of Rio Grande de Santiago Delta is constituted by three topoforms: 1) Delta plain (alluvial) of Santiago river with the use of the agriculture soil, 2) saline coast with lakes, mangrove vegetation and aquaculture-fishery use and 3) system of coastal ridges with sandy soils, halophyte vegetation and cultivations (^{INEGI, 2013}).

Geomorphological landscapes

The interpretation of geomorphological units was realized from the tridimensional model of land from aerial photographs, supported by digital orthophotos, digital model of elevation (DME) (^{Priego et al., 2010}), geological, edaphological, vegetation and soil use thematic maps (^{INEGI, 2013}). Maps were digitalized using the Geographical Information System ArcGIS 10.3. With them, the hierarchized geomorphological caption was designed, using the physiographic model previously defined by ^{INEGI (2013)} as a frameshift, the geomorphological environment corresponding to areas dominated by tectonic blocks of sedimentary rocks without marked geoforms of other environments; Great geomorphological landscape based on the physical description of its geology, macro relief and climate as proposed by ^{Robertson et al. (2013)}; finally the geomorphological landscape that adds coverage and soil use to these characteristics (^{González et al., 2009}). The criterion of separation of geomorphoedaphological units proposed by ^{Zinck (2012)} and applied in Mexico by ^{Bautista et al. (2015)} and ^{Zavala et al. (2016)} was used.

Geomorphoedaphological landscapes

To characterize geomorphoedaphological units, 43 sites were described through soil profiles (^{FAO, 2009}) and were added to two already described by INEGI, for a total of 45 detailed descriptions: a profile for each 18.88 km². There are units with one to five soil profiles described according to their size, heterogeneity and availability (^{Elbersen et al., 1986}; ^{SEMARNAT, 2002}; ^{Soil Survey Staff, 1993}). Samples were analyzed in laboratory; mechanical composition, apparent density, pH, electrical conductivity, organic matter, interchangeable cations and cationic interchange capacity (^{SEMARNAT, 2002}). With field and laboratory data, the described profiles were classified, the World Reference Base (WRB) International Standard for Soil Classification System (^{IUSS, Working Group, WRB, 2015}) and integrated to the geomorphoedaphological map and caption of the study area.

Soil distribution pattern

Soil distribution patterns of the study area were analyzed from the clustering of Reference Soil Groups (RSG) and their I primary and II supplementary qualifiers of the International Standard for Soil Classification System (^{IUSS, Working Group, WRB, 2015}) and for types of relief (geostructures, geomorphological environments and geomorphoedaphological landscapes).

Geomorphoedaphological landscapes

In the municipality of San Blas, Nayarit, 16 geomorphological landscapes were identified, distributed into three geomorphological environments and two geostructures (Table 1).

The municipality of San Blas, Nayarit, presented two types of climate (AW₁ and AW₂), both warm-subhumid, dark rocks of basaltic igneous were found in the Neovolcanic Axis; in the Coastal and Delta Plain accumulations of sediments mainly sands and limos which have been depositing over time, these delta deposits have harrowed the ancient coastal line until making the current external continental platform (^{Curray & Moore, 1964}), the distribution of these properties, of present vegetation and RSG was presented in Table 2.

For each one of the geomorphoedaphological landscapes, the location and identification of the soil profiles described in the study (Figure 2) were described below. Primary and supplementary qualifiers were shown in Table 3 for the edaphological identification of landscapes.

Figure 2 Geomorfoedaphological landscapes and soil distribution regularities in the municipality of San Blas, Nayarit. 

Soils of the geomorphological environment of the Pacific Coastal Plain

This ecosystem was located in the extreme south of the Pacific Coastal Plain, it presented three great landscapes, Delta Plain made by sediments from the Santiago River, Littoral Ridges Plain originated during sea regressive step and, Saline Plain with Coastal Lakes, influenced by seawater intrusion into the continent.

Great landscape: Littoral ridges Plain. Two geomorphoedaphological landscapes were identified in this ecosystem.

A- Beach and coastal dunes. Found at the limits between sea and continent, these were mounds of sandy sediments accumulation transported by the sea and winds, with a 10-20 % slope, presented a development of Arenosols Sodic (id of the profiles: SBN13 and SBN14), with pH from neutral to highly alkaline and very low OM content.

B- Littoral cords or beach ridges. Were elevated residual materials deposited in the form of beach ridges, which were dragged and deposited by the sea and Santiago river; in this landscape, soils of the Arenosols Eutric type were found (id of the profiles: SBN15 and SBN16), slightly saline, with pH from neutral to moderately acid and very low to medium OM content.

Great landscape: Saline coastal plain with coastal lakes. This ecosystem presented a direct intrusion of seawater by means of tidal movement through tidal mouths and channels; as well, seasonality of rainfalls provoked a mixture and increase in inundation zones that introduced salinity in these, which were generally the lowest parts of the land. Three geomorphological landscapes were identified.

C- Ordinary tidal inundation plain with coastal estuary and lakes. In this landscape, the saline intrusion occurred toward the continent through tidal mouths and channels or estuaries. Daily tidal inundation reached a part of the alluvial plain and littoral ridges of lower altitude, in some sectors, made lakes with this regime of daily inundations and development of mangrove vegetation. In this landscape, Solonchaks Subacuatic, Sodic, Fluvic, Arenic, Humic were identified (profiles SBN11 and SBN20).

D- Extraordinary tidal inundation plain. Corresponded to a portion of the plain with inundation of high tides during the period from April to June, coinciding with the period of higher heat and evaporation. In this landscape, salts come to the surface and were shown as whitish spots on the surface of the soil. Solonchaks Thidalic, Fluvic, Chromic and Arenic profiles were identified (profile SBN03).

E- Seasonal tidal inundation plain. Once the rain season in the region established, from June to October, precipitation and river runoffs contributed to tides, by inundating a wider sector of the plain with brackish water. In this landscape, two Reference Soil Groups were identified: Arenosols Sodic (SBN04) and Fluvisols Subaquatic, Sodic, Arenic (profiles SBN12 and SBN17).

Great landscape: Delta plain of Santiago river. Plain integrated by alluvium of Santiago river which originates in Ocotlán in Chapala lake, as well as by small streams which drained directly to the sea (El Palillo and El Solito). It has to be pointed out that Santiago river currently presents a side network of protection for avenues in the lower part of the basin and a system of great dams in waterfall in the higher part (El Aguamilpa, El Cajón, La Yesca and Santa Rosa), that restrain the major part of the sediments. Four geomorphological landscapes were identified in this region.

F- Low plain with fluvial-marine influence. It made a large extension of flatland with sectors where the saline wedge of marshland or saline phreatic water influenced landscape. In this geoform, there were Fluvisols with salinity and without salinity and Cambisols. Four Fluvisols were identified, two of them Sodic, Cromic, Ocric (profiles SBN22 and SBN23), moderately saline to saline, and no saline in the others two. The pH was mainly moderately alkaline and OM content was medium in the higher part and low in the lower part of the profiles. Cambisols Eutric, Cromic and Arenic (profile SBN02) were registered in turn with a moderately acid pH, without presence of salts and low OM content.

G- Medium alluvial plain of overflow. These were plains of inundation in possibly covered by riverbeds; diverse Reference Soil Groups were found, such as Fluvisols, Cambisols and Phaeozems. Fluvisols Endoskeletic, Eutric, Arenic were neutral, without salts and medium OM content, presented small rock fragments in their inside (profile SBN08), Fluvisols Sodic, Ochric (profile SBN01) without salts with a pH from neutral to moderately alkaline and low OM content. A Cambisols Eutric was registered as well with characteristics without salts and a moderately acid pH (profile 21641) and a Phaeozems Luvic Siltic in zones which still conserved natural vegetation of tropical forest.

H- High alluvial plain. They constituted the most ancient and the highest part of the sediments deposited in Santiago river Delta, with more time for clayish mineral formation. In this landscape, a Luvisols Háplic, Siltic (profile SBN27) soil was classified, without salts, moderately acid and medium OM content.

I- Flooding riverbed terraces. These were terraces of fresh alluvium located close to the current flooding riverbed. Terraces were made by sediments when the flow of Santiago river were not regulated yet. Reference Soil Groups found in this landscape were Fluvisols Sodic, Siltic (profiles SBN24, 25 and 26), they were deep soil with frank texture, without salts, with sodium accumulation, pH oscillating in ranges between moderately acid to moderately alkaline as established by the Official Mexican Standard NOM-021, with medium OM accumulation in the higher part of the profile and low OM content in the rest of the horizons.

Soils of the geomorphological environment of Nayarit Neovolcanic Belts

This ecosystem of Nayarit volcanic belts makes part of the northwestern extreme of the Neovolcanic Axis of Mexico, in the municipality of San Blas, Nayarit, and presented a great landscape, mountains with steep hillsides.

Great landscape: Volcanic mountains with steep hillsides. It presented mountainous landscapes with soils derived from basaltic rocks, mostly black and reddish, composed by young soils in primary process of formation of very thin soils with scarce clay (sialitization process) even very developed soils in formation of clay (Acrisoles). Seven geomorphological landscapes were identified, in this group, clay formation in soils was higher than in the rest of the land, it presented high to very high OM contents, without salts and with acid pH due to the high alteration of parent rock and its low saturation of bases.

J- Structures with isolated elevations. In this landscape, Phaeozems Luvic (profile SBN30) was distributed, presenting soils with frank texture, without presence of salts, moderately acid, with higher clay content in horizon B relative to A; and with very high OM content, dark and well defined granular structure. The formation of this soil occurred under the influence of OM contribution in conditions of medium sub-deciduous forests, with Phaeozems Luvic soil unit, indicating a certain instability in soil formation and an horizon B Argic is starting to set up, corresponding to Luvic qualifier.

K- Conglomerate and lomerios surfaces. In this landscape, fragments of rocks of the conglomerates took part in soil formation and in addition, it is a type of relief with low stability in time to make evolved soils. Regosols, Cambisols and Phaeozems were identified. SBN09 profile was classified as Regosols Skeletic, Colluvic, Eutric, Arenic, very slightly saline, with moderately acid soils and with medium category in OM accumulation; SBN10 profile, as Phaeozems Luvic Chromic, without salts, neutral pH and with high OM content; finally 21663 profile as Cambisols Eutric.

L- Intra-mountainous valley in basic rocks landscape. It is a depression that made a plain inside a mountainous landscape, through the deposit of sediments aggregated by gravity. In this geoform, Cambisols Rhodic, Eutric, Clayic, Humic, Colluvic (profile SBN30) were identified, without salts, moderately acid, poor in saturation of bases and medium OM content. In this landscape, with presence of some intra-mountainous valleys, made of basic rocks, a certain stability in soil formation is expected. Therefore Cambisols Rhodic were present, which are soils that in some classifications were named as fersialitic, since there is accumulation of free iron by a more advanced weathering, leading to the formation of a red to brown-reddish horizon B (^{Hernández et al., 2015}). As these are low reliefs, there are transport materials in the profile (colluvios) and OM was accumulated in pasture conditions.

M- Low mountains of basic rocks. It was constituted by mounds with smooth slop of volcanic origin. Main RSG is Luvisols Chromic, which is found in small hills (mounds), above all in the higher and more stable part of the relief. It is representative of this geoform because in current climatic conditions, the weathering leads to the formation of a profile type ABC, but with clay leachates (Argic horizon), brown-reddish for the quantity of free iron released from its formation on rocks with basic composition. Three soil profiles were reported: SBN32 profile as Luvisols Haplic, Arenic; Aticama1 profile (Luvisols Rhodic) and Aticama2 profile (Luvisols Chromic, Siltic), classified as Luvisols, presented a frank texture, without salts, with moderately acid pH and high to medium OM content in the first horizons. Moreover, Las Palmas profile was classified as Luvisols Chromic, soils with a texture a little more clayish were found, without presence of salts, moderately acid and with very high to medium OM content. As well, other soils were found at minor scale as Cambisols, Phaeozems and Leptosols. For instance, SBN18 profile was classified as Cambisols Fluvic, Distric, Arenic, without presence of salts, moderately acid, poor in sum of changeable bases and medium OM content.

N- Basalt lava flows of different height. These are lava flows of dark igneous volcanic rocks, rich in iron and magnesium. In this landscape, soils of Acrisols, Phaezems, Luvisols, Regosols and Cambisols types were found. However, in this landscape, the main produced soils were Luvisols Chromic. Jalcocotán profile was classified as Luvisols Chromic, frank clayish, humic without salts, with moderately acid pH and a high OM accumulation; these soils were deep with a high number of horizons. Another profile described in Jolotemba was classified as Luvisols Gleyic, Chromic, Clayic, Humic, Colluvic, without presence of salts, with a dark color and high OM accumulation. As well, the profile of INEGI 21671 was analyzed, classified as Luvisols Chromic, Clayic (that results of fersialtic composition according to ^{Hernández et al. (2006}, ²⁰¹⁵⁾ that were cultivated with mango plantation (Mangifera indica). In addition, in stable reliefs where soil had a higher evolution, Luvisols evolved to Acrisols, these occurred when soils have obligatory a horizon Bt (Argic) of alluvial clay accumulation, saturated in bases in Bt, with illite-type clay, smectite and vermiculite, but as time passed they get washed and became desaturated in bases in horizon Bt, turning into kaolinite-type clays. For example, there is SBN06 profile, classified as RSG Acrisol; Unit of Acrisols Chromic, Siltic, frank without presence of salts, highly acid; it is a soil poor in sum of bases and presenting medium OM content. As well, Phaeozems Luvic (profile 21668) were present, where natural vegetation was still maintained.

O- Basalt lava flow with cineritic cones. Flows normally occurred from openings located throughout fractures developed on the sides of cineritic cones, being the accumulation of pyroclasts (solid fragments) expulsed from volcano conduit around the crater; predominant soils were Phaeozems and Cambisols, with fragments of volcanic rocks (skeletal). SBN19 and 78049 profiles were classified as Phaeozems Skeletic, Colluvic, both without presence of salts, with neutral pH, poor in bases and with high OM content. SBN21 profile was a Cambisols Skeletic, Eutric, Colluvic, and did not present salinity in all the profile, from neutral to moderately acid, rich in bases in the first horizons and poor in Deep, with low to medium OM content.

P- Volcanic structure of basic rocks. These are structures of volcanic origin, made by basalt rocks, in this landscape, Phaeozems Skeletic were identified represented by SBN07 profile, they did not present salts in their profile, neutral pH, poor in changeable bases, with medium OM accumulation. SBN34 and SBN35 profiles registered in stable forms of the relief were classified as Acrisols Rhodic, Humic, and SBNIB01 profile was Acrisols Abruptic, Chromic, Humic, Clayic; these are highly acid soils with less than 50 % of saturation of bases, horizon B Argic and less than 16 Cmol per 100 g in clay, and with a high OM content in conditions of stable vegetation without cultivation. It has to be pointed out that in this landscape, stable conditions of relief were created, very acid Acrisols soils, with horizon B Argic and degree of saturation by bases less than 50 %. Similarly to other parts of mountainous regions with warm climate, these soils were created in stable parts of the relief, above all on basic and intermediary rocks (mainly basalt). In these conditions, soil formation is automorphic, generally in washing conditions, therefore no salts were present in none of the created soils.

Soil distribution patterns

Eight Reference Soil Groups were identified, which were clustered into three soil distribution patterns for their type and origin of relief and for their qualifiers. The first one was constituted by soils affected in their formation by seawater intrusion into the continent (geomorphological landscapes A, B, C, D and E); the second one by soils derived from fluvial affluence of Rio Grande de Santiago (F, G, H and I) and; the third one by soils formed in mountainous reliefs with automorphic formation from basaltic rock (J, M, N, O, Q and P) (Figure 2 and Table 3).

RSG of the first cluster were Arenosols, Solonchaks and Fluvisols with high contents of sand, sodium and subaquatic. They were related to sea transgressive phase on the alluvial plain of Santiago river during Holocene; as well, to the coastal regressive step with formation of littoral ridges and lagoon-estuary system (^{Curray et al., 1969}). Arenosols and Solonchaks soils in the study area coincided with those reported by ^{Bautista et al. (2015)} and ^{Zavala et al. (2016)} in littoral and marsh geomorphological environments, excepted Histosols (^{Zavala et al., 2016}), Regosols and Leptosols (^{Bautista et al., 2015}). In this region, the principal determinants that defined these landscapes were texture, salinity, inundation, slop, relief and groundwater tables, coinciding with ^{Zavala et al. (2016)} where fluvial inactivity and Surface water mantle were annexed.

The second pattern of distribution was constituted by Fluvisols, Cambisols, Luvisols and Phaeozems with qualifiers Siltic, Sodic and Ochric. They corresponded to the different levels of the alluvial plain of Santiago river formed during the late Pleistocene; Luvisols were developed in the highest and most ancient part of the plain, Cambisols in the medium part of the plain and Fluvisols in the lowest part of the plain and on the river edges. The formation of Phaeozems was maintained in the sectors where natural vegetation of tropical sub-deciduous forest was conserved. In this region, there were no coincidences with those reported in the homologue environment Pseudopalustre of ^{Bautista et al. (2015)} in the state of Yucatán, since RSG they established were Vertisols, Leptosols and Stagnosols; while the Karstic environment coincided in the presence of soils of type Cambisols and Luvisols; in fluvial plains with ^{Zavala et al. (2016)} where Fluvisols and Cambisols were described.

The third distribution pattern of RSG was Regosols, Cambisols, Luvisols, Acrisols and Phaeozems with Cromic, Skeletic, Eutric, Luvic and Rhodic, Clayic, Colluvic and Humic qualifiers. Similarly to RSG presented by ^{Zavala et al. (2016)} where Alisols, Acrisols and Luvisols prevailed; on the other hand, it coincided with ^{Bautista et al. (2015)} in the description of these geoforms (Tecto-Karstic) of lomerios with different height, slop and volcanic origin, soils described by these authors were Leptosols, Vertisols, Luvisols and Cambisols; it was in agreement with the fact that they were the most ancient soils with the highest clay accumulation, humidified and with volcanic origin. Topography of Plio-Quaternary (^{Demant et al., 1976}) and Eocene (^{Bautista et al., 2015}) volcanic buildings defined soils distribution from minor to major development of Argic horizon, ^{Zavala et al. (2016)} added Cutanic and Clayic qualifiers. In other cases, the permanence of natural vegetation conserved in the soil a highly humidified horizon with presence of Phaeozems. In the municipality of San Blas, dominant soils in the mountainous region were Luvisols and Cambisols, similarly to those described in Tabasco (^{Zavala et al., 2016}).

The obtained results set the basis for demonstrating that soil formation in a determined region was due to the interaction of factors that take part in its formation, showing different forms in its distribution, which is the principal essence of Soil Geography, according to ^{Dobrovolsky (2006)}. According to ^{Puchulu & Fernández (2014)}, soils presented a high spatial variability as a consequence of the interaction of factors that conditioned the place and it was reflected in its development and composition; which in turn is expressed as a distribution pattern. ^{Jáuregui et al. (2018)} agreed with these criteria as well, who mentioned that the soil is a natural entity presenting a structure and characteristics continuously varying in space and time, giving a high diversity of soils as a result.

In addition, it has to be considered that several authors demonstrated the importance of soil analysis by means of geomorphological landscapes in the agricultural use, which were translated to soil limiting factors (^{Sánchez et al., 2013}, ^{Solís et al., 2014}; ^{Zavala et al., 2016}). These analyses showed the potential for different uses, such as agroforestry, aquaculture, conservation and urbanistic; which in turn contribute to the expression of factors restricting the capacity of general use of landscapes (^{Zavala et al., 2016}).

Geomorphological landscapes described in this study for the municipality of San Blas were similar to the landscapes reported by ^{Bojórquez & López (1997)} in the municipality of Tuxpan, Nayarit; and to landscapes established by ^{Zavala et al. (2016)}, excepted karstic terraces in the state of Tabasco; to the landscapes established in the northern plain of the state of Nayarit by ^{Bojórquez et al. (2006)} and ^{González et al. (2009)}; to the physiographic provinces in the study performed by ^{Puchulu & Fernández (2014)}, inside of the province of Tucumán, Argentina, however, in this last study, soils were developed in unalike conditions, making soil classification completely different and presenting no coincidence among them. ^{Murray et al. (2012)}, in their area of analysis, presented a general division of landscapes, but the same decision criteria were taken as in the present study for geomorphological environments, 1- Saline coastal plain with coastal lakes and 2- Delta coastal plain. Many of the present results confirmed and reinforced what was reported by ^{Bojórquez et al. (2006}, ²⁰⁰⁷ and ²⁰⁰⁸⁾, since it represents the base for delimiting landscapes which were integrated in the great landscapes previously mentioned.