Danilo López-Hernández

Rev. Fac. Agron. (LUZ). 2022, 39(1): e223916

ISSN 2477-9407

DOI: https://doi.org/10.47280/RevFacAgron(LUZ).v39.n1.16

Crop Production

Associate editor: Dra. Ana Gonzalez

Keywords:

Bray I

Acid soils

P adsorption

Solubilisation

Dissolution of Monte Fresco phosphate rock and their effects on phosphorus fractionation in

Venezuelan soils

Disolución de la roca fosfórica Monte Fresco y sus efectos sobre las fracciones de fósforo en suelos

venezolanos

Dissolução da rocha fosfórica de Monte Fresco e seus efeitos sobre frações de fosforo em solos

venezuelanos

Universidad Central de Venezuela, Instituto de Zoología

y Ecología Tropical, Centro de Ecología Aplicada, Apdo.

47058, Caracas 1041A, Venezuela.

Received: 08-03-2021

Accepted: 08-11-2021

Published: 15-02-2022

Abstract

Incubation tests analysed the reaction of Monte Fresco phosphoric

rock (PR) with nine Venezuelan soils representative of different agro-

ecological conditions, and contrasting physical-chemical and mineralogical

characteristics linked to the PR dissolution process. The soils presented

different capacities to dissolve the PR; response, generally associated with its

intrinsic characteristics. The highest PR dissolution value (ΔP) was found in

the soil Iguana (50 mg P.kg

-1

soil), soil with appropriate properties to induce

this process: acidic pH and low content of total and available P, followed with

intermediate values (7-22 mg P.kg

-1

soil) for Barinas and Casupal with acidic

pH and moderate content of total P. The other soils had low dissolution rates

(1.4-3.0 mg P.kg

-1

soil) and higher P content. Finally, Veguitas and Bajo Seco

soils with pH ≥5.6, and high total and available P contents and exchangeable

calcium, showed no PR dissolution. The process of dissolving the PR during

incubation is complex, it is activated with the presence of hydrogen ions

around the fertiliser but can be affected by enzymatic and microbiological

events as PR interacts with the soil, so that the dynamics of PR dissolution

uctuate.

This scientic publication in digital format is a continuation of the Printed Review: Legal Deposit pp 196802ZU42, ISSN 0378-7818.

Rev. Fac. Agron. (LUZ). 2022, 39(1): e223916. January - March. ISSN 2477-9407.2-6 |

Resumen

Mediante ensayos de incubación se analizó la reacción de la

roca fosfórica (RF) Monte Fresco con nueve suelos venezolanos

representativos de diferentes condiciones agroecológicas y con

contrastantes características sicoquímicas y mineralógicas ligadas

al proceso de disolución de la RF. Los suelos presentaron diferentes

capacidades para disolver la RF; respuesta, en general, asociada a sus

características intrínsecas. El mayor valor de disolución de la RF (ΔP)

se encontró en el suelo Iguana (50 mg P.kg

-1

suelo), caracterizado por

tener pH ácido y bajo contenido de fósforo total (Pt) y aprovechable.

Siguieron, con valores intermedios (7-22 mg P.kg

-1

suelo) los suelos

Barinas y Casupal con pH ácidos y contenidos moderados de P-total.

Otro grupo de suelos, con mayor contenido de P presentaron bajos

índices de disolución (1,4-3,0 mg P.kg

-1

suelo); mientras que los

suelos Veguitas y Bajo Seco con pH ≥5,6 y altos contenidos de P

total, disponible y calcio intercambiable, no mostraron disolución de

la RF. El proceso de disolución de la RF durante la incubación es

complejo, se activó con la presencia de los hidrogeniones alrededor

del fertilizante, pero puede ser afectado por eventos enzimáticos

y microbiológicos a medida que la RF interacciona con el suelo,

de manera, que la dinámica de la disolución de la RF presenta

uctuaciones.

Palabras claves: Bray I, suelos ácidos, adsorción de P, solubilización

Resumo

Os testes de incubação analisaram a reação da rocha fosfórica

(RF) de Monte Fresco com nove solos venezuelanos representativos

de diferentes condições agroecológicas, e de características físico-

químicas e mineralógicas ligadas ao processo de dissolução de

RF contrastando. Os solos apresentaram diferentes capacidades

para dissolver o RF; resposta, geralmente associada com suas

características intrínsecas. O maior valor de dissolução de RF (ΔP) foi

encontrado no solo Iguana (50 mg P.kg

-1

solo), solo com propriedades

adequadas para induzir esse processo: pH ácido e baixo teor de P

total e utilizável, seguidos de valores intermediários (7-22 mg P.kg

solo) dos solos Barinas e Casupal de pH ácido e teor moderado de

P total. Os demais solos apresentaram baixas taxas de dissolução

(1,4-3,0 mgP.kg

-1

solo) e maior teor P. Por m, os solos Veguitas e

Bajo Seco com pH ≥5,6, e alto teor total e disponível de P e cálcio

intercambiável, não apresentaram dissolução de RF. O processo de

dissolução da RF durante a incubação é complexo, ativado com a

presença de hidrogenias ao redor do fertilizante, mas pode ser afetado

por eventos enzimáticos e microbiológicos à medida que a RF interage

com o solo, de modo que a dinâmica da dissolução de RF utua.

Palavras-chave: Bray I, solos ácidos, adsorção de P, solubilização

Introduction

In highly weathered environments, phosphorus (P) appears as a

limiting element for plant and animal production because it is xed

by amorphous iron and aluminium oxides and hydroxides that abound

in the soil prole (López-Hernández and Burnham, 1974; Brenner et

al., 2019). To counteract these low levels of available P, it is necessary

to use appropriate doses of phosphorous fertilisers that are supplied,

either as soluble sources, of high cost, due to their pretreatment,

or as insoluble phosphate rocks (PR), of lower value, mainly only

applicable in the case of acid soils (Rajan et al., 1996; Cicek et al.,

2020).

The reactivity of PRs increases with soil acidity (hydrogen ion

concentration around the PR granule), generally associated with

relatively high levels of exchangeable Al (high Al saturation); so that,

in soils with a pH greater than 5.6, the PR practically do not supply

available P to the crops. On the contrary, the low saturation of calcium

and phosphates in solution, characteristic of acid soils, as well as a

high content of organic matter in the soil, favour the solubilisation

of PRs (Rajan et al., 1996). On the other hand, a high phosphate

retention capacity in the soil can also facilitate PR solubilisation,

although this P, once released from the rock, can be quickly retained

by the solid adsorbent matrix and do not enter in solution (López-

Hernández, 1977; Rajan et al., 1996; Romero and López-Hernández,

2018). Regarding total P levels, soils with medium phosphate levels

are considered more suitable for the application of PR than soils

extremely decient in phosphates (Rajan et al., 1996; Romero and

López-Hernández, 2018).

PRs have been directly applied in many previous trials in different

soils and for different crops in Venezuela (Sequera and Ramírez,

2013). None-the-less, that the agronomic and economic effectiveness

of Venezuelan phosphate rocks has been well studied (López de R.

et al., 1994), there have been little research on the phosphate rock-

soil interaction, an aspect of agronomic interest that would allow

determining: in which soils, the application of phosphate rocks

would be more efcient. An evaluation as this kind revise particular

importance in the case of Venezuelan PR Monte Fresco (PRMF), due

to its proven reserves, and its potential to be used, under adequate

treatments, as source of soluble phosphorous fertiliser (Casanova,

2007).

The objective of this work is to study the reaction (dissolution) of

the Monte Fresco phosphate rock in nine Venezuelan soils that present

contrasting physicochemical and mineralogical characteristics, the

chosen soils are representative, at the national level, of areas with

different agroecological conditions (table 1). The experimentation

includes incubation experiments where the reaction of the PR with

the chosen soils is analysed.

Materials and methods

Nine contrasting soils in physicochemical characteristics linked to

the phosphate rock dissolution process: pH, exchangeable aluminium

and calcium, total and available phosphorus, organic carbon, P

adsorption capacity and cation exchange capacity (CEC), were used.

The soil samples come from uncultivated soils located in different

areas of the country (table 1), and correspond to the surface horizon (0-

15 cm) of composite samples formed from subsamples. For chemical

determinations, the samples were air-dried and then sieved to obtain

the fraction of soil less than 2 mm. The routine methodologies for soil

characterisation correspond to those used at INIA, Maracay (Romero

and López-Hernández, 2018; López-Hernández and Romero, 2019).

In the incubation experiments, the Monte Fresco phosphate rock,

located in the state of Táchira, Venezuela, was used. MFPR has a total

P content of 9.3% and a solubility in citric acid of 0.71%, this PR has

an apatite content of 64% (Pérez and Smyth, 2005).

This scientic publication in digital format is a continuation of the Printed Review: Legal Deposit pp 196802ZU42, ISSN 0378-7818.

López-Hernández. Rev. Fac. Agron. (LUZ). 2022, 39(1): e2239163-6 |

Table 1. Geographical location and classication of the soils

studied.

Soils Location Classication

Bramón

Estación Experimental

Bramón, Táchira

Typic Tropudult

Mantecal Mantecal, Apure Fluventic Ustropepts

Barinas Pie de Monte, Barinas Oxic Paleustalfs

Casupal Norestes llanos de Monagas Oxic Paleustults

Palmeras Guárico Typic Paleustults

Iguana II Santa María Ipire, Guárico Plinthic Paleustults

Iguana

Estación Experimental

La Iguana, Guárico

Ustoxic Quartzipsamment

Veguitas

Guanare-Masparro,

Portuguesa

Typic Ustropepts

Bajo Seco

Estación Experimental

Bajo Seco, Miranda

Typic Humitropepts

Source: Author

Determination of the phosphate adsorption index (PAI)

The PAI was determined using the Bache and Williams method,

which consists of determining a point of the adsorption isotherm

(López-Hernández, 2016). This adsorption point was obtained by

stirring 1 g of soil with a solution of 0.0025M KH

and 0.02M

KCl for 18 h in a 1:20 ratio. At the end of the stirring period, the

suspension was ltered or centrifuged, and the P content in the

supernatant solution was analysed using the Murphy and Riley

photocolorimetric method. Adsorption values (x) were expressed in

mg P/100 g soil, and the nal concentration of P in solution (C) in

µmol P.L

-1

. The Bache and Williams index was calculated as x/log C.

The mineralogical analysis was performed using the X-ray

diffraction technique on plates prepared with the clay fraction

(Reynolds and Moore, 1989).

Incubation of soils with PR

Portions of approximately 500 g for each of the soils studied

were fertilised with doses equivalent to 300 mg P.kg

-1

of PRMF.

Subsequently, 50 g of each fertilised soil were placed in seven glass

jars and incubated at 100 % available humidity and at an approximate

temperature of 25 °C. The incubation periods correspond to 1, 3, 7,

15, 30, 60 and 100 days. Control samples (without phosphate rock)

were simultaneously incubated under the same conditions.

Extraction and determination of available P after incubation

At the end of the incubation period, available P extractions

were carried out in triplicate (4 g of soil) using the Bray I method

according to Romero and López-Hernández (2018). The P in solution

was determined by the photocolorimetric method of Murphy and

Riley reviewed by López-Hernández (2016). The difference, between

the level of P extracted from the treated soil and the P extracted in

the control, corresponds to the dissolved P (ΔP) of the PR. The P

that exceeds the control in soils fertilised with PR comes from the

dissolution of the rock, thus, ΔP can be considered as an indirect

estimator of dissolution and allows inferences to be made about the

degree of reaction of the PR (Rajan et al., 1996).

Changes in available phosphorus fractions as a function of

incubation time

Triplicate samples of two of the chosen soils (Iguana and Bramón)

were fertilised with doses of 300 mg P.kg

-1

of PR Monte Fresco

and incubated in glass jars at 100% available humidity and at an

approximate temperature of 25 °C for 3, 15 and 30 days. At the end of

each incubation period, the soils underwent a partial P fractionation

according to a modication of the Hedley method (López-Contreras

et al. 2007). The extracted phosphorus in the different fractions was

measured by the classic method of Murphy and Riley.

Statistical analysis

The Statistical Analysis System (SANEST) was used,

corresponding to nine independent samples (soils) where seven

treatments (times) are compared. Analysis of variance and comparison

of means were performed by Duncan’s bilateral test with a probability

value of P≤0.05. The relationship between PR dissolution values and

other soil properties was established using Pearson’s correlation

coefcient.

Results and discussion

As can be seen in tables 2 and 3, the soils presented very different

physicochemical characteristics. In general, medium textures

predominate, although there are three soils (Casupal, Iguana and

Iguana II) with sandy textures (table 2). The Bramón and Mantecal

soils presented the lowest pH (4.1 and 4.2, respectively); another

group had a pH of 4.6-5.5, while the samples from Veguitas and Bajo

Seco had a pH of 5.8 and 6.1, respectively. The organic carbon (CO)

content was highly variable (0.30-6.54%): the Iguana and Casupal

soils presented very low CO values (0.30-0.33%), with low values

were Iguana II (0.75%), Palmeras (0.78 %) and Mantecal (1.04%),

while Barinas, Veguitas and Bramón presented an intermediate

CO content (1.27-1.85%); a high content of CO (6.54%) was only

presented by Bajo Seco (table 2).

Veguitas, Bajo Seco and Bramón soils are characterised by high

levels of total and available P (Bray I), while Iguana, Iguana II and

Casupal presented low values of total and available P, the rest of the

soils maintain intermediate values (table 2). The highest P adsorption

indices were recorded in Bramón, Mantecal, Bajo Seco and Palmeras,

the rest presented low values (table 2).

Table 2. Main chemical characteristics of the soils analysed.

Soils pH %OC

P total

(mg.kg

-1

)

P Bray

(mg.kg

-1

)

Adsorption

Index

(x/ log C)*

Texture

Bramón 4.1 1.85 468 15.2 23.6 FA

Mantecal 4.2 1.04 328 4.6 13.6 F

Barinas 4.6 1.27 266 4.5 8.0 FAa

Casupal 4.7 0.33 211 5.7 6.6 aF

Palmeras 4.7 0.78 275 2.5 11.8 F

Iguana II 5.3 0.75 190 1.5 4.6 Fa

Iguana 5.5 0.30 163 1.8 5.7 a

Veguitas 5.8 1.34 955 23.7 5.7 F

Bajo

Seco

6.1 6.54 601 31.8 10.6 FAa

*x= mg.P.100g

-1

soil; C=µmol.P.L

-1

. Source: Author

The CEC were high in Bajo Seco and Veguitas soils, very low

in Iguana and Casupal, the rest of the soils analysed presented

intermediate values (table 3). Bajo Seco and Veguitas registered high

values of exchangeable Ca and Mg, as did the Bramón soil (despite

its low pH), the rest of the soils presented low levels of exchangeable

Ca, as expected, since they are well weathered soils (tables 1 and 3).

Bramón, Mantecal and Barinas presented the highest contents of

exchangeable Al and total acidity, while Casupal, Palmera and Iguana

II maintained intermediate values, and the lowest values of these

chemical parameters were found in Iguana, Veguitas and Bajo Seco,

this information is in agreement with the highest pH values associated

with these soils (table 2).

This scientic publication in digital format is a continuation of the Printed Review: Legal Deposit pp 196802ZU42, ISSN 0378-7818.

Rev. Fac. Agron. (LUZ). 2022, 39(1): e223916. January - March. ISSN 2477-9407.4-6 |

Table 3. Exchangeable bases, total acidity and cation exchange

capacity (CEC) of the soils analysed.

Soils Ca

CEC

Total

acidity

cmol.kg

-1

Bramón 2.50 1.22 0.33 0.23 0.9 0.8 6.6 1.7

Mantecal 1.50 1.48 0.65 0.85 0.4 1.1 6.0 1.5

Barinas 0.50 0.48 0.06 1.00 0.5 1.1 5.0 1.6

Casupal 0.75 0.48 0.05 0.69 0.2 0.3 2.4 0.5

Palmeras 0.75 1.72 0.39 0.54 0.3 0.7 3.9 1.0

Iguana II 1.25 1.48 0.46 0.69 0.3 0.5 3.8 0.8

Iguana 0.25 0.22 0.19 0.23 0.1 0.1 1.2 0.2

Veguitas 4.25 2.98 0.19 4.08 0.1 0.1 8.9 0.2

Bajo

Seco

10.50 3.48 0.11 5.77 0.1 0.2 19.8 0.3

Source: Author

The mineralogical composition of the soils (table 4) evidenced

the dominance of kaolinite as the main phyllosilicate, except for

Bramón, where vermiculite predominates, and Veguitas and Bajo

Seco, where micas dominate.

PRMF dissolution values and their relationship with soil

properties

The PR dissolution values (ΔP) at 100 days of incubation

corresponded to three categories (table 5). A group with the highest

PR dissolution capacity (DC) represented by Iguana, Casupal and

Barinas; the highest DC value was found in Iguana (50.0 mgP.kg

-1

a soil that showed appropriate properties to induce this process:

acidic pH and low content of total and available P. Barinas and

Casupal with lower DC than Iguana (7 and 22 mg.kg

-1

, respectively)

presented also acidic pH, but intermediate contents of total and

available P. Another group of soils (Palmeras, Bramón, Mantecal

and Iguana II) with low DC indices (1.4-3.0 mg.kg

-1

) and different

physicochemical properties, occupied a second category, while

soils with pH higher than 5.6 (Veguitas and Bajo Seco) were located

in a third DC category, represented by negative ΔP values, which

indicated that in that incubation period, the Bray I reagent extracted

a higher proportion of available P from the control soil. The high

proportion of P extracted from the control soil is in agreement with

the high levels of P in these soils (table 2).

However, It is important to highlight the low dissolution

values of Bramón, Iguana II and Mantecal soils, since they are

soils characterised by having acid pHs and signicant levels of

total acidity (0.8-1.7 cmol.kg

-1

, table 3), therefore, with potential

to dissolve PR. Any potential dissolution of the rock would be

counteracted in Bramón and Mantecal by the high levels of

total and available P. However, the Iguana II soil had a marginal

PR dissolution, which is surprising, since its physicochemical

characteristics are very similar to those of Iguana, the soil that

recorded the highest ΔP value (table 5). The only difference found

between both soils was the presence of halloysite and traces of

vermiculite in the mineralogical component of the Iguana soil

(Table 4), and a slightly higher natural fertility in the Iguana II soil

(table 3). The Veguitas and Bajo Seco soils presented pH above 5.6,

high contents of total P, available P and exchangeable calcium, so

it was not expected, in these soils, greater PR dissolution (Romero

and López-Hernández, 2018; Rajan et al., 1996).

Table 4. Mineralogical composition of the soils studied.

Soils Kaolinite Quartz Mica Vermiculite Feldspars Others

Bramón ++ ++ + ++++ nd G

Mantecal ++++ ++++ ++ ++ 0.25+ nd

Barinas ++++ 0.3 + 0.3+ +

G(+)

Casupal ++++ 0.5H ++++ Tr nd 0.1+ G, Esm.

Palmeras ++++ + + + 0.25+ G,P,Cl

Iguana II ++++ +++ 0.5+ nd nd Esm.

Iguana +++ H ++++ 0.5+ + 0.25+ nd

Veguitas +++ +++ ++++ nd + G,P,Cl

Bajo

Seco

++ 0.5+ +++ nd nd G,Pa,Gi

H= Halloysite; P= Pyrophyllite; Cl= Chlorite; Pa= Paragonite; G= Goethite; Gi= Gibbsite; Esm= Smectite; Tr= Traces; nd= no determined.

Table 5. Values of phosphorus extracted with the Bray I solution (mg P.kg-1) in unfertilized soils (without RF) and treated with PR (with

RF).

Soils Without PR With PR

P Bray with PR- P Bray without PR

(ΔP)*

Iguana 2.5 52.5 50.0

Casupal 8.0 30.0 22.0

Barinas 2.5 9.5 7.0

Palmeras 2.2 4.2 2.0

Bramón 2.0 2.2 2.0

Mantecal 4.5 7.5 3.0

Iguana II 1.6 3.0 1.4

Veguitas 3.8 2.5 -1.3

Bajo Seco 2.0 1.8 -2.0

*The difference between the level of P extracted from the treated soil and the control corresponds to the dissolved P (ΔP) of the PR. Source: Author

This scientic publication in digital format is a continuation of the Printed Review: Legal Deposit pp 196802ZU42, ISSN 0378-7818.

López-Hernández. Rev. Fac. Agron. (LUZ). 2022, 39(1): e2239165-6 |

Previous analysis of the use of PR Monte Fresco carried out by

López et al. (1994) for the Palmeras soil, noted a slight increase

in the P and Ca content of the soil after treatment with PR, and a

good residual effect on the yield of Andropogun gayanus after the

third year of application, which indicated a moderate dissolution of

PR Monte Fresco in this soil. Likewise, Pérez (1995) presented a

comparison of various extraction methods of available P in PRs with

different solubilities using also Palmeras soil. From Pérez´s results

it was inferred that, unlike, what was reported in this study, there

was a greater dissolution of PR Monte Fresco compared to other

soils located in the region, none-the-less, the levels of available Ca

in the Palmeras sample used by Pérez (1995) were lower than the

value reported here.

Dynamics of PR dissolution in soils

In table 6, the average ΔP values were recorded over an

incubation period of 1-100 days. Although the information did not

show a clear behaviour pattern, a tendency was observed for the ΔP

values to increase from 1-15 days of incubation for the Bramón,

Casupal, Palmeras, Iguana and Veguitas soils, which then slightly

decreased from 30-60 days, but with strong increases at 100 d in

Iguana and Casupal. In the case of the Mantecal, Barinas and Iguana

II soils, the averages were quite close along the experimental period,

with few signicant differences during incubation. Finally, the Bajo

Seco and Veguitas soils showed large uctuations in the levels of P

from dissolved from the PR throughout the incubation period, with

some negative values during incubation.

The PR dissolution process is complex and it is activated by

the presence of hydrogen ions around the fertiliser granules, which

contribute to the solubilisation of insoluble apatites (Morillo et

al., 2007). This process, essentially chemical may also involve

enzymatic and microbiological events that occur as PR interacts

with the soil in an appropriately humid environment. Such is the

case of the release of hydrogen ions or organic acids due to the

activity of P-solubilising bacteria (PSB), which could induce

greater PR solubilisation (Mora et al., 2017; 2019; Hunt et al.,

2007). Thus, as the incubation process occurred in the tested soils,

there was a redistribution of P in the different fractions affected

by these chemical and microbial dissolution-immobilisation events.

A more rigorous analysis of the changes in the most available

P fractions was carried out by means of PR fertilisation (300 mgP.

kg-1) of the Iguana and Bramón soils (gures 1 and 2, respectively)

and observing the changes in P-fractions throughout the incubation

period. During incubation, changes in the levels of P-resin,

P-bicarbonate, P-microbial, Pi and Po-NaOH were observed.

Table 6. ΔP values (mgP.kg

-1

) extracted with the Bray I solution

t different incubation times.

In the Iguana soil, the most available fractions of P (P-resin

and P-NaHCO

) had a signicant increase in the rst 15 days of

incubation, while the P-microbial and Pi-NaOH did not undergo

greater modication, on the contrary, Po-NaOH decreased

signicantly in that period (gure 1). For the Bramón soil, in

general, there was a tendency to increase P values throughout the

incubation period (3-30 days) in the P-resin, Pi and Po-NaOH and

P-microbial fraction (gure 2). This last fraction presented quite

high values at 30d (70 mg P.kg

-1

), very possibly related to an intense

microbial activity associated with the high content of organic

carbon (Ravindran and Yang, 2015), and the adequate availability

of nutrients (Ca and Mg) existing in that soil (table 3). Thus, the

conjunction of all these processes throughout the incubation helped

to explain the uctuations in the dissolution values.

ΔP (mgP.kg

-1

) during incubation time

Soils 1d 3d 7d 15d 30d 60d 100d

Iguana 1.8a* 9.0b 10.0b 25.0c 20.0c 10.0b 50.0d

Casupal 6.0a 11.5b 9.0ab 25.0c 5.5a 11.0b 22.0c

Barinas 8.3b 6.8b 3.8a 10.0b 8.3b 11.3b 7.0b

Palmeras 3.0a 3.8a 2.8a 7.8b 6.8b 4.3a 2.0a

Bramón 2.0b 4.0b 3.0b 10.0d -2.0a 5.5c 2.0b

Mantecal 0.5b 1.8b -3.0a 2.8b 3.3b 1.0b 3.0b

Iguana II 1.8a 1.3a 0.0a 1.5a 0.7a 3.5a 1.4a

Veguitas -1.5b -1.5b -1.5b 6.0d 2.0c 3.0c -1.30b

Bajo

Seco

10.0d -2.0b 9.0d -11.0a 0.0b 3.0c -2.0b

*Means followed by different letters between different times differ signicantly

at 5% by Duncan’s two-sided test. Source: Author

Figure 1. Changes in available P fractions during incubation,

Iguana soil. Means followed by different letters differ

signicantly.

Figure 2. Changes in available P fractions during incubation,

Bramón soil. Means followed by different letters

differ signicantly.

A correlation analysis of the dissolution value (ΔP) at 100

days of incubation with the different properties of the analysed

soils showed, in general, low correlation coefcients for the soil

characteristics and PR dissolution (table 7). All the parameters

analysed, except P adsorption, tend to negatively affect the

dissolution, however, only the total P content reached signicance

(P<0.10); the CEC and the P Bray I approached, although without

reaching the level of signicance. Undoubtedly, the small number

of soils analysed due to logistical reasons affected the signicance

of these results. Herrera and Casanova (1997), in tests with several

PR, point out that it was not possible to detect a single characteristic

of the rocks that explains the differences between the amounts of

phosphorus extracted with Bray, it is noteworthy that they also

worked with a reduced number of soils (5).

This scientic publication in digital format is a continuation of the Printed Review: Legal Deposit pp 196802ZU42, ISSN 0378-7818.

Rev. Fac. Agron. (LUZ). 2022, 39(1): e223916. January - March. ISSN 2477-9407.6-6 |

Table 7. Pearson correlation coefcient between the PR dissolution index (ΔP) and the different soil properties; ns and *, non-signicant

and signicant at 0.1%, respectively.

pH Exchangeable Al Exchangeable Ca % C P total P Bray I CEC IBW

Pearson correlation coefcient

-0.001

-0.255

-0.444

-0.372

-0.645* -0.506

-0.531

0.250

Source : Author

Conclusions

The soils of the selected agro-ecological zones presented

different behaviours regarding the ability to dissolve the Monte

Fresco phosphate rock. In general, this response was associated with

the intrinsic characteristics of the soil, such as: pH, content of total

and available P and exchangeable calcium, although a signicant

statistical association was only found with the content of total P. The

Iguana soil, with a high capacity to dissolve PR due to its acidity and

low P content, generates a good PR dissolution, while the Bajo Seco

and Veguitas soils, less acidic and with high contents of total and

available P, induce a poor or no dissolution of the rock.

Acknowledgement

The nancial collaboration of the Institute of Zoology and Tropical

Ecology and the Faculty of Agronomy of the Central University

of Venezuela is appreciated, this work was carried out with the

invaluable participation of the MSc. G. Romero, who, unfortunately,

could not be located for his inclusion as a co-author of the article.

Literature cited

Brenner, J., Porter, W., Phillips, J.R, Childs, J. Yang, X. and Mayes, M. (2019).

Phosphorus sorption on tropical soils with relevance to Earth system

model needs. Soil Research 57, 17–27. https://doi.org/10.1071/SR18197

Casanova, E. 2007. Efecto de rocas fosfóricas naturales y modicadas sobre

la cantidad y calidad de pastos introducidos en Venezuela. Agronomia

Tropical 57, 271-280.

Cicek, H., Bhullar, G.S., Mandloi, L.S. Andres, C. and Riar, A.S. (2000). Partial

acidulation of rock phosphate for increased productivity in organic and

smallholder farming. Sustainability 12, 607. https://doi.org/10.3390/

su12020607

Herrera, T. y Casanova, E. (1997). Efecto de las características de suelos y rocas

fosfóricas sobre el fósforo disponible. Venesuelos 5, 34-39. Corpus ID:

101184690

Kellogg, L. Bridgham, S. and López-Hernández. D. (2006). Organic phosphorus

mineralization. A comparison of isotopic and non-isotopic methods. Soil

Science American Journal 70, 1349-1358. DOI: 10.2136/sssaj2005.0300

Hunt, J.F.T., Ohno, T., He, Z., Honeycutt, C.W. and Dail, D.B. (2007). Inhibition

of phosphorus sorption to goethite, gibbsite and kaolin by fresh and

decomposed organic matter. Biology Fertility Soils 44, 277-288.

López-Contreras, A.Y., Hernández-Valencia, I. and López-Hernández, D. (2007).

Fractionation of soil phosphorus in organic amended farms located on

sandy soils of Venezuelan Amazonian. Biology Fertility Soils 43, 771-

777. http://dx.doi.org/10.1007/s00374-006-0162-x

López de R. I., López, M., Sánchez, A., Nieves, L. y Wiedenhofer, H. (1994).

Respuesta del pasto Andropogon gayanus a la roca fosfórica en dos suelos

ultisoles del estado Guárico. Agronomía Tropical 44(1), 81-100.

López-Hernández, D. and Burnham, C.P. (1974). The covariance of phosphate

sorption with other soil properties in some British and Tropical soils.

Journal Soil Science 25, 196-206.DOI:10.1111/J.1365-2389.1974.

TB01116. XCorpus ID: 97707902

López-Hernández D. (1977). La Química del Fósforo en Suelos Ácidos. Ediciones

de la Biblioteca (EBUC). Universidad Central de Venezuela. Caracas.

123p

López-Hernández, D. (2016). Soils with hardened laterites are they really high

P-sorbing? Ciencia 24, 78-186.

López-Hernández, D. y G. Romero. G. (2019). Cambios en las fracciones de P en

suelos por la adición de roca fosfórica Monte Fresco a diferentes periodos

de incubación y contenidos de humedad. Bioagro 31, 13-22.

Mora, E., Toro, M., Flores, E. and López-Hernández, D. (2017). Plant growth

promoting abilities of phosphate solubilizing bacteria native from a high

P sorbing Ultisol. Annals Advance Agricultural Science 1(1), 1-10. DOI:

10.22606/as.2017.11001

Mora, E., López-Hernández, D. and Toro, M. (2019). Arbuscular mycorrhiza

and PGPR applications in tropical savannas. In: Zúñiga, D., Ormeño,

E. y González A.F (Eds.) Microbial probiotics for agricultural systems.

Advances in agronomic use. Springer. ISBN: 978-3-030-17596-2. 169-

178.

Morillo, A., Sequera, O. y Ramírez. R. (2007). Roca fosfórica acidulada como

fuente de fósforo en un suelo ácido con o sin encalado. Bioagro 19, 161-

168.

Pérez, M. J. (1995). Comparación de cuatro métodos extractantes de fósforo

disponible en suelos tratados con fuentes de fósforo de diferentes grados

de solubilidad. Agronomía Tropical 45(4), 507-526.

Pérez, M.J. y Smyth, T.J. (2005). Potencial agronómico y eciencia agronómica

de tres rocas fosfóricas de diferente composición mineralógica. Revista

Facultad Agronomía (LUZ). 22, 214-227.

Rajan, S.S., Watkinson, J.H. and Sinclair, A.G. (1996). Phosphate rocks for

direct application to soils. Advance in Agronomy 57. 77-159. https://doi.

org/10.1016/S0065-2113(08)60923-2

Ravindran. A. and Yang, S.S. (2015). Effects of vegetation type on microbial

biomass, carbon and nitrogen in subalpine mountain forest soils. Journal

of Microbiology, Immunology and Infection 48 (4), 362-369. DOI:

10.1016/j.jmii.2014.02.003

Romero, G. y López-Hernández, D. (2018). Evaluación de métodos para la

disolución de la roca fosfórica Monte Fresco. Bioagro 30, 151-156.

Reynolds Jr., R.C. and Moore, D.M. (1989). Principles and Techniques of

Quantitative Analysis of Clay Minerals by X-Ray Powder Diffraction.

Oxford University Press, New York, 332-337.

Sequera, O. y Ramírez, R. (2013). Roca fosfórica acidulada con ácido sulfúrico

y tiosulfato de amonio como fuente de fósforo para frijol en dos tipos de

suelo. Bioagro 25(1), 39-46.