RESULTS AND DISCUSSION 1. Experiment 1

% Rev. P = E value (ppm)/ Fixed P (ppm)*100

The isotopic parameters were determined according to Fardeau’s methodology on isotopic exchange kinetics considering times 10 and 100 minutes. The intensity (Cp), quantity (E₁) and capacity (E₁/Cp) parameters were evaluated as well as the fixation index (r₁/R), [12, 13].

TABLE I. TREATMENTS FOR EXPERIMENT 2 T0 = check

T1 = 100 ppm P as TSP T2 = 250 ppm P as TSP T3 = 500 ppm P as PR T4 = 1000 ppm P as PR

T5 = 100 ppm P as TSP + 500 ppm P as PR T6 = 100 ppm P as TSP + 1000 ppm P as PR

3. RESULTS AND DISCUSSION 3.1. Experiment 1

The results of available P in the soils studied as measured by the chemical extraction methods are presented

in Table II. The correlation coefficients among the methods were 0.68 for Olsen vs N. Carolina, 0.07 for Olsen vs Bray 1, and 0.49 for N. Carolina vs Bray 1. The N. Carolina method characterized in a better way the status of P in the Chilean and Venezuelan soils, which did not happen with the Bray 1 method.

To quantify the P retention in some soils, the Langmuir adsorption model was used and the adsorption strength (K) and the maximum adsorption capacity (B) were measured (Table III). With regard to these para-meters, 3 soils coming from Chile (N. Braunau, Frutillar and Osorno) and one soil coming from Venezuela (Yaritagua) were compared in relation to phosphorus adsorption characteristics (Table III).

The Chilean soils have a very high P fixing capacity with maximum P adsorption values greater than 1000 mg P/kg soil. The Chilean soils also exhibit higher fixing strength compared to the Venezuelan soil. In relation to isotopic exchangeable P, (E value), the Chilean soils have a low value of isotopic exchangeable P without added P (Table IV). There is a progressive increase in E value, under increasing fertilizer rates, particularly at 500 and 1000 ppm P (Table IV).

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TABLE II. AVAILABLE P AS MEASURED BY CHEMICAL EXTRACTION METHODS Available Phosphorus (mg/kg)

Soil PH Olsen N. Carolina Bray 1

N Braunau 4.15 1.94 4.96 0.47

Fresia 4.48 1.29 2.34 ---

Frutillar 4.31 8.90 17.38 ---

El Pao 4.48 1.94 8.01 6.65

Chaguaramas 5.06 4.63 13.38 11.81

El Tigre 5.03 3.85 15.90 21.90

Huilma Fallow 4.95 5.79 5.55 0.65

Huilma Conifer 4.99 19.28 26.53 13.15

Huilma Wheat 4.98 7.45 9.97 1.02

Osorno 5.46 3.27 4.13 0.28

Yaritagua 6.43 6.26 19.03 8.77

TABLE III. PHOSPHORUS ADSORPTION PARAMETERS Soil B (mg P/kg soil) K

Yaritagua 296 0.032

N. Braunau 1505 43.5

Frutillar 1156 14.7

Osorno 2069 0.39

TABLE IV. "E" VALUE (PPM) E1 VALUE (ppm) P rate (ppm)

Venezuela Yaritagua soil

Venezuela N. Braunau soil

Chile Osorno soil

Chile Frutillar soil

100 6.6 --- 1.58 0.11

250 13.0 --- 1.31 1.77

500 18.3 15.4 248 3.53

1000 24.1 54.8 141 21.3

2000 34.0 24.9 173 70.5

In the Nueva Braunau and Frutillar soils, the reversible P (%) with higher P rates reached 5.5 %, while for the Yaritagua soil the reversible P fluctuated between 8% and 12%, according to the P rates applied (Table V).

The Nueva Braunau and Frutillar soils have a very low percentage of reversible P, due to the high P-fixing capacity, (Table VI) facing the Yaritagua soil, which has a progressive increase as the added P rates increased.

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Table VII shows the change in available P (Olsen method) according to the different treatments.

An important increase of available P-Olsen was shown in relation to the check, due to the application of both sources. The highest results were obtained with TSP at the 250 ppm P and the mixture of 100 ppm P as TSP and 1000 ppm P as PR (Table VII).

TABLE VII. OLSEN P AT 30 DAYS OF INCUBATION

P Treatments Olsen P

In relation to the water-soluble P source (TSP), there is a response according to the applied increasing P rates. This indicated there was a good solubilization of P and thus an increase of available P as the dosis increases.

It is important to mention that the high P fixing capacity of the soil (higher than 70%) may influence the low response to P applications. It is possible that a great part of liberated P, coming from soluble and less

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soluble sources, could be quickly fixed by the soil and potentially diminishes the response to P application.

The relevance of this assay has to be found not only in the P enrichment in the soil due to the treatments, but also in the contribution of other nutrients like the calcium present in TSP and PR. Obviously, both nutrient improved the general conditions of soil fertility.

The P retention (Blakemore) confirms even more the low available P values obtained by the Olsen method for each one of the treatments. From Table VIII, we may conclude that the soil has a high P retention capacity, and most of the applied P to the soil is quickly fixed. There is no variation between the values coming fromthe soil without fertilizer and the values coming from the soil with P fertilizers.

The values between the soil alone and the higher P rates did not show important variations like the case of available P values by the Olsen method for the same treatments. P retention decreases in treatments T2 (250 ppm of P as TSP) and T5and T6, which are mixtures of TSP and PR.

The intensity factor was low for the check treatment, below the 0.02 mg/L that is considered nornmal for plant growth. The Cp values improved significantly with the increasing P rates of application within each source. An increase of Cp was observed as the applied P rates increases, except for T2 treatment (250 ppm of P as TSP) where Cp decreased (Table IX). In relation to Cp values, it increases a little in T1, T2 and T3with regard to T0. It is important to point out the highest value in the intensity factor (Cp) was obtained in the T6 treatment. This situation means that the mixture of P sources in that treatment showed a higher response to phosphates fertilizers application. A very low retention index (r₁/R) was found for all the treatments, thus confirming the very high P-fixing capacity of the soil. This would imply that the P-sorption capacity strongly controlled P activity in the soil solution. The combination of very low P concentrations in the soil extractand very high P fixation in this soil resulted in very high quantity (E values) and capacity ( E₁/Cp) factors. Abnormally very high E values were obtained (Data not reported). Specific protocols for application ofthe isotope exchange method should be developed for these conditions.

TABLE VIII. P RETENTION ACCORDING TO BLAKEMORE [1]

Treatment P Retention (%)

T0 75.0 T1 75.3 T2 73.2 T3 75.2 T4 75.2 T5 73.4 T6 71.3

TABLE IX. INTENSITY FACTOR (Cp) Treatment Cp X10^-2

mg/L T0 1.45

T1 7.33 T2 5.56 T3 7.61 T4 10.6 T5 10.7 T6 21.6

179 TABLE X. PERCENTAGE OF P DERIVED FROM FERTILIZER

Treatments Pdff%

T0 --- T1 59.4 T2 34.5 T3 56.9 T4 61.9

T5 59.2 T6 77.0

The %P derived from fertilizer [12] showed pronounced differences between the treatments (Table X).

The highest percentage of P derived from fertilizer was obtained with 100 ppm P rate as TSP (T1) and 1000 ppm P as PR (T4), [12]. The mixture of TSP and PR is the best treatment as it is shown in Table X.

Both the % of Pddf and the Cp values increased with increasing P rates of application. There was also an increase in available soil P-Olsen, as shown in Table VII..

4. CONCLUSIONS

The P retention percentage (Blakemore method) of the Perquenco soil was very high. The P application as PR or TSP did not reduce significantly the P retention of this soil. Nevertheless, it exists a positive effect whenhe P sources are applied as mixtures, resulting in a slight decrease of the P retention percentage.

Available P (Olsen method) in this soil increased from 3.6 ppm P for the check to a maximum of 20 ppm P for the 250 mg P/kg as TSP, reaching medium to high values according to the extraction method utilised.

The intensity factor was low in the check treatment but increased to satifactory levels with the P fertilizer treatments. The fixation index was very high leading to an overestimation of the quantity and capacity factors. Particular care must be taken in utilizing this isotope technique in soils with low P concentration in solution and high P-fixing capacity.

ACKNOWLEDGEMENTS

This work was conducted under the research contract CHI-7499 in the frame of the FAO/IAEA Coordinated Research Program “The use of nuclear and related techniques for evaluating the agronomic effectiveness of phosphate fertilizer, in particular rock phosphate” for which we are very grateful.

REFERENCES

[1] SADZAWKA, M.A., Métodos de análisis de suelos. Instituto de Investigaciones Agropecuarias (Chile) Est. Exp. La Platina (Santiago). Serie La Platina N° 16 (1990),130 p.

[2] BARROW, N.J., The description of phosphate adsorption curves. J. Soil Sci. 29 (1978), 447-462.

[3] PINO, I., PARADA, A.M., ZAPATA, F., NAVIA, M., LUZIO, W., Comparative study on P uptake and utilization from P fertilizers by Chilean wheat genotypes in volcanic soils (in this publication, 2001).

[4] MELLA, A., KÜHNE, A., Sistemática y descripción de las familias, asociaciones y series de suelos derivados de materiales piroclásticos de la zona central - sur de Chile. En: Tosso, J. (ed.).

Suelos volcánicos de Chile. Instituto de Investigaciones Agropecuarias. Ministerio de Agricultura. Santiago. (1985), 549-716.

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[5] FARDEAU, J.C., Le phosphore assimilable des sols: sa représentation par un modèle fonctionnel à plusieurs compartiments. Agronomie 13 (1993) 317-331.

[6] FARDEAU, J.C., GUIRAUD, G., MAROL, C., Bioavailable soil P as a key to sustainable agri-culture. In: Proceedings of a symposium “Nuclear techniques in soil- plant studies for sustainable agriculture and environmental preservation” IAEA.STI/PUB/947 (1995) 131-144.

[7] FARDEAU, J.C., Dynamics of phosphate in soils. An isotopic outlook. Fertilizer Research 45 (1996) 91-100.

[8] FARDEAU, J.C., GUIRAUD, G., MAROL, C., The role of isotopic techniques on the evaluation of the agronomic effectiveness of P fertilizers. Fertilizer Research 45 (1996) 101-109.

[9] PINO, I., URBINA, C., LUZIO, W., CASAS, L., Retención de fósforo en dos suelos derivados de cenizas volcánicas. Nucleotécnica. 3 (4) (1983) 47-50.

[10] PINO, I., VANDERDEELEN, J., BAERT, L., Kinetics of phosphate adsorption in a soil derived from volcanic ash. Turrialba 23 (3)(1973) 291-296.

[11] PINO, I., URBINA, C., CASAS, L., Evaluación de la capacidad de retención de fósforo en suelos derivados de cenizas volcánicas. Nucleotécnica 4 (6) (1984) 43- 47.

[12] FARDEAU, J.C., P-32 isotopic exchange kinetics. Experimental procedure, interpretation of results. Application to the prediction of the efficiency of P fertilizers. Working paper presented at the Third Research Coordination Meeting of the FAO/IAEA Coordinated Research Programmes. “The use of Nuclear and related techniques for evaluating the Agronomic Effectiveness of Phosphate fertilizers, in particular Rock Phosphate”. Report D1-RC-542.3 (1997).

[13] NOVOZAMSKY, I., VAN DIJK, D., LEE, J.J., HOUBA, V.J.G., Automated determination of trace amounts of phosphate in soil extracts using malachite green. Commun. Soil Sci. Plant Anal. 24 (9&10) (1993) 1065-1077.

PART IV