Effects of Preplant Phosphoric Acid Treatment on P Retention of Tuffs of the Canary Islands

Transcription

1 Effects of Preplant Phosphoric Acid Treatment on P Retention of Tuffs of the Canary Islands C.D. Hernández, A.R. Socorro and M.C. Cid Instituto Canario de Investigaciones Agrarias Apartado La Laguna, Tenerife Spain B. Santos and D. Ríos Servicio de Agricultura y Desarrollo Rural, Cabildo de Tenerife. Plaza España s/n Santa Cruz de Tenerife Spain Keywords: substrate, soilless medium, volcanic cinder, lapilli Abstract Several types of volcanic tuff are found in the Canary Islands, where they are popular with growers as a cheap and abundant alternative for soilless culture. Textures and composition vary according to type, but all have in common a high, buffered and a P sorption capacity of agricultural importance. Four tuffs, two coarse black unweathered tuffs and two fine red weathered (alterated) tuffs, from four different localities, were selected to evaluate changes in P sorption after a preplant treatment with phosphoric acid to achieve. Granulometry,, and exchange capacity (CEC) were determined; neutralization curves showed that both red tuffs needed more acid to correct. Treated tuffs were fertigated during five weeks with a standard nutrient solution to determine short-term effects on the solution P content retention; rockwool slabs were used as control. Drainage was collected daily, and phosphorus content was determined. In red tuffs, there was a strong initial P release that gradually declined and was practically null at the end of the trial. The black tuffs did not change the P level of the solution during the first week, but afterwards they exhibited an increasing sorption inversely proportional to the decrease in the P level of the solution. Results showed a strong relationship between weathering (significantly more than particle size alone) and the amount of P sorbed from the nutrient solution. However, the present trial shows that preplant phosphoric acid treatment alone is not enough to sustain the necessary P level of the nutrient solution. For the black tuffs studied, the solution needs extra P (particularly for high demand crops), and for the red tuffs, part of the phosphoric acid should be replaced by nitric acid in order to avoid the strong initial release of P that unbalanced the nutrient solution composition. INTRODUCTION Basaltic tuffs, also known as volcanic cinder and lapilli, abound in the Canary Islands and have long been popular among horticulturists as a cheap, easily obtainable medium for soilless culture. Currently, over 300 hectares of protected crops, mainly tomato and roses, use local tuff as sole substrate. While tuff is a viable alternative to synthetic and/or imported substrates in terms of cost, it has two drawbacks in soilless culture: an alkaline and a moderate tendency to retain phosphorus (Luque, 11, Cid et al., 1). The active components of tuffs, volcanic glass and secondary materials (clays and minerals), and the proportions of each ultimately depend on the degree of alteration or weathering undergone in each case, and in this sense, P retention has been linked to the quantity of volcanic glass (Silber et al., 14; Cid et al., 1) and insoluble free Fe and Al (Nanzyo et al., 1). In turn, in soilless culture, the range of retention is further affected by the of the fertigation solution itself, whereby the more acid solution, the greater the P sorbed by the tuff (Galin and Singer, 1; Casey and Bunker, 10; Silber et al., 14). Phosphorus retention will obviously affect nutrient uptake and subsequently influence the development and yield of crops with high P requirements (Bar Yosef and Imas, 1), but it becomes a crucial issue in young plants of any species, as unsatisfied P demand will result in poor root development. The aim of this work was to evaluate the effectiveness (and durability) of phosphoric acid preplant treatment of local tuffs, to correct high in order to avoid the initial strong P sorption in this substrate. Proc. IS on Soilless Cult. and Hydroponics Ed: M. Urrestarazu Gavilán Acta Hort. ISHS 200 4

2 MATERIAL AND METHODS Four tuffs from three different quarries were selected: two black, BB and BM (from Montañas Negras), and two red, RM (from Las Magarzas, Montaña Birmagen) and RT (from Montaña Talavera). Compared to the black tuffs, both RM and RT were finer and more weathered (as seen by Fe oxidation which, together with clay content, gives these tuffs their red color). Particle size was determined, and and CE were measured in a 1:2 (v/v) water extract. P retention was measured using Blakemore et al. (11) method for volcanic soils, and cation exchange capacity (CEC) at was determined following the method of Bower et al. (12). Data are presented in Table 1. The control treatment was slabs of rockwool (RW). Commercial H3 PO 4 (% w/w, density 1.3 g/cm 3 ) was used to correct tuffs (Cid et al., 1). Correct dosage was calculated according to the neutralization curves of each tuff, obtained using 200 ml of tuff, a 2:1. (v/v) tuff:phosphoric solution, an equilibration period of 24 hr, with two replications. The experiment was set up under normal growing conditions in an unheated plastic greenhouse, in Valle Guerra (Tenerife). The experimental design was randomized blocks, with 3 replications, with six containers/tuff and two containers of RW per replicate. Twenty-eight-litre polyethylene containers were used for each treatment. All containers were saturated with standard nutrient solution (Table 2); in the tuff treatments, phosphoric acid was added in the quantities required by each to achieve. After 4 hr, all treatment were drained and washed with fresh nutrient solution; was measured and ranged between. and in all cases. All treatments were then fertigated with the nutrient solution for five weeks to determine if this pretreatment was capable, at least in the short term, of controlling P sorption and the subsequent drop of phosphate in the solution. Drainages of each experimental unit were collected daily, amalgamated per treatment and maintained at -20ºC to prepare a weekly sample for analysis. P in drainage solution was determined as vanadomolibdic acid complex and measured at 420 nm. RESULTS AND DISCUSSION Preplant Phosphoric Acid Dosage Black tuffs needed low quantities of phosphoric acid: 0.4 and 0. ml/l solution were enough to reach, and remain stable, in BB and BM respectively (Fig. 1). Both red tuffs, with their higher degree of weathering and larger percentages of fine particles, needed over 2 ml of H 3 PO 4 to reach (lower dosages allowed the to rise almost immediately (well within 4 hr). This higher and buffer capacity of red tuffs compared with black ones was also noted by Silber et al. (14) in Israeli tuffs. Pretention/Release The black tuffs did not change the P level of the solution during the first week, but afterwards exhibited an increasing sorption until the fourth week, as shown by the decreasing P levels of the drained solutions (Fig. 2). In the last week of the trial, BB and BM leachates had 2% and 30% less P than that applied in the nutrient solution values that are significantly higher than those of the red tuffs. In red tuffs, there was a strong initial P release (resulting from the high doses of phosphoric acid applied to correct ) that gradually declined and was practically null after three weeks (Fig. 3). Subsequent to that peak, P sorption was stable, although significantly higher in the RT tuff. At the end of the trial, RM retained only about 4 % of the P added to the nutrient solution, whereas RT retained 12 %. Results showed a strong relationship between weathering (significantly more than particle size alone) and the amount of P sorbed from the nutrient solution. This is consonant with the findings of Silber et al. (14) for tuffs from the Golan Heights. It should be noted that black tuffs, with their typically high content of unalterated (unweathered) volcanic glass, with incompletely bonded Si, Al, and Fe, and their porous structure, create a large hydroxylated surface (Casey and Bunker, 10, Davis and Kent, 10) which, although outside of the scope of this trial, does have agricultural implications: the acid of nutrient 00

3 solutions causes the tuff surfaces to become protoned and positively charged, and thus prone to sorbing phosphate anions. This was suggested by Cid et al. (1) and indeed occurred in the present trial. In general, the results show that preplant phosphoric acid treatment alone is not enough to sustain the necessary P level of the nutrient solution. For the black tuffs studied, the solution needs extra P (particularly for high demand crops), and for the red tuffs, part of the phosphoric acid should be replaced by nitric acid in order to avoid the strong initial release of P that unbalanced the nutrient solution composition. Literature Cited Bar Yosef, B. and Imas, P. 1. Phosphorus fertigation and growth substrate effects on dry matter production and nutrient contenrs in greenhouse tomatoes. Acta Hort. 401: Blakemore, L.C., Searle, P.L. and Daly, B.K. 11. Soil bureau laboratory methods: A method for chemical analysis of soils. N.Z. Soil Bur. Sci. Rep. 10A (revised). Casey, W.H. and Bunker, B. 10. Leaching minerals and glass surfaces during dissolution. In: Hachella, M.F. and White, A.F. eds. Mineral-water interface Geochemistry. Mineralogical Society of Amer. Washington D.C Cid, M.C., Socorro, A.R. and Zieslin, N. 1. Changes in nutrient solution caused by volcanic cinder media of soilless greenhouse roses in the Canary Islands. Acta Hort. 424: Davis, J.A. and Kent, B.D. 10. Surface complexation modelling in aqueous geochemistry. In: Hachella, M.F. and White, A.F. eds. Mineral-water interface Geochemistry. Mineralogical Society of Amer. Washington D.C Galin, T. and Singer, A. 1. Changes in tuff lapilli from the Golan Heights resulting from their use as growth medium. Soilless Culture 4(2): Luque, A. 11. Retention of ions by volcanic sand used in hydroponic cultures. Acta Hort. 12:-1. Nanzyo, M., Takahashi, T., Sato, A., Soji, S. and Yamada, I. 1. Dilute acid-soluble phosphorus in fresh air-borne tephras and fixation with an increase in active aluminium and iron. Soil Sci. and Plant Nutr. 43(4): 3-4. Silber, A., Bar-Yosef, B., Singer, A. and Chen, Y. 14. Mineralogical and chemical composition of three tuffs from northern Israel. Geoderma 3(2): Tables Table 1. Physical and chemical properties of tuffs used in the experience. Alteration C. I. Tuff Color (2) EC (2) P retention (3) CEC (4) degree (%) (ds/m) (%) (cmol c /kg) RM Red High RT Red High BB Black Null BM Black Very low (1) Coarseness index: Percentage of particles > 1 mm (w/w), (2) Determined in extract 1:2 by volume. (3) Blakemore et al. (11), (4) Exchange capacity at, Bower et al. (12) Table 2. Composition of the nutrient solution. mg/l Ca Mg (1) K Na (1) N-NH 4 N-NO 3 P-PO 4 Fe (2) EC (1) From the irrigation water. (2) Rest of microelements were also added at standard concentrations. 01

4 Figures BB Tuff BM Tuff RM Tuff RT Tuff Fig. 1. Neutralization curves of tuffs with phosphoric acid. Black tuffs: BB and BM from Montañas Negras (Santiago del Teide, Tenerife). Red tuffs: RM from Las Magarzas, Montaña Birmagen (El Rosario, Tenerife), and RT from Montaña Talavera (Santa Cruz, Tenerife). 02

5 P-PO 4 (% Rw) BB BM P-PO 4 (% Rw) RM RT Time (weeks) Time (weeks) Fig. 2. Changes in P-PO 4 content in the drainage solution from tuffs, expressed as percentages respect to the control (rockwool leachate). Black tuffs: BB and BM from Montañas Negras. Red tuffs: RM and RT from Las Magarzas and Montaña Talavera. 03