L. Andreu

Phosphorus fertilizer recovery from calcareous soils amended with humic and fulvic acids

Precipitation of Ca phosphates negatively affects recovery by plants of P fertilizer applied to calcareous soils, but organic matter slows the precipitation of poorly soluble Ca phosphates. To study the effect of high molecular weight organic compounds on the recovery of applied P, a mixture of humic and fulvic acids was applied to calcareous soils with different levels of salinity and Na saturation which were fertilized with 200 and 2000 mg P kg−1 as NH4H2PO4. Recovery was measured as the ratio of increment in Olsen P-to-applied P after 30, 60 and 150 days, and associated P forms were studied using sequential chemical fractionation and 31P NMR spectroscopy. Application of the humic-fulvic acid mixture (HFA) increased the amount of applied P recovered as Olsen P in all the soils except in one soil with the highest Na saturation. In soils with high Ca saturation and high Olsen P, recovery increased from < 15% in the absence of amendment to > 40% at a 5 g HFA kg−1 amendment rate (30 days incubation and 200 mg P kg−1 fertilizer rate). This is ascribed to inhibition of the precipitation of poorly soluble Ca phosphates, consistent with the sequential chemical extraction (reduction of the HCl extractable P) and P concentration in 0.01 M CaCl2 (1:10 soil:solution ratio) extracts. 31P NMR spectra revealed that in non-amended samples, most spectral shifts were due to poorly soluble P compounds (carbonate apatite); on the other hand, at the 5 g HFA kg−1 rate, significant amounts of amorphous Ca phosphate and dicalcium phosphate dihydrate (DCDP) were identified. The increase in the recovery of applied P due to HFA reveals a positive effect of the application of organic matter as soil amendments on the efficiency of P fertilizers and also explains that manures and other organic sources of P were more efficient increasing available P than inorganic P fertilizers in calcareous soils.

Spatial and temporal distribution of soil water balance for a drip-irrigated almond tree

Relatively little information is available on the spatial distribution of soil water under drip irrigation, and how it is affected by root distribution, emitter placement and irrigation amounts. We hypothesize that variables such as emitter position relative to the active roots as well as irrigation amount and frequency will affect the soil water regime in general, and specifically the spatial and temporal changes in soil water content as controlled by root water uptake and leaching. A better understanding of these interrelationships will provide alternative means for proper and efficient drip irrigation water management practices. Moreover, the present study will provide an extensive database which can serve as input for analytical or numerical modelling of drip-irrigated trees. We present the results of a field study in which the soil water regime of a surface drip irrigated almond tree is investigated. The experimental site (6.6 m X 4.8 m) was intensively instrumented with tensiometers and neutron probe access tubes to infer the three-dimensional distribution of soil water and root water uptake during the irrigation season. Drainage fluxes were estimated from measured hydraulic head gradients and hydraulic conductivity data. Unsaturated hydraulic conductivity were determined from in situ measurements by the instantaneous profile method, and in the laboratory using the multi-step outflow method. The water balance results showed that the applied water was not sufficient to match the actual tree water use by evapotranspiration, causing soil waler depletion around the tree as the irrigation season progressed. Moreover, soil water content data demonstrated temporal changes in the water uptake patterns. The temporal occurrence of leaching justifies the need for soil water measurements in the scheduling and design of drip irrigation systems. 0 1997 Elsevier Science B.V.

The cumulative effect of three decades of phosphogypsum amendments in reclaimed marsh soils from SW Spain : 226Ra, 238U and Cd contents in soils and tomato fruit

Phosphogypsum (PG), a by-product of the phosphate fertiliser industries, has been applied as soil amendment to reduce Na saturation in soils, as in the reclaimed marsh area from SW Spain, where available PG has a typical fingerprint of 710+/-40 Bq kg(-1) of (226)Ra, 165+/-15 Bq kg(-1) of (238)U and 2.8+/-0.4 mg kg(-1) of Cd. This work was focussed on the cumulative effects of PG amendments on the enrichment of these pollutants in cultivated soils and plants (Lycopersicum esculentum Mill L.) from the area studied, where PG has been applied since 1978 at recommended rates of 20-25 Mg ha(-1) every 2-3 years. A field experiment was conducted over three years to compare activity concentrations of (226)Ra ((214)Pb) and (238)U ((234)Th) in non-reclaimed soils, reclaimed soils with no additional PG application, and reclaimed soils with two additional PG applications. A non-significant effect of two PG amendments (in three years) was observed when compared with non-amended reclaimed plots. Nevertheless, a significant (p<0.05) enrichment of (226)Ra was observed in the surface horizon (0-30 cm) of reclaimed plots relative to deeper horizons and also when compared with the surface horizon of non-reclaimed soil (p<0.05), thereby revealing the cumulative effect of three decades of PG applications. Furthermore, the effect of a continuous application of PG was studied by analysing soils and tomato fruits from six commercial farms with different cumulative rates of PG applied. Cadmium concentrations in tomatoes, which were one order of magnitude higher than those found in tomatoes from other areas in South Spain, were positively correlated (r = 0.917) with (226)Ra-concentration in soils, which can be considered an accurate index of the cumulative PG rate of each farm.

Application of the model MACRO to water movement and salt leaching in drained and irrigated marsh soils, Marismas, Spain

Abstract This paper presents an application of a two-domain model of water flow and solute transport in macroporous soil (MACRO) to field experiments in drained and irrigated, saline heavy clay soils under cotton. Model predictions are compared to detailed measurements of the soil water balance and leaching of chloride to field drains made during a 17-day period following two successive sprinkler irrigations. The model was able to reproduce the measured drain hydrograph and the observed response of the water table, providing calibrated, rather than directly measured, saturated hydraulic conductivity values were used. At the end of the first irrigation, the soil profile was fully recharged, with the water table only 10 cm below the surface. In the 2 weeks following irrigation, soil water extraction by the crop was largely restricted to the upper 30 cm of soil, presumably due to excessive salt concentrations in the subsoil. Indeed, the model simulations indicated a reduction in transpiration below the potential rate after only 1 week, with an accumulated water deficit of 60 to 70 mm. A strong dilution of the chloride concentrations during peak drain discharges was observed and was also predicted by the model. This dilution was related to the rapid infiltration of irrigation water of low salt concentration in the cracks. In two flow domains, the model precisely matched observed leaching losses of chloride during peak drain discharges, but underestimated by ≈ 25% the accumulated loss of chloride 1 week after the irrigation. This was entirely due to an underestimation of chloride concentrations in the outflow during the late stages of recession. During such recession flows, the chloride concentrations in the drain outflow (≈ g·l−1) were most likely influenced by inflows of saline shallow groundwater from surrounding areas. In one flow domain, the model predicted qualitatively similar patterns of drain outflow and chloride leaching. Nevertheless, the model performed less well when bypass flow was not taken into account, with chloride leaching overestimated during peak discharges, despite a total drain outflow which was 10 mm less than in the two-domain case.

Fertilizer Phosphorus Recovery from Gypsum-Amended, Reclaimed Calcareous Marsh Soils

Gypsum amendments in marsh soils may affect the recovery of applied P fertilizer as the addition of Ca and sulphate alters the P sorption and release capacities of soils through their effects on P adsorption and precipitation processes. The effect of gypsum on P recovery was studied in a laboratory experiment where gypsum was added at 0, 6, 20, and 100g kg -1 to six representative marsh soils from SW Spain fertilized with 200 or 2000mg Pkg -1 applied as NH 4 H 2 PO 4 . Recovery was measured as the increase in Olsen P after 30, 60, and 150 days. P forms were also studied at these times, using sequential chemical extraction. Recovery of applied P decreased with increase in initial Olsen P (P status), Ca saturation, and sulphate concentration in the 1:1 soil extract. Less than 10% of applied P was recovered as Olsen P from the soils with highest P status and Ca saturation (LB2, LB5) after 30 days at the lower fertilizer P rate. P recovered from soils with an initial low Olsen P and high Na saturation (LB3 and LB4) was much higher (more than 50% in LB4 at 30 days at the two P rates). The addition of gypsum increased recovery of applied P (measured as Olsen P) in soils with a high degree of reclamation (high Olsen P and high Ca saturation): in LB2 and LB5, the recovery was double in samples amended with gypsum at 100g kg -1 than in nonamended samples at 150 days. By contrast, recovery of applied P was decreased by gypsum in soils with high Na saturation and an initial low P status. The effect of gypsum on recovery in high P - high Ca soils is related to an interference of sulphate with sorption processes that reduces short-term P sorption and increases the proportion of applied P that precipitates as soluble (bicarbonate extractable) phosphates in the long run. The effect of gypsum on recovery in low P (high Na saturation) soils is due to the increased P sorption capacity resulting from increased Ca saturation. Gypsum amendments increase the efficiency of fertilizer P, contributing to enhanced productivity of freshly reclaimed saline and sodic marsh soils.

Calcium‐ and iron‐related phosphorus in calcareous and calcareous marsh soils: Sequential chemical fractionation and 31p nuclear magnetic resonance study

Abstract Phosphorus (P) forms in soils determine the amount of P available for crops and the potential for this element to be released to water. Sequential chemical fractionation can provide some information about major P forms in soils, and allow one to distinguish iron (Fe)‐related phosphorus from calcium (Ca)‐bound P. The 31P nuclear magnetic resonance (NMR) spectroscopy has been used in the identification of organic P, precipitated Ca‐phosphates, and aluminum (Al)‐related P in acid soils. Three calcareous soils and four calcareous marsh soils were used in this study. These two types of soils differ in the nature of iron oxides, which are the main P sorbent surfaces. The ratio of low crystalline to high crystalline iron oxides is higher in marsh soils than in calcareous soils as a consequence of the special genesis and conditions of the soil (reduction‐oxidation cycles). Such a ratio is related to the proportion of occluded P in low crystalline oxides relative to that of high crystalline oxides. Citrate‐bicarbonate extractable P (CB‐P) in the fractionation schemes can be ascribed to adsorbed P and high soluble calcium phosphates. CB‐P is correlated with the sum of P fractions in all the soils, thus indicating that the amount of the P that can be easily released is related to the rate of P enrichment of the soil. The 31P NMR spectral data reveal that hydroxyapatite is the dominant P form in the soils studied. This is consistent with the fractionation data, where acid‐extractable P is the main P fraction. The spectra also provide some information about the amount of total inorganic P and Ca‐phosphates in calcareous soils.

Application efficiency of micro-sprinkler irrigation of almond trees

Abstract Micro-sprinklers are becoming a preferred irrigation method for water application in orchards. However, there is relatively little data available to support a particular irrigation scheduling method. The objective of this study is to quantify the components of the water balance of an almond tree under micro-sprinkler irrigation. For that purpose, an experimental plot around an almond tree with an area of 2.0 m X 2.0 m without vegetation, representing about one quarter of the wetted area of the micro-sprinkler was instrumented with neutron access tubes, tensiometers and catch cans. Twenty-five access tubes with catch cans were distributed in a square grid of 0.5 m × 0.5 m, to a depth of 0.9 m. Eight pairs of tensiometers were installed at depths of 0.825 and 0.975 m within the experimental plot. During a 7-day period in August, 1995 the plot was sprinkler-irrigated on three days, and water application rates and uniformity coefficients were calculated for each irrigation event. Neutron probe readings at 15 cm depth increments and tensiometer readings were taken 4 to 6 times daily. Results showed large evaporation losses during and immediately after the irrigations. Evaporation losses of the wetted area was estimated to be between 2 and 4 mm/irrigation event. Consequently, application efficiencies were only 73–79%, the wetting of the root zone was limited to the 0–30 cm depth interval only, the soil profile was depleted of soil water, and daily crop coefficient values at days between irrigation events were between 0.6 and 0.8. The study recommends irrigation in the evening and night hours, thereby largely eliminating the evaporation losses that occur during daytime irrigation hours.

Occupational dosimetric assessment (inhalation pathway) from the application of phosphogypsum in agriculture in South West Spain.

Phosphogypsum (PG) has been traditionally applied as Ca-amendment in saline marsh soils in SW Spain, where available PG has 710+/-40Bqkg(-1) of 226Ra. This work assesses the potential radiological risk for farmers through 222Rn exhalation from PG-amended soils and by inhalation of PG-dust during its application. A three-year field experiment was conducted in a commercial farm involving two treatments: control and 25tPGha(-1) with three replicates (each 0.5ha plots). The 222Rn exhalation rate was positively correlated with potential evapotranspiration, which explained 67% of the variability. Statistically significant differences between the control and PG treatments were not found for 222Rn exhalation rates, and mean values were within the lowest quartile of the typical range for 222Rn exhalation from soils. Airborne dust samples were collected during the application of PG and sugar-beet sludge amendments. The highest PG-attributable 226Ra concentration in the dust samples was 3.3x10(2)microBqm(-3), implying negligible dose increment for exposed workers.