Use of dolomite phosphate rock (DPR) fertilizers to reduce phosphorus leaching from sandy soil

Manipulating geological phosphorus resources for improved production and environmental outcomes during plant establishment

Phosphorus (P) fertilisers are under scrutiny due to resource constraints and environmental impacts. Simple rock phosphate (RP) modifications with acids and co-applied with microbial inoculum could offer sustainable alternative P fertiliser products. We evaluated the effects of acid-treated rock phosphate (RP) in combination with fungal inoculum on plant establishment, environmental impacts (nutrient leaching) and soil quality in a 5-month pot trial. The treatments were evaluated in a clayey Vertisol and a silty Acrisol using cotton (Gossypium hirsutum) as a model plant. The RP treatments - apart from the unmodified and HCl products - were effective in promoting plant establishment with two of the microbial formulations superior to conventional P fertilisers by an average factor of 2 in both soil types (p < 0.05). All RP products restricted P leaching compared with conventional P fertilisers (p < 0.05), by an average factor of 5 for diammonium phosphate (DAP) in both soil types and 3 for the triple superphosphate TSP (only in Acrisol). Nitrate leaching from all treatments was high although much lower from the RP treatments compared with the conventional fertilisers towards the end of the establishment trial, by an average factor of 1.5 (p < 0.05). Ranking analysis revealed that some RP treatments showed evidence for improved ongoing soil quality, including decreased P leaching and soil acidification risks. Microbial analysis showed complex interactions between treatment and soil type. Nonetheless, inoculum persistence at the end of the plant establishment phase was observed for all pots analysed. Our results demonstrate that relatively simple modifications to RP could pave the way for developing sustainable P fertilisers.

Process Behavior and Product Quality in Fertilizer Manufacturing Using Continuous Hopper Transfer Pan Granulation—Experimental Investigations

Fertilizers are commonly used to improve the soil quality in both conventional and organic agriculture. One such fertilizer is dolomite for which soil application in granulated form is advantageous. These granules are commonly produced from ground dolomite powder in continuous pan transfer granulators. During production, the granulator’s operation parameters affect the granules’ properties and thereby also the overall performance of the fertilizer. To ensure product granules of certain specifications and an efficient overall production, process control and intensification approaches based on mathematical models can be applied. However, the latter require high-quality quantitative experimental data describing the effects of process operation parameters on the granule properties. Therefore, in this article, such data is presented for a lab-scale experimental setup. Investigations were carried out into how variations in binder spray rate, binder composition, feed powder flow rate, pan inclination angle, and angular velocity affect particle size distribution, mechanical stability, and humidity. Furthermore, in contrast to existing work samples from both, pan granules and product granules are analyzed. The influence of operation parameter variations on the differences between both, also known as trajectory separation, is described quantitatively. The results obtained indicate an increase in the average particle size with increasing binder flow rate to feed rate and increasing binder concentration and the inclination angle of the pan. Compressive strength varied significantly depending on the operating parameters. Significant differences in properties were observed for the product and the intermediate (pan) samples. In fact, for some operation parameters, e.g., binder feed rate, the magnitude of the separation effect strongly depends on the specific value of the operation parameter. The presented concise data will enable future mathematical modeling of the pan granulation process, e.g., using the framework of population balance equations.

Activated dolomite phosphate rock fertilizers to reduce leaching of phosphorus and trace metals as compared to superphosphate

Use of water-soluble phosphorus (P) fertilizers such as superphosphates (SP) has been increasingly concerned due to the loss of P and trace metals such as cadmium (Cd) into the environment. Activated phosphate rock holds promising as an alternate due to non-destructive and slow release nature. The objective of this study was to assess the leaching potential of P and trace metals from newly developed activated dolomite phosphate rock (HA-DPR and SLS-DPR), as compared to water-soluble fertilizers, including superphosphate (SP), diammonium phosphate (DAP) and original dolomite phosphate rock (DPR). Two representative agricultural soils (Alfisol and Spodosol soil) were sampled at the 0-20 cm depth from two farms for the column leaching experiments. Seven leachings were conducted at day 1, 3, 7, 14, 21, 28, and 56, respectively, and a total of 1050 mL of deionized water (equivalent to half year's rainfall in the Indian River area) was applied for leaching. Leachates were collected from each leaching event and analyzed for water-soluble P and metals. Activated DPR application led to an increase in soil pH by 1.4-1.7 units, whereas SP resulted in a decrease in soil pH by 0.4-1.6 units. P leaching from activated DPR were 33-61 times less than that from SP or DAP. HA-DPR and SLS-DPR treatments reduced cumulative trace metals in leachate by 1.3-12.3 times for the Alfisol soil, and 1.4 to 8.4 times for the Spodosol soil, respectively. These results indicate that activated DPR fertilizers are more environmentally friendly than water-soluble fertilizers.

Characteristics of vegetable crop cultivation and nutrient releasing with struvite as a slow-release fertilizer

Struvite precipitation is an effective method to remove and recover ammonia and phosphate from livestock wastewater. Struvite has properties similar to those of the traditional ammonium-phosphate fertilizer, which does not burn the roots owing to its slow-release characteristics. Struvite is an effective fertilizer as its nutrient-releasing rate is very slow. But the release rate of ammonia and phosphate in soil depends on the size of crystals. In this study, the nutrient-releasing pattern of three types of struvite crystals and liquid fertilizer was compared using soil column. X-Ray fluorescence spectrometry was conducted to investigate the potential use of struvite as a fertilizer. Various struvite crystalline fertilizers were evaluated for their fertilizer performance by cultivating potted vegetable crops. The nitrogen removal efficiency of zeolite-seeded struvite was higher than that of no seed struvite. The ammonia nitrogen removal efficiency was more than 99% irrespective of the kind of zeolite. The soil column test revealed that nutrient releasing from liquid fertilizer and zeolite-seeded struvite recovered from livestock wastewater was 11 and 63 days, respectively. Struvite recovered from livestock wastewater contained more than 20% (w/w) potassium oxide; however, the concentration of heavy metals, such as copper and zinc, was very low. Therefore, we considered that the synthesized struvite using livestock wastewater has high value as fertilizer. The recovered struvite was effective under appropriate concentrations to cultivate all the applied vegetable crops in this study.

Phosphorus recovery through adsorption by layered double hydroxide nano-composites and transfer into a struvite-like fertilizer

Phosphorus (P) recovery from waste streams is attracting more attention due to the twin problems of aquatic eutrophication and global phosphorus scarcity. Layered double hydroxide mineral materials are promising phosphate adsorbents due to their excellent anion-exchange abilities and large surface area. In this study, the adsorption isotherms estimated the saturation capacity of calcined layered double hydroxide (LDH) nano-composites for phosphate as 100.7 mg-P/g-adsorbent. Ion exchange is proposed as the major mechanism in the sorption process. The commonly utilized desorption method via sodium hydroxide was found to cause recrystallization of LDH particles in increased crystal size based on the dissolution-reprecipitation (D-R) mechanism. Ammonia solution was used to desorb P from LDH adsorbent. However, struvite crystallization occurred during the desorption process because struvite has a higher precipitation tendency than LDH with sufficient NH₄⁺ ions. The comparison of P leaching behavior among P-adsorbed LDH, ammonia-treated LDH, and pure struvite samples demonstrates that ammonia desorption probably weakened the interaction between phosphate species and LDH cationic layers, thus enhancing the bioavailability of P in LDH adsorbents. The ammonia-treated LDH has a high potential to serve like struvite as a P slow-releasing fertilizer.

Phosphorus Availability and Release Pattern from Activated Dolomite Phosphate Rock in Central Florida

In this study, novel technology was developed to convert dolomite phosphate rock (DPR) into slow release P fertilizers. The DPR was powdered to <100 mesh and activated with organic molecules under optimal reaction conditions. As compared to original DPR, available P, estimated by water-soluble P released from the three DPRs activated with three types of organic molecules, increased by 6.86, 3.32, and 7.46 times, respectively. Phosphorus supply from the activated DPRs was greater than that of superphosphates (SP). Use of activated DPRs resulted in a significant increase in plant biomass of maize (Zea mays) and millet (Pennisetum glaucum) (succeeding crop), as compared to original DPR, and displayed better effectiveness than monoammonium phosphate or SP. The XRD and FTIR analyses showed that activation did not change the crystal structure of DPR, but the interactions between organic molecules and the P-bearing minerals stimulated P release from DPR.

Arsenic-hyperaccumulator Pteris vittata efficiently solubilized phosphate rock to sustain plant growth and As uptake

Phosphorus (P) is one of the most important nutrients for phytoremediation of arsenic (As)-contaminated soils. In this study, we demonstrated that As-hyperaccumulator Pteris vittata was efficient in acquiring P from insoluble phosphate rock (PR). When supplemented with PR as the sole P source in hydroponic systems, P. vittata accumulated 49% and 28% higher P in the roots and fronds than the -P treatment. In contrast, non-hyperaccumulator Pteris ensiformis was unable to solubilize P from PR. To gain insights into PR solubilization by plants, organic acids in plant root exudates were analyzed by HPLC. The results showed that phytic acid was the predominant (>90%) organic acid in P. vittata root exudates whereas only oxalic acid was detected in P. ensiformis. Moreover, P. vittata secreted more phytic acid in -P and PR treatments. Compared to oxalic acid, phytic acid was more effective in solubilizing PR, suggesting that phytic acid was critical for PR utilization. Besides, secretion of phytic acid by P. vittata was not inhibited by arsenate. Our data indicated that phytic acid played an important role in efficient use of insoluble PR by P. vittata, shedding light on using insoluble PR to enhance phytoremediation of As-contaminated soils.

Phosphorus leaching from a sandy soil in the presence of modified and un-modified adsorbents

Phosphorus (P) leaching from a sandy soil was investigated in the presence of modified and unmodified clay minerals and nanoparticles (NPs). Compared with control soil, amended soil with NPs had the highest percentage of P retention than amended soil with clay minerals. Among the adsorbents used, the highest percentage of P retention was produced by Al₂O₃-chitosan while the lowest percentage of P retention was by zeolite. Data measured for P leaching after using adsorbents were used to predict P leaching using transport model. PHREEQC model was able to model P leaching from control and amended soil. After leaching, P values in control and amended soil were fractionated by a sequential extraction procedure. Concentration of P in Ca-bound fraction (HCl-P) after application of modified and unmodified clay minerals and NPs (except TiO₂ and Al₂O₃) increased and decreased, respectively. Saturation indices (SIs) and P speciation were assessed using the Visual MINTEQ version 2.3 program. According to the SIs, leaching P from control and amended soil with different adsorbent was controlled by dissolution of hydroxyapatite. The results indicated that used adsorbents can reduce P leaching from the sandy soil. Thus, retention of P by amended soil reduced a risk in terms of groundwater contamination with P.

Struvite-based fertilizer and its physical and chemical properties

This study describes a method to formulate struvite fine powder into pellets that are easy to spread on agricultural land. To evaluate the quality of produced pellets, some chemical and physical properties commonly measured for fertilizers were tested. The findings indicated that the salt index and heavy metal content ofstruvite pellets were significantly lower than those of commercial NPK fertilizers. In addition, the percentage of nutrient released from struvite pellets after 105 days was in the range of 9.6-23.2, 8.4-26.7 and 11.3-32.6% for nitrogen, phosphorous and magnesium, respectively, which is considerably lower than that of commercial NPK fertilizer. Among different formulations between struvite crystals and binders, starch and bentonite were the most efficient in agglomerating struvite powder, leading to an increase in the crush strength to over the recommended limit of >2.5 kgf for fertilizer hardness.

Sources of nutrients impacting surface waters in Florida: a review

The promulgation of numeric nutrient criteria for evaluating impairment of waterbodies in Florida is underway. Adherence to the water quality standards needed to meet these criteria will potentially require substantial allocations of public and private resources in order to better control nutrient (i.e., nitrogen and phosphorus) releases from contributing sources. Major sources of nutrients include atmospheric deposition (195-380 mg-N/m(2)/yr, 6 to 16 mg-P/m(2)/yr), reclaimed water irrigation (0.13-29 mg-N/L, 0.02 to 6 mg-P/L), septic systems (3.3 × 10(3)-6.68 × 10(3) g-N/person/yr, 0.49 × 10(3)-0.85 × 10(3) g-P/person/yr) and fertilizer applications (8 × 10(6)-24 × 10(6) mg-N/m(2)/yr). Estimated nitrogen loadings to the Florida environment, as calculated from the above rates are as follows: 5.9 × 10(9)-9.4 × 10(9) g-N/yr from atmospheric deposition, 1.2 × 10(8)-2.6 × 10(10) g-N/yr from reclaimed water, 2.4 × 10(10)-4.9 × 10(10) g-N/year from septic systems, and 1.4 × 10(11) g-N/yr from fertilizer application. Similarly, source specific phosphorus loading calculations are also presented in this paper. A fraction of those nutrient inputs may reach receiving waterbodies depending upon site specific regulation on nutrient control, nutrient management practices, and environmental attenuation. In Florida, the interconnectivity of hydrologic pathways due to the karst landscape and high volumes of rainfall add to the complexity of tracking nutrient loads back to their sources. In addition to source specific nutrient loadings, this review discusses the merits of source specific markers such as elemental isotopes (boron, nitrogen, oxygen, strontium, uranium and carbon) and trace organic compounds (sucralose, gadolinium anomaly, carbamazepine, and galaxolide) in relating nutrient loads back to sources of origin. Although this review is focused in Florida, the development of source specific markers as a tool for tracing nutrient loadings back to sources of origin is applicable and of value to all other geographical locations.

Recovery of struvite from animal wastewater and its nutrient leaching loss in soil

Contaminants in swine wastewater were recovered in the form of struvite, a crystal of magnesium ammonium phosphate (MAP), using a newly designed process, and the leaching loss of MAP in soil was examined. The continuous flow process was operated under optimal conditions: 1.0 molar ratio of magnesium (Mg) addition with respect to orthophosphate (OP) and an aeration rate of 0.73 L/L min. Five treatments were performed with three replications for soil pH and nutrient leaching loss tests. It was found that 93% of the OP in the swine wastewater was crystallized, and the MAP crystal formation was verified by X-ray diffraction (XRD) and scanning electron microscope (SEM) analyses. The analyses revealed that the pattern of pH change and N leaching losses for MAP-treated soil were remarkably different from those for fused super phosphate (FSP)-urea-treated soils. The pH levels for the control and FSP-urea-treated soils after a five-week experiment were unchanged or slightly decreased, whereas an increase in pH was observed in the MAP-treated soils. Leaching loss of N was higher in FSP-urea treatments, with MAP treatments showing N losses of only 1.93 and 2.05%, respectively, while FSP-urea treatments showed N losses of 7.82 and 6.47%, respectively, during the same period. Phosphate (P) leaching was very slow in both MAP- and FSP-treated groups.

Use of amendments to reduce leaching loss of phosphorus and other nutrients from a sandy soil in Florida

GOAL, SCOPE AND BACKGROUND

Transport of P from agricultural land contributes to the eutrophication of surface waters. Soil amendment is considered one of the best management practices (BMPs) to reduce P loss from sandy soils. Laboratory column leaching experiments were conducted to evaluate the effectiveness of different soil amendments in reducing P leaching from a typical sandy soil in Florida.

METHODS

The tested amendments were CaCl2, CaCO3, Al(OH)3, cellulose, and mill mud, and applied at the rate of 15 g/kg for a single amendment and each 7.5 g/kg if two amendments were combined. Leaching was conducted every four days for 32 days, 250 mL of deionized water being leached for each column per leaching event. Leachates were collected from each leaching event and analyzed for reactive P, PO4-P, and macro and micro-elements.

RESULTS AND DISCUSSION

Except for the soils amended with CaCl2, or CaCl2+CaCO3, reactive P and PO4-P leaching losses mainly occurred in the first three leaching events. Phosphorus leaching from the soils amended with CaCl2 or CaCl2+CaCO3 was less but more persistent than that of other amendments. Reactive P leaching loss was reduced by 36.0% and 40.4% for the amendments of CaCl2, and CaCl2+CaCO3, respectively, as compared with chemical fertilizer alone, and the corresponding values for PO4-P were 70.8% and 71.9%. The concentrations of K, Mg, Cu, and Fe in leachate were also decreased by CaCl2 or CaCl2+CaCO3 amendment. Among the seven amendments, CaCl2, CaCO3, or their combination were most effective in reducing P leaching from the sandy soil, followed by cellulose and Al(OH)3, the effects of mill mud and mill mud + Al(OH)3 were marginal.

CONCLUSIONS

These results indicate that the use of CaCl2, CaCO3, or their combination can significantly reduce P leaching from sandy soil, and should be considered in the development of BMPs for the sandy soil regions. Recommendations and Outlook. Most agricultural soils in south Florida are very sandy with minimal holding capacities for moisture and nutrients. Repeated application of fertilizer is necessary to sustain desired yield of crops on these soils. However, eutrophication of fresh water systems in this area has been increasingly concerned by the public. Losses of P from agricultural fields by means of leaching and surface runoff are suspected as one of the important non-point contamination sources. The benefits and effectiveness of soil amendment in reducing P losses from cropping production systems while sustaining desired crop yield need to be demonstrated. Calcium chloride, CaCO3, or their combination significantly reduce P leaching from sandy soil, and should be considered in the development of BMPs for the sandy soil regions.