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Introduction Total coffee production in the world in 2013 was 8.7 million Mg of coffee beans. Coffee must be processed after harvest to separate beans from the husk, and the post-harvest process can generate different types of residues. Knowing that 50 % of the harvested coffee is husk, it is important to consider that these by-products can contribute to environmental problems if not disposed of properly. There is a need to find alternatives to the piling of coffee husk as a means of disposal at coffee farms. The objectives in this experiment were to characterize five types of coffee processing residues and assess their value as potassium (K) fertilizer by examining K release. Results Chemical characteristics of coffee residues varied with processing method, such as wet and dry processes. Application of residues to soil columns followed by 40 weeks of simulated leaching increased total K content in the soil for all treatments based on application rate, except for enriched and three-year composted coffee husk (3ycomp). Coffee processing residues had higher concentrations of K leached compared with the control with no K application, but only 3ycomp was higher than other coffee processing residues. Total K released from coffee residues increased linearly with application rate, but the decomposition of the coffee residues was low, which suggest that K release from coffee processing residues is not related to decomposition. Conclusion Post-harvest coffee processes impacted concentrations of K, nitrogen, carbon, cellulose, hemicellulose, lignin, phenol, pH, and electrical conductivity among the coffee processing residues. The K release was high (over 90 %), but it did not depend on the type of coffee processing residue, and thus, the residues can be a substitute for a mineral source of K applied to soils. The use of coffee processing residues as source of K did not prevent K losses by leaching. Some coffee residues resulted in greater K leaching than mineral KCl applied at the same rate.

Eutrophication is a major problem in lakes and rivers throughout the world. One such system is Lake Macatawa, located in West Michigan, which hydrologically connects to Lake Michigan. Lake Macatawa and its watershed suffer from excess phosphorus and sediment loads. The total maximum daily load for the lake calls for a total phosphorus (TP) reduction of 75%, which would reduce the water column total phosphorus concentration from 125 μg/L to 50 μg/L. Understanding how P moves through this landscape, into Lake Macatawa, and ultimately to Lake Michigan and the St. Lawrence Seaway, is critical to managing and controlling P runoff. A potentially significant source of P to Lake Macatawa occurs through agricultural tile drainage. Various best management practices (BMPs) have been implemented in the Macatawa watershed to reduce P loading, especially surface runoff, but their overall effectiveness has been limited. Electric arc furnace (EAF) slag, a waste product from the steel industry, can chemically bind P and has been used previously in agricultural settings. Three iron slag filters were installed at the end of agricultural tile lines in the Macatawa watershed and evaluated to assess their effectiveness in removing P, while also monitoring for the presence of potentially toxic chemicals leaching from the slag. After 1 year of slag filter performance, both SRP (soluble reactive phosphorus) and TP decreased in the tile drain effluent: percent reductions of soluble reactive phosphorus and TP ranged from 7.4% to 57.3% and 59.5–76.5%, respectively. Absolute concentrations of TP were reduced to between 100 and 329 μg/L, which still exceeds the 50 μg/L goal for Lake Macatawa. Concentrations of toxic metals, polycyclic aromatic hydrocarbons compounds, and cyanide all were at levels below drinking water standards. Our preliminary conclusions are that the installation of these filters should be targeted to areas where tile drain effluent P levels are very high (SRP > 250 μg/L) to obtain an optimal cost/benefit ratio. While they are not a panacea, when installed in combination with other BMPs (Best Management Practices), EAF slag filters may play an important localized role in reducing P to Lake Macatawa and farther downstream.

Pollution control structures may be adapted to enhance the removal of nutrients and pesticides from water. Charcoal-like material is known to sorb organic compounds in solution, including atrazine, a herbicide used to control broadleaf weeds, which persists in soils and can migrate from soils to water bodies, ultimately affecting water quality. The primary goal of this study was to investigate if activated charcoal cloths (single-weave, SW; double-weave, DW; and knitted, KT) are more efficient to remove atrazine from aqueous solutions than a cloth without activated charcoal (control) currently used in pollution control structures. The approach consisted of sorption kinetics, flow-through, and desorption/degradation experiments using 50 and 1000 μg L-1 atrazine solutions. Results showed that within 30 min of contact time, the control sorbed from 22.6 to 36.1% of atrazine in solution; whereas the activated charcoal cloths sorbed from 76 to 99% of atrazine in solution (SW > KT > DW). The flow-through experiments showed that the SW sorbed 88.2, 76.1, and 52.2% of atrazine at the contact time of 0.75, 5, and 10 min, respectively. After 28 days of incubation, previously sorbed atrazine on the SW cloth did not degrade and

Previous studies have shown that excess phosphorus (P) uptake by maize can lead to a decreased grain yield. Part one of this study demonstrated that maize experienced luxury consumption of P in three phases of P uptake. The objective of this work was to further explore how P uptake indirectly impacts the uptake of other nutrients and their translocation within the plant to explain the yield penalty associated with luxury P consumption. Three maize hybrids were grown under optimal conditions using sand-culture hydroponics for precise control of the root environment. Plants were grown to maturity with six different P concentrations followed by biomass and nutrient partitioning analysis of various maize parts. All non-P nutrients achieved maximum grain content at P uptake levels that coincided with the maximum grain yield, while the partitioning of K, Mg, Mn, B, N, S, and Fe into other non-grain tissue continued with further P uptake. With luxury P consumption beyond the point corresponding with maximum grain yield, the N, S, Fe, Cu, and Zn grain content significantly decreased along with the grain yield. With luxury P consumption, Cu, Zn, and Fe accumulated in the roots. Grain production with luxury P uptake may have been limited by P-inhibited translocation of Cu, Zn, and Fe from roots to grain. This decrease in translocation did not prevent further non-grain tissue growth since those nutrients were not as limiting as they were for grain. Data suggest that these micronutrients limited protein production, which was evident from the decrease in grain N and S content and concentration that coincided with the decrease in grain yield concomitant with luxury P uptake.

Development of a more precise and process-based tool for making phosphorus (P) recommendations requires detailed understanding of plant P uptake needs. Future adaptation of a nutrient uptake model for this purpose must utilize a mass-balance approach. The objectives of this study were to determine the minimum P uptake mass required for achieving maximum grain yield of maize and to evaluate plant P partitioning over a range of P uptake. Three maize hybrids were grown under optimal conditions using sand-culture hydroponics for precise control of the root environment. Plants were grown to maturity with six different P concentrations followed by biomass and nutrient partitioning analysis of various maize parts. Phosphorus uptake occurred in three phases with two steps of luxury consumption; (i) increased uptake with increased grain yield and total biomass until maximum grain yield was attained at 580 mg P uptake, (ii) further P uptake with increase in total biomass until 730 mg P uptake, but with decrease in grain yield; and (iii) additional P uptake with little to no increase in total biomass and continued decrease in grain yield. Luxury consumption of P implies that excess P fertility is an economic drag for grain production.

Abstract Phosphorus (P) removal structures are a new best management practice for filtering dissolved P in non‐point drainage from legacy P soils through use of P sorption materials (PSMs). Structures must be designed according to characteristics of the site (hydrology and constraints) and PSMs to be utilized, as well as user‐defined goals (P removal, lifetime, and flow rate), making it a cumbersome process. A freely available P Transport Reduction App (P‐TRAP) allows users to quickly produce a custom design or evaluate a hypothetical or existing structure. The software includes a library of P removal flow‐through curves for many different PSMs conducted under various conditions of inflow P concentration and retention time. Design output includes the necessary PSM mass and orientation, pipe requirement, and a table of annual P removal. The software enables conservationists and engineers to quickly compare cost and efficiency among possible designs. Phosphorus Transport Reduction App (P‐Trap) software is used to evaluate and design site‐specific P removal structures based on (a) user inputs regarding site and P sorption material (PSM) characteristics and (b) P removal performance goals.

Soil phosphorus (P) solubility and kinetics partly control dissolved P losses to surface water and uptake by plants. While previous studies have focused on batch techniques for measuring soil P desorption kinetics, flow-through techniques are more realistic because they simulate P removal from the system, akin to runoff, leaching, and plant uptake. The objectives were to measure soil P desorption by a flow-through technique at two flow rates and several batch methods, and utilize both for understanding how flow rate impacts the thermodynamics and kinetics of soil P desorption. Desorption obeyed first-order kinetics in two different phases: an initial rapid desorption phase followed by a gradual release. Desorption was limited by equilibrium and the kinetics of physical processes as demonstrated by an interruption test. Dilution-promoted desorption occurred with increasing cumulative volume, which increased desorption rate via first-order kinetics. The batch tests that simulated cumulative solution volume and time of flow-through were similar to the flow-through results; however, the batch methods overestimated the desorption rates due to less limitations to diffusion. Fast flow rates desorbed less P, but at a greater speed than slow flow rates. The differences were due to contact time, cumulative time, and solution volume, which ultimately controlled the potential for chemical reactions to be realized through physical processes. The interaction between these processes will control the quantity and rate of desorption that buffer P in non-point drainage losses and plant uptake.

The purpose of this special issue is to explore current challenges and develop a better understanding of the processes that control dissolved phosphorus (P) removal among P removal structures [...]

Several structural, treatment, and management approaches exist to minimize phosphorus (P) transport from agricultural landscapes (e.g., cover cropping and conservation tillage). However, many of these practices are designed to minimize particulate P transport and are not as effective in controlling dissolved P (DP) losses. Phosphorus removal structures employ a P sorption material (PSM) to trap DP from flowing water. These structures have been successful in treating surface runoff by utilizing aluminum (Al)-treated steel slag, but subsurface tile drainage has never been tested with this material. The goal of this study was to evaluate the performance and economics of a ditch-style P removal structure using Al-treated steel slag for treating agricultural subsurface drainage discharge. The structure treated subsurface drainage water from a 4.5 ha agricultural field with elevated soil test P levels. Overall, the structure removed approximately 27% and 50% of all DP and total P (TP) entering the structure, respectively (i.e., 2.4 and 9.4 kg DP and TP removal). After an initial period of strong DP removal, the discrete DP removal became highly variable. Flow-through analysis of slag samples showed that the slag used to construct the structure was coarser and less sorptive compared to the slag samples collected prior to construction that were used to design and size the structure. Results of this study highlight the importance of correctly designing the P removal structures using representative PSMs.

Blind inlets are implemented to promote obstruction-free surface drainage of field depressions as an alternative to tile risers for the removal of sediment and particulate phosphorus (P) through an aggregate bed. However, conventional limestone used in blind inlets does not remove dissolved P, which is a stronger eutrophication agent than particulate P. Steel slag has been suggested as an alternative to limestone in blind inlets for removing dissolved P. The objectives of this study were to construct a blind inlet with steel slag and evaluate its ability to remove dissolved P, nitrogen (N), and herbicides. A blind inlet was constructed with steel slag in late 2015; data from only 2018 are reported due to inflow sampling issues. The blind inlet removed at least 45% of the dissolved P load and was still effective after three years. The dissolved P removal efficiency was greater with higher inflow P concentrations. More than 70% of glyphosate and its metabolite, and dicamba were removed. Total N was removed in the form of organic N and ammonium, although N cycling processes within the blind inlet appeared to produce nitrate. Higher dissolved atrazine and organic carbon loads were measured in outflow than inflow, likely due to the deposition of sediment-bound particulate forms not measured in inflow, which then solubilized with time. At a cost similar to local aggregate, steel slag in blind inlets represents a simple update for improving dissolved P removal.

The reduction of dissolved phosphorus (P) transport to water systems is of critical importance for water quality. Phosphorus sorption materials (PSMs) are media with high affinity for dissolved P, and therefore serve as the core components of P removal structures. These structures can intercept dissolved P in surface and subsurface flows, before discharge into water bodies. While the P removal ability of PSMs has been extensively studied, lesser is known about the capacity to regenerate and recover P from P-saturated PSMs. This article evaluates a methodology to recover the P removal ability of aluminum- and iron-rich P-saturated PSMs. A series of flow-through experiments were conducted, alternating between P sorption (0.5 and 50 mg L − 1 P) and desorption with potassium hydroxide (KOH; 5 or 20 pore volumes [PV]), varying residence times (0.5 min and 10 min), and number of recirculations (0, 6 and 24). Across two cycles of sorption-desorption, Alcan, Biomax and PhosRedeem showed an average P recovery of 81%, 79%, and 7%, with standard deviation of 10%, 21% and 6%, respectively. The most effective regeneration treatment was characterized by the largest KOH volume (20 PV) and no recirculation, with up to 100% reported P recovery. Although KOH at 5 PV was less effective, the use of recirculation did increase P recovery. The lifetime of Al/Fe-dominated PSMs in P removal structures can be extended through feasible regeneration techniques demonstrated in this study, for both high and low P concentration scenarios.

Atrazine is one of the most common broad-leaf herbicides used in the world. However, due to extensive use for many years, atrazine often appears in surface and groundwater. Atrazine transport is inhibited by degradation or sorption to soil components, especially organic matter. Biochar is a charcoal-like material produced from pyrolysis of biomass. Due to the amount and type of functional groups found on biochar, this product has shown potential for sorption of atrazine from solution. There is an interest in developing best management practices utilizing biochar to filter atrazine from non-point drainage with pollution-control structures such as blind inlets. The objective of this study was to explore the kinetics and thermodynamics of atrazine sorption to biochar using two different approaches: flow-through sorption cells and isothermal titration calorimetry (ITC). Twenty-five milligrams of an oak (Quercus spp.)-derived biochar was suspended in water and titrated 25 times (0.01 mL per titration) with atrazine at three different concentrations, and by a single titration (0.25 mL), with heat of reaction directly measured with ITC. A benchtop atrazine sorption study that simulated the titration experiment was also conducted. A continuous flow-through system was used to quantify the impact of contact time on atrazine sorption to biochar. Atrazine sorption to biochar displayed both exothermic and endothermic signals within each titration, although the net reaction was exothermic and proportional to the degree of sorption. Net enthalpy was −4,231 ± 130 kJ mole−1 atrazine sorbed. The existence of both exotherms and endotherms within a single titration, plus observation of an initial fast reaction phase from 0 to 300 s followed by a slower phase, suggested multiple sorption mechanisms to biochar. Results of flow-through tests supported kinetics observations, with the 300 s contact time removing much more atrazine compared to 45 s, while 600 s improved little compared to 300 s. Based on flow-through results, annual atrazine removal goal of 50%, and typical Midwestern U.S. tile drainage conditions, a pollution-control structure implementing this biochar sample would require 32 and 4 Mg for a design utilizing a contact time of 45 and 300 s, respectively. Future work is necessary for estimating degradation of atrazine sorbed to biochar.

Occasionally, the classic understanding of the effect of pH on P uptake from soils is questioned through the claim that maximum P uptake occurs at a pH much lower than 6.5−7. The purpose of this paper was to thoroughly examine that claim and provide a critical review on soil processes that control how soil pH affects P solubility and availability. We discuss how individual P retention mechanisms are affected by pH in isolation and when combined in soils, and how both real and apparent exceptions to the classic view can occasionally occur due to dynamics between mechanisms, experimental techniques (equilibration time, method of soluble P extraction, and pH adjustment), and plant species that thrive under acidic conditions. While real exceptions to the rule of thumb of maximum P availability at near neutral pH can occur, we conclude that the classic textbook recommendation is generally sound.

Mehlich-3 (M3) is one of the most common agronomic and environmental phosphorus (P) extractants for determining P fertilizer requirements and the potential for non-point source pollution. Understanding how soil properties impact M3 extractability can improve our ability to properly use this soil test. The objectives of this study were to investigate the impact of soil pH on P extractability by M3 and water in different soils containing equal total P, and to ascertain information about mechanisms of M3-P extraction. Soil pH at four field sites was previously adjusted to a range of approximately 4.5–7.5. Soils (Grant, Dale, Teller, Easpur) were characterized, and P was extracted with M3 and water. Extraction of Mehlich-3 P decreased 40% to 55% with increasing pH, which was potentially due to changing P forms, partial neutralization of extractant pH, and consumption of extractant fluoride (F−) by non P-containing calcium (Ca) minerals. Water-soluble P (WSP) increased with increasing pH up to pH 6–7. Mehlich-3 P and WSP were not positively correlated except for one soil type. Mehlich-3 P is best utilized with WSP as indicators of quantity and intensity, respectively. Use of M3-P alone at pH < 5.5 may overestimate solubility. Further research should examine the suitability of M3-P at pH > 7.

Agricultural drainage ditches can deliver high loads of phosphorus (P) to surface water. Installation of filter structures containing P sorbing materials (PSMs), including gypsum, is an emerging practice that has shown promise to reduce these P loads. The objective of this study was to evaluate what effect soil amendment with gypsum would have on soil P concentrations and forms in a laboratory incubation experiment. Gypsum was saturated at two levels with P, and applied to a silt loam and a sandy loam at two rates. The treated soils were incubated in the laboratory at 25°C, and samples were collected on eight dates between 0 and 183 days after amendment. Spent gypsum application did not significantly increase soil water-extractable or Mehlich 3 P when applied at typical agronomic rates. This appears to be a viable strategy to remove P from agricultural drainage waters but does not appear to provide any additional P fertilizer value.