A study was conducted to evaluate the potential of struvite as a P amendment in three contrasting soils. In incubation studies, the P content of the solution increased with increasing amounts of struvite application.
Dissertation structure
The chapter also includes general conclusions on the ability of struvite as an alternative P fertilizer and gaps for future research.
Introduction
Urine separation and utilisation as a fertilizer material
Studies conducted by Ronteltap et al. 2007) showed that most heavy metals such as cadmium (Cd), cobalt (Co), chromium (Cr), copper (Cu), nickel (Ni) and lead (Pb) precipitate as carbonates and hydroxides (Udert et al. , 2003; In South Africa, studies conducted by Mnkeni et al. , 2008) used urea as a conventional fertilizer and urine was applied based on the recommended N.
Challenges of using urine as a fertilizer material
The solid state of struvite allows easy handling in terms of storage and transport (Etter et al., 2011). The production of struvite has further been shown to reduce the pathogen risk normally associated with handling human urine (Decrey et al., 2011).
Struvite as an alternative urine management strategy
The choice of the Mg source plays a decisive role in determining the quality and the cost of production of struvite and therefore the cost of the final product (Etter et al., 2011). According to Ronteltap et al. 2010), the main parameters affecting struvite particle size are pH and temperature.
Benefits of using struvite as an alternative management strategy: Effects of struvite
The nutrient content of urine is subject to variation depending on factors such as handling and diet of the producers and therefore there is no consistency in the nutrient content of the urine. During struvite precipitation, N recovery can be increased to 60% of the total initial N by the use of zeolite and/or activated carbon (Ganrot et al., 2007) with zeolite being preferred due to cost-related issues.
Struvite use as fertilizer and its effects on crop growth and nutrient uptake
2011) explained these observations similarly, but in contrast to Antonini et al. 2012), as the latter considered struvite solubility in water to be the main driving parameter of efficacy, while Cabeze et al. 2011) believe that the lower purity (presence of other nutrients) of struvite is the main driving force behind the positive results of struvite. Similar observations regarding struvite potential were made by Ryu et al. 2012) who showed that there is an improvement in the growth of Chinese cabbage with an increase in the application rate of struvite (Figure 2.8).
Soil properties that affect the effectiveness of P released from struvite
- pH
- Ionic strength
- Organic matter
- Iron and aluminium oxides and clay mineralogy
In general struvite solubility is inversely related to pH, i.e., it decreases with an increase in pH (Doyle and Parsons, 2002). As the pH increases there is an increase in the concentration of phosphate ions and a decrease in the concentration of ammonium due to the reduction of protons in the system. As the pH decreases; The electrostatic potential at the P adsorption plane increases, leading to an increase in P affinity and thus adsorption.
Struvite solubility in a pure system increases with an increase in ionic strength, due to the consequent decrease in the effective concentration of the component ions of struvite (Bhuiyan et al., 2007). The presence of counterions such as Na+ or K+ or any other cations that increase with an increase in electrolyte concentration makes the adsorption surface less negative and therefore improves phosphate adsorption by increasing the electrostatic potential on the adsorption surface. 1980) concluded that an increase in ionic strength is similar to a decrease in pH which also increases. the electrostatic potential in the plane of adsorption. 2008) also made a distinction regarding the contribution of various cations in the enhancement of P-adsorption, whereby it was observed that there was an enhancement in P-adsorption when the electrolyte was higher in K+ than Na+. Soil pH plays an important role in organo-metal interaction, as OM strongly adsorbs to metals at low pH and the adsorption decreases with an increase in pH.
Several researchers have observed an increase in P availability in the presence of OM, and it is on the basis of this mechanism that Sibanda and Young (1986) proposed a decrease in P adsorption in the presence of organic matter.
Conclusions
Soils
- Soil analysis
- Soil characteristics
As expected, Ia had a high concentration of both DCB and oxalate-extractable Fe and Al. The exchangeable acidity of Se was low and the same as that of Cf. The Ia had a loam texture, while the Se was a clay loam and the Cf a loamy sand.
Struvite
- Struvite analysis
- Struvite properties
At higher rates, less struvite dissolves due to the sufficiently high pH and higher solid:solution ratios, resulting in less ammonium hydroxide being formed with less effect on the final pH. This view is perhaps further confirmed by the strong correlation between Na and EC (Table 3.4). Furthermore, there is a positive relationship, as determined using Microsoft Excel 2010, between EC and the amount of struvite applied and there is a strong negative correlation between pH and EC (tables 3.3 and 3.4).
EC on struvite solubility, there is very limited information on the effect of struvite on pH and EC, although they are considered to be the main variables controlling struvite solubility (Doyle and Parsons, 2002; Bhuiyan et al., 2007). The drop in pH with increasing struvite deposition amount (Table 3.3; r2 = 0.86;, Table 3.4) can be explained by its limited solubility at higher deposition amounts, as element concentrations increased at a decreasing rate in relation to the deposition amount. This idea may be further supported by the pH value at 0.1 g L-1, as it was the highest, and solubility should be the highest at this application rate.
This perhaps further explains the release of the elements from struvite, which Massey et al. 2009) have shown that struvite solubility decreases with an increase in pH.
- Introduction
- Materials and methods
- Sorption studies
- Sorption isotherms
- Incubation experiments
- Results
- Adsorption parameters
- Patterns of P release from struvite in different soils during incubation
- Patterns of magnesium release from struvite in different soils during the second
- pH changes during first incubation
- Discussion
- Conclusions
Further information on the chemical mechanism of the sorption can be derived from the Langmuir equation (Bubba et al., 2003). The equilibrium solution concentrations for Se and Cf were different from those of Ia due to the known high sorption properties of Ia. Buffer index, the slope of the tangent of the Freundlich equation; PBC (phosphorus buffer capacity) - slope of the tangent of the linear equation Q (mg kg-1) versus C (mg L-1); Q 0.2,0.1 - initial amount of P required to maintain the P concentration in the soil solution at levels = 0.2 and 0.1 mg L-1, respectively; Kf – the distribution coefficient.
During the first incubation, solution P increased with an increase in the amount of struvite in all three soils, especially in the Ia and Cf (Figure 4.1). The sandy texture of the Cf explains the lower P sorption parameters compared to the Ia and Se. The rapid increase in pH in the Ia by 0.6 pH units at the highest struvite dosage correlates well with the change in soil P concentration.
The affinity of the soils for P, represented by Kf, was in the order Ia > Se > Cf.
- Materials and Methods
- Pot experiment
- Chemical analysis of plant tissue and soil
- Statistical analysis
- Results
- Effect of struvite application rate on maize plant height, leaf area and drymatter yield
- Maize shoot phosphorus content as a function of different struvite application rates 55
- Effect of struvite on soil solution P content at harvest
- Effect of struvite application rate on soil nutrient composition at harvest
- Discussion
- Conclusions
Tissue P did not respond to struvite application rate or to SSP application in Se (Figure 5.2). In contrast to the dry matter yield results, tissue P increased with application rate beyond half the recommended amount in Cf, while SSP was similar to struvite at the recommended amount in Se. The uptake of other elements on Ia and Se was higher in SSP-treated pots compared to the struvite, except for Mn and Fe, which were highest in the struvite treatment at the recommended rate in Ia (Table 5.1).
In Cf, intakes of N, K, Na, Zn, Cu and Fe were highest at half the recommended rate in the struvite treatment while Ca, Mg and Mn were highest in the SSP treatment. When struvite was applied at half the recommended rate, soil P solubility in Se was similar to that of Cf and the trend was similar to that of P uptake (Figure 5.3). The concentration of Ca, Mg, Zn, Mn and Cu was higher in the SSP treatment, K was higher in the treatment with twice the recommended struvite rate for Ia.
In the Cf, K and Mn were highest in the SSP while Ca and Mg were highest in the struvite treatment at twice the recommended dose (Table 5.2).
Introduction
Agricultural implications of struvite as an alternative P source
In the three soils, recoverable soil P at harvest was different for the struvite-treated pots compared to the SSP treatments. In the Ia, the amount of soil P in struvite-treated pots was greater than in SSP-treated pots; in the Cf, the P content in the soil was the same in all treatments; and in the Se, soil P content at harvest in struvite treatments was lower than that where SSP was applied. Among all treatments and all soils, the highest dry matter yield was in the SSP-treated Se, and this could be explained by the combination of high inherent fertility and unrestricted dissolution of SSP compared to struvite (Massey et al., 2009). ).
In the struvite treatments, the lower yield in Se compared to Cf could be explained by the low solubility of struvite in Se with pH 7.1 than Cf with pH 6.3. The uptake of Zn, Cu, Mn, Ca and K was highest in Cf, while Ia had the lowest uptake of macro- and micronutrients. This may in Ia explain the high amount of extractable soil P in the struvite-treated pots at harvest compared to SSP-treated pots.
In the Se, the soil P content was a function of the poor solubility of the struvite in this higher pH soil.
Recommendations for the future work
The incubation studies suggest that P in the Cf is mainly controlled by dissolution/precipitation reactions rather than adsorption/desorption reactions, and that its pH was not limiting for struvite dissolution. The use of human urine as organic fertilizer in okra (Abelmoschuus esculentus) production in southeastern Nigeria. 2007) Recovery of N and P from human urine by freezing, struvite precipitation and adsorption to zeolite and activated carbon. 2000) An economic and environmental evaluation of the potential for replacement of recovered phosphorus.
Kanabo, I.A.K., Gilkes, R.J (1988) The effects of moisture regime and incubation period on the decomposition of North Carolina phosphate rock in soil. 2011) Human urine as a source of alternative natural fertilizers in agriculture A flight of fantasy or a feasible reality. Preliminary study of pH effect in the application of Langmuir and Freundlich isotherms to Cu-Zn competitive adsorption.
2008) Evaluation of human urine as a nutrient source for selected vegetables and maize under tunnel house conditions in the Eastern Cape, South Africa. 2009) Evaluation of source separated human urine (ALW) as a source of nutrients for banana cultivation and impact on quality parameter. 2009) Phosphorus dissolution from animal bone carbon in 12 soils. 2012) Factors controlling the interaction of phosphate with iron oxides.
