Research Projects

Research Projects

Chemical and physiological aspects of various forms of phosphorus fertilizers

Dr. Ran Erel
Start: Jan 2017 –  End: Jan 2019

General: Phosphorus (P) is an essential macronutrient with a central role in numerous structural and biochemical plant processes. Due to strong P fixation to soil constituents, P is limiting crop production in many arable soils and P fertilizer application is frequently needed to achieve high productivity (Hinsinger 2001; Lynch 2007). Unfortunately, rock phosphates that are mined for elaborating P fertilizers are a finite and depleted resource, and thus their prices are expected to further rise in the coming decades (Cordell et al. 2009). Plants have developed numerous P acquisition means to access and release soil P. In the course of modern plant breeding some of those adaptive traits have been lost (Wissuwa et al. 2009). Therefore, there is a need to develop future P fertilization strategy with superior P fertilizer utilization and high P recovery.

P in soils. Although the total amount of P in the soil may be high, it is often present in unavailable forms (Schachtman et al. 1998). Hence, in most soils, P is strongly bound to soil particles or found in the organic form. In acidic soils P predominantly reacts with iron and aluminum, and in alkaline soils with calcium to form poorly soluble complexes (Holford 1997). These P pools are not readily available for plants roots hence merely small fraction of soil P is available. Thus, in many agricultural systems the application of P to the soil is necessary to ensure plant productivity. However, the recovery of applied P by crop plants in a growing season is very low, because in the soil more than 80% of the P becomes immobile and unavailable for plant uptake because of adsorption, precipitation, or conversion to the organic form (Holford 1997; Vance et al. 2003). This result in P loading of agricultural land that may lead to hazardous run-off from P-loaded soils which causing eutrophication and hypoxia of lakes and marine estuaries of the developed world.

Several mechanisms allowing release of sparingly available P fixed in the soil. The main acknowledged means are: pH modification, competition on adsorption sites and phosphatases activity. Soil pH adjustment has large potential to affect P availability (Devau et al. 2011b; Hinsinger 2001; Hinsinger et al. 2003), and acidification is frequently measured in response to P deficiency. Additional way of releasing P is by competing on sorption sites. Several organic ions where shown to be enhance P solubility. Plants roots exudation of carboxylates was reported to potentially increase the solubilization of P compounds under P deficiency (Bolan et al. 1994; Veneklaas et al. 2003), depending on the carboxylate and soil type (Duputel et al. 2013). Of the organic acids, citric, oxalic and malic acids are the most frequently referred to for their potential effect on the rhizosphere (Hinsinger et al. 2003). A large portion of soil P occurs in organic forms, and a common response of plants and microorganisms to P-deficiency is the synthesis and exudation of acid phosphatase enzymes (Richardson et al. 2011; Simpson et al. 2011), while alkaline phosphatase enzymes are being produced and released only by microorganisms (George et al. 2002). Sustainable management of P in agriculture requires to discover mechanisms that enhance P acquisition and exploit these adaptations to make plants more efficient at acquiring P, and advance fertilization management schemes that increase soil P availability. In this context, improved understanding of P cycling in the soil is desirable.

P fertilizers: In large portion of arable soils P is limiting plants growth (Lynch 2007) and thus, P fertilization is required for profitable agrosystem. Chemical P fertilizers are derived from phosphate rock. Two popular forms of P fertilizers are: single superphosphate (SSP) and triple superphosphate (TSP) which produced by reacting fine phosphate rock with liquid sulfuric or phosphoric acid. These high P and water soluble sources of P are also good source for calcium and sulfur nutrition (Chien et al. 2011).  When TSP or SSP dissolves in the soil solution, it is hydrolyzed and H3PO4 is released causing temporal lower soil pH.

Second group of water-soluble P fertilizers are produced from phosphoric acid and include: MAP, MKP and poly-P. The various forms of P fertilizers may affect P availability and consequently, P acquisition by plants roots. Yet, the interaction between P application form and chemical processes (i.e. soil P distribution, fixation dynamic and P availably) are highly dependent on environmental conditions and thus, are not simple to predict. Few of the factor acknowledged to play a role in soil P dynamics: clay mineralogy, soil texture, pH and moisture (Devau et al. 2011a; Marschner 2011). The mechanisms governing P availability of the various P fertilizers are not adequately understood. On top of that, plants roots significantly involve and interact with these processes adding extra level of complexity (Hinsinger 2001; Hinsinger et al. 2009). Only a limited amount of research has been reported in the literature regarding the agronomic effectiveness of different fully acidulated P fertilizers in terms of source (e.g., SSP, TSP and MAP). Most published reports on this topic have focused on water-insoluble and partially water soluble P sources as rock P (Chien et al. 2011). Exploring the chemical and physiological mechanisms altering P availability in given environmental conditions will lead to better P management and enhance fertilizer utilization efficiency by predicting the most suitable P formulation and application methods.

Polyphosphate: Polyphosphates (poly-P) composed of two or more orthophosphates (ortho-P) units condensed to linear or cyclic chains sharing oxygen atoms. In soils, poly-P are subject to hydrolysis to form several ortho-P or pyrophosphate. Poly-P based fertilizers are commonly used in agriculture (McBeath et al. 2007b). While various forms of poly-P are commonly applied fertilizers, ortho-P is the chief readily available form for plants. Poly-P hydrolysis typically occurs within few days or weeks subsequent to soil poly-P application with estimated half-life of 2-3 weeks (McBeath et al. 2007a). Conversely, poly-P is very stable in solutions as long as pH and temperature are not extreme (McBeath et al. 2007a). The hydrolysis reaction of pyrophosphate is shown below:

P2O7-4 + H2O → 2HPO42-

Several factor affects poly-P hydrolysis rate, such as chain length, pH, temperature, enzymes activity and soil moisture (Dick and Tabatabai 1986; Hons et al. 1986). In a detailed study, Torres-Dorante et al. (2005) demonstrated the substantial effect of soil type on hydrolysis dynamics and adsorption processes. In comparison to ortho-P, poly-P structures are less readily precipitate in soils and thus, often referred as “slow-released” P fertilizers (Dick and Tabatabai 1987). This chemical property have the potential to increase P mobility in the soil and hence, may elevate P acquisition by plants roots (Torres-Dorante et al. 2006). Indeed, several studies indicated superior P up take or growth in response to poly-P compared to ortho-P fertilizer application (references within (Torres-Dorante et al. 2005)). Yet, numerous studies found no significant effect on P source type (e.g.(Engelstad and Terman 1980; Goh et al. 2013; Ottman et al. 2006)) or inconsistent response depending on poly-P type, soil type and the experiment duration (Dick and Tabatabai 1987). Furthermore, indirect benefits of poly-P utilization, mainly due to micronutrients chelation, may easily overshadow the direct impact. Overall, these studies are generally descriptive and lack mechanistic explanation.

Inorganic poly-P is naturally occurs in soils as a result of P polymerization by soil microorganisms (Harold 1966). Poly-P can accumulate to high concentrations in the microbial cells, these P pools probably used for storage and regulation of cytoplasmic pH  (Pick et al. 1990). Thus, given the right conditions, soil ortho-P can be converted to poly-P by microorganisms (Pepper et al. 1976).

Various chemical processes occurring following poly-P application to the soil. Soil solution pH was temporally increased subsequent to poly-P application while soil organic matter and micronutrient levels decreased (Lindsay 1979). In fact, poly-P is a powerful ligand for cation and micronutrients (McBeath et al. 2009). Overall, understanding the environmental factors that interact with chemical processes of hydrolysis, transport and precipitation are highly relevant for predicting P availability and developing superior fertilization strategy for given environment.

Controlled release P fertilizer. Due to the strong sorption and fixing of P in soils, a promising emerging strategy is development and utilization of coated or controlled released P fertilizer (CRF). In general, CRF have the potential to reduce significantly environmental threats while maintaining high crop yields of good quality (Shaviv 2001). Although coated fertilizers were developed a long time ago, due to economic and practical considerations this technology has little impact on agriculture. The soil chemistry of this polymer-coated P fertilizer published in the peer-reviewed scientific journals and questions remain unanswered regarding the mechanisms of reducing P fixation by the CRF. Yet, several field studies demonstrated the positive effect of coated P compare to water soluble MAP (Chagas et al. 2015; Chien et al. 2009; Pauly et al. 2002).

Plants response to P. Roots responses to environmental alternation by rhizosphere or/and morphological adjustment (Hinsinger 2001; Hinsinger et al. 2009; White et al. 2013). As P being poorly available and mobile in soils, total root length is often considered as an important feature of root system architecture to efficiently acquire soil P (Pagès 2011), while mycorrhizal symbiosis may provide an alternative option (Richardson et al. 2011), especially under low P supply. Roots systems exhibiting the largest root length, surface area and branching would be expected to exhibit a greater P acquisition efficiency. Thus, theoretically, factors stimulating root growth, branching and generally increasing the absorbing surface area will indirectly elevate P fertilizers utilization. Phosphate source was previously shown to alter roots growth (Torres-Dorante et al. 2006) and thus, the potentially accessible P pool. Yet, there are small number of studies examining such possible indirect effect.

Second root strategy for P acquisition is by enhancing bioavailability of poorly labile forms of P. In this regard, exudations of phosphatases enzymes by roots catalyze P hydrolysis and elevates P availability. Both plants and microbial oriented enzymes are participating in P hydrolysis. Phosphorus deficiency was shown to stimulate phosphatases exudation (Engelstad and Terman 1980), thus, under stressed conditions PUE is expected to increase.