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Organic amendment addition enhances phosphate fertiliser uptake and wheat growth in an acid soil.


Phosphorus (P) is a major plant nutrient, applied as an inorganic fertiliser to much of the industrialised world to sustain agricultural productivity. However, the effectiveness of P fertiliser application is compromised in acid soils, due to increased P sorption through solid phase adsorption and precipitation reactions (White 1980). This is of global concern, as soil acidity affects up to 50% of the potentially arable land throughout the world (vonUexkull and Mutert 1995).

A large body of international literature supports the role of organic amendments in increasing the availability of soil P in agricultural systems (Iyamuremye and Dick 1996; Erich et al. 2002; Guppy et al. 2005b). The proposed mechanisms driving this effect include chelation of metal cations (e.g. [Al.sup.3+]) by organic and phenolic acids (Hue et al. 1986; Hu et al. 2005), competition for P sorption sites (Hue 1991; Violante et al. 1991), mobilisation of P from poorly soluble fractions (Martinez et al. 1984; Bolan et al. 1994), and increased solution pH (Ritchic and Dolling 1985; Hue 1992). However, the response of soil P to amendment addition can vary substantially, as organic amendments may be derived from several source materials of variable composition.

In addition to the effect of organic amendments on background soil P concentrations (endogenous P), the application of amendments in the presence of inorganic phosphate fertiliser may also affect the solubility, and hence the potential plant availability of the applied P. Organic amendment addition may either increase or decrease P concentrations in solution, depending upon not only the composition of amendments added, but also the form of P fertiliser applied. Phosphate sorption studies have shown that the form of P fertiliser is important due to interactions between the associated cations (e.g. [Ca.sup.2+], N[H.sub.4.sup.+]) and solid-phase exchange surfaces, and the relative changes in solution pH associated with fertiliser dissolution (Schefe et al. 2007). Incubation studies have also demonstrated that a biologically active organic material with a high cation exchange capacity can adsorb a large proportion of applied P fertiliser, while application of a highly humified material can increase solution P concentrations in an acid soil (Schefe et al. 2008a).

Short-term sorption and incubation experiments can explain many of the processes which may be occurring between soil, organic amendments, and P fertilisers. However, in order to determine the agronomic influence of organic amendments on P dynamics, plant-based validation is required. Therefore, a pot experiment was conducted to determine the effect of organic amendments and phosphate fertilisers on wheat growth in an acid soil. The premise for this experiment was that the addition of organic amendments will increase the plant uptake of P fertilisers, resulting in increased plant growth and increased tissue P uptake per unit of P applied. It was further hypothesised that fertiliser--amendment interactions would be influenced by the form of P fertiliser, and amendment type applied.

Materials and methods

Soil and amendment characterisation The soil used for this experiment was a bleached eutrophic yellow Dermosol (Isbell 1996), sourced from the Department of Primary Industries--Rutherglen Centre, Victoria, Australia. This acidic soil ([pH.sub.water] 4.4, [pH.sub.Ca] 3.8) had been under a phalaris-based (Phalaris aquatica) pasture for the past 15 years, receiving annual additions of superphosphate. A Dermosol was selected as it is the dominant soil type within the strongly acidic agricultural zone in Victoria (Isbell et al. 1997). A single sample was collected from the 0-0.15 m depth and air-dried.

Lignite and compost were chosen as commercial organic amendments of differing origin, composition, and maturity. The compost product was largely derived from green vegetable waste, which had undergone a rapid composting process and was still biologically active ([pH.sub.water] 8.2). In contrast the lignite product, sourced from the Latrobe Valley in Victoria, was originally plant material, transformed into a highly humified low rank coal, resulting in a biologically stable product ([pH.sub.water] 5.4). The compost and lignite were dried at 40[degrees]C and sieved to <2 mm diameter.

The analytical data for the soil, lignite, and compost are presented in Table 1. Determination of electrical conductivity (EC), pH in water, pH in Ca[Cl.sub.2], Colwell P, and cation exchange capacity (N[H.sub.4]OAc) were analysed as described in methods 3A1, 4A1, 4B2, 9B 1, and 15D3 of Rayment and Higginson (1992). A high-frequency induction furnace (LECO Pty Ltd) was used to measure total soil C. Total soil P was determined by perchloric digestion before analysis by inductively coupled plasma-atomic emission spectrometry (ICPAES; Spectro Analytical Instruments Pty Ltd, Kleve, Germany). The soil had a phosphate buffering capacity (PBC) of (110 mg/kg)/([log.sub.10] [micro]g P/L), which is classed as very high (Moody and Bolland 1999). Solid-state [sup.13]C nuclear magnetic resonance (NMR) analysis of the organic amendments (Bruker 300MHz NMR spectrometer; USA) showed that the lignite contained significant proportions of aromatic and aliphatic groups, while the compost largely comprised carbohydrate and aliphatic compounds (Schefe et al. 2008b).

Experimental design and layout

Large rectangular tubs (0.6m by 0.18m by 0.26 m) were used for this experiment to maximise the space available for root growth, and to allow banding of fertilisers within a seed row, consistent with best practice (Engelstad and Terman 1980). Drainage holes (8 mm) were drilled in the base of each tub and covered with porous cloth.

A layering approach was used in the distribution of soil and amendments in this experiment. The base of the tub was covered with a layer of sand (45 mm) to aid drainage. This sand was covered by 150 mm of soil, and sieved to <6.5mm, which removed large clods while maintaining soil aggregation and drainage. The average bulk density of this layer was 1.3 g/[cm.sup.3], which reflects the average bulk density measured in cropping paddocks in the local area. This layer was then covered by a 'topsoil' layer (40 mm) of soil and/or amendments, with amendments incorporated into the topsoil mix before adding to the tub. Lignite and compost was applied on an equivalent C basis, with 1% and 2.5% lignite and compost treatments equating to the addition of 1% and 2.5% total C to the soil, respectively. The final topsoil [pH.sup.Ca] of the 2.5% lignite and 2.5% compost treatments was 4.14 and 6.65, respectively, with [EC.sub.1:5] values of 86.4 and 497 [micro]S/cm, respectively.

Pots were positioned in the glasshouse with day/night temperatures of 20[degrees]C/16[degrees]C on a 12-h rotation. All treatments received 2 L of distilled water to simulate the 'settling in' of amendments before sowing. Water addition was repeated 3 times over the next 7 days to stimulate microbial activity and organic matter decomposition. The topsoil was then allowed to dry for several days before preparation of the seedbed.

The wheat variety chosen was Triticum aestivum L. cv. Egret (Fisher and Scott 1983; Delhaize et al. 1993). All seeds were pre-germinated for 24 h on moist cotton wool in an incubator at 25[degrees]C at constant light before sowing. One row of 12 seeds was sown (40 mm apart, 20 mm deep) into each tub. Fertiliser granules comprising triple superphosphate [TSP; Ca([H.sub.2]P[O.sub.4].[H.sub.2]O)] and di-ammonium phosphate [DAP; [(N[H.sub.4]).sub.2]HP[O.sub.4]] (Pivot Pty Ltd) were banded with the seed row at rates of 0.05, 0.111, and 0.278 g P per treatment, equating to rates of 5, 10, and 25 kgP/ha. All P and soil amendment treatments were replicated 3 times in a randomised block design.

All treatments received regular watering with distilled water throughout the experiment, with soil moisture maintained at a level non-limiting to plant growth (between 10 and 60 kPa), as measured by a moisture probe (Theta Probe, Campbell Scientific Pty Ltd).

Plant sampling

Plant height and growth stage (Zadoks et al. 1974) were measured when >75% of the plants in the most advanced treatments were at the 2-leaf, 4-leaf, and 6-leaf growth stages. This was done by measuring every plant in each replicate, then averaging these values to obtain 1 value per replicate. All plants were harvested when the most advanced treatments reached the 6-leaf stage (40 days after sowing). The shoots were cut off at the base and dried at 60[degrees]C for 48 h. The dried shoots were then weighed before being ground, digested in perchloric acid, and analysed for tissue elemental concentrations (P, K, S, Ca, Mg, Na, Cu, Zn, Mn, Fe, Al, B) by ICPAES. Although all elements were analysed, only results from elements of interest (P, A1, Ca, Mg, Mn) are reported. Nutrient uptake is presented on a per ha basis, in order to relate nutrient uptake with the rate of fertiliser applied, which is also displayed on a per ha basis. To do this, nutrient uptake was calculated on a per plant basis, and then multiplied by the number of seeds sown over 1 ha.

Statistical analysis

Analysis of variance (ANOVA) was used to determine the effect of organic amendments (untreated soil, 1% lignite, 2.5% lignite, 1% compost, and 2.5% compost) and P fertiliser treatments (TSP and DAP at 3 rates) on plant height and growth stage throughout the experiment, and shoot DM and nutrient uptake as measured at the end of the experiment (GENSTAT 2005).


Plant growth--no P applied

The addition of organic amendments to soil with no P added significantly increased plant height at the 6-leaf stage (P < 0.005; Fig. 1). The greatest increase in growth relative to the untreated control was in the 1%o and 2.5% compost treatments. There were no differences in tiller number (Table 2) or dry matter (DM) (Fig. 3) between amended treatments in the absence of applied P.


Plant growth P applied as TSP or DAP

The addition of P significantly increased plant height (Fig. 1) and DM (Fig. 2) in all soil treatments (P < 0.05). Tiller number also increased in the lignite-amended treatments (P< 0.05; Table 2) upon P addition.

The effect of the form of P applied (TSP or DAP) and the rate at which it was applied (5, 10, and 25 kg P/ha) was then determined within each amended treatment. In the untreated soil there were no differences in plant growth between the TSP and DAP treatments at each application rate. At the 2- and 4-leaf measurements of the 1% lignite treatment, plant height was greatest in the TSP 25 kg P/ha (P< 0.05) compared with the other treatments, while at the 6-leaf stage there were no differences in plant height between TSP and DAP in the 1% lignite treatments. Shoot DM (Fig. 2) and tiller number (Table 2) in the 1% lignite treatment also increased significantly with the higher P rate of 25 kg P/ha for both TSP and DAP (P< 0.01). In the 2.5% lignite treatment, plant height at the 6-leaf stage was significantly less in the TSP 5 kg P/ha than the other added P treatments (P< 0.001), while tiller number was significantly increased at 25 kgP/ha, for both TSP and DAP (P<0.05). In the 1% compost treatment, plant height was increased in the TSP 25 kgP/ha at the 4- and 6-leaf measurements (P<0.05). The source of P was significant in determining shoot DM in the 2.5% compost treatment, with increased shoot DM in the DAP-applied treatments compared with the equivalent rates of TSP (P < 0.05).


An additive effect was observed between soil amendment and P application, with plant height and DM significantly greater in the 1% and 2.5% lignite and compost treatments when applied in conjunction with P, compared with when P was added to untreated soil (P<0.05). This effect was seen in both the TSP and DAP treatments. In addition, an increased relative response was measured in the 1% lignite and compost treatments compared with the 2.5% treatments, with the lower amendment additions having as great an effect on plant growth as the larger amendment addition of 2.5%.

Plant nutrient uptake

Addition of organic amendments in the absence of P had a significant effect on plant uptake of nutrients (Figs 3, 4). When no fertiliser P was added, plant P uptake was increased with the addition of lignite and compost, with P uptake greatest in the 2.5% compost treatment (P < 0.05; Fig. 3). Plant uptake of Ca, Mg, Al, and Mn also increased with compost addition (P< 0.05; Fig. 4).

The addition of organic amendments significantly increased P uptake compared with the untreated soil when combined with DAP addition at 25 kg P/ha (P< 0.001; Fig. 3). However, there were no differences in P uptake between the amended treatments at the high rate of DAP addition. The addition of TSP resulted in a plateau of plant P uptake for all treatments except the 1% lignite treatment, with no significant differences in P uptake between the 5 and 25 kg P/ha application rates of TSP.

The addition of organic amendments increased the uptake of Ca (Fig. 4). In the 1% lignite treatment, both TSP and DAP addition increased Ca uptake at 25 kg P/ha addition (P< 0.05), while only DAP addition increased Ca uptake in the 2.5% lignite and both compost treatments (P< 0.05). Plant uptake of Mg was only increased in the lignite treatments at the high P application rates of both TSP and DAP (P<0.05). Mn uptake was only significantly increased in the lignite treatments with 25 kg P/ha DAP addition, compared with the same P rate in the untreated soil (P<0.05). High variability in Ai uptake negated any treatment effects.


The lack of difference in shoot height at the first growth measurement (2-leaf) indicates that until this time the plant was largely drawing on seed reserves for nutrients, and that roots had not yet accessed the fertiliser band (Fig. 1). It was only after this point that differences in shoot height became significant.


The rate at which TSP or DAP was added to the untreated soil did not significantly influence plant DM. This lack of effect suggests that of the P fertiliser applied, most was sorbed by P-reactive sites, a result which correlates to the low pH and high P-buffering capacity of this soil. This result is supported by the relative lack of P uptake in the untreated soil.

An increase in shoot DM was measured when P was added to the 1% and 2.5% compost treatments (no added P), compared with the untreated soil (P< 0.05), a result which is most likely due to a number of different factors occurring concurrently. The plant growth of the nil-P, compost-amended treatments was almost equal to the added P treatments of the untreated soil, indicating that the compost may be contributing valuable nutrients for plant uptake (Jakobsen 1995; Hue and Sobieszczyk 1999). Metal phytotoxicity issues associated with an acid soil would also be reduced with compost addition (Hue and Amien 1989), due to increased pH and the complexation of metal ions such as Al and Fe (Staunton and Leprince 1996). Endogenous soil P may also be solubilised through biochemical processes (Schefe et al. 2008b). However, previous researchers have noted the P sorption capacity of compost, due to the high number of P-sorptive sites in the cation-rich environment (Guppy et al. 2005a).

The addition of the lignite had a positive effect on plant height and dry matter, with its effects largely attributable to the contribution of organic ligands for P substitution or soluble metal-bridging reactions (Chen and Schnitzer 1976; Hue 1991). The reality of P availability in the presence of organic amendments (compost and lignite) within a dynamic, aerobic system is likely to be a compromise between sorption and solubilisation, with the dominant processes driven by the presence of inorganic (fertiliser) P, and the chemical form in which it is presented. The high soil solution P concentrations associated with granule dissolution will drive sorption of P onto solid phase surfaces, while P solubilisation will dominate upon low soil solution P concentrations.

As the TSP granules dissolved, a highly acidic P-saturated solution (c. pH 1.5) moved from the granule into the surrounding soil, which may have caused the dissolution of soil Fe, AL, and Mn (Lombi et al. 2004). These cations may then come into contact with the highly saturated P solution, resulting in metal-phosphate complexes, which then re-precipitate, removing P from solution (Lehr et al. 1959; Lindsay and Stephenson 1959a, 195%)o Such processes may account for the relative lack of P uptake in all TSP treatments. These reactions may also involve the TSP-derived Ca, which may re-complex with P in the soil environment, or may be available for uptake by the plant. The addition of lignite or compost may also result in the resolubilisation of TSP-derived phosphate and cations, previously sorbed upon granule dissolution. Such a process is more likely to occur in the compost-amended soil, with the high compost pH compensating for the acidifying effect of the TSP. Solubilisation of P through biological and chemical processes would significantly contribute towards increased concentrations of both endogenous and applied P available for plant uptake (Lobartini et al. 1998).

The dissolution of DAP granules resulted in the movement of N[H.sub.4.sup.+] out of the granule in association with the phosphate ions. This N[H.sub.4.sup.+] may have displaced soil cations, such as Ca and Al, which then complexed with the phosphate and reprecipitated (Khasawneh et al. 1974; Sample et al. 1979). These complexes may dissolve/solubilise through a significant pH shift of the soil solution, due to the high localised pH of DAP in solution. Previous incubation experiments showed that the increased solution pH associated with DAP addition to lignite-amended soil was an important factor in increasing soil solution P concentrations (Schefe et al. 2008a), which may also relate to the increased plant P uptake observed in the current study. Increased solution P concentrations are likely due to increased P solubility and the formation of soluble organo-metallic phosphate complexes, thus reducing the amount of precipitated P (Riggle and von Wandruszka 2005).


The 1% lignite and 1% compost treatments resulted in plant DM responses that were equal to, or greater than, those measured in the 2.5% treatments. This effect has been noted previously, with lower doses of humic substances being shown to have a positive effect on plant growth, while higher doses may inhibit plant growth (Ayuso et al. 1996). The addition of even 1% C as lignite or compost may be considered an excessive amount in an agricultural system, from which a reduction in plant growth may have been expected. However, the threshold quantity of organic amendments above which growth inhibition may occur is dependent upon the initial soil organic C concentrations (Lee and Bartlett 1976). As this soil was very low in C (1.3%) it is feasible that a relatively large quantity of C may be added before plant growth declined. An equivalent growth response with 60% less C added (1% C v. 2.5% C) is a positive result, as it suggests that significant DM gains may be made at a lower C application rate than used in this experiment. Further reductions in application rates may be gained through banding of amendments in rows aligned with fertiliser application bands, thus ensuring maximum contact between C and P, and enhancing the potential for increased P uptake. Such reductions in C application rates will enhance the economic viability of organic amendment application, which is an important consideration in broadacre cropping due to the high transportation cost per unit of C applied.


Fertiliser applications of 20 kg P/ha are generally accepted as the required P application rate for grain production on similar, acidic soils in north-eastern Victoria. However, the addition of organic amendments with 5 or 10 kg P/ha resulted in shoot DM and plant P uptake significantly greater than that achieved with the untreated soil at 10 or 25 kg P/ha. Therefore, the addition of organic amendments (particularly lignite), applied in conjunction with DAP, has the potential to significantly reduce the quantity of inorganic P fertiliser required to achieve the same plant growth. Further study is required to assess whether this growth response translates to improvements in grain yield at feasible agronomic and economic rates of addition. If this result does translate into improved yield, this result is of agricultural significance, as a reduction in fertiliser usage will have important environmental and economic implications for the Australian cropping industry.


This work was funded through a GRDC research scholarship, with additional funding from Monash University and the Department of Primary Industries, Victoria (DPI). Thanks to Ken Wilson for his considerable assistance in the establishment of this experiment.

Manuscript received 22 February 2008, accepted 6 August 2008


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C. R. Schefe (A,E), A. F. Patti (B,C), T. S. Clune (A,D), and W. R. Jackson (B)

(A) Future Farming Systems Research Division, Department of Primary Industries, Rutherglen Centre, RMB 1145, Rutherglen, Vic. 3685, Australia.

(B) Centre for Green Chemistry, PO Box 23, Monash University, Clayton, Vic. 3800, Australia.

(C) School of Applied Sciences and Engineering, Monash University, Churchill, Vic. 3842, Australia.

(D) North East Water, PO Box 863, Wodonga, Vic. 3689, Australia.

(E) Corresponding author. Email:
Table 1. Chemical characteristics of the soil and organic materials as
determined by wet chemistry methodology

Parameter Soil Lignite Compost

Soil pH (in [H.sub.2]O) 4.43 5.38 8.21
Soil pH (in Ca[Cl.sub.2]) 3.82 4.67 7.81
[EC.sub.1:5] (dS/m) 0.09 0.29 3.39
Total C (%) 1.3 40 19
CEC ([cmol.sub.c]/kg) 2.64 28.3 50.4
Total P (mg P/kg) 230 <50 (A) 2900
Colwell P (mg P/kg) 27.1
PBC (mg/kg)/([log.sub.10] 110
 [micro]g P/L) (B)

(A) Below detection limits.

(B) Phosphate buffering capacity, method 9J1 (Rayment and
Higginson 1992).

Table 2. Tiller number at the 6-leaf growth stage

Within soil treatment, means followed by the same letter are not
significantly different (P>0.05) between fertiliser treatments. There
were no significant differences between soil treatments within each
fertiliser type/rate

Soil treatment Nil P 5 kg P/ha

Untreated soil 0.00 0.71 0.16
1% lignite 0.03a 0.96ab 1.73bc
2.5% lignite 0.15a 0.82ab 1.03abc
1% compost 0.44 1.20 1.61
2.5% compost 0.44 1.40 1.15

Soil treatment 10 kg P/ha 25 kg P/ha

Untreated soil 0.82 0.99 0.93 0.99
1% lignite 1.21bc 1.20bc 2.22c 2.51c
2.5% lignite 1.33abc 1.55bc 1.93bc 2.23c
1% compost 1.41 1.73 1.84 2.15
2.5% compost 0.95 1.75 1.10 1.79
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Author:Schefe, C.R.; Patti, A.F.; Clune, T.S.; Jackson, W.R.
Publication:Australian Journal of Soil Research
Article Type:Report
Geographic Code:8AUST
Date:Dec 1, 2008
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Identifying fertiliser management strategies to maximise nitrogen and phosphorus acquisition by wheat in two contrasting soils from Victoria,...
Wheat roots proliferate in response to nitrogen and phosphorus fertilisers in Sodosol and Vertosol soils of south-eastern Australia.
Plant availability of phosphorus from fluid fertiliser is maintained under soil moisture deficit in non-calcareous soils of south-eastern Australia.

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