Phosphorus and phosphorus stratification

Take home message

• Phosphorus (P) stratification can impact on the critical P requirements of grain crops. In northern NSW and Queensland placing P at 20cm below the soil surface resulted in significant grain yield responses (~13% increase in wheat) (Bell et al., 2016). These findings are worth further consideration in other regions of the Australian grain belt particularly where soil P values in the 10 - 30cm layer are very low (Colwell P < 5mg/kg soil) and surface soils (> 10cm) experience extended periods of low soil moisture that limits P uptake by roots.
• Testing the extent of P stratification on-farm (surface 0–10cm and subsoil 10–30cm) can assist in P budgeting as the lack of subsoil P can be offset by higher concentrations of surface P in most soils provided soil moisture conditions are adequate for P uptake.
• The extent to which P stratification impacts grain yield is influenced by; (i) the Colwell P values for 0–10cm and 10–30cm layers, (ii) the probability of poor crop P uptake due to low soil moisture at 0–10cm, and (iii) the crop type. The current understanding of these components for southern cropping systems is inadequate to provide any precise recommendations.

Background

In the grains-growing areas of south-east Australia, the bulk of plant nutrients in labile form usually occurs in the topsoil, with much lower amounts present in the subsoil. It is becoming increasingly evident that in environments where the nutrient-rich topsoil is prone to drying, nutrient uptake by crops is likely to be adversely affected despite the availability of water in the subsoil. This is likely due to impeded root growth in the dry topsoil or reduced diffusion of immobile nutrients to plant roots or both. Despite the numerous studies on vertical nutrient stratification, there is still limited information on the effectiveness of subsoil nutrition on yield productivity and also the efficiency of nutrient use. This paper reviews P cycling and budgeting in grain production systems with an emphasis on P stratification and the resulting consequences it has on crop P demand.

Phosphorus cycling

Soils of Australia in their native state are deficient in P with some exceptions through northern NSW and Queensland (e.g. Vertisols) which have only been depleted in more recent times through cropping. Consequently, advisers aim to ensure P fertiliser has been added in amounts that are approximately equivalent to the amount of P exported in grain plus other losses such as unrecovered P in stubble and soil. Phosphorus fertiliser that is added to the soil primarily goes into the ‘soil reserve’ where the P binds to soil, a process referred to as P sorption or fixation. Fixation occurs when P reacts with other minerals to form insoluble compounds and becomes unavailable to crops. An important factor controlling P fixation is soil pH as shown in Figure 1. There are three peaks of P fixation. The two highest peaks occur in the acid range of pH 4 and 5.5, where P precipitates with iron (Fe) and aluminium (Al). It is very difficult to supply sufficient P for crop needs when P solubility is being controlled by Fe and Al. The third peak occurs in alkaline soils around pH 8.0 when P is precipitated primarily by calcium (Ca). This fixation is relatively weak, and it is generally more economical to apply more P fertiliser than adding amendments to acidify the soil (Figure 1).

Figure 1. The effect of soil pH on phosphorus availability.

Plant available P in soil solution is predominantly present as dihydrogen phosphate (H₂PO₄^-) or as hydrogen phosphate (HPO₄^-2) in more neutral and alkaline soils. Various estimates indicate approximately 70–80% of P fertiliser added in the crop year becomes part of the soil reserve (Price 2006). The soil P reserve can be described further however for the purpose of this paper it’s important to simply acknowledge that within the soil P reserve there is different bonding of P that influences the short- and long-term plant available P (Figure 2). For example the soil reserve is made up of (1) sorbed P (P held on the surface of fine clay particles), (2) secondary P minerals (freshly bounded Fe, Al and manganese (Mn) phosphates [acid soils] and Ca and magnesium (Mg) phosphates [alkaline soils]) and (3) primary P minerals (age and crystallised Fe, Al, Mn, Ca and Mg phosphates). The soil P reserve (Figure 2) in P adequate soils (Table 1) provides the largest percentage of crop nutrient requirements in any one year which is estimated at approximately 30–80% (Price 2006, Mcbeath et al 2012). Phosphorus fertiliser can directly provide approximately 20–30% of crop requirements (Price 2006) with available P from stubble making up approximately 9–44% (Noack et al 2012) and roots approximately 21–26% (Foyjunnessa et al 2016).

Figure 2. Soil phosphorus cycling in winter cropping systems.

Table 1. Colwell P (mg /kg soil) values for 90 and 95% of maximum grain yield for various crop and soil type combinations extracted from the BFDC database.

n.b. Estimated Colwell P critical values for chickpea, faba bean, lentil and broadleaf lupins are not available from the BFDC database due to no or insufficient data. Similarly, not enough data exists for feed barley, field pea, canola and narrow leaf lupin to provide specific soil type estimates of Colwell P critical values. Where states are nominated under ‘Location’ this refers to the state where most of the experiments (not necessarily all) were conducted.

Phosphorus Buffering Index (PBI)

The PBI test measures the P sorption of the soil. This is the process by which soluble P becomes adsorbed to clay minerals and/or precipitated in soil and it determines the partitioning of P between the solid and solution phases of the soil. A high PBI therefore results in a greater tendency for P sorption compared with a low PBI. Consequently, P sorption capacity of soil influences the availability of P to plants and can be useful for determining Colwell P critical values. Figure 3 shows the relationship between PBI and Colwell P critical for wheat. Usually large changes in PBI values are required to change crop critical P values. Examples of this are provided in Table 2 calculated from Moody (2007). In addition, estimates are also provided from Bell et al (2013) which are quantified from a large data set in the Better Fertiliser Decisions Cropping database.

Figure 3. Effect of phosphorus buffering index on critical Colwell-P (0–0.10 m) required for 90% maximum grain yield of wheat. Critical Colwell P = 4.6 x PBI^0.393 (Moody 2007).

Table 2. Estimated 90% critical Colwell P soil values (mg P/kg soil) for wheat grown in soils with differing PBI (Moody 2007 and Bell et al 2013).

Figure 4. Grain yield response of canola across a range of soil types. Data taken from the BFDC.

Figure 5. Grain yield response of wheat on Red Chomosol soils of NSW. Data taken from the BFDC.

Critical Colwell P soil test values

The critical soil test range is the soil P status, often measured as Colwell P, that will ensure 90-95% maximum crop production. This range may differ for different crop species and soil types that have a different PBI value or the level of soil moisture and degree of surface P stratification. The latter two factors are likely explanations for differences in critical values between years where species and PBI are fixed. Additional factors may include the type of equations used, as small changes in slope near the asymptote (e.g. maximum yield) can make large changes to soil critical values on the x axis.

An analysis of data from the BFDC database using Mitscherlich equations indicates 90 and 95% critical values for canola across soil types are estimated at 22 and 27mg P/kg soil using Colwell P at 0-10cm soil depth (Figure 4). The same comparisons for wheat on Red Chromosol indicates a Colwell P critical value of 35 and 42mg P/kg soil (Figure 5) and for wheat on Vertisol a Colwell P critical value of 25 and 35mg P/Kg soil (Figure 6). Using the Mitscherlich equation provides a slightly higher estimate of critical value than those estimated directly from the BFDC database (Table 1) that use quadratic equations to estimate critical P, however there is sound general agreement between the values calculated with different equations.

Figure 6. Grain yield response of wheat on Vertisol soils of NSW. Data taken from the BFDC.

The sampling depth of P has a significant effect on its critical value. For example, data from the BFDC national wheat data set showed that across soil types, sampling at 0-5cm, 0–10cm and 0–15cm resulted in Colwell P critical value variation from 31 and 36, 24 and 32 and 15 and 20mg/kg for 90% and 95% of maximum grain yield, respectively.

Industry standard practice is to sample at 0–10cm however, knowledge of P concentration in the 10–30cm layer can be very informative in P budgeting. The differences in critical Colwell P for sampling depth may have resulted from (i) differing soil P status at deeper un-sampled depths, (ii) dilution and P stratification effects with greater soil sampling depth and (iii) different crop recovery of P from different depths of soil. The point raised here which will be examined in more detail later in this paper, is that P status at un-sampled depths can contribute to a reduction in wheat critical values as shown above.

Phosphorus budgeting

Phosphorus is exported in grain and recycled in stubble and roots provided the stubble component is retained. Phosphorus in wheat grain ranges from 2.7–3.9kg P/t while in canola seed the range is 3.9–7.8kg P/t (Table 3). Phosphorus in stubble for wheat and canola ranges from 1.0-3.0kg P/t and 2.0–4.0kg P/ha, respectively. Root P concentrations in wheat and canola ranges from 1.5–3.0 and 2.0–2.5kg P/t, respectively.

Table 3. Concentration of phosphorus (kg/t) for wheat and canola grain samples selected from NVT sites. Values are expressed on a dry weight basis (Norton 2012; 2014).

Approximations of P used for P budgeting in wheat include grain P export (2.7–3.6kg P/t) plus stubble P not accessible to the following crop (0.4–0.8kg P/t) plus soil losses (0.3–0.7kg P/t grain production) which provides an estimated 3.6–5.5kg P required/t of grain production. Similarly, for canola seed P export (4.0–6.5kg P/t) plus stubble P not accessible (0.6–1.0kg P/t) plus soil losses (0.3–0.7kg P/t grain production) which provides an estimated 6.1–10.2kg P required/t of grain production. On a per hectare basis the export of P for wheat and canola is approximately the same assuming canola has half the water use efficiency for grain production as wheat. These budgets are estimates, and therefore, must be assessed and adjusted by tracking soil P values to determine if soil test values are increasing (over estimate of P budget), decreasing (under estimate of P budgeting) or remaining within the critical 90 and 95% range (P budget balance). After several years of soil testing and adjusting P inputs it is possible to ensure relatively stable soil P test values.

Phosphorus savings after drought

A recent meta-analysis (He and Dijkstra 2014) demonstrated that drought stress decreases the concentration of P in plant tissue, and several studies have shown that drought can decrease nutrient uptake from soil. Decreases in nutrient uptake during drought may occur for several reasons, including the reduction of nutrient diffusion and mass flow in the soil. Drought can also decrease nutrient uptake by affecting the kinetics of nutrient uptake by roots, however this effect is less well studied.

In cropping systems starter P is important for (i) early root development which assists the plant in exploring the greater soil P reserve and (ii) early head development when potential grain number is set (e.g. at or just prior to DC30).

Many P experiments have shown responses to starter P however, P savings can be made after drought especially where (i) December P export in grain is lower than P inputs at sowing and (ii) soil Colwell P values are equal to or greater than soil critical values. In these circumstances one third of historical average P inputs can be used down to a base level of 3–4kg P/ha. As an example, if wheat target yield for 2019 is estimated at 3t/ha and the P budget is estimated to be 3.6–5.5kg P/t of grain production then we have a P budget of 10.8–16.5kg P/ha or 49–75kg/ha mono-ammonium phosphate (MAP) fertiliser. If a medium value of 62kg/ha MAP (13.5kg P/ha) was assumed as our standard P budget this could be reduced by two thirds down to 18.6kg / ha of MAP or 4.1kg P/ha following the dry 2018. At this rate, the MAP granules are placed in-row at approximately 3.5–4.5cm spacings when using 25cm tyne spacing. Wheat sowing rates of 50–65kg/ha are likely to place seed at approximately every 2–2.5cm in-row while a full MAP rate of 62kg/ha provides an in-row granule spacing of approximately 1.0–1.2cm.

Phosphorus stratification

There are several reasons why P is often highly stratified near the soil surface, including; (i) ‘native’ Australian soils were deficient in P and farming systems, have for the most part, applied P in the top 0–10cm of soil, (ii) P is highly reactive in soils binding with Fe, Al and Mn at low pH and Ca at high pH as well as bonding with small clay particles, consequently P is not readily leached in most soils, (iii) farming systems have shifted from intensive cultivation prior to sowing to no-till or minimum-till systems and this has reduced soil mixing, and (iv) P in stubble retained systems is recycled to the soil surface.

An example of a stratified soil sampled in July 2017 is provided in Figure 7 (Armstrong et al 2017). In this example the ‘plant row’ has a Colwell P of approximately 55mg P/kg soil in the 0–2.5cm section and increases to approximately 62mg P/kg soil in the 2.5–5cm section, which reflects the fertiliser drilled at sowing. At 5–10cm the Colwell P value drop to approximately 24mg P/kg soil and declines to approximately 10mg P/kg in the 10–15cm layer. The sampling ‘near row’ has no fertiliser spike in the 2.5–5cm section (e.g. approximately 37mg P/kg ‘near row’ compared to approximately 62mg P/kg ‘plant row’ in the 2.5–5cm section). The ‘middle row’ (inter-row) section shows very high Colwell P at the soil surface (approximately 133mg P/kg soil on the 0–2.5cm section). This is most probably because the tyne on the ‘plant row’ has thrown P rich surface soil into the inter-row space (e.g. middle row).

Figure 7. Vertical and horizontal stratification of P measured as Colwell P at the long-term Hart experiment. Samples taken in 2017 (Armstrong et al 2017).

The calculated Colwell P at 0 - 10cm on the plant row is approximately 40.5mg P/kg (Figure 7). The question is; how much of this does the plant root access given sowing depth of around 5cm and frequent drying of surface soil. In this scenario, let’s assume the plant does not access P in the top 0–2.5cm, and therefore, the estimated Colwell P at 0–10cm reduces to 27.4mg P/kg soil. In most cases, this is still adequate P for 90% of maximum yield (Table 2) however, it highlights a number of very important issues including what is the relative efficiency of P access at different depths and soil moisture. In the above example (Figure 7) if we assume a 0–10cm Colwell P was 30mg P/kg instead of 40.5mg P/kg and the same proportion of P stratification is applied with no access to P in the 0–2.5cm layer then the Colwell P value becomes approximately 20mg P/kg soil. In this contrived scenario, crop yield may be limited.

While the above scenarios are simplistic (e.g. zero P access in the 0–2.5cm section whereas low uptake efficiency is more likely in the 0-2.5 cm section), the point is clear that highly P stratified soils have the potential to limit yield particularly where P is high stratified in the 0–2.5cm layer and this layer is subject to frequent drying. Figure 8 demonstrates the principle that P uptake can be limited by soil moisture and soil P status. This evidence supports the theory that a frequently drying surface soil with adequate subsoil moisture may respond to deeper placement of P. However, this needs to be tested with wheat in southern NSW soil and climatic conditions before any conclusive statements can be made.

Figure 8. Phosphorus uptake in roots of maize to different soil moisture and soil phosphorus levels.

Phosphorus placement at sowing

Compared with broadcasting or banding fertiliser P with seed, the placement of P at 2–6cm below seed has shown significant yield increases in 14 scientific studies (wheat: Alston 1976; Nable and McConnell et al 1986; Webb 1993; Sander and Eghball 1999; Singh et al 2005; Wilhelm 2005; canola: Grewal et al 1997; Hocking et al 2003; Wilhelm 2005; lupin: Jarvis and Bolland 1990, 1991; Crabtree et al 1998; Brennan 1999; Crabtree 1999; Scott et al 2003) and no significant increase in five scientific studies (Hudak et al 1989; Reeves and Mullins 1995; Bolland and Jarvis 1996; McCutcheon and Rzewnicki 2001; Vyn and Janovicek 2001). All of these studies placed P at depths less than 15cm and some of these studies had starting soil P values below crop critical values.

In at Nebraska a P placement study determined the optimum P placement depth as 11.9cm (Figure 9). Research in WA by Bolland and Jarvis (1990) found wheat yield was increased by approximately 20% when the fertiliser was placed at 9cm below the soil surface compared to 3cm in the first year of sowing single superphosphate. In the second year, superphosphate placed at 13cm depth in the previous year increased grain yield by approximately 60% in lupins compared with freshly drilled fertiliser at 3cm deep.

Figure 9. Effect of different depths of P placement at sowing on winter wheat yield in Nebraska (McConnell et al 1986).

Deep P

More recent research has focused on deeper placement (20cm) of P as MAP at 50cm row spacing in northern NSW and QLD. Figure 10 (Bell et al 2016) shows results from Dysart in Queensland where deep P was drilled in 2013. The zero deep P rate represents deep drilling at 20cm with no deep P applied (i.e. ripping effect) while the farmer practise represents no deep drilling and no deep P. The percent increase from the zero deep P rate to the best deep P response in each consecutive year was 17% (Year 1), 11% (Year 2), 7% (Year 3) and 59% (Year 4). In Year 2 (11% increase) and Year 3 (7% increase) nitrogen limited maximum yield production (Figure 10). Consequently, the P responses for Year 2 and Year 3 may be considered conservative. In each treatment 6kg/ha of P was applied at sowing and this plus the soil reserve P was not expected to limit potential yield (Figure 10). A summary of deep P results (data not shown) indicates deep P applied as MAP at 20kg P/ha provided an average of 13% yield increase in wheat yield, 11% increase in chickpea grain yield based on 10 and 4 crop years of research, respectively.

Figure 10. Deep P drill in Year 1 (2013) at a depth of 20cm and row spacing of 50cm and the subsequent grain yield response over four consecutive years at Dysart QLD for sorghum and chick pea. No additional deep P was applied in subsequent years and annual P at sowing was 6kg/ha.

Future research

It is often assumed that because P requirements for crops have been extensively studied both in and outside Australia that all required knowledge for crop production is known. This is certainly not the case for modern cropping practices where subsoil P (0–30cm) is being exported in grain and redistributed on the soil surface via stubble. This process increases the degree of P stratification where soil P is very low in the 10–30cm layer (< 5mg P/kg soil) and high in the 0–5cm layer (e.g. 35mg P/kg soil). Other factors that contribute to P stratification include shallow placement of P at sowing, and for tyned implements, soil throw into the inter-row. In these circumstances surface drying events in the 0–5cm layer may limit grain production. Exceptions to this stratification process occur where P is leached in low PBI soils or where deep cultivation occurs which mixes the soil.

Conclusion

Phosphorus placement below seed at sowing is most likely to provide yield benefits compared to P placed with seed.

Testing the extent of P stratification on-farm (surface 0–10cm and subsoil 10–30cm) can assist in P budgeting as the lack of subsoil P can be offset by higher concentrations of surface P in most soils provided soil moisture conditions are adequate for P uptake.

Deep P placement (20cm) in northern NSW and QLD in winter dry and summer wet conditions are providing more insights into deep P responses however, these findings cannot be directly applied to cropping zones where rainfall is non-seasonal or Mediterranean in distribution because the frequency and duration of soil drying in the P rich 0–5cm layer is different and will impact on responses to deep P placement.

Reference

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Acknowledgments

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the author would like to thank them for their continued support. In addition the authors would like to thank all the companies and organisations that make the BFDC database possible http://www.bfdc.com.au/interrogator/acknowledgements.vm.

Contact details

Graeme Sandral
NSW DPI, Wagga Wagga Agricultural Institute
Phone: 02 6938 1807
Email: graeme.sandral@dpi.nsw.gov.au
Twitter handle: @gsandral