Toward a Better P Nutrition Package: Diagnosing P Status and Application Strategies to Improve Fertiliser Response

| Date: 11 May 2010

Mike Bell1, David Lester2,Phil Moody3 and Chris Guppy4
Agri-Science Qld, DEEDI, 1Kingaroy, 4610 Qld and 2Leslie Research Centre, Toowoomba, 4350 Qld; 3DERM, Indooroopilly, 4066; and 4 UNE, Armidale 2351.
Take home message
The current approach to overcoming P limitations to crop yield is facing challenges, especially in black and grey cracking clay soils. Depletion of native fertility reserves and continued reliance on P reserves not measured by traditional soil tests creates difficulties for farmers and advisors devising P fertilizer application strategies. Additionally, the relative P depletion of subsoils which are relied upon when surface soils are dry represents a growing challenge to ensure crops are able to obtain adequate P. Soil testing strategies should include periodic assessment of the P status of subsoil layers (10-30cm), and include a measure of some of the important soil P reserves by undertaking BSES-P tests as well as the traditional Colwell P. While clearly defined critical values of subsoil P are not available as yet, the size and distribution of Colwell and BSES-P pools in these layers will provide an indication of potential problems and the need for closer examination of P fertiliser strategies. Preliminary results suggest that once subsoil P becomes limiting, yield losses can become substantial and major changes to P application strategies will be needed.     
P removal and profile stratification
Negative P budgets were suggested across the northern grains region by the National Land & Water Audit in 2001, and have since been confirmed in most crops and regions in the recently-completed DAQ00084 project, with the exception of some systems where crop yields were well below expectations due to drought (eg. Moree-Narrabri and North Star in Figure 1).
 
Figure 1: Average regional P budgets for crops sampled between 2006 and 2008. Surplus or deficit was calculated from P inputs in fertilizer minus P removed in grain.
 
What this figure doesn’t show is that as well as general rundown of soil P reserves, depletion is occurring largely in the subsoil layers of the soil profile. The top 10cm layer receives any P returned in crop residues, as well as the starter P fertiliser (except perhaps in deep-sown chickpea crops), but as P is largely immobile in clay soils this is where these inputs stay. By contrast, P is being removed from deeper layers to meet crop demands, especially during dry periods (eg. all last winter in many areas!). Recent studies from the Incitec Pivot long term trial site at Colonsay on the inner Darling Downs have shown that ca. 50% of the net P removal in soils receiving no P fertilizer occurred in the 10-60cm layer, with the vast majority coming from 10-30cm. Even applications of starter P fertilizer at 20 kg P/ha/crop had no impact on soil P decline below 10cm.
Crop P uptake and response to starter P
Why should we worry about profile depletion? The extent to which crops rely on starter P to obtain their P needs varies with species and season, but recent results from work conducted in the SQ Farming Systems project (CSA00013) has found that in most cases starter P (regardless of the rate used) generally only supplied 1-2 kg P/ha to the crop P uptake of the current crop, with the rest supplied from the soil profile (residual P from residues of crop and previous fertilizer applications in the 0-10cm layer, and soil P reserves below that). This makes sense when we think of the volume of soil enriched by the starter P fertilizer compared to the overall volume of soil tapped by the crop root system at different growth stages.
 
      

Figure 2: Diagram of starter P placement and root activity at either (a) the seedling stage or (b) when the crop is flowering and accumulating nutrients and biomass at high rates. 
                                    
 
Starter P applied in or near the seed trench ensures rapid early contact of a small, relatively inefficient root system with a high concentration of plant available P. This ensures a high plant P concentration in these early growth stages, especially during floral initiation and establishment of grain number, and is important for establishing a high yield potential. However, the plant only needs 1-2 kg P/ha to do this, as there is only limited biomass and hence a limited crop P demand. The crop then goes on to grow a more extensive root system to meet the demands of a crop that is growing and approaching peak rates of dry matter accumulation, with the pattern of P accumulating closely mirroring that of biomass accumulation. The starter band and the proportion of the root system in it quite quickly becomes a very small proportion of the P supply system, and so to satisfy crop P demand, the crop will need either a (relatively lower) concentration of P around a much larger proportion of the root system, or as depicted on the right hand diagram, a number of enriched zones accessible to a significant proportion of the crop roots. The proportion of the crop root system that needs to access P-rich soil in these later growth stages is unclear.
 
This highlights the role played by native soil P reserves, or conversely the risks associated with their depletion – especially in the subsoil. The main cropping soils in the northern grains region had variable P reserves even before cropping commenced, ranging from substantial to marginal (Figure 3). Subsequent continued net P export and relative subsurface depletion/surface enrichment have pushed some soils to the stage where native reserves are not enough to meet crop demands and achieve high water and nitrogen use efficiency, even when starter P is used. This creates real issues for growers and advisors – which soils have insufficient profile P reserves  to meet demands, and once we know such soils exist, what P application strategies can be implemented to overcome the problem?
 
Figure 3: Variation in P reserves (indicated by the additional P extracted as BSES-P, compared to the traditional Colwell P test) across commercial fields in the southern Qld grain belt for 0-10 and 10-30cm depths. The line indicates the 1:1 line, with sites on that line having no additional P reserves.
Soil testing strategies
Traditionally when testing to determine crop P requirement, a 0-10cm soil sample was taken and analysed for Colwell P and PBI. However, given the depletion and stratification discussed above, as well as the existence (or otherwise) of additional slow-release P reserves that can be detected using the BSES-P test (Figure 3), Colwell-P alone is unlikely to provide all the information to make an informed decision. Of particular note should be the lack of correlation between the two soil P tests, with this most obvious in samples from below 10cm.
 
While data is still being collected on the rates at which these BSES-P reserves can become available to plants (ie. over days, weeks or long fallows), current observations suggest subsoils with quite low BSES-P levels (ie. < 30 mg/kg) are still able to meet the demands of a well developed root system (ie. a trickle of P from many roots accessing a large soil volume). However, there are plenty of subsoils across the region where soil P (both Colwell and BSES) is low and crops seem to contain marginal-low P contents. For example, grain P data from wheat, barley and sorghum collected across the region in DAQ00084 suggest that 20-25% of all crops were marginal-low in P, while the figures for chickpea crops were much higher. The implications of this from a productivity perspective are unclear, but early results are suggesting that some significant yield gains may be possible on these soils.
Low profile P and management responses
Core trials have been established on soils with marginal-low soil P in the 0-10cm layer (13-18 mg/kg Colwell P), combined with low Colwell P (3-5 mg/kg) and BSES P (8-14 mg/kg) in the 10-30cm layer. Different fertiliser P application strategies have been employed and crops of wheat and sorghum grown so far. The 2009 wheat crop was grown under conditions of minimal in-crop rain, while the 2008/09 sorghum crop was grown under favourable moisture conditions. Grain yield results (Table 1) suggest that in these conditions of low available P, there are substantial yield advantages that can be obtained from improving crop P status beyond that from using traditional starter P approaches. Interestingly, while those results are both accompanied by significant increases in P concentrations and uptake, they were also accompanied by increased crop N uptake. In the case of the wheat crop, the biomass N concentration increased markedly in the high P treatment, suggesting better recovery of soil N and (if moisture had been available) a significantly higher yield potential. The results in these trials contrasted with some others on sites with similarly low P, but where soil N availability was not sufficient to meet the demands of crops where adequate P was available.
 
Table 1: Biomass production and grain yields of sorghum (cv. MR43) and wheat (cv. Walloroi) in response to different P application strategies on low subsoil P sites. Crop N and P accumulation in biomass and grain are also shown.
Crop
P treatment
Biomass
Grain yield
Dry matter
(kg/ha)
N content
(kg/ha)
P content
(kg/ha)
Yield
Kg/ha (12%)
N content
(kg/ha)
P content
(kg/ha)
Sorghum 2008/09
No P
8330
100
8.2
3480
54
5.2
Starter
(6P)
9710
110
9.8
3690
58
5.8
Starter plus deep or shallow bands; 1m spacing (40P)
10650
130
11.5
4040
NA
NA
Starter plus deep and shallow bands + enriched topsoil (120P)
13330
155
19.2
4670
71
10.1
 
Wheat 2009
No P
4660
64
4.7
2105
50
4.3
Starter
(5P)
5450
65
4.4
2020
49
4.2
Starter plus deep or shallow bands; 0.5 or 1m spacing (40P)
6210
94
7.5
2300
58
5.2
Starter plus deep or shallow bands + enriched topsoil (120P)
6720
118
11.2
2380
64
6.8
 
Results such as these in Table 1 clearly demonstrate that when soil P is low, especially in the subsoil layers, additional bands of pre-applied P fertilizer in a one-pass operation are relatively ineffective at overcoming the P limitation. It is only when multiple bands (preferably at spacings of <50cm) are applied in different positions in the soil profile that significant P responses have been recorded. Indications are that it is the greater volume of enriched soil that is the key to overcoming these P limitations, rather than the high P rates used in our trials, so multiple bands of much lower rates (possibly using fluid P to increase the volume of enriched soil) clearly need investigation.
 
While these results suggest that once soils have become depleted in subsoil P significant yield gains can be achieved by improving soil P status, there are a large number of practical issues that need to be resolved. Aside from the issues of the cost of additional P fertiliser, key questions that need answers include the volume of enriched soil needed to optimise crop performance and the resulting placement strategies (including form of P fertilizer) to achieve these objectives - without sacrificing precious soil moisture (and planting opportunities) in the process.
 
One advantage that applies in the black and grey Vertosols is the relatively low (<150) Phosphorus Buffer Indices (PBI) that contribute to the good residual value of applied P fertiliser from previous crop seasons. It is our hope that relatively low rates of P fertilizer can be applied opportunistically into deeper layers of the soil profile (eg. with deep-sown chickpeas, or when the profiles are very dry), with the eventual effect of enriching enough of the soil volume to overcome P limitations. Many of these questions are to be addressed in the current nutrient management project which is supported by both the grains and cotton R&D Corporations in the coming years.
Contact details
Mike Bell
Bjelke Petersen Research Station
PO Box 23, Kingaroy Qld 4610.
Ph: 07 4160 0730
Fx: 07 4162 3238
Email: email