Improving fertilizer decisions for P on the Western Downs

Author: Mike Bell, University of Qld (Gatton) and David Lester DAF (Toowoomba) | Date: 31 Jul 2015

Take home message

Crops access P from both the topsoil (0-10cm) and subsoil (especially the 10-30cm layer, where there are lots of roots). Supplying adequate P for early growth stages (starter P) is essential for early vigour and to set grain number and yield potential in grain crops, while P in deeper soil layers helps to meet the demand to grow the crop – especially when topsoils dry out and crops live on stored soil moisture. Low subsoil P can be a substantial yield limitation in all but the toughest seasonal conditions.

Seasonal rainfall will affect the relative importance of topsoil and subsoil P, while crop species and seasonal moisture influence the yield response to starter P and deep P bands. Responses to deep bands are maximized when there is a rainfall event after establishment, to allow secondary root and tiller development, provided there is subsequently enough water and available N to allow that higher yield potential to be achieved. While residual benefits of deep P bands will persist for a number of years, the economics of deep P banding will be influenced by seasonal conditions in the years following application.


As a rough rule of thumb, crops require a minimum of 1.5-2.0 kg P/t biomass production and remove about 3.0-3.5 kg P/t grain, although crops will accumulate great quantities and remove more in grain if P is available. Early in the growing season biomass P concentration is higher, with this growth stage critical for setting yield potential in grain crops. Depending on the ratio of grain: total DM, crops can remove 50-80% of the P in harvested product, with the residue returned to the surface soil where it will stay unless incorporated with tillage. In short, the bigger the crop the higher the P requirement, and the more marginal the P status the greater the proportion of total P removed in grain.

Subsoil P has declined through un-replenished P uptake, shallow fertilizer placement and a lack of tillage, with the greatest declines in the 10-30cm layer. Soil testing for readily (Colwell P) and slowly (BSES P) available soil P reserves is an essential guide for making fertilizer decisions, with both the 0-10cm layer (to determine starter P needs) and the 10-30cm layer (deep P applications) needing testing. Deep tests need to be undertaken infrequently, as results don’t change quickly, but knowledge of the P status of the subsoil is essential before making fertilizer decisions.

Where would we expect P responses?

Soil testing of both topsoil (0-10cm) and subsoil (10-30cm) layers is the key to determining whether a response to applied P (starter or deep P or both) could be expected. While we are currently working to refine these fairly broad soil categories, and to explore crop requirements, our best estimates for the Vertosols appear in Table 1 below.

Briefly, deep P will generally deliver a growth response if (i) Colwell P is <10 mg/kg in the 10-30cm layer, AND (ii) BSES P in that same layer is <30 mg/kg. If Colwell P in that layer is >10mg/kg responses are unlikely, but if Colwell is <10 mg/kg and BSES P is 30-100 mg/kg, you are in an area of uncertainty, as we can’t know the solubility of all the minerals measured in the BSES test. If that is the case, try a test strip.

Table 1. Critical P values and their relationship to P fertiliser decisions in northern Vertosols


Soil test interpretation
Colwell P BSES P Fertiliser decision
Do I need to apply starter P?
(0-10cm depth)
<20 mg/kg N/A Definitely
20-30 mg/kg
>30 mg/kg
N/A Unlikely
Do I need to apply deep P?
(10-30cm depth)
>10 mg/kg
<10 mg/kg  30-100 mg/kg
<10 mg/kg
<30 mg/kg

If the subsoil is low, what size responses would we expect?

If we had to summarise all our trial experience to date, we would conclude that

  • The frequency of grain yield responses to starter P would be in the order of wheat > long fallow sorghum, mungbean and chickpea > short fallow sorghum. The magnitude of the responses varied, but typically ranged from 0-10%, except in a mungbean crop where soil P was very low and responses were much larger in relative terms.
  • All crops (wheat, chickpea, sorghum and mungbean) have shown responses to deep P application.
  • Responses to deep P, assuming other nutrients like K and S were sufficient, have averaged ~20% in crop yields in the 1st and 2nd season (few sites have been monitored any longer). Some responses have been larger (50-70%) under particular agronomic (e.g. heavy nematode pressure) or climatic (limited or no effective in crop rainfall) circumstances. , but in dry seasonal conditions without good secondary root development they have been more like 5 - 10%.
  • The strongest responses to deep P in grains (wheat and sorghum) occurred when post-planting rainfall allowed the establishment of secondary roots and tillers, which are the main pathways to plant P uptake and increased yields. In wet years when the topsoil was readily accessible, or in extremely dry years, responses were more limited. Chickpeas have been variable, with both strong responses in low rainfall years and also quite marginal ones, although consistent responses were still recorded in wetter seasons. The differences in dry years would seem to lie in how low the crop P status was and how effectively the crop accessed the deep bands.

How often would we expect to see these responses?

Clearly there is an effect of seasonal moisture availability and in-crop rainfall on yields and fertiliser responses, and we can loosely classify seasons into three basic types with characteristics that influence crop response to deep P. We have loosely termed these as

  • Dry starts - those years with little or no effective rainfall from planting until after tillering. Secondary root growth and tillering are seriously affected and there is limited uptake of P from bands;
  • Wet start/dry finish - those with enough rain to ensure good early growth, secondary root development and tillering but later stress ensuring a strong reliance on subsoil moisture; and
  • Wetter years– no severe crop stress, with an expectation that more regular rainfall will ensure the top soils have plenty of active roots (although we may be battling foliage diseases in the hope of securing high yields).

The frequency of these types of seasons obviously varies (winter v summer, and region to region). In the Roma district, the frequency of ‘dry start’ years is unchanged by soil water holding capacity and is a fairly consistent 20-25% of years in both summer (sorghum) and winter (wheat or chickpeas) planting windows. There are differences between crops, seasons and soil water holding capacity in the frequency of seasons classed as ‘Unstressed/little water stress’ and those that ran into stress later in the growing season (‘good start, dry finish’). Most obvious is the effect of water holding capacity (soils with a bigger bucket run out of water later in the season less frequently, especially in chickpeas), but summer sorghum has a slightly lower frequency of stressed crops than winter wheat.

Collectively, these seasonal moisture conditions impact on yield potential (see Figure 1 below) as well as on the crops ability to respond to deep placed P fertilizer. Average grain yields vary 2-3 fold between season types, with the grain crops (both summer and winter) showing the greatest yield fluctuations. 

Figure 1. Simulated yields of chickpeas, wheat and sorghum in ‘dry start’, ‘wet start/dry finish’ and ‘no stress year’ at Roma for soils with 120mm or 240mm PAWC that were at least 2/3 full at sowing.

Figure 1. Simulated yields of chickpeas, wheat and sorghum in ‘dry start’, ‘wet start/dry finish’ and ‘no stress year’ at Roma for soils with 120mm or 240mm PAWC that were at least 2/3 full at sowing.

The yield losses due to low P incurred in the different seasonal types are clearly pivotal in any analysis of the costs and benefits of ensuring adequate P nutrition. The estimates we have come up with are preliminary and based on (in some cases very) limited data. However we think the discounts or yield reductions shown in Table 3 below are realistic for soils where we are very confident P responses will be obtained.  We have presented these as total yield response to applied P (the sum of starter and deep P), although we would note that responses to starter P range from 0% to 10% of yield potential, with most consistent responses in higher yielding seasons. Under the right seasonal conditions, overall P response can be much greater. These estimates of P responses can be used to work out the value of lost yield if low soil P is not addressed adequately (ie. right rate, right place and right time).

Table 2. Estimates of relative yield losses that would be experienced if fertiliser P was not applied for different soils and season types.  These effects are due to a combination of starter P and deep P, and would reduce the simulated potential yields such as shown in Figure 1. 

Season PAWC Wheat Sorghum Chickpeas
Dry start 120mm 5% 5% 5%
Wet start, dry finish 10% 10% 15%
No serious water stress 15% 15% 10%
Dry start 240mm 10% 10% 30%
Wet start, dry finish 25% 25% 25%
No serious water stress 15% 15% 10%

As these loss factors are percentages, it is fairly obvious that regions/soil types with higher potential yields will deliver a larger yield benefit if low P is addressed. Conversely, in sites or seasons growing low yielding crops, the payoff for deep P applications (eg. 10-20% yield increase) is likely to be limited, as water or some other factor is a much greater yield constraint.

What is the residual value of applied P?

This is currently part of the focus of the existing trial program in UQ00063, with responses to different P rates assessed in both the initial crop year and subsequent seasons through a crop rotation. We know from work at IPL long term fertiliser trial sites like Colonsay and Tulloona that roughly half of the net P removal in our cropping soils has come from the subsoil (Wang et al., 2007), so we assume that future fertiliser programs will ultimately be a mix of starter and periodic deep P applications if they are to be sustainable.

Our experience in soils with low Phosphorus Buffer Indices (PBI) across the region (ie. most of the major cropping soils) is that residual value of applied P is excellent and covers multiple crop years. The longest trials we have monitored have been for 5-6 years, but most are 2-4 crop seasons and counting and we continue to see residual benefits of deep P, with an example shown from Capella in Figure 2 below. This trial was established to explore the additive effects of P, K and S fertilizers on a site that was low to marginal in all three nutrients, but as data illustrate, fixing the P limitation has been the most consistent effect, with an additive effect of K appearing from time to time. In this case P was applied in 2011 and 15-20% yield responses have been recorded to P or combined P and K applications in each crop year. The largest actual yield response to applied P (500 kg/ha chickpeas) was in the wetter season of 2012. The site has just had an additional sorghum harvest from the 2014/15 season.

Figure 2. Actual grain yield responses to P, K and S fertilizers, alone or in combination, at a site near Capella. Fertilizer was applied

Figure 2. Actual grain yield responses to P, K and S fertilizers, alone or in combination, at a site near Capella. Fertilizer was applied

Experiences in the west

We have had a number of trials in the Lundavra, Westmar and Meandarra districts in the last couple of winters, with the residual benefits being assessed in the current season at all sites. Typically our trials look at the relative impact of both starter and deep-applied P, with the object being to ensure P is supplied in a position in the soil profile where the roots can access it, regardless of the seasonal conditions.

Examples of the type of responses are shown for a chickpea crop at ‘Trenmore’, Meandarra from last year. Dry matter data showed a small response to starter P and a much larger response to ‘deep P’, with the growth response increasing to ~50% with increasing P rate (deep bands were placed 50cm apart). However, in what was a classic ‘dry finish’ season with a couple of inopportune frosts thrown in, that large growth response did not result in any consistent grain yield response above the 10-15% (100-150 kg/ha) resulting from the deep ripping operation itself. This was extremely disappointing, and was typical of the tough seasonal conditions of the last couple of winters in this part of the region (marginal planting moisture, lack of follow up rain to promote tillering and deep P acquisition and minimal effective in season rain to prevent late stress). Unfortunately, as discussed earlier and illustrated in Figure 1, these weather conditions are not uncommon in this region, and represent a significant challenge to getting reliable returns from investing in deep P application. Responses to starter P have also been variable and generally decline as deep P rates increase (i.e. as overall site P status improved). However in most instances in these western areas the majority of the grain yield response could have been achieved with the starter P application.   

Figure 3. Actual biomass and grain yield responses to starter and deep P fertilizers in a chickpea crop at Meandarra in the 2014 winter season. Data are shown as actual biomass or grain yield, and as the response to P treatment in actual (delta DM or GY, kg/ha) or relative (% increase or decrease) terms, compared to the farmer reference with starter P.

Figure 3. Actual biomass and grain yield responses to starter and deep P fertilizers in a chickpea crop at Meandarra in the 2014 winter season. Data are shown as actual biomass or grain yield, and as the response to P treatment in actual (delta DM or GY, kg/ha) or relative (% increase or decrease) terms, compared to the farmer reference with starter P.

What does this mean for future P application strategies in the Roma region?

Our trial program is continuing and we will sample more sites and seasonal conditions to build our understanding of factors governing the size of the crop P response in different regions. However results to date would suggest that the frequency and size of grain yield responses to applied P most commonly encountered in regions like Roma, with lower/more variable rainfall, will limit the reliability of returns on investment from deep P. Starter P will provide a more consistent return on investment at this time, although this situation will most likely change as we further erode our subsoil P reserves and become more reliant on fertilizer P inputs to meet plant demand. We hope our expanding database of experimental sites will provide a more accurate assessment of the soil test situations which meet these conditions and warrant the additional expense of deep P applications. In the interim, growers and advisors who encounter low subsoil P in soil tests are encouraged to use test strips to explore the size and longevity of crop responses in their own fields.


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, and the authors would like to thank them for their continued support.

Contact details

Prof Mike Bell
University of Queensland
Plant Science Building (8117A), Gatton Campus
(07) 5460 1140
0429 600 730

GRDC Project code: UQ00063, UQ00078