Changing nutrient management strategies in response to declining background fertility

The economics of deep phosphorus use

Authors:

Mike Bell (QAAFI), David Lester, Brendan Power, Andrew Zull and Howard Cox (DAFFQ), Guy McMullen (NSW DPI) and Jim Laycock (IPL)

GRDC code:

UQ00063, CSA00036

Phosphorus (P) is an increasingly important fertiliser nutrient needed to support productivity and profitability of northern region cropping systems. It is a nutrient that is needed in small amounts (but at high plant tissue concentrations) to support the establishment of grain numbers at floral initiation in grain crops, very early in crop development. This is why the practice of using starter P fertilisers (placed with or very close to the seeding trench and hence the developing seedling root system) evolved. But P is also needed in increasingly larger quantities in later stages of growth to establish a high tiller density (in cereals), to help develop a vigorous root system and to grow biomass and ultimately fill grains (in all species). That later season P has traditionally come from native subsoil P reserves, but as we remove more and more in harvested grain the need to introduce fertiliser P sources to replenish that is becoming more urgent. As one can imagine, the placement of starter P fertiliser to meet the demands of a young seedling with a very small root system will be quite different to that needed to meet demands of a well established plant living on subsoil moisture during flowering and grain filling.

Some soils were low in available P reserves from the start and have required P inputs from early in their cropping history. However for many others the need to apply P is relatively new or still non-existent, due to either high native P fertility or relatively short crop histories. As a result, we in the north are still working out where and how best to use P in our fertiliser programs. This is a major contrast to southern and western systems where soil P was in its native state almost uniformly low, and long term P fertiliser use now has growers questioning when to stop (or at least reduce rates). In many ways the northern grains region (NGR) is at the opposite end of the P fertility continuum to the rest of Australia, in that we are starting to ramp up P fertiliser use.

Another reason we are different is how we farm. With the possible exception of parts of the central west of NSW on lighter soils, soil moisture storage during fallows and subsequent extraction and use during a crop season are as important, or in many cases more important, than in-season rainfall for achieving a profitable yield result in most of the NGR. Last winter (2013) was an extreme example, where many crops in Qld and NE and NW NSW were successfully grown on little or no effective in-crop rainfall. While not always to this extent, subsoils and root activity in them are the keys to our success in most northern cropping seasons. However it is not just water we extract from those subsoils, with availability of nutrients in those same layers essential to sustain growth of plants. Nutrients removed from those subsoil layers have to be replenished if we wish to keep successfully farming subsoil moisture, and while some nutrients like nitrogen (N) and sulfur (S) can be moved back into those layers as soil water stocks replenish, this transfer method doesn’t work for immobile nutrients like P and potassium (K). Replacing subsoil P and K requires either placing fertiliser into those layers directly, or moving fertilised topsoils deeper into the profile with some sort of inversion tillage. The latter is generally not a popular or feasible option in our largely zero tillage, heavy clay soils, given our reliance on stubble cover for water infiltration and the often unfavourable chemical characteristics of subsoils that would be brought to the soil surface.

This then generates a series of questions around (i) whether we would expect to get a crop response to applied P; (ii) how and when that P should be distributed across the soil profile; and (iii) perhaps most importantly, does it pay and when! We explore all three issues, using experimental data and case studies, in combination with APSIM modelling using regional climate data. We have used this climate data to explore historical variation in crop productivity in different regions, in addition to using seasonal rainfall distributions to classify season types and their frequency. The latter has proven to influence the magnitude of response to P application in experimental results, and so will be an important influence on the expected profitability and payback periods as well as the risk of the deep P placement.

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.

Of course, a soil test that is lower than a critical concentration or range does not guarantee a response to applied fertiliser. Rather, these critical values or ranges are indicators of a probable response with favourable seasonal conditions and agronomic variables. You will also note there are some pretty broad differences between responsive and definitely non-responsive soils, with this large grey area where we are currently researching to better refine soil test-crop response guidelines.

What response would we expect in yield in different seasons?

This is clearly a work in progress, but we are starting to be able to make some general observations from trial data and existing work done in the NGR. We will restrict this study to soils where we are confident we will get a response to applied P. That is, for Colwell P <25 mg/kg in the 0-10cm layer in the case of a starter P application, and when Colwell P is <10 mg/kg and BSES P is < 30 mg/kg in the 10-30cm layer in the case of deep P. Key points are as follows –

• 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.
• 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 acquisition and increased yields. In wet years when the topsoil was readily accessible, or in extremely dry years, responses were more limited. By contrast, the strongest responses in chickpeas were in years with little effective rainfall, especially up to pod loading, although consistent responses were still recorded in wetter seasons.

Except for wheat, there is very limited data on responses to starter P by crops in the NGR and there is no historic data on responses to deep P for any crops – other than what we have generated in the last few years. As a result, the representation for different crop species, soil types and seasons is somewhat limited. However we would suggest there are three basic types of seasons, with characteristics that influence crop response to deep P. We have loosely termed these as

Type 1 seasons, or ‘dry’ starts - those years with little or no effective rainfall from planting until after tillering;

Type 2 seasons, or the ‘wet start/dry finish’ years - those with enough rain to ensure good early growth, secondary root development and tillering but serious later stress ensuring a strong reliance on subsoil moisture; and

Type 3 seasons or ‘wet’ 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), and we have used the climatic record from key sites across the NGR to estimate the frequency of occurrence. We have also used APSIM to generate yield distributions for these seasons on high (240mm) and low (120mm) soil plant available water storage capacity (PAWC) and different starting profile moisture contents (full, 2/3 full or 1/3 full). These simulations were used to estimate average yields of our four key crops (wheat, chickpea, mungbean and sorghum) in each season type, with these averages used to estimate the potential yields from which losses due to P deficiency can be calculated.  These data are shown for wheat and chickpea, sown on a 2/3 full moisture profile in soils with either a 120mm or 240mm PAWC, in Table 2.

A few key points to make about this analysis are (i) the frequency of Type 1 seasons (dry starts) is relatively low (5-10% of years) except in the central Highlands of Qld, where it ranges from 30-40% of years; (ii) there is a strong interaction between Type 2 late stress seasons and soil type/PAWC, with the higher PAWC reducing the frequency of late stress seasons by an average of 30% (wheat) to 40% (chickpeas). Average yields in those less frequent Type 2 seasons were still 800 (chickpeas) -1200 (wheat) kg/ha higher in the high PAWC soils; (iii) Similar effects were evident in the summer crops (not shown), although the frequency of Type 1 seasons was much higher (average of 40% for sorghum and 21% for mungbean) than in winter crops. Higher PAWC reduced the average frequency of late stress summer seasons by an average of 16% (sorghum) to 30% (mungbeans), with average yields in those late stress seasons 1550 (sorghum) and 500 (mungbeans) kg/ha higher in the high PAWC soil.   

The yield losses due to low P incurred in the different seasonal types are clearly pivotal in any such analysis of the costs and benefits of ensuring adequate P nutrition. The estimates used in this analysis 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 have been used to work out the opportunity cost of lost yield if P applications are not made and placed in the right part of the soil profile. These values are used to estimate the costs and benefits of P fertiliser application. We have used average prices for each crop: $200/t for sorghum, $250/t for wheat, $400/t for chickpeas and $700/t for mungbeans. Clearly, differences in yield potential due to PAWC will have a major impact on the absolute yield response to applied P

How much do we need to apply and 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.

In our analyses here we have assumed deep P application rates of either 20 or 40 kg P/ha (eg. 100 or 200 kg MAP/ha), with responses available undiminished over a 3 (20P) or 5 (40P) year cropping period.  While most of this will be a ‘new’ fertiliser input in some areas, in others at least part of this P may be able to be diverted from what is currently being applied in starter fertilisers. Our experience suggests that, provided metering systems are adequate to ensure an even fertiliser distribution along the seeding trench (easier with liquid P forms), effective rates of starter P can be as low as 3-4 kg P/ha in row spacings of 100cm or greater (eg. summer sorghum) and 5-6 kg P/ha row spacings of 25-50cm (wheat and chickpeas). The response from higher rates of starter P is likely to be marginal, and although available to roots in subsequent seasons (provided topsoils are wet), could probably be used more efficiently as part of deep applications.

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 these large P applications into subsoils is excellent and covers multiple crop years. Papers in the 2012 GRDC Updates have shown applications of 40 kg P/ha providing consistent yield responses of 15-20% over 6 consecutive crop seasons at Brookstead. Additional P uptake in crop biomass was as high as 10 kg P/ha in some seasons, but the rate of removal of fertiliser P in grain was often only 20-25% of that found in biomass, so the continued yield responses were consistent with a mass balance that suggested residual P should be available. The proportion of the applied P still in the subsoil is debatable, given the quantities returned to the soil surface as residue.

What does it cost to deep place P?

We are indebted to growers in central and southern Qld for providing estimates upon which to base these figures. These operators have used purpose-bought strip tillage equipment or modified planters capable of deep sowing chickpeas to place fertiliser at ca. 20cm depth. Both sets of equipment were 12m wide and placed fertiliser at 50cm or 75cm band spacings (we would recommend bands 50cm apart or closer if possible to get the best response), and operated at 7-8 km/h, using about 60-65L/h in diesel. We have estimated total application costs at between $30 and $40/ha, which are added to costs of MAP or a mix of MAP and KCl (if both P and K are limiting) that vary from $750-$850/t bulk. Thus the cost of deep placement of 100 kg/ha product is estimated at $105-115/ha after every 3^rd crop, or $180-$210/ha for 200 kg/ha after every 5^th crop, with this longer duration based on results from our trials at Brookstead (see next section). Allowing for the interest on borrowing to afford those outlays over the residual period would raise the total cost of deep placement by 25% (3 year) or 40% (5 year), assuming borrowing at 8% interest. We have assumed these costs are additional to what is being incurred now (including any use of starter P). We think this is reasonable given possibly reduced costs associated with lower starter P rates but an increased N fertiliser cost associated with higher yield potentials in the cereal crops.  

So does it pay to deep place P?

Simulations

We have used the simulated potential yields (as illustrated for winter crops in Table 2) and the discounts that would be incurred from low P (Table 3) to estimate the average responses to applied P for each crop in each season type. Then for each location and PAWC soil we have calculated an average yield response across the climate record which is weighted for the different frequencies of each season type. For example, on a 240mm PAWC soil in Gunnedah growing wheat, there were 6% Type 1 seasons with an average yield of 2.5 t/ha, 30% Type 2 seasons with an average yield of 3.3 t/ha and 65% of Type 3 seasons with an average yield of 5.4 t/ha. Using these figures and the relative yield losses that would be incurred without adequate P in each season type shown in Table 3, we estimate the average annual loss in wheat yield resulting from inadequate P availability on a responsive soil would be 840 kg/ha.

We have made similar calculations for each crop, soil type and location, and assembled some 5 crop rotation sequences (Fig. 1) which show the estimated cumulative lost production ($/ha) that we predict would be overcome by a 200 kg/ha deep MAP application at a cost of $180-$210/ha plus any allowances for borrowing costs. Data clearly show greater returns from applying P on a higher PAWC soils (reflecting the greater productivity of such soils in most seasons), and to some extent higher returns on summer only or mixed summer-winter crop rotations. The latter would appear to be due to the relatively high returns generated by mungbeans valued at $700/t, and given the quality/price uncertainties with this crop, those findings should be treated with caution.

Perhaps the key finding is that positive returns were generated from deep placement of P in responsive soils in most situations – the exceptions being on low PAWC soils with summer-dominant rotations in Walgett and Condobolin (not commonly grown anyway), and exclusively winter crop rotations at Emerald.  On low PAWC soils these returns barely exceeded costs of application and would represent a relatively poor return on investment, but in high PAWC soils returns typically ranged from $2-$5 for each dollar invested in deep P application.

How does this line up with experimental results

These simulated results can be contrasted with actual case studies from various sites conducted at locations across the region and followed for at least 2 consecutive crops.

• At Brookstead, our longest running site, we produced an additional 2100 kg/ha sorghum and 1200 kg/ha wheat over 6 crops from 2006/7 to 2011, which would have returned an extra $720/ha.
• At Capella we produced an extra 900 kg/ha chickpeas in 2012 and an extra 200 kg/ha wheat in 2013, which would be worth $410/ha for the first 2 crops after application using average crop values.
• At Gindie we produced an extra 600 kg/ha sorghum in 2011/12 and an extra 500 kg/ha chickpeas in 2013, which would return an extra $320/ha for the first 2 crops after application using average crop values.
• At Jandowae we produced an extra 0.5 t/ha wheat in a dry 2009 winter, followed by an extra 1.5 t/ha sorghum in 2010/11 which would return an extra $425/ha for the first 2 crops after application. Unfortunately a following chickpea crop was lost in the wet 2011 winter.
• At Wondalli we produced an extra 500 kg/ha sorghum in 2008/09 followed by an extra 650 kg/ha wheat in 2011, which would return an extra $263/ha for the first 2 crops after application using average crop values.
• At Biniguy we produced an extra 1.2 t/ha of wheat in 2011 followed by an extra 1 t/ha sorghum grain in 2011/12, returning an extra $500/ha.

Assuming similar application costs at each of these sites, all results support the conclusion that deep placed P would likely provide returns to growers well in excess of the application costs. The fact that most sites achieved those returns within 2 crop seasons after deep P application may at least partly reflect the string of wetter seasonal conditions in recent years, although the results from Jandowae, Capella and Gindie all included dry seasons in at least one of the crops followed.

The experimental program is continuing, and further data will be collected to give greater surety of financial returns from adoption of deep P placement, and a better understanding of the impact and longevity of different deep P rates. For those considering deep P applications in their cropping program, a few key factors need to be considered –

• Make sure you test the paddocks first, to ensure a P response is likely.
• Make sure P isn’t the only limiting nutrient – secondary limitations to yield from K or S can limit an expected P response.
• If you are raising grain crop yield potentials above what is normally experienced, make sure there is adequate N available to achieve that higher yield goal.
• If you are diverting some of the P fertiliser used in starter applications to periodic deep P applications, make sure the rate you apply is still adequate to meet seedling P demand, with the key factor likely to be uniformity of fertiliser distribution along the seeding row.

Contact details

Dr Mike Bell
Queensland Alliance for Agriculture and Food Innovation (QAAFI), Univ of Qld
Ph: 07 4160 0730
Fx: 07 4162 3238
Email: m.bell4@uq.edu.au