Should I apply phosphorus at sowing?

By Cropping Adviser Tim Bartimote and Mixed Farming Adviser Rohan Leach

With input prices for the 2022 winter crop season already quite high there is potential to consider reducing fertiliser rates at sowing to reduce costs. In some high fertility situations, there is the temptation to leave out starter fertiliser altogether. This article will investigate in what situations we can reduce or even eliminate our rates of starter fertiliser.

Phosphorus (P) is an essential nutrient for healthy root and shoot growth. Adequate P nutrition for the plant results in early seedling vigour and better root development. This allows for better uptake of soil moisture and nutrients leading to more robust plants. Therefore, it is particularly important to ensure adequate P nutrition when plants are germinating, as late application of P fertiliser may miss the key requirement period among seedlings.

Phosphorus and our crops

Sowing is the best time to apply P in crops. Being an immobile nutrient, P will typically not move from where it is placed or naturally residing in the soil. Soil moisture near the P is also required for plants to gain access to it. Hence, if P is not found with the seed, plants will need to go searching to meet their P requirements. This can be problematic if seedlings are spending their limited resources in search of more P rather than pushing newly grown roots down into moisture. Further, if P is shared between root growth and setting tillers, then there will likely be a yield penalty due to slow initial development and a lack of tillers.

This can still be seen even in more fertile soils. Those with a Colwell P above 25 may consider leaving out starter fertiliser or drastically reducing usual input, as it is deemed there is sufficient P for crop establishment. However, while a soil test will tell you the average level of P, it does not mean that the P is evenly distributed across the paddock, or across the top 10 cm. Incorporating some P with seed will still benefit initial plant development and provide this uniformly across the paddock.

Planning to broadcast P later in the season could be argued as another potential option, in the circumstance that prices drop. However, much of the benefit would be lost for yield potential if this occurs later than the first month after sowing. Accessing P on or near the soil surface is dependent on moisture availability. If moisture is limited in the topsoil, then P uptake would be restricted.

The Phosphorus Bank

When we look at P fertility in our cropping systems, we can use the analogy of our soils as a bank for P. We can deposit more P into the bank in the form of fertilisers (synthetic or organic) and we withdraw from the bank every time we harvest a crop. To put some numbers on this analogy, if we ‘deposit’ 80kg of Monoammonium phosphate (MAP) around 17.5kg of P goes into the soil bank. If we grow a 5 tonne wheat crop and send it to the silos, we withdraw anywhere between 15-19kg of P in the form of grain. A small amount of P is tied up in the bank (think fees and taxes) through binding to soil particles and we are roughly in the same position we started at the start of the growing season.

This rate of fertiliser where we aren’t depositing significantly more than we are withdrawing is called our P maintenance rate. It depends on soil type and annual rainfall and can be roughly calculated by historical average yields. For example, maintenance rates in regions averaging 2.5 t/ha wheat crops are roughly in the vicinity of 40kg of MAP while the example above shows where maintenance rates are for 5 t/ha average yield.

If producers want to reduce their rates of fertiliser, high levels of soil fertility are essential, i.e., a high bank account. You are relying on soil P reserves to make up the difference from your reduced input. If soil P levels are below the critical value (in the central west around 25-35 mg/kg Colwell P in the top 10cm of soil) dropping our fertiliser rates below our historic input levels carries a very high risk of reduction in yield as we are simply limited by low P.

It therefore makes sense if some producers want to trade this resource like any other. We can draw on soil P when fertiliser prices are high and then deposit more than is needed when those prices drop again. However, like all trading there is the increased risk that comes from speculating on markets. Some may consider it cheap insurance to simply use maintenance rates of P fertiliser and not be dependent on market fluctuations.

It is therefore critical to soil test our paddocks so that we have a thorough understanding of our fertility and to help plan and budget how much fertiliser is needed.

Risk vs Reward of Starter Phosphorus in High P Soils

For those who have remarkably high rates of soil fertility, above 60 mg/kg Colwell P, you could reduce or eliminate the need for starter fertiliser in the short term. Recent pricing for MAP at the writing of this article is $1,500/t ex gst ex farm. A jaw dropping increase compared to previous seasons. Due to this price hike some producers who have these highly fertile soils have mentioned that they will be dropping rates of MAP back to 30 kg/ha ($45/ha) or simply using none at all.

It is clear what a producer will gain from opting out on applying some (30kg/ha) starter fertiliser, roughly $45/ha to spend elsewhere. At the same time though, will the producer potentially lose out by not putting fertiliser with the seed? In other words, is there an opportunity cost in yield by removing all phosphorus out of the sowing trench?

Using wheat as an example, the current price for APW1 is roughly $390/t ex gst on-farm. Therefore, if by adding fertiliser a producer gains a further 115kg/ha of APW1 wheat in yield then this will cover the cost of fertiliser at sowing. An increase in yield then above this number is profit and great justification for still adding some MAP. However, if MAP is added and the gain in yield is less than 115kg/ha, then the producer would have been better not to add any fertiliser. Likewise, if by leaving out MAP, the crop yield drops by more than 115kg/ha then the producer has lost a significant amount of money as an opportunity cost. What then is the likelihood of making this yield gain?

Being a niche issue there has not been an exhaustive amount of data on this topic. Furthermore, out of the handful of trials conducted, some arrive at different conclusions. 12 trials which ranged from 1963 to 2009 suggest that starter P in highly fertile soils (>62 mg/kg Colwell P) didn’t have a significant yield impact. To access these trials yourself, you can find them on the Better Fertiliser Decisions website. While another range of trials in the southern Central West conducted between 2007 and 2022 seem to suggest that even a small amount of P (6 kg/ha) does have a significant impact on yield.

Management could explain this difference in results. Soil samples typically show averages and do not identify significant highs and lows in the top 10 cm. Excessive stratification could mean there are elevated levels of P in the top few centimetres of soil while there is a significant lack of P below. A sample from this soil could still provide a suitable average (>60 mg/kg) yet leave plants deficient and soils responsive to the addition of P. Moving from more consistent cultivation to a no-till system could explain why some high P soils are still responsive. Since the soil is no longer being mixed together through cultivation and achieving a more consistent spread of phosphorus.

Therefore, on paddocks where Colwell P has been accurately identified to be greater than 60 mg/kg. It would make sense, considering current fertiliser prices, to at least reduce starter fertiliser rates in this year's wheat crop. And depending on your confidence of the consistency of P in the soil, even considering leaving it out altogether. However, in the long term I would not recommend continuing this practice and suggest the application of a maintenance rate when the decision is more economical. Continually mining the soil of P will eventually reduce average P levels significantly.

For more information on the importance of P in our soils and making fertiliser decisions at sowing, contact your local LLS ag adviser on 1300 795 299.

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