Making more of phosphorous and nitrogen were two of the presentations at the Teagasc national tillage conference in January. These focused on the potential for improvement in the way we try to optimise the amount of nitrogen we apply to specific fields and also on the importance of phosphorous as a nutrient in different crops.

The challenge of nitrogen rate

The rates of nitrogen recommended for use on crops are a result of research at different sites over time. The amount to use, or the likely response from nitrogen, originates from testing various rates at a number of sites over a number of years. The results can be summarised as a single curve showing the optimum rate but in reality this is the average of a number of curves produced at different sites and in different years.

At the conference, Richie Hackett of Teagasc asked if can we be more specific about the optimum for an individual field? Can we find the specific response curve for a crop before it is harvested? Can we find a different way of making recommendations that would be more accurate more often?

In reality, some fields might require less in a given year while others might require more. However, because N cannot be removed after it is applied, any alternative system would have to apply less initially with the option to top up later in the season.

The big challenge in a given field in a given year is that one can never know if additional nitrogen would have given more yield or if the same yield could have been achieved with less nitrogen. Grain quality requirements must be balanced here also.

Soil mineral N

One of the major uncertainties associated with total N supply is the amount of mineral nitrogen in the soil at the start of the growing season. Richie said that this can be measured but the process is both difficult and demanding at farm level. The efficiency of N use, which can vary either side of 60%, is also uncertain. It is difficult to successfully minimise these variables on a field level before fertilisation begins.

With these variables in mind, Richie conducted an experiment which made an initial estimate of the N requirement of the crop with the intention of topping up lower rates at some point later in the season to ensure that the crop maintained an adequate N status.

The amount of N that should be in a crop has been well researched and an optimum value has been established for the different stages of crop growth. N status can also be difficult and awkward to measure conventionally but there is an increasing number of tools available which may be capable of doing this remotely and in a timely fashion.

Examples include satellite, drones, N-sensors and handheld leaf monitors. In theory, these have the potential to guide crop N status and optimum N application rate.

N curves

To test this theory, Richie worked with spring barley at a site where 170kg N/ha was a well-proven optimum. Three treatments compared either 90 or 120 or the full 170kg N/ha by the tillering stage. Any additional N for the lower rates was applied at GS31/32. By this stage, monitoring indicated that rates at and below 120kg N/ha had below optimum N status, which meant that deficiency was likely.

The research showed that moderate deficiency, when identified by GS30/31, could be fully undone by the application of additional N at that time. But where the N status was not identified until GS37/39, full yield recovery did not occur where only 90kg had been applied during tillering. However, full yield recovery did occur where only 120kg had been applied by the tillering stage.

This emphasised the fact that crops need a basic minimum amount of early N to build yield potential. If this is not supplied, the occurrence of lower-than-optimum N status late in the season cannot be reversed to deliver potential yield.

Richie concluded that alternative methods to help assess optimum N rate may have a place in improving the targeted N rate. However, it will need more research and could add cost to production. But it could also help to produce more yield and/or lower nitrogen use while providing environmental benefits.

Phosphorus power

Phosphorous (P) is especially important to drive early growth and access to adequate P can easily be seen in early plant growth (see picture). At the conference, David Wall from Teagasc, Johnstown Castle, presented results from recent research which showed the responsiveness of winter wheat and spring barley to P application.

David reported that 60% of soil tests have lower-than-optimum phosphorous but responsiveness to P application is not always a straight line effect. Rate evaluation trials also showed different response optimums which varied from 20 to 50kg P/ha on Index 1 soils. But David stated that higher soil fertility indices produced higher yields and required less applied P to do this.

Soil fertility is an important consideration in driving yield in spring barley and generally low soil P indices cannot be compensated for by applying more P in the year of production. Hence the general recommendation to build soils up to the optimum Index 3 level where only maintenance is required. Where P fertility is low, the application of surplus P is advised to help build the soil.

It is still worth noting that even in high P index soils there can occasionally be a significant response to applied P. While “normal” might indicate no response to applied P in an Index 4 soil, soil temperature during early spring growth can influence root access to P. So in cold springs, the addition of even a minimal amount of P can make a big difference.

David reported one such example. In the cold and late spring of 2013, one Index 4 soil returned a 32% yield response to a 20kg P/ha dressing. So rate and timing judgements can still come down to the individual grower in a specific year.

P in winter wheat

Responses from P-rate trials on winter wheat showed much more variable response on both Index 1 and Index 2 soils. David indicated that winter wheat seems to be less dependent on freshly applied P and seems to be poorly responsive, even on some Index 1 sites. He suggested that part of the reason for this was that the winter crop is growing slower during establishment and that it has more time to forage for nutrients before it reaches peak growth at stem extension.

Another factor here has to be the greater foraging capacity of wheat roots compared with winter barley. Its ability to access P from the soil complex, which might not be available to either the soil test or a winter barley crop, should mean that it is naturally less responsive to P. But I would certainly not say that the same applies to winter barley as this crop needs good fertility to thrive.

In this regard, David concluded that high P applications were not able to equal the benefit of having a high soil index to begin with.