Hard to believe

5 Nov 2014

I’m not fond of museums, but I am a nightmare when I actually visit one. I read everything and so make very slow progress. I remember my family hauling me out of the Elvis Presley museum in Las Vegas because I had barely progressed beyond about 20% of the exhibits whilst they had done the whole lot and had had a cup of coffee.

The same slow progress occurred a few days ago. We were invited to a ‘do’ just down the road at the Imperial War Museum at Duxford and were given the freedom to see the exhibits for most of the day. Needless to say, I went round just one of the many hangars. In this there was one of the two prototypes of Concorde. It seems that it did its cold weather testing above the Equator. This really sounded counter-intuitive so I read on. Apparently the coldest temperatures at 50,000 feet are above the Equator and the warmest temperatures at that height are above the North Pole!

Issues that have occurred in my day job have sometimes sounded counter-intuitive but again have been true. The prime example was when NIAB TAG, in its most recent 60 trials, collated the nitrogen response curves for feed wheat and found that the economic optimum nitrogen doses did not appear to decrease with increasing Soil Mineral Nitrogen (SMN) levels in the soil. In fact, the best predictor of the optimum dose for the yield of feed wheat in individual trials was to apply the average optimum dose identified in the 60 trials rather than follow any of the current recommendation systems.

I must admit that this worried my colleagues and I because it seemed cFertiliser spreadingounter-intuitive and so we read published papers which contained the results of similar trials. True enough, the overall conclusions were the same for situations similar to those in our trials. Interestingly, this also included the database used for the current edition of RB209.  It should be noted that all our trials were done on long-term arable soils where there had been no recent history of organic manure use; we did not have sites on true sands or the true silts. None of our sites had SMN levels above 100 kg N/ha. Please note that I am talking about feed wheats where the level of protein is immaterial.

Our results have been a continuing fascination to me. Subsequently, I developed an alternative recommendation system and tested it against the results of our trials and also the trials carried out by other organisations. It gave almost the same level of accuracy of prediction for the optimum dose in each trial as applying the average dose identified in our database.

The alternative recommendation system I developed was not very novel because it was the same as that used in RB209 except that SMN was assumed to be used at 50% efficiency rather than the assumption of 100% as used in RB209. This simply resulted in the recommended N dose being reduced by 15 kg/ha between index 1 and 2 and also between 2 and 3 rather than the 30 kg/ha in RB209. There is now so much information to suggest that the efficiency of use of SMN is well below 100% that this evidence can no longer be ignored in the next edition of RB209.

Recently we have re-opened an internal email debate on our results and discussed them with a soil scientist who has penned many peer-reviewed papers on nitrogen application to wheat. This correspondence released the genie of canopy management.

Remember, canopy management in wheat? It was the vogue for many years and much research funding was spent on it. It argued that wheat yield was not related to nitrogen dose but was determined by using the nutrient to build an optimum crop canopy to trap solar energy efficiently. I think it failed because the SMN was assumed to be used at 100% efficiency and because by the time the size of the final dose of N could be calculated, it was often too late in the season for it to be fully exploited by the crop.

Let’s go through the maths of canopy management but assume that SMN is used at 50% rather than 100% efficiency:

  • To trap sunlight efficiently in wheat there needs to be a Green Area Index (ratio of the green surface area of the crop per m2 to one m2 of soil surface) of at least 6 by around ear emergence;
  • Each unit of GAI requires around 30 kg N so the total of N required in the crop by around ear emergence is 180 kg N/ha;
  • There is already some N in the crop by the spring, typically around 30 kg N/ha;
  • There is also some in the soil (SMN), typically around 50 kg N/ha which we now assume is used at 50% efficiency by the crop;
  • This means the N demand to be met by applied bag N is 180 kg N/ha less the 30 kg N/ha already in the crop and the 25 kg N/ha from the soil;
  • Therefore the demand for N from the fertiliser is 125 kg N/ha;
  • RB209 assumes that bag N is used at 60% efficiency and so the dose needed to satisfy the crop requirement of 125 kg N/ha is that number divided by 0.6;
  • This equates to a dose of applied bag N of 208 kg/ha.  

This is extraordinarily close to the average economic optimum dose of 205 kg N/ha in our 60 trials when one kg of N costs the same as 5 kg of wheat. The optimum dose of bag N increases as the price of N gets relatively cheaper. This demonstrates that achieving a more complete canopy, either earlier in the season and/or during grain fill, is worthwhile when the cost of N falls relative to the price of feed wheat.

I recognise that I’ve ignored N from rainfall and from net mineralisation in this calculation. Much of the rainfall that is received by the crop occurs by the early spring (when N availability in the soil and crop is assessed) and net mineralisation is limited in the situation of our trials. In the context of our trials, these are relatively background constants and so do not affect the level of reduction in N recommended between indices 1 and 2 and between indices 2 and 3.

It is comforting that canopy management for a typical situation suggests the same optimum dose as the average of our 60 trials. However, some soil scientists maintain that canopy management should not be considered when predicting the optimum N dose of very high yielding crops. They assume that as yields rise, grain N (protein) gets diluted and there must be a limit to this dilution. All I can say is that our trials have shown that doses of around 205 kg N/ha can support surprisingly high yields of wheat. In fact, our results intimate that only a very slight upward adjustment of N dose is needed for yields of feed wheat above 10–11 t/ha. This is because although the N removed from the field in very high yielding feed wheat can be higher, such crops are much more efficient in taking up and utilising nitrogen than average yielding crops.

I’m sure that there must be logical reasons for the temperatures at 50,000 feet to be at their lowest above the equator. Not to be outdone, we’re slowly developing a better understanding of what drives the optimal doses of N for feed wheat. Whilst this results in the improved predictability of the ‘average’ situation, our database suggests that there is a large and unpredictable variation between sites in the efficiency of use of both SMN and applied nitrogen. Trying to establish reasons for such a variation remains a significant challenge to researchers.      

Please remember that milling wheats are a different kettle of fish because additional nitrogen, above that required for yield, is necessary in order to meet the specified grain N (protein) content for higher yielding crops.

Apologies for the length of this blog being as tortuous as my progress round Duxford....