Choose trusty sentinels

15 Feb 2013

“Use careful watch; choose trusty sentinels”

[Richard III to the Duke of Norfolk]

My father was a farmer, county councillor and history nut. The Leicestershire County Council offices were in the middle of the city and he used the designated car-park at Greyfriars. He would have been tickled pink if he’d known that he’d parked his car over the grave of Richard III. Richard IIII’m sure he must have read the biographies but I’m not sure how he viewed Richard; a bad king or a good king. Modern thinking suggests that he was a good king and Shakespeare, either intentionally or unintentionally, got it wrong. It’s interesting how views change over time.

The same could be said about the views on soils and plant nutrition. It’s easy to assume that soil is just there and a medium for various uses, such as growing crops and burying monarchs. However, as everyone now recognises, it is a living and an enormously complex medium; so complex that we still have only an incomplete understanding.

For instance, some soil scientists have held the view that the amount of soil mineral nitrogen measured in the spring has a dominant influence on the dose of nitrogen to apply to an individual wheat crop but examination of the trial databases doesn’t really support this view. The trials database assembled to help guide deliberations on the content of the most recent edition of RB209 (the Fertiliser Manual) shows that for any given value of measured soil mineral nitrogen, the optimum applied dose of nitrogen for feed wheat can vary by a factor of three. There is a long way to go in improving nitrogen recommendations.

The constancy of the presence of soil has also implied that some of the other guidelines for nutrition advice are also solid and unchallengeable. For instance, the advice in RB209 is to maintain the level of plant available phosphate at Olsen P Index 2.  This seems inviolable and so it comes as a shock to discover that this information was derived largely from only two soil types.

As a result there is an HGCA-funded project that is checking whether Index 2 is the correct soil index to maintain for cereals and oilseed rape over a range of soil types. The project has yet to be completed but the results so far generally suggest that this is correct. However, there have been higher yields on plots at Index 3 in trials on sites where poor soil structure may have inhibited the ability of the crops to scavenge for phosphate.

There is one soil type in the project that isn’t behaving like the others. It’s a Cotswold’s brash soil, where it hasn’t been possible to maintain the high soil indices that were created on the other soil types by using large initial doses of triple superphosphate fertiliser. Here, yields have been maintained by soil indices below 2 but there hasn’t been, so far, responses to the application of fresh phosphate at such low indices. This is probably due to the extremely high levels of calcium in the soil making the phosphate less available to plants as measured by the Olsen P technique used in the laboratories. So after some untangling of the results there may be some specific guidelines for calcareous soils in the next edition of RB209. 

The results of this project confirm that phosphate supply to the soil is not a straightforward process. Freshly applied phosphate remains available to plants for a few months but gradually much of it becomes, at least temporarily, locked up in ‘pools’ that have low availability to plants. However, in most UK well structured soils, at Index 2 the amount in the plant available ‘pools’ is sufficient for unrestricted crop growth even if fresh phosphate is not applied.

This is not the same the world over. In many, if not most Australian soils they have to apply some phosphate on an annual basis because that applied to previous crops is not sufficiently available.

This reminds me of some old correspondence I read when I was working in Australia last year. The letters were between a woman who had married and emigrated to farm in Western Australia in the 1920s and her mother in the UK.  It was really heart-breaking stuff about bringing up an increasing number of children whilst initially living in a tent and then in a very small house (hut).  Added to that was the pain of homesickness (no Skype in those days), the hardships resulting from what nature can throw at you in Australia and the land rights battles with neighbours. However, there was a happy ending of sorts. She came back for a prolonged visit to the UK in 1951 and her son wrote to her whilst she was here. He said that he had been to a local field day and a soil scientist had told him all about the benefits of annual applications of phosphate. This, according to his letter, would result in a golden future for the family. I think you have to be a true romantic to believe that!