Some thoughts on DGT from the Community…

At a recent meeting (December 2014), we discussed the dependence of the relationship between soil P availability and plant response on the soil P test that is used.

Our rationale:

Identifying appropriate methods for quantifying P, in addressing different research questions, remains a challenge. Comparing between soil P analysis methods, in addition to soil properties and experimental variables, limits the progress we can make towards understanding how different conditions affect P dynamics. Whilst a globally-applicable test to quantify each P fraction is unattainable, questioning our rationale for the measures we select is essential. Furthermore, it is useful to consider what an alternative measure would mean in terms of the results we acquire and how we interpret them.

We discussed the following paper:

Six L, Smolders E, Merckx R. 2013. The performance of DGT versus conventional soil phosphorus tests in tropical soils—maize and rice responses to P application. Plant and Soil, 366, 49–66.

We focused the conclusion:

For predicting yield response to applied P, an intensity measure (DGT) was most effective for maize, whereas conventional quantity measures (e.g. Olsen P) were most effective for rice.

This suggested that rice does not depend on diffusion of P in the soil (as measured by DGT). Compared to maize, rice has a greater ability to acquire P, via: secretion of organic acids to solubilise non-labile P, symbiotic associations with mycorrhizal fungi, and a more efficient root system for P uptake. Thus availability of soil P depends on the crop species.

In response, we considered the question:

  • How to identify the most representative soil P tests for specific soil-plant systems, without laborious preliminary experimentation or requiring numerous (costly) analyses?

Our main points were:

  • The standard test for plant-available P for UK soils is Olsen P. How did this arise? From a comprehensive review of the available tests and selection of the most replicable and appropriate measure? Assessed for what purpose and soil-plant system?

Key outcomes:

  • Six et al. (2013) raised three criteria for soil P tests. If these are satisfied by numerous different tests, does it matter which we select? This paper and our discussion suggested it does, in which case, what other criteria are we applying to our judgement?
  • How to rectify the perhaps contradictory aims for greater harmonisation of methods (at laboratory group/ university/ regional/ global scales) and for soil-plant system specific measures?
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The spatial variability of soil phosphorus- what controls it?

The recently published paper ‘Spatial variability of soil phosphorus in the Fribourg canton, Switzerland’ by Roger et al. (Geoderma, 2014) was discussed at our first meeting.

To summarise, a spatial investigation of the different phosphorus forms (total, organic and available) across 250 sites (FRIBO network) was carried out. Their results suggested that within agricultural soils the highest mean values of available P were found in cropland (2.12-81.3mg kg-1), whilst mean total P values were found to be most abundant in permanent grasslands (1186 mg kg-1).  It was also suggested that available P appeared more sensitive to extrinsic factors (land use) than total P. They also suggested that the study in China by Liu et al. (Geoderma, 2013) reinforced their hypothesis that environmental conditions (such as temperature and precipitation) have less of an impact than farming practices (e.g. fertilisation), which leads to this difference in P abundance among land use. But what do you think??

This study is another, however, that highlights the importance of ‘legacy P’ (historic land use) when attempting to tackle the diffuse agricultural P pollution problem. Perhaps we may ask whether these results are the same in other systems? This study may be a coarse resolution, but is geostatistics a positive way forward? And can we use this information to move forward – will it call for more localised policies and practices?

A “rockin'” phosphorus trip to Rothamsted Research

Last week we joined Dr. Martin Blackwell (Rothamsted Research), Dr. Ben Turner (Smithsonian Tropical Research Institute) and others at North Wyke (Rothamsted Research), Okehampton, Devon to talk more phosphorus at the Partnering Award Meeting!

We joined the group on Wednesday (The M6 as per usual was a particular challenge… ) down at North Wyke after they had spent the beginning of the week at Rothamsted Research, Harpenden. This trip was definitely a blast from the past for Prof Haygarth as he spent 15 years working at North Wyke!

For a bit of background here, for those of you who don’t know… North Wyke is unique research facility with its own research farm allowing for studies on both arable and grassland systems, it is certainly quite a place!

We had introductions and a tour of the facilities, being typical scientists getting excited over tubes and machines that to anyone else would just look bizarre and/or useless (check out the pictures).

The Thursday included breakout sessions where we delved more into the ever expanding world of phosphorus. With oxygen isotope tracing possibilities in soil and water, and ideas flying on how we might better utilise residual soil P (definitely watch this space), it was a brain buzzing day!

Friday the Lancaster crew (Prof Haygarth, Daniel Blackburn, Kirsty Ross and Hannah Wright) faced the M5/M6… an epic journey consisting of a lot of bad singing…

So there was a lot of science and brainstorming but there was also a lot of “rockin’ out”… specifically by Prof Haygarth and Dr Hawkins… #Nutbush. Make of it what you will!

A fantastic few days, many thanks to everyone involved, you can definitely expect some exciting science in the future from this group! Next time, Panama!