Workshop: Soil phosphorus – feeding your soil without breaking the planet

The weather turned out to be absolutely glorious for our gardening workshop about soil phosphorus on the 15th of May. So much so that we had to abandon the overheated polytunnel and seek shelter under the trees in our ‘outdoor classroom’ area.

As the participants trickled in, they were directed to place their jam jars with garden soil samples on the testing table, next to a mysterious array of tubes, funnels and chemicals. Then, we settled into our chairs to listen to Courtney Giles, a soil scientist from The James Hutton Institute, who gave us a brief introduction to soil phosphorus.

She told us that phosphorus is a critical nutrient required for plant growth. However, the primary sources of phosphorus for food production are finite, mineral resources, which impart a large carbon footprint to extract and distribute around the globe. The over-application of phosphorus fertilizers can be damaging to our streams and lakes, often leading to the formation of harmful algal blooms and poor water quality. She finished off by giving some tips on how to provide phosphorus to our garden plants without relying on the mineral sources. The main messages here seemed to be – cow manure:) It turns out that vegetarian diet also reduces both, our carbon and phosphorus footprints. The introduction was followed by a few interesting questions from the audience. To listen to the introduction (and the lovely bird chorus in the background) in full see the video linked below.

Courtney went on to point out that minimising our carbon and phosphorus footprints when fertilising our garden starts with understanding of the quality of our soil – if we test for nutrient levels we can avoid overapplication of fertilisers. This signalled a transition to the, much anticipated, hands-on part of the workshop where we tested our soil samples for levels of bioavailable phosphate (or phosphorus form available for plants to take up) and pH (or soil acidity).

Acidity testing was included as it is important for many reasons. Each plant has their own pH requirements but most plants require fairly neutral pH of around 6.5-7. Soil pH also affects availability of phosphorus for take up by plant roots – it is poor when soils are too acidic or alkaline. Royal Horticultural Society has a good outline of gardening pH basics here.

We used a very basic ‘guerrilla’ soil test kit that Courtney put together for us, but if you would like accurate data on your garden soil you should use a proper soil testing kit (available in garden centres and online). For those who don’t trust their own chemistry skills, professional soil testing services are available. The James Hutton Institute has one, and so does RHS.

Courtney’s guerrilla testing process involved lots of jar shaking, maracas style, to mix the test ingredients. The mood turned rather festive and there was time for a chat and sampling of the cakes. (Thank you, Jessie, Margaret and Kaska for supplying your wonderful home baking again)

Some of us also tried our hand at the Phosphorus Game which Courtney developed with her colleagues at the James Hutton Institute. It turned out to be a rather fun, albeit somewhat competitive, way to find out what interactions influence availability of phosphorus to plants in soil. Who would have known that nematodes help release phosphorus bound in bacteria for plant use? You can download your own copy of the game here – it is a perfect addition to a summer garden party for kids and adults alike!

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After all the excitement, it was hard to turn our attention back to checking our samples against standardised indicators provided by Courtney. We were relieved to find out that it was all good news – it turned out that our soils’ pH was not far from neutral and that soil phosphorus levels were within the range recommended for growing fruit and veg (see below). Even the horrible sandy soils from Tayport river flats seemed to have just about enough phosphorus, with Discovery compost levels predictably going off the charts.

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You can read more about nutrient efficiency and the research that Courtney does as the James Hutton Institute here or in the materials related to the June’s Linking Environment And Farming (LEAF) Technical Day event at Balruddery farm.

Well – this was all a lot of fun:) Can’t wait for our next workshop with Gabrielle Flinn from Buglife on working with nature to create a vibrant and healthy garden. Hope to see you there on Sunday, 12th June at 11am sharp. And bring the kids along!

 

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