You might think that loading your soil with compost, manure, or woodchip can only be a good thing, enriching it and adding to its ability to nourish crops? We thought the same – until Dr Andrew Ross took a closer look at our soil. It turns out that we’ve got a problem. Our soil has too much phosphorus and rapidly declining levels of potassium.
How did we get here? And, more importantly, what do we do next? Andrew Ross takes up the story.
Improving water retention with organic matter
When we were offered the Highbridge Community Farm field way back in February 2010, I took samples of the soil and analysed them in the College lab where I taught. They had between 4-6% organic matter in them. This was a fairly good level that held lots of nutrients for the plants.
But over the next few years, as we came across droughts and water shortages, we realised that more organic matter in the soil would act like a sponge and hold more water. So we chucked on the manure and compost and woodchip and our local council’s recycled green waste called Progrow. The organic matter level rose to 13% and we just about doubled the water holding capacity of the soil (about 90 litres per square metre in the top 30 cm when the soil is at capacity).
Then came the first problem. Lots of swedes and turnips died. It turned out that the Progrow had raised the pH of the soil to 7.5-8. At that high pH level, the plants couldn’t absorb boron and they died of a shortage. We’ve managed to get the pH down to 6.9-7.2 and everything grew well again.
The phosphorus problem
In 2019, we decided to have some professional soil tests done. Since 2019, we have had the soil analysed every two years and plan to repeat the analysis in October this year. You can read about these analyses in a previous article here.
These soil tests revealed level 8 phosphorus. The scale goes up only to 9 and the target level for growing vegetables is level 3 so you can tell that we had a problem.
Figure 2 summarises the phosphate results for each set of tests. Plants don’t require lots of phosphate and, very often, fertilisers can contain a lot more phosphate than they need. Over the last five years, our plants have taken up some phosphate but not sufficiently to lower the levels in the soil. This is because of small additions of manure, compost or woodchip which seem to keep the phosphate level more or less steady.
When crops like potatoes and tomatoes grow, they take up quite large quantities of phosphate as Figure 3 shows, but most of the other crops take up relatively little. If we keep adding materials to the soil, the phosphate level could rise further into the danger level of 9.
Just four barrow loads more of compost
We have now worked out that if we added 4 barrow loads of compost (weighing 100 kg dry weight and containing 1% Nitrogen (N), 1% Phosphorus (P), 1% Potassium (K), which is typical of compost), then that load would contain 1 kg each of nitrogen and phosphorus and potassium. Spreading that over 200 m2 of a plot and mixing it with the soil to a depth of 30 cm will add 5g of each nutrient per sq m or 16.67 mg per litre of soil.
So putting just 4 barrow loads of compost could raise the phosphate level by 16.67 mg. If that had been done over plots 5-8 in October 2023, that would have raised their phosphate level to 279.7 which is dangerously close to the threshold of level 9 which is 280! And that is why we are trying to add as little compost or manure or woodchip to our plots as possible at this time.
We know that if the phosphate concentration rises to level 9, plants can show calcium deficiency. They show this with browning and dying of new growth at the tips of leaves and roots and a reduced ability to absorb the micronutrients Iron and Zinc. (The photo below shows shows iron deficiency in a plant which is characterised by strong yellowing of young leaves.) In addition, there can be poor seed- and fruit production and a greater susceptibility to disease. These are not things that we want to happen and why we must continue to aim at reducing soil phosphate.
The potassium issue
We have the opposite issue with potassium. Plants take up lots of potassium from the soil. and some is washed down out of the soil by excessive rainfall, particularly if the soil is not covered with growing plants. Studies indicate that potassium lost by drainage from soil is approximately 0.5 g or 500 mg per square metre for every 100 mm of through drainage. We had 1000 mm of rain at the farm last year. That is one of the reasons why many teams are planting the cover crop Phacelia over the winter period. Phacelia keeps the soil covered and protects it from heavy rains which leech nutrients down, while pulling up washed-down nutrients with their deep roots.
You can see from our results in Figure 5 that the potassium levels have gone down steadily from level 6 to level 5 then to level 4. By October this year, they could be as low as level 2+ which is as low as we would want them to go.
So, we might need to add potassium to some crops this summer or next year if we begin to see nutrient deficiencies in any of our crops. The photo below shows how plants respond to low potassium levels. Leaves can turn yellow, especially at the tips, or the margins will crinkle and curl. Then they might go brown and the tissue might die. Plants appear stunted and have poor flowering or fruiting.
Choosing a fertiliser
The best fertiliser on the list is muriate of potash (potassium chloride), while potassium sulphate (0.0.50), potassium magnesium sulphate (0.0.22) and potassium nitrate (13.0.44) have large quantities of potassium.
But are these fertilisers organic? And does that matter?
Of the other definitely-organic fertilisers in the table, seaweed fertiliser looks to have the greatest quantities of potassium while having low levels of phosphorus, so that may be the best alternative. And that’s a decision for HCF in the future.
Thanks for reading.
As a community, we’re grateful for Andrew’s commitment to the science behind growing. The chemical analysis of our soil allows us to understand what’s happening in it and to explain the consequences for our plants. We can change the recommendations for the growing teams to give our soil the best chance to thrive. Ongoing testing will show whether we’re back on the right track and whether any more adjustments are needed.
Let us know your thoughts and questions.
Some useful references: