To grow good crops, we need healthy, balanced soil. Andrew Ross writes about our soil tests and how the results from these and our local weather are informing the way that we will manage the plots over the next year or two.
Image credit: Steve Grundy
Three sets of analyses
Over the past five growing seasons, we have had 3 sets of soil analysis: April 2019, April 2022 and October 2023, whereby 20 small samples from 4 plots (1-4, 5-8 etc) were grouped together and analysed.
What have we discovered?
Organic growers are encouraged to have high levels of organic matter in their soils because it acts as a sponge holding large quantities of nutrients and water. Over the years, we have been piling on the manure, compost and woodchip and so we have raised our organic matter levels to around 12-15%.
The analyses show that our nutrient levels are high which is generally good, but the big problem has become our soil phosphate level.
Phosphate levels have stayed stubbornly around 254 mg/l which is way too high and really needs to be reduced to below 100 mg/l. Phosphorus (P) is not directly toxic to plants but, at high levels, it can inhibit the uptake of iron and zinc. This year, we have been seeing raspberries and parsnips showing signs of iron deficiency.
Another feature of high phosphate levels is that, attached to soil particles, they can run off into rivers especially in wet weather and cause eutrophication (excessive richness of nutrients in the water). Phosphates from farming are damaging the rivers like the Itchen that run into the Solent and so both Eastleigh Borough Council and Natural England have phosphate mitigation strategies in place.
What will we do about the high level of phosphate?
These high levels of phosphate mean that we are going to:
- Continue with the ban on adding manure to the plots
- Stop adding woodchip to the plots
- Continue to make our own seed and potting-on compost so that we don’t bring more phosphate on to the site.
During a growing season, most crops will take up between 13-75 mg/l of phosphate (which we take away and eat). More will be removed from the plots to the compost bins in crop residues and weeds. But relying on reducing phosphate by this method alone could take several years to achieve healthier levels. In the meantime, other substances will become depleted. So, we will be trying a few other ways as well.
- Growing winter cover crops such as Phacelia which will add extra organic matter to the soil but not extra phosphorus.
- Growing winter crops like leeks and kale which will remove more phosphorus.
- Growing the nitrogen-fixing field beans or winter tares (a green manure) during the winter which, if cut down before the beans develop, will add extra nitrogen and organic matter without adding more phosphorus.
- Hoping that, over time, some phosphorus will be converted to the more stable form called “rock phosphate” that has a very limited solubility in the soil.
Levels of nitrogen (N), potassium (K) and magnesium (Mg) will inevitably fall so we might have to add to the soil inorganic fertilisers such as ammonium nitrate, urea, potassium sulphate, magnesium sulphate and possibly wood ash (which contains lots of K and Mg but some P (phosphorus) as well) to keep up the levels of other plant nutrients.
Rainfall and our management of the plots over winter
No-one can have failed to notice the amount of rain we have had this year. The average annual rainfall at Highbridge is 802 mm or 802 litres per square metre. A few days ago, we exceeded this amount for 2023 and the cumulative total is already nearing 850 mm. And we still have the normally-wet months of November and December to go!
While this reduces the amount of watering that we have had to do, it can make life difficult too. Root crops like potatoes, carrots and parsnips are more difficult to dig. The soil can become compacted by our trampling on it. Crops go mouldy quicker.
If we get lots more rain this winter on our sodden, waterlogged soil, nutrients can run off the surface or be leached down through the ground.
Minimising run off
To minimise this, all teams are being encouraged to grow plants over the winter on all plots. The plants will help to intercept the rain, reduce surface compaction and runoff and pull back up into the plant roots the nutrients that could otherwise be leached.
More ways to enrich our soil
As well as our winter crops like kales, leeks and parsnips, several teams have been planting the green manure called Phacelia. In the Spring, if we don’t have any severe frosts that kill it, the Phacelia can be cut off, chopped up on the soil surface and left to the worms to pull the decaying bits into the soil. This adds organic matter to the soil.
Other teams who will be planting potatoes next Spring have been planting “spud mix” – a mixture of mustard and radish. Next Spring, the mustard and radish can be dug up and immediately turned straight into the ground. This releases a chemical which kills the wireworms that would otherwise make holes in potatoes.
It is now getting too late to plant Phacelia, but we are hoping that the weather will be kind to us and give us a window of a few weeks. This will allow teams that missed the Phacelia sowing an opportunity to plant field beans or winter tares. These have the additional benefit of being nitrogen fixers as well as soil cover crops.
Soil testing has given us valuable insights into the state of our soil and the opportunity to rebalance in a natural way. We’re also doing all we can to mitigate the effects of recent heavy rainfall. What do you do to keep your soil as healthy as possible?