Why arable farmers should target 5% soil organic matter

Targeting a soil organic matter level of 5% and maintaining a neutral pH can help optimise crop nutrition and promote good soil husbandry.

Organic matter is the soil’s main nutrient store, and there is a clear relationship between this and nutrient potential.

Current research reveals about 50% of UK arable soils have an organic matter level less than 5%, highlighting major scope for improvement.

See also: Expert advice on creating biologically dynamic soils

“More organic matter means more potential for stored nitrogen, phosphorus, potassium, and other micronutrients,” explains soil data analyst Jon Telfer of Lancrop laboratories.

To put this it into perspective, a 1% organic matter contains about:

• 1t/ha of nitrogen
• 0.5t/ha of phosphorus (expressed as P2O5)
• 2.5t/ha of potassium (expressed as K2O)
• 0.3t/ha of sulphur (expressed as SO3)
• 200kg/ha of manganese
• 25kg/ha of zinc
• 10kg/ha of boron
• 5kg/ha of copper
• 10t/ha of carbon

Availability has limits

When it comes to nutrient availability, lab analysis reveals a linear increase up to 4-5% soil organic matter (SOM), before it starts to plateau.

“Beyond this point, you don’t gain additional “bang for your buck” from increasing soil organic matter levels from a nutrient perspective. In other words, a 5% SOM level delivers maximum return for most nutrients” explains Jon.

Jon Telfer © Ellie Dearlove

Nitrogen is the exception as the nutrient availability curve flattens off around 6-7% SOM, so extra gains in organic matter levels here can be beneficial.

A healthy, biologically active arable soil typically releases 110-120kg N/ha per year.

© Tim Scrivener

“You can feed microbes an all-you-can-eat buffet, but they still have a metabolic limit to how much nutrition they can process,” says Jon.

Using respiration testing, C:N ratios and organic carbon, the mineralisation potential of a soil can be estimated.

Phosphorus behaves slightly differently. After about 5% SOM, lab tests can show a decline in P availability. This is not because P isn’t there and not available, but because it’s tied up in organic forms that the standard lab tests struggle to detect.

“Grassland soils often sit closer to 10% organic matter. For me the message is clear: arable farmers should build it, and grassland farmers should use it,” says Jon.

Neutral pH for soil biology

Not only does soil organic matter act as a major nutrient store, but it also buffers pH.

Different groups of soil organisms thrive in different pH ranges, but most beneficial biology such as earthworms, protozoa and bacteria perform best at neutral conditions.

“Nitrifying bacteria thrive around pH 7, while earthworms prefer pH 7-8 and nitrogen-fixing organisms are most active at pH 8,” says Jon.

Growers should aim for a neutral pH to promote soil biology – this is what turns nutrient potential into nutrient availability.

Too high or too low pH undermines biological activity, reducing and disrupting nutrient availability which can lead to crop deficiencies.

“A deficiency of any single nutrient is enough to limit crop performance. Crops need adequate nutrition,” says Jon.

Crops can be nutrient deficient without expressing visible symptoms, but this hidden hunger can lead to a potential yield loss. Where there is deficiency with visible symptoms, plant growth or yield can be significantly hindered, says Jon.

Growers need to get the basics right and not react to deficiencies when yield potential has already been lost.

Phosphorus

Phosphorus is one of the trickiest nutrients to manage. One in five UK arable soils is deficient in P. In grassland, the figure is closer to 40%, which is surprising given the level of organic returns.

Phosphorus is a tricky element to pin down as it tails off rapidly out of a neutral pH zone.

“It is highly reactive. It is the only nutrient that directly drives biochemical energy transfer in plants. It can store energy, and because it wants to interact with other chemistry so readily, it is very sensitive to soil conditions,” says Jon.

Move outside the neutral pH zone and P quickly becomes locked up.

“You may have a soil P index of 2 but in acidic soils with a pH of 5 it becomes insoluble and not plant available.”

In this case P binds with iron and aluminium, becoming insoluble. Similarly, in alkaline, calcareous soils P forms strong calcium triphosphate bonds, again unavailable to plants.

“This shows you can have a perfectly respectable soil P index, but if soil pH is not neutral much of that P is compromised.

“It all has an impact. Growers and agronomists need to look at these things together. It’s no good looking at one line of a report in isolation,” he says.

Potassium

Potassium tells a similar story – 2% of arable soils have a K index of 0, while just over 20% have an index of 1.

“Our lab analysis shows less than one in 10 soils are on target for both optimum P and K levels in soils, so there is big room for improvements,” notes Jon.

Potassium exists as a positively charged cation, residing on negatively charged exchange site in soils such as fine clay particles and organic matter.

This charge is effectively what stops K from washing away, but in order to become plant-available, K must be displaced into soil water by a more dominate cation.

Cations come with a strict order of strength of absorption, starting with sodium as the weakest followed by, ammonium, potassium, magnesium and calcium as the most dominate.

Where calcium levels are low, most commonly in low-pH soils, K mobility and availability is restricted. Once again, an index 2 result can be misleading if pH and exchangeable calcium are ignored.

Typical leaf nutrient concentrations

Use of tissue testing to monitor and manage crops can help tailor crop nutrition.

Plants across a range of species typically require nutrients in different proportions.

While nutrients such as nitrogen are required in large quantities, secondary and other micronutrients are equally as important – they are just required in smaller amounts.

“Using timely tissue testing, and growth stage information, we can identify potential deficiencies, at hidden hunger stage, and address them before they impact further on crop production,” says Jon Telfer.

Tissue testing isn’t just a trouble shooting tool to use when problems appear, but can be employed to manage a good crop into an even better one, he says.

Jon Telfer was presenting at the AHDB Smart Nutrition roadshow, exploring how growers can address hidden nutrient imbalances