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A major UK survey has confirmed significant variability in crop nutrients and soil quality – making it harder to grow crops cost-effectively. Variable soil quality hampers efficient crop production

• Big differences in our soil quality

• Challenge to sustainable future

A major UK survey has confirmed significant variability in crop nutrients and soil quality – making it harder to grow crops cost-effectively.

UK soil has far greater variability in its structure, nutrient make-up and ability to utilise nitrogen than previously thought, according to the five-year study of almost 50,000 individual fields carried out by NRM.

The finding means farmers striving to boost productivity against a backdrop of greater environmental and sustainability demands are doing so with sub-optimal resources, says NRM analyst Sajjad Awan.

“It’s been a real eye opener just how variable our soils are. Some of this is undoubtedly being driven by climate change with the wetter, milder winters and drier, hotter summers now experienced affecting soil microflora and biology significantly.

The study findings suggest a need to reshape some aspects of farm management, says Dr Awan.

Although this might sound daunting, some simple steps can address many of issues, he adds.

Soil carbon

Carbon content is a good guide to the overall organic matter of soils. Although carbon levels are low on many arable farms, they can be improved – and there are many beneficial aspects of doing so, says Dr Awan.

“Generally speaking, we’re seeing arable land containing 24% less soil carbon than that used for livestock, but within all soils there is tremendous variation in this.”

In arable soils, for example, NRM CarbonCheck shows the range is from 28t/ha to 178t/ha for total carbon stock measured at 30cm with the median being 86t/ha. For livestock this is about 113t/ha which is close to that of horticultural soils at 122t/ha.

“If you’re growing crops on land in the lower range of that scale, you will be struggling. Soil carbon content affects all manner of things including moisture retention, overall soil health and nitrogen use efficiency.

“Using Farmyard manure (FYM) and other organic materials can help– and so can reducing the number of cultivations, disturbing the soil less and avoiding soil compaction.”

Reducing fertiliser

While producers increasingly realise the importance of soil testing ahead of fertiliser applications, few appreciate the significance of testing nutrient offtake values in the grain post-harvest, says Dr Awan.

“Knowing how much nitrogen, phoshape and potash is contained within the grain can give you a vital early indication of fertiliser needs for the following season. Again, our analysis shows this varies considerably from year to year.”

The extent of these changes is highly significant, says Dr Awan. Grain nitrogen offtake measured using NRM GrainCheck at harvest 2022 was around 4% lower than in 2021.

“At a time when nitrogen prices are so high, understanding precisely what nitrogen is in the soil in the shape of soil mineral nitrogen (SMN) and the total soil nitrogen supply (SNS) can help save on fertiliser bills.”

Drier summers are also creating a greater number of soils with indices below 2 for phosphate and potash. Where summer droughts were experienced, there were 13% more soils below index 2 in the drier years than in an average year.

Drought conditions

“Mineralisation of phosphorus in the soil is much reduced under drought like conditions and this is reflected in our analysis with the distribution curve visually skewed towards the lower indices.

“Increasing soil organic matter helps soils retain moisture better and hold key nutrients more effectively whilst building vital resilience to future droughts.

“Minimising ploughing and disturbance of the soil will help get the biology working to its full potential as will the use of FYM and other organic sources of nutrition.”

Variable manure

But the NRM analysis shows the contribution of farmyard manure and slurry to farming systems is much more variable than previously thought with some applications contributing just 3% of that from others.

“The differences are of a magnitude I don’t think anybody was expecting. Focusing on N, P and K alone shows a 20t/ha application ranges from £100 to £3300 in terms of nutrient value.

“It’s another great example of how working with average values and making management decisions based on these can be so damaging to production and profitability.”

Limitations

Analysis shows that a quarter of UK arable land is below index 2 for magnesium. This means many growers are trying to maximise production with a hugely important nutrient exerting severe limitations on this.

“Magnesium is essential in the production of the chlorophyll that drives photosynthesis, enzymatic activation and protein synthesis. It’s as least as important as phosphate and potash yet all too often gets overlooked.

“A greater emphasis on individual testing, whether it be soil organic matter, soil nutrient status, nitrogen off-take in harvested crops or for a whole host of other analytics, has got to become a routine part of future farm management.”