Soil fertility and the role of soil respiration

3 May 2023

Two of our sustainability commitments are dependent on healthy soil - to increase milk from forage by 10 per cent and to sequester 100,000 tonnes of CO2 per annum by 2025. Lancrop Laboratories Business Development Manager Hannah Shirt describes how aspects of soil fertility are connected and the role soil respiration plays in understanding soil’s potential.

Healthy soil is made up of 25% air, 25% water, 40-45% minerals and 5-10% organic matter, which is why soil testing isn’t just about determining soil’s nutrient status. Some differences in plant growth, yield and quality can be due to the soil biology, which is not assessed by standard soil testing. To establish a soil’s true status, testing needs to incorporate all three facets of fertility – chemical, physical and biological.

Chemical analysis

Chemical fertility includes pH, nutrients and C.E.C. (cation exchange capacity), with the most important aspect of nutrient use efficiency being soil pH. Soil analysis accurately measures the acidity, neutrality or alkalinity which has a direct impact on the availability of essential nutrients such as N, P and K as well as micronutrients.

It is important to know the potential of your soil’s ability to provide essential nutrients. This is why soil analysis for pH, P, K and Mg has been mandatory under the Farming Rules for Water since March 2018. Compliance with this legislation and high fertiliser prices mean efficient fertiliser recovery has never been more important.

To identify any potentially limiting factors in your soil, it is worth checking this broad spectrum of nutrients, both for deficiencies and where an excess of one may antagonise another.

On grazing land, trace element guidelines are reported for livestock health rather than plant health. This can highlight dietary mineral shortfalls potentially responsible for livestock disorders and are usually confirmed with fresh tissue analysis or forage mineral assay.

Physical assessment

Physical fertility includes soil structure, texture and compaction, so testing shouldn’t be limited to laboratory measurements. Regular field assessments enable structure to be checked, compaction removed and ensure drains are working correctly.

The Law of the Minimum

Scientist Justus von Liebig’s theory, the Law of the Minimum describes how plant growth relies on the scarcest nutrient resource, rather than the total amount available. Or put simply, a chain is only as strong as its weakest link. If there is a deficiency of one factor, it can limit growth, yield and even quality, and illustrates how oversupply of a nutrient can be a waste.

Optimising soil biology

Biological fertility incorporates organic matter and soil biology. Soil is a living biological eco-system containing many organisms, including bacteria, delicate strands of fungal hyphae, mites, centipedes and the all- important soil engineers, earthworms. They all need a suitable habitat in which to thrive, ie. a well- structured, free-draining soil with good pH.

Soil respiration

Like us, this aerobic life breathes in oxygen and breathes out carbon dioxide, so by assessing ‘soil respiration’, the amount of biological life within a soil can be determined.

The Solvita Soil Test measures the amount of CO2 released from a soil by its respiring micro-organisms. In a laboratory test, the soil is dried, sieved and milled to 2mm as standard, effectively putting the microbial population to sleep. Once water is added to 50% air filled porosity, the soil is placed in a sealed container for 24 hours, with a gel strip changing colour when activated by CO2. This process mimics the conditions needed for mineralisation and causes a ‘CO2 burst’.

The test result is used to estimate the soil’s microbial activity and nutrient cycling capability. If it’s too low, it suggests potential issues are impacting that microbial activity. It’s like the canary in the mine.

To gain the most from soil, we need to look after the life within it. When biology is the foundation of an environment it also brings structural stability, reducing the soil loss through wind and water erosion. When all these optimum conditions are met the nutrients can be mineralised from the reservoir of organic matter.

To find out more about the Solvita Soil Test, call 01759 305116 or go to