The Potash Development Association has confirmed that many soils are currently in an extremely poor structural state.
This is a direct consequence of the heavy rains that have fallen consistently for most of the past 12 to 18 months.
Persistent water logging will bring about a deterioration in soil health. A healthy soil is often represented as being 45% mineral component (sand, silt and clay), 5% organic matter, 25% water and 25% air.
As it dries, the amount of water in the soil reduces and the amount of air increases. However, when it rains, these air spaces are filled with water which pushes air out, moving it from aerobic to anaerobic conditions.
Under natural drainage, when a soil reaches field capacity a proportion of the larger air spaces remain, as these have drained, however saturation is when all the air spaces are filled with water.
In this situation there is obviously a lack of oxygen (anaerobic conditions) for the biology in the soil to utilise, and therefore it will not function as effectively and over time will become depleted.
The longer soils are in this state, the more depleted they are likely to become.
The soil microbial biomass is fundamental to soil function and structure as they help to decompose plant and animal residues and organic matter, releasing carbon dioxide, plant available nutrients and exudates (glomalin) which helps bind soil particles together.
They also help to maintain connectivity between pore spaces in the soil, improving the flow of available nutrients through the soil and allowing improved gaseous exchange to get rid of CO2.
Saturated fields will also start to lose soil structure as soil aggregates are loosened and soils slump. Slumped soils become ‘massive’ in structure when they dry out, becoming very tight and hard to work.
Although clay soils may naturally restructure as the wetting and drying process swells and shrinks the clay particles creating cracks in the soil, this is on a macro level and does not repair all the damage done.
Cultivated soils are generally most at risk of slumping, however, to varying degrees this can occur on all soils.
One of the best methods to stop this process occurring is to maintain living roots in the soil, however the difficult autumn establishment last year meant that rooting was compromised and will not have been as effective at protecting soils.
Poor soil structure is very evident this year as a result of the conditions and cultivation decisions are challenging.
Rooting
Effective rooting is critical to plant success, becoming even more essential in more challenging seasons.
It is uncertain how the 2025 season and particularly this autumn will turn out, but crops grown on poorly structured soils will clearly be compromised.
Slumped soils have a lack of pore spaces, reducing poor space connectivity, which gives a soil a higher bulk density.
Bulk densities can vary by soil type and cultivation methods, but typically anything over 1.6 g/cm3 can impact root growth.
Clearly a restriction to rooting will have a detrimental impact on nutrient uptake, as a smaller root system is able to explore a smaller area of soil from which to pick up nutrients.
Nutrient availability
Alongside rooting, a high soil bulk density can also restrict microbial activity and biochemical processes which are crucial for nutrient availability.
Fewer pore spaces also mean lower available water, and considering nutrients are taken up out of soil solution, a lower volume of water means a smaller proportion of available nutrients.
A reduction in the connectivity between pore spaces will restrict the movement of nutrients within the soil further reducing their availability.
Waterlogged, anaerobic soils will have higher rates of denitrification, leading to an increase in nitrous oxide emissions and a lower soil nitrogen supply.
After harvest is a typical spot for routine soil analysis. Although soils have been very wet for most of the year, the recent dry conditions have reversed this and soils have been quite dry towards the surface, which is where soil sampling is typically conducted (0-15 cm).
It is worth considering the state of soils when analysing the results as soil analysis does not take bulk density into account, as the sample is effectively ground down, so results from this summer may lead to higher levels of anticipated nutrient availability than is likely in reality where cultivations are able to alleviate any compaction issues.
This could be of importance for those soils that are borderline, and could be of greater importance when considered in conjunction with a reduction in rooting.
Cultivation history is also important when considering nutrient availability through the soil profile, as soil sampling is typically taken to 15cm in tillage soils, aimed at being representative of the full plough depth.
Where low disturbance or no-till practices are employed, there will be a build-up of nutrients close to the soil surface and soil analysis results will tend to overestimate the supply of phosphate and potash.
Sampling to the full plough depth (25-30cm) in these situations may help to give a more representative result.