Organic matter, manures and amendments
N.B. Please be aware of the relevant regulations which affect your farm, in terms of:
- N application restrictions (e.g. NVZ)
- Application of waste materials to land
- Storage requirements for different organic amendments (esp. potential leaching of nutrients, potential for persistent herbicides present in certain types of manures)
These are not addressed in these notes.
Soil organic matter and soil organic carbon
- The solid fraction of soil (which ideally should be around 50 % of the total volume) comprises minerals (sand, silt, clay) and organic matter (see Soil structure).
- Organic matter is composed of carbon-based molecules, which can also include hydrogen, oxygen, nitrogen and sulphur: see Figure 1 as an example molecule.
- The carbon within these organic molecules is labelled ‘organic carbon’;
- Hence, soil organic carbon (SOC) is a component of soil organic matter (SOM) but they are NOT THE SAME THING.
- The SOM content of a soil is often approximately double the SOC content,
- i.e. SOM (% w/w) ~ 2 * SOC (% w/w).
- Although this is only an approximation and the exact relationship varies by soil, it is a useful rule of thumb for relating the values.
- If SOC is measured and there is a requirement to quote SOM (or vice versa), the following relationship is used to calculate the missing value:
- SOM (% w/w) = 1.72 * SOC (% w/w)
- This is the relationship used by the labs to calculate SOC where SOM is measured by loss on ignition. However, it is important to remember that this is only an approximation and the accuracy of the calculated value will vary depending on the soil.
Figure 1. An organic molecule: carbon forms part of the structure.
Types of soil organic matter
- Organic matter can broadly be classified into three types:
- Active / transient – days /months timescales, rapid nutrient provision, short-term aggregate protection;
- Slow / temporary – years / decades timescales, nutrient provision;
- Passive / persistent – decades / centuries timescales, CEC and water-holding capacity, stabilised within soil matrix.
- More details of these three types are presented in Table 1 and this presentation.
Table 1. Types of organic matter: active, slow and passive.
Benefits of soil organic matter
- Organic matter benefits soil in many different ways, influencing biology, chemistry and physics of soil health, including (but not limited to):
- Boosting microbial activity and diversity (potential for pathogen suppression);
- Improving root health and ease of growth (e.g. penetration through well-structured soil);
- Resistance against compaction, increased porosity à more aerobic soil à beneficial to microbes and roots;
- Better water movement including increased plant-available water and reduced waterlogging;
- Greater soil moisture window for potential cultivations: soil is more resilient against damage at lower and higher moisture contents.
- Improved provision (storage and availability) of nutrients for plants and microbes.
- Potatoes are among the crops most likely to benefit from applications of organic materials, due to smaller root system benefiting most from:
- Overall soil ‘health’ (microbial activity; invertebrates);
- Additional nutrients;
- Increased water-holding capacity;
- Improved soil structure.
Figure 2. Potatoes are among the crops most likely to benefit from applications of organic materials. Results from Hijbeek et al. 2017, shown in this presentation.
- Higher SOM content benefits potatoes before planting, during crop establishment, and while the crop is growing:
- Structural resilience against intense mechanical interventions required for potatoes;
- Resilience against erosion of bare soil;
- Easier root penetration;
- Release of nutrients through the season;
- Improved water retention.
Assessing the economics
- Costs of organic amendment application can be fairly easily quantified (material, time, fuel, etc); but quantifying benefits is more challenging:
- Potential yield increase?
- Ease of working soil
- Nutrients not accounted for?
- Longer cultivation window
- Less erosion?
- Better water availability to crop and less flooding?
- HOWEVER, the benefits of organic matter in soil are real and important, even if attaching a monetary value is challenging.
Analysis of soil organic matter content
- Ideally samples for spatial comparison should all be taken on the same day.
- Do not take samples when soil is especially wet or dry, or within 1 month of cultivation.
- Avoid sampling sites which have received organic material in the previous 3 months.
TO BE ADDED
Uncertainty in measured values
TO BE ADDED
- It is important to repeat SOM/SOC analyses every 4 to 5 years to build up a trend in the value over the medium to long term: see Practical considerations for increasing SOM content.
- When comparing results from one site at different times, ensure the same analytical method was used and results give the same information (i.e. SOC or SOM, don’t mix the two). Ideally, results should be from the same lab (we recognise this isn’t always practically possible).
- Mark sampling location using GPS to enable exactly the same location to be used in future.
- Ensure good records are kept of sampling locations and methods, to ensure consistency.
Nitrogen: type and content
- Nitrogen content in organic amendments (e.g. slurries, composts, manures, digestates) vary:
- Different materials tend to have different N contents, and
- Within a given type of material there will be variability.
- Compost can be particularly variable.
- Measuring actual N content of the material you are using (up to date for that batch) is important to enable correct calculation of the amount of N applied. If using literature values (including RB209), be aware that the accuracy is limited.
- Different (chemical) forms of N behave differently in soil. They are labelled ‘inorganic’ and ‘organic’ in a chemical sense, not in terms of ‘organic’ or ‘conventional’ farming. Their origin can be from synthetic fertiliser or from an organic material but the chemical form of N is what determines its behaviour once it is mixed with soil. For example, manure may contain similar proportions of both organic and inorganic forms of N.
- Organic N is nitrogen which is part of an organic molecule, i.e. part of SOM. This requires mineralisation (chemical transformation to ammonium or nitrate) before it is available to plant.
- Ammonium (NH4+) and nitrate (NO3–) are inorganic forms of N. Ammonium is positively charged so can bind to cation binding sites and be retained in soil to some extent. Nitrate is negatively charged and is not retained in soil: it moves vertically and horizontally through the soil profile at the same rate as the water in which it is carried. This is important when considering potential leaching.
- Readily available nutrients are available to plant but also available for losses to air and water: consider timing of application relative to likely uptake (best applied to strongly growing crop where practical).
Reasons for applying organic materials
- It is helpful to consider the reason for applying an organic amendment. Is the application driven by available materials, or a desire to have an effect on the soil? Whichever is the driving factor, understanding the potential impact of different types of materials can help optimise decisions about what to apply where.
- Organic amendments for soil improvement (composts, perhaps manures)
- Likely to apply larger amounts of material
- Particularly relevant for lighter soils
- Can provide slow-release nutrients
- Take care re build up of P and K; consider potential plastics in compost.
- Organic amendments for fertiliser value (digestates, slurries, manures)
- Calculate amount based on N content of material
- Rapid-release nutrients including N, P, K: may be able to reduce applications of synthetic fertilisers applied
- Minimal impact on long-term SOM build up
- Bear in mind potential for losses of readily available nutrients (especially N) to air and water.
- See table of characteristics presented in Figure 3 and further information in this presentation.
Figure 3. Characteristics of organic materials.
Practical considerations for increasing SOM content
What happens to applied OM?
- After application to soil, organic materials start to interact with their new environment:
- Chemical oxidation: literally the oxygen in the air/soil starts to break down the material;
- Biological breakdown: microbes in the soil start to ‘eat’ the material, giving off CO2 as a waste product and releasing nutrients including N, S and P.
- Rate of breakdown depends on the applied material, e.g. C to N ratio, liquid/solid; and also depends on environmental factors:
- Contact with air speeds up oxidation so incorporation into the soil profile can slow breakdown;
- Compaction can significantly reduce rate of oxidation, e.g. straw pan discovered within field a year later in an anaerobic area of soil;
- Warm, wet conditions favour microbial activity so breakdown is faster.
- Some organic material will remain in the soil and this contributes to the store of SOC.
- In terms of carbon sequestration, different models require different residence times but to be counted as ‘stored’, it would need to remain for a relatively long time, at least a year.
Rates of SOC build up
- Since organic material is partially broken down after application, only a proportion of it will remain for the medium/long-term. Therefore SOC build up will not be as fast as it would be if all applied carbon remained in the soil.
- Data from 15 European long-term field experiments suggested that with annual applications of 10 t/ha FYM, SOC could be expected to increase by around 0.06 % per year.
- Other UK and European literature sources quote values between 0.01 and 0.05 % increase in SOC per year.
- In other words, SOC build up is VERY SLOW.
Notes on organic matter, manures and amendments
Managing soil organic matter in rotations containing potatoes 2021 (Understanding soil organic matter; managing soil organic matter for soil resilience in potato rotations)
Organic materials for potatoes: what, when and why 2020 (Importance in potato rotations; measurement of soil organic matter; benefits of increasing soil organic matter)
Organic matter for potatoes 2020 (The importance of organic matter and building up organic matter in soil)
Organic manures 2012 (Effects of organic manures on soil nematode populations)
PRG Notes 1996 Nutrients in slurry (Measuring nitrogen and potassium in animal slurries by an electrical conductivity test)
PRG Notes 1995 Organic manures (Availability of nitrogen from cattle and poultry manures)