History tells us that every time global grain prices take a significant rise, fertiliser prices follow. This fact alone should be sufficient to encourage farmers to depend less on what will nearly always be a relatively costly input.
Using organic fertiliser sources is one of two ways to decrease dependence on artificial fertilisers. Nitrogen-fixing crops and rotation is the other, but that’s for another article on another day.
All organic materials are potentially useful for tillage land. Some are higher in nutrient content, others will be higher in carbon.
All these products also bring benefits for soil health because they feed the soil biology, which is critical to the optimum functioning of soils if they are to be highly productive.
Put simply, it should be pH and biology first, fertility second. A healthy soil will have better structure, higher mineralisation, grow bigger bulk, impact on soil and foliar diseases, require less fertiliser and have higher yield potential.
As well as the food for the trillions of organisms in the soil, they all carry varying amounts of a range of nutrients – N, P, K and carbon, as well as micronutrients.
Variability in and between products
Products differ in their nutrient and dry matter content and these must be taken into consideration for use. Nutrient and carbon content influence cost and value so it is important that users know exactly what they are applying. But the benefits of organics go far beyond just nutrients.
There are generic values for nearly all organic fertiliser options but there is no guarantee that is what you are getting unless you test and analyse.
It is also worth noting that having variety in the organic materials added to land is a further help to the biology and health of the soil.
Having the ability to mix and match organic products to fertility or biology requirements is better long-term than using just one product on any land.
So combinations of poultry litter, slurry, compost, straw, etc, can be very useful for any soil. Think of your own stomach and the variety of things you like to eat.
Sampling and testing
Because of the inherent variability, sampling and testing are very important when using organic manures. The figures given here for each source are merely guides to nutrient content, but every sample will be different.
Slurry will vary hugely depending on the feeding regime and how water is handled.
For example, slurry from bulls being fattened would most likely be high in phosphorus, while slurry from animals on a mainly silage diet would have a higher potash content.
Most laboratories that analyse soil samples will also test for nutrient content in manures. But the key to getting accurate analysis is to have a representative sample. A spear sampler used for silage testing could be used to sample farmyard manure, while slurry should be fully agitated first.
Great care is needed when sampling slurry. A lab will require approximately 2l of slurry to determine its nutrient content.
Sampling is difficult with bulky materials that cannot be uniformly mixed, but a test is only as good as the sample. Take sub-samples from the top, bottom and centre of every pile and do this for as many piles as possible. Mix these together and take sub-samples for analysis.
Don’t let the sample dry out because you need accurate dry matter content to estimate the quantity of nutrients being applied.
Slurry and farmyard manure FYM samples generally cost from €100 to €120 per sample. Ensure that the lab provides results for the available nutrients and converts the results into kilograms or units.
Value is inextricably linked with cost and benefit. Their nutrient content can be valued in the short and long term. The benefits are unquestionable, providing products are applied at sensible rates. Value is the relationship between the costs associated with application, the savings in fertiliser and diesel and the benefit to yield over time.
Almost all products cost money to organise and apply. Slurries and poultry products need to be incorporated post-application but this can double as a stale seedbed or straw incorporation pass.
The overall cost is generally balanced against the quantity and value of the nutrients but you must remember that they do much more and different products have different strengths and weaknesses.
The big challenge for many is to get access to a few different products. Straw out, dung in, is the easiest for some but the value of dung/FYM is becoming more appreciated in intensive grassland situations.
Organic product characteristics
Farmyard manure is often a widely available source of organic manure on mixed farms. Tillage farmers with livestock use their own straw to bed animals and the farmyard manure produced is extremely valuable to the soil. While it provides some of its P and K slowly over time, it has major benefits for soil structure and in helping to build up soil organic matter.
Nutrient content will vary with different types of farmyard manure and the amount of straw used. Teagasc puts its average nutrient content at N – 1.35kg/t; P – 1.2kg/t; K – 6kg/t.
Cattle slurry is another readily available source on mixed farms. Again, it can be variable and consideration should be given to the diet the animals were on. Slurry from a fattening house will differ greatly from that where dry cows are being housed.
The use of low-emission slurry spreading equipment can reduce N loss as ammonia. Applying at cool temperatures will also help to reduce losses. Speedy incorporation will dramatically reduce losses and farmers should aim for incorporation within 48 hours of spreading.
Teagasc estimates the average nutrient content of cattle slurry at N – 2kg/m3; P – 0.6kg/m3; K – 3.3kg/m3 but this can vary massively with diet. Dry matter content is around 7%.
Slurry application will help to improve soil organic matter level, microbial activity and soil structure.
This is a really valuable source of nutrients for tillage farms if it is available locally. Again, the type of animals and their diet will influence its makeup and testing can be useful here.
Teagasc estimates the average nutrient content of pig slurry at N – 2.1kg/m3; P – 0.8kg/m3; K – 1.9kg/m3. Average dry matter is around 4%.
The P and K in pig slurry will be available quicker than P in farmyard manure because it is all ground up finer to begin with. Quick incorporation will again help to reduce N losses and it will also help soil health.
Digestate is a nutrient-rich substance produced by anaerobic digestion that can be used as an organic fertiliser. It consists of material that is not fully digested and dead microorganisms. All the nutrients in the feedstock will be present in the digestate but they will be considerably more available.
Like other organic matter sources, the nutritional value of digestate will vary depending on the diet provided to the AD plant – see Table 1.
Here food-based digestate (unseparated) has higher N content but lower P and K content than farm-based digestate.
Farm-based digestate, which are generally based on slurry and forage values, have higher dry matter but lower total and available N. Digestate is also packed with trace elements.
Spent mushroom substrate
Mushroom substrate is a composted organic material produced from wheat or other cereal straw, animal manure and gypsum.
When the substrate no longer produces commercially viable yields of mushrooms, it is considered as “spent mushroom substrate” (SMS) although it still contains many nutrients.
SMS is a soil conditioner which releases nutrients slowly over time and helps improve soil structure, health and organic matter. However, its high salt content can inhibit plant growth following regular application of high rates. Its low bulk density makes it costly to transport.
Its total nutrient content is put at 8kg N/t, 1.5kg P/t and 5-8kg K/t, with dry matter between 33% and 36%. The N content is mainly organic, with only about 20% available (1.6kg N/t) in the year of application. The P is regarded as being 100% available on Index 3 and 4 soils but only 50% on Index 1 and 2 soils.
Be cautious of hydrogen sulfide gas (H2S) when handling stored spent mushroom medium.
Compost generally comes in two forms – compost produced from green material (grass cuttings, leaves, etc) and compost produced from a combination of green material and food/animal byproduct.
It is a good source of organic matter and a soil conditioner. Nutrients are released slowly and their concentration is very variable. Plastics and other contaminants can be an issue. Compost has high dry matter at 60%+ and average nutrient content is shown in Table 2.
There are two main sources – municipal or sewage sludge and industrial sludge. Concern remains about the use of municipal sludge on land used for food production but this mainly relates to unquantified fears about things such as human hormone levels or medical drugs or their metabolites.
Municipal sludge is a valuable source of organic matter and nutrients where it can be managed properly. However, its use is prevented in quality assurance schemes.
It is best used with spring cropping where the N can be taken up quickly and providing bagged fertiliser is reduced proportionately.
Nutritional value is variable but an “average” sample would be around 3.5%N, 1.2%P, 1%K and 50%C. This makes it a very strong source of N and P with a lot of carbon on board.
The nitrogen is virtually all available immediately, while phosphorus availability can vary between 30% and 50% but it is all available in time.
Industrial sludges are more variable but they will come with an analysis profile and application is controlled by environmental control processes.
This involves many different products and a range of dry matter levels. Dry matter and nutrient concentration can vary so it is important to know what you are spreading. However, all sources tend to be high in nitrogen and so application rates need to be treated with care on an annual and multiannual basis.
Its nitrogen content is regarded as being about 50% available in year one, which means more residual nitrogen in the soil over time and this builds with repeated applications. The Teagasc average available nutrient content is shown in Table 3.