Putting a value on organic nutrients inside the farm gate

With prices of chemical fertiliser back on the rise amid issues with natural gas supply, farmers should be looking inside the farm gate as to what they can do to add nutrients back to land in as cost-effective and environmentally sustainable a way as possible. For livestock farmers, organic fertilisers like slurry and farmyard manure should be used as efficiently as possible.

The value of organic fertiliser

Teagasc conducted a study in 2021, with the results published in March 2022. The value attributed to organic fertilisers was based on the value of N being €2.29/kg, P being €3.84/kg and K being €1.48/kg. The nutrient value, as well as the relative monetary value, of some common types of organic fertilisers are listed in Table 1.

The N value for cattle slurry listed in the table is based off of total N of 2.4 kg/1,000 gallons, with 40% of N being taken up when slurry is spread with LESS (dribble bar/trailing shoe), giving a value of 9 units/1,000 gallon approximately.

Nutrient availability

The major difference in nutrients from organic v chemical fertiliser applications is in plant nutrient uptake. Due to the nutrients being available in different forms in organic fertilisers, the actual value will depend on the plant uptake of these nutrients.

Nitrogen

The N content in organic fertilisers is normally present in two forms: ammonium, which is readily available for plant uptake, and N in organic compounds, which are less available to plants. Approximately 40-60% of the N content of slurries is ammonia-based. However, the availability of ammonium is highly variable, with uptake depending mainly on the weather conditions at the time of application and method of application. Application in cool moist weather (eg spring rather than summer) and spreading using LESS equipment will lead to greater N uptake by the plant and less ammonium lost to the atmosphere. Where cattle slurry is spread with a splash plate compared to LESS equipment, the N value of the slurry falls from 9 units/1,000 gallons to 5 units/1,000 gallons due to loss to the atmosphere.

P and K

The availability of P and K from organic manures is dependent on the soil’s P and K status. When soils are index 1 or 2 for P, the P availability in the organic manures is deemed to be 50%. For low-index soils, it is recommended that organic manures are only used for a maximum of 50% of the P application as a result of this lower availability. When the soil P status is medium to high (index 3-4), organic fertilisers can be used to supply 100% of the crop P requirement. Similarly for K requirements, organic manures can be used for 75% of K requirements on index 1 and 2 soils, whilst they can be used for 100% of the K requirement on high index soils.

Maximising value

Savings from using organic fertilisers can only be made where the chemical fertiliser application is adjusted accordingly.

While the given organic manures have listed values, the actual value to your farm may be different. Cattle slurry and pig slurry are valued relatively similar (€43 and €50, respectively). However, 68% of the value of cattle slurry is due to its high K content, while only 35% of the pig slurry value is attributed to K. In this sense, farmers with low K index soils would see better value for money in applying cattle slurry.

Where slurries are used, nitrogen uptake will always be higher where LESS application is used.

However, good practice regarding correct weather conditions (overcast, light drizzle) will also aid in reducing loss of N in the form of ammonium.

With organic fertilisers being a natural product, values vary considerably. Manures and slurries from animals fed intensive diets will have higher nutrient values, as will stock fed silage harvested from land with high P and K indices. The best way to know the value of your organic fertiliser is through testing it.