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Organic Manures

Value of Organic Fertilisers

Organic fertilisers are a valuable source of N, P K, & S and can replace synthetic fertilisers and reduce production costs. To make fertiliser savings it is important to make adjustments to crop nutrient requirements for the nutrients supplied in the organic fertiliser.  Then select a suitable synthetic fertiliser to supply the balance of the crops N, P, K & S requirements.

With increases in fertiliser prices over the last 12 months, the value of organic fertilisers have increased.  Organic fertilisers are a very good option to supply major crop nutrients at lower costs.  In addition, valuable organic matter / carbon supplied in the organic fertiliser.   The addition of organic fertilisers especially to tillage soils or fields intensively cut for grass silage bring many benefits from the supply of valuable soil carbon to feeding soil biology to improving soil health and increasing crop yield.

Calculating the value

The value of various organic fertilisers can be calculated by multiplying the content of available plant available nutrients, particularly nitrogen (N), phosphorus (P) and potassium (K), by the chemical fertiliser cost of each element respectively. The chemical fertiliser cost of N, P and K will depend on the price of individual fertilisers.

Based on a range of fertiliser products and prices in April 2022, the average vakue per kg of nutrient was ~ €2.83 per kg N, €4.55 per kg P, and €1.69 per kg K. Alternatively, the value of nutrients can be calculated based on current prices of products such as CAN (27% N) or Urea (46% N) for N, Super P (16% P) for P & Muriate of Potash (MOP 50% K) for K but values tend not to be as accurate.

Available nutrients vs. total nutrients

The relative value of organic fertilisers should be calculated based on the nutrients that are plant available at time of application and will replace chemical fertiliser, rather than the total nutrient content. Any organic fertiliser will contain an amount of total N, P and K per tonne or m³. However, the amount of total nutrients cannot be assumed fully available and equivalent to the fertiliser value. Since the nutrients will be present in many different forms, the fertiliser value will depend on the amount of nutrient that is actually available for plant uptake similar to what is in bag fertiliser.

Nitrogen (N) availability

The N content in organic fertilisers is normally present in two forms: (i) ammonium, which is readily available for plant uptake; and (ii) N in organic compounds, which are less available to plants. Therefore, the amount of ammonium relative to the total N content will determine the N availability in the organic fertiliser in the year of application. In general, slurries and poultry manures will contain more ammonia (approximately 40 – 60 % of the total N content) compared with solid manures such FYM or spent mushroom compost (normally 20 to 30% of the total N content). The availability of ammonium N depends mainly application method, speed of incorporation and weather conditions at the time of application. Application in cool, moist weather conditions (e.g. spring rather than summer) will increase the amount of ammonium N that is taken up by the crop and therefore increases the overall N recovery of the organic fertiliser thus reducing fertiliser N.

Phosphorus (P) and potassium (K) availability

The rate of release of P and K from organic fertiliser depends on the soils P and K status (Index). When soils are at P, index 1 or 2 the P in organic fertilisers is deemed to be 50% available, therefore it is recommended to only supply 50% of P crop requirement with organic fertilisers and the remaining 50% with artificial fertiliser P. When the soil P status is medium (Index 3), organic fertilisers can be used to supply 100% of the crop P requirement. Similarly, in the case of crop requirement of K, it is advised that only 75% of the crop requirement should be applied as organic fertiliser on soils with low K status (Index 1 & 2), whereas organic fertilisers can supply 100% of crop requirement on soils with K Index 3.

Value of organic fertilisers

Table 1 shows indicative monetary values of some common organic fertilisers based on current estimates of chemical fertiliser prices. The values shown are estimates of the value of 1 tonne or 1,000 gallons of each organic fertiliser type, assuming that the availabilities of N are achieved, and that there is a requirement for all the nutrients being supplied. For example, if cattle slurry was being applied to a field with no K requirement, then the proportion of the value attributable to available K content would need to be excluded, since there would be no chemical fertiliser saving. The values shown in the table need to be adjusted for any costs associated with transport or spreading or storage of the organic fertiliser.                             

Available Nutrient Content & Guide Value (€) of Organic Fertilisers 2022


Components of value

It is important to consider the value of each individual nutrient in addition to the overall total value of the organic fertiliser. For example, pig slurry and cattle slurry appear to be similar in terms of total value (€11 to 13 / m³). However, figure 1 shows the proportion of the total value of cattle and pig slurry that is attributable to each nutrient. In the case of cattle slurry, 69% of the total value comes from the K content. With pig slurry, since the available P and N contents is higher, and the K content is lower, the N and P make up a larger proportion of the value.



Figure 1. Proportion of the total value of cattle and pig slurry that is attributable to N P and K contents.

Maximising potential for fertiliser savings

Potential savings from using organic fertilisers will be maximised by following two principles. Firstly, plan the applications of all fertilisers in such a way that nutrients are not being applied in excess of crop demands. Balancing organic and chemical fertiliser applications so that the N, P and K are supplied on the correct proportions is crucial in order to ensure maximum value for money from fertiliser. Nutrients supplied that are not required by the crop or the soil is an additional cost on the farmer.

The second principle is to apply the organic fertiliser at a time, and in a manner (LESS) that maximises the nutrient availability. This is of particular significance to N recovery from organic fertilisers with high ammonia contents, such as slurries and poultry manure. Ammonia losses to the air are highest when soil and air conditions are dry and warm. As a guide, spring application is most desirable, as crop demands are higher than in autumn, and the weather is normally cooler than in summer. Application method is crucial for liquid slurries as it will increase N recovery at time of application plus it ensures a more precise delivery of the nutrients across the spread width.

Cattle slurry on grassland

The N fertiliser replacement value of cattle slurry on grassland is normally between 15 and 40%, depending on application method (Splash plate v LESS) and on the weather conditions and timing of application. Application of slurry using bandspreading, trailing shoe or injection methods decreases the losses of ammonia and increases the N fertiliser value.

The nutrient content of cattle slurry can be highly variable, and is affected by many factors such as animal type, animal diet and dilution of slurry with dirty water or rainwater. Guideline N, P and K fertiliser replacement values for cattle slurry are shown in Table 2. Note that slurry dilution (which can be approximated based on judgements of relative dilution with water) has a dramatic effect on the assumed N, P and K value, while application timing and method only have an effect on N.  It is good practice to have slurry tested for dry matter % (DM%), N, P & K. This will provide valuable information on the nutrient profile of the organic fertiliser and ensure it is used most efficiently.

 Table 2.  Available N, P & K values of Cattle at different Dry Matter (DM)% in springtime by LESS application techniques

Dry matter % (Slurry description)

N kg/mᶟ

(units/1,000 gals)

P kg/mᶟ

(units/1,000 gals)

K kg/mᶟ

(units/1,000 gals)
2 (very dilute) 0.4 (4) 0.21 (2) 1.4 (13)
4 (watery) 0.7 (6) 0.35 (3) 2.1 (21)
6 (typical) 1.0 (9) 0.5  (5) 3.5 (32)
7 (thicker) 1.1 (10) 0.6  (6) 4.0 (36)

Note – On index 1 & 2 soils reduce slurry P availability by 50% & reduce K availability by 10%  

Decision making

When planning organic fertiliser applications for the year ahead, slurry should be considered in light of three simple rules of ‘where’ to spread, ‘when’ to spread, and ‘how much’ to spread:

  1. Where: nutrient distribution around the farm should be determined by P and K requirements (insofar as is possible considering buffer margins and soil trafficability and land suitability restrictions).
  2. When: once distribution around the farm is decided based on P and K requirements, the timing of application should be planned so that the N fertiliser value can be maximised.
  3. How much: the rate of slurry application should be based on crop requirements, particularly of P and K. Tanker calibration to ensure accurate application rate is also essential.


  • Rising fertiliser prices are increasing the relative value of alternative nutrient sources such as organic fertilisers
  • Significant costs savings can be made by using organic fertilisers to replace or complement chemical fertiliser
  • Potential savings need to be adjusted for transport, storage and spreading costs
  • Savings are highest when organic fertilisers are applied in accordance with crop needs, and in a manner that maximises the plant availability of nutrients, particularly N
  • Consider the individual nutrients that contribute to the overall value
  • Where to spread: apply to fields with P and K requirements
  • When to spread: apply at a time that minimises potential losses of N as ammonia (Cool, moist, overcast conditions)
  • Apply with LESS technology to maximise N recovery at time of application

Suppliers of Slurry Hydrometers

Martin Sykes
Phone: 0044 1348 881686
Mobile: 07875 088391
Email: martinsykes@farmline.com
Address: Ty Barti ddu, Little Newcastle, Haverfordwest, Pembrokeshire, SA62 5TD, UK