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Organic Manures

Value of organic fertilisers

The value of any organic fertiliser is a function of two things:

  1. the current price of chemical fertiliser N, P and K
  2. how much chemical fertiliser the organic fertiliser can replace

Organic fertilisers are only as valuable as the chemical fertiliser that can be saved by using them. If you are importing organic fertiliser without making adjustments in chemical fertiliser applications, then the organic fertiliser will not be saving you any money.

Volatile chemical fertiliser prices in recent years have resulted in equally volatile organic fertiliser value. This can complicate decisions of whether or not to import organic fertilisers onto the farm. In addition to fluctuating fertiliser prices, there has also been an increasing number of organic fertiliser types becoming available including various solid and liquid organic materials with different N P and K contents and availabilities.

Calculating the value

The value of various organic fertilisers can be calculated by multiplying the content of valuable 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 fertiliser prices in November 2021, average chemical fertiliser price per kg of nutrient was approximately €2.18 per kg N, €3.69 per kg P, and €1.33 per kg K. Alternatively, the value of a nutrient can be calculated based on current price of an individual fertiliser such as CAN or Urea for N, or Muriate of Potash for K.

Available nutrients vs. total nutrients

The relative value of organic fertilisers should be calculated based on the nutrients that are plant available 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. However, the amount of total nutrients cannot be assumed to be 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.

Nitrogen 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 less than 25% of the total N content). The availability of ammonium N depends mainly on the weather conditions at the time of application. Application in cool moist weather (e.g. spring rather than summer) will increase the amount of ammonium that is captured by the crop and therefore increases the overall N recovery and organic fertiliser value.

Phosphorus and potassium availability

The rate of release of P and K from organic manures depends on the soils P and K status. When soils are at P, index 1 or 2 the P in organic manures is deemed to be 50% available, therefore it is recommended to only supply 50% of P crop requirement with organic amnures and the remaining 50% with artificial fertiliser P . 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 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 3-4.

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 2017
Organic Fertiliser TypeN kg/m3
(units/1,000 gal)
P kg/m3,6
(untis/1,000 gal)
K kg/m3
(units/1,000 gal)
Value €/m3
Or (€/1,000 gal) 4,5
Liquid Manures

Cattle (6% DM(SI605,2017)1

2.0 (18) 0.8 (7) 3.5 (32) 12.8(58)
Cattle (6% DM) (Actual) 2 1.0 (9) 0.6(5) 3.5 (32) 9.4(43)
Pig (4% DM) 2.1 (19) 0.8 (7) 1.9 (19) 11.1 (50)
Solid Water 0.48 (4) 0.08 (0.7) 0.6 (5) 2.3 (10)
Solid Manures N kg/t 1
P kg/t
K kg/t
Value €/t
Dunstead Manure 1.4 (3) 0.9 (2) 4.2 (8) 12.9
Farm Yard Manure 1.35 (3) 1.2 (2) 6.0 (12.0) 16.6
Broiler/deep litter 14 (28) 6.0 (12) 18.0 (36) 82
Layers (30% D.M) 6.85 (14) 2.9 (6) 6.0 (12) 36
Layers (55% D.M) 11.5 (23) 5.5 (11) 12.0 (24) 65
Turkeys 14 (28) 13.8 (28) 12.0 (24) 103
Spent Mushroom Compost
1.6 (3)


8.0 (16)


¹ Nitrogen availability based on Nitrates Directive SI 605, 2017 (Cattle slurry total N of 5.0kg & 40% availability).  Conversion - kg by 2 = units

 2 The actual value of N in Cattle slurry (Green Book) is approx. 9 units/1,000 gallon (Based on total N of 2.4kgN/m3 @ 40% N availability by LESS application). 

Spring application of organic manures is required to maximize N recovery.  Manures should be tested to determine manure nutrient content

3 Incorporation of high N manures within 2 to 6hrs after application assume 50% N availability

4 Value of N = €2.29/kg. P = €3.84/kg, K = €1.48/kg for 2021 (Nutrient values based on price / volume of range of fertiliser products).

5 Cost of spreading & transport not included.  6Reduce P availability to 50% on P index 1 & 2 soils. 

7 Values under units/1,000gals or per ton have been rounded to closest unit.                                                                                                                                                                       

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 (€10.50 / t, or €48 / 1,000 gallons). 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, 68% 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 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.

Cattle slurry on grassland

The N fertiliser replacement value of cattle slurry on grassland is normally between 5 and 30%, depending on application method and on the weather conditions and timing of application. Application of slurry using bandspreading, trailing shoe or injection methods also 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.

Typical N, P and K fertiliser replacement values of cattle slurries with varied levels of dilution


Application Dilution
ApplicationFertiliser Value
(units/ 1000 gallons)
Timing Method NPK
7 None Spring SP* 6 5 30
TS/BS* 10
Summer SP 3
5 1/3 water,
2/3 slurry
Spring SP 5 4 28
Summer SP 2
3 2/3 water,
1/3 slurry
Spring SP 3 2 17
Summer SP 1
*SP = Splash Plate
*TS/BS = Trailing Shoe/Band Spreader

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)

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