Question 1

Is slurry really free?

Accepted Answer

No — slurry has a real cost per kg of nutrient when you account for spreading time, fuel, machinery wear, and the opportunity cost of storage. A typical cattle slurry spread costs £25–£40/ha in tractor and equipment time. At 4% DM with 2.3 kg N/t, spreading at 20,000 litres/ha (20t) delivers 46 kg N — making the spreading cost alone around £0.65/kg N before any other costs. This calculator makes that comparison explicit so you can value your slurry accurately against bag nitrogen.

Question 2

What is available nitrogen (N available %)?

Accepted Answer

Not all the nitrogen in organic manures is immediately available to crops. Some is locked in organic form and releases slowly over months or years. The RB209 availability percentage is the proportion of total N that the crop can access in the year of application. Cattle FYM typically releases only 25% of its total N in the first year; pig slurry releases around 55%; digestate liquor can release 70% or more because most of its N is in ammonium form. This calculator uses total N for cost comparison — for fertiliser planning purposes always work with available N.

Question 3

What are the RB209 nutrient values based on?

Accepted Answer

The RB209 (AHDB Nutrient Management Guide) nutrient values are derived from a large dataset of analytical results for each manure type. They are averages — your actual manure will vary depending on animal diet, water dilution, storage method and housing system. For accurate fertiliser planning you should have your manure or slurry analysed by an accredited laboratory. The values in this calculator are suitable for cost comparison and initial planning but should not be used for precise nutrient management without actual analysis.

Question 4

Why does cost per kg N vary so much between manure types?

Accepted Answer

The cost per kg N depends on three things: the N content of the material, the application cost, and how much you need to apply to hit your target rate. High-DM materials like broiler litter (30 kg N/t) need far less volume per hectare than dilute cattle slurry (2.3 kg N/t at 4% DM), so spreading costs are lower per unit of N even if the per-hour spreading cost is similar. This is why broiler litter often shows as the cheapest organic N source per kg when spreading costs are factored in.

Question 5

Should I use cost per kg N or cost per kg P/K to make spreading decisions?

Accepted Answer

For most spring applications on arable and grassland, cost per kg N is the primary metric — nitrogen drives yield and is the nutrient applied in largest quantities. However, if your fields are at low P or K index, the P and K value of organic manures can be significant. Cattle FYM at 3.5 kg P/t is replacing 3.5 kg of triple superphosphate equivalent — worth £1.50–£2.00 per tonne of FYM at current fertiliser prices. Factor all three nutrients into your decision for the most complete picture.

Manure type	N (kg/t)	P (kg/t)	K (kg/t)	N available %
Cattle FYM	6	3.5	8	25%
Cattle FYM (stored under cover)	7	3.5	8	25%
Cattle slurry (4% DM)	2.3	1.1	3	35%
Cattle slurry (6% DM)	2.8	1.5	3.5	35%
Cattle slurry (8% DM)	3.7	2	4.5	35%
Cattle slurry (10% DM)	4.3	2.5	5.2	35%
Pig FYM	7	7	5	25%
Pig slurry (4% DM)	3.5	2	2.5	55%
Pig slurry (6% DM)	4.5	2.8	3	55%
Broiler litter (fresh)	30	20	18	50%
Broiler litter (stored)	25	17	15	50%
Layer manure (fresh)	16	14	8	40%
Layer manure (stored)	13	12	6	40%
Broiler slurry	9.5	6.5	5.5	55%
Sheep FYM	7	3	8	20%
Horse manure	5.5	2.5	6.5	15%
Digestate (food-based whole)	5.5	1.5	3.5	65%
Digestate (food-based liquor)	3.5	0.5	2.8	70%
Digestate (slurry-based whole)	3.2	1.2	3	65%
Digestate (slurry-based liquor)	2.2	0.8	2.5	70%
Compost (green waste)	8	3.5	7	10%
Compost (food and green waste)	9	4	8	10%
Sewage sludge (liquid)	3	4.5	0.3	50%
Sewage sludge (cake)	25	35	1	20%

Nutrient Cost Comparator

RB209 Organic Manure Nutrient Reference

Frequently Asked Questions