ARCHIVE: Fertiliser Manual (RB209)

Allowing for the nutrient content of biosolids

The same principles apply as have been described on page 61 for livestock manures.


20 t/ha of digested cake is applied in autumn before winter wheat (8 t/ha grain yield, straw baled), grown on a medium soil following a previous cereal crop. The sludge is rapidly incorporated. The soil is at P Index 2 and K Index 2-. Where the digested cake is surface applied in the autumn, allowing for biosolids nutrients saves up to £243/ha. This potential saving will be less where soil P or K Indices are above maintenance levels.





Financial Saving (£/ha)

1. Estimated total nutrients in digested cake (kg/t)




Analysis provided by biosolids supplier or typical values from tables
on pages 75 and 76.




2. Estimate available nutrients in digested cake (kg/t)        
Nitrogen (see page 75)


Phosphate and potash (see page 77)




3. Nutrients supplied by digested cake that are equivalent to
inorganic fertiliser (kg/ha)

20t/ha supplies 220 kg/ha total nitrogen and 33 kg/ha of crop available N




Potential saving from biosolids use      


4. Nutrient requirements for winter wheat (kg/ha)        
Section 4 of this book




5. Inorganic fertiliser needed for the wheat crop (kg/ha)        
Stage 4 minus Stage 3




Actual saving for next crop from manure use      


6. Surplus digested cake phosphate for subsequent crops that are
equivalent to inorganic fertiliser (kg/ha)
 See note e      
Stage 3 minus Stage 4  



Saving for subsequent crops from biosolids use      


a. Nitrogen availability is 15% of total N (see table on page 76)
b. Total phosphate and potash content used in calculations to maintain soil P and K Indices.
c. Nutrients required for maintenance of soil reserves (soil P Index 2 and K Index 2-)
d. Saving for next crop plus value of surplus biosolids phosphate which will contribute to the nutrient requirement of future crops.
e. Some additional nitrogen will be available in the 2nd year (20 kg/ha) and 3rd year (10 kg/ha) after application – see page 76.
Assumed fertiliser costs: Nitrogen 60p/kg; phosphate 60p/kg; potash 60p/kg.

Heavy metals

Biosolids products contain heavy metals but at lower concentrations than in the past. On certain soils, for example copper deficient soils, biosolids can correct a trace element deficiency. However in the majority of situations, long-term accumulation of heavy metals in the soil is the more important issue. For biosolids additions, there is a statutory requirement to analyse topsoils for heavy metals before land spreading to ensure that concentrations are below maximum permissible soil levels, and to control annual additions of metals. Limits for soil concentrations and permitted rates of addition of heavy metals are given in Defra Protecting Our Water, Soil and Air: A Code of Good Agricultural Practice.

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