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20. Biosolids and the need for Potash (524.97K)
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20. Biosolids and the need for Potash

Published June 2007

Biosolids (Sewage Sludge)

Biosolids (sewage sludge) provide valuable plant-available nutrients and have useful soil conditioning properties. They are subject to regulations which require that the application rates of specific heavy metals and their concentrations in soils are not exceeded, that disease risks to stock and humans are minimised and that applications should match the requirements of crops. Rates of applications are therefore important and so is the balance of nutrients. Water companies go beyond the regulations by agreement with stakeholders. Potential contaminants have been greatly reduced over the years by cooperation with industry, with their removal at source, and by legislation that has prevented the manufacture and sale of hazardous substances. Biosolids provide useful quantities of nitrogen and phosphate, but only modest amounts of potash and magnesium because these elements are quite soluble and are washed out in the treated water. Lime stabilised biosolids are a useful liming material.

Because most plant species require a greater total supply of potash than any other nutrient it is vital to complement biosolids with appropriate applications of potash fertiliser.

Typical nutrient content of biosolids*

DM Nitrogen Phosphate Potash Sulphur Magnesium
% total available total available total K2O total* SO3 total MgO
Digested Liquid kg/m3 4 2.0 1.5 0.7 Trace 1.1 0.3
Digested cake kg/t 25 7.5 see 9.0 4.5 Trace 6.0 1.3
Thermally dried kg/t 95 35 below 45 22 Trace 25 6.0
Lime stabilised kg/t 40 6.0 8.0 4.0 Trace 8.0 2.0
* % availability uncertain


Autumn Winter Spring Summer
(Aug-Oct) (Nov-Jan) (Feb-April) use on grassland
% total N available to next crop Sandy/
All soils All soils
Surface applied
Digested liquid 5 20 25 40 50 no
Digested cake 5 10 10 10 15
Thermally dried 5 10 10 10 15
Lime stabilised 5 10 10 10 15
Soil incorporated 6 hours after application
Digested liquid 5 20 20 45 55 not
Digested cake 5 10 10 15 20
Thermally dried 5 10 10 10 15
Lime stabilised 5 10 10 10 15
Deep injected (25-30cm)
Digested liquid 5 10 15 35 65 65

These figures provide a guide to typical contents but these vary widely and reference should be made to your water operator.
Source: DEFRA Fertiliser Recommendations RB 209 8th edition.

Imperial conversions
kg/m3 x 9 = units/1000 gal
kg/t x 2 = units/t

Sludge Regulations 1989

These regulations enact the European Directive (on the protection of the environment, and in particular of the soil, when sewage sludge is used in agriculture, 86/278/EEC). This was the first soil protection directive. The main provisions of the 1989 Regulations require that:

  • All soils must be sampled and the results must be available before the first application of biosolids, and be resampled thereafter when heavy metals could have accumulated.
  • Biosolids must not be applied to agricultural land when the concentrations of certain heavy metals in the soil (lead, cadmium, mercury, copper, zinc, and nickel) are greater than specified limits, according to soil pH.
  • The rate at which heavy metals in biosolids may be added to soil is restricted.
  • Biosolids must not be applied if the soil pH is below 5.0.
  • Restrictions on use of different materials and harvest intervals for different crops must be adhered to, see Safe Sludge Matrix, page 3.
  • Biosolids application rates must take account of the fertiliser needs of the rotation; the use of biosolids must not impair soil quality nor pollute ground or surface waters.

The farmer or landowner must provide the biosolids producer/supplier with relevant information about the application of other materials to the land. The biosolids producer/supplier must provide information to the farmer or landowner about the nutrients and heavy metals applied in the biosolids.


Statutory Management Requirement No. 3 (SMR 3) requires compliance with the Sludge (Use in Agriculture) Regulations 1989 and the Code of Practice for Agricultural Use of Sewage Sludge. HMSO, London 1996. The biosolids supplier will ensure that SMR 3 is observed and also SMR 4 (Nitrate Vulnerable Zones), as far as biosolids are concerned.

Use of biosolids can be considered to contribute to the requirement to maintain the land in ‘Good Agricultural and Environmental Condition’ (GAEC) because it improves soil structure, builds soil organic matter and increases water infiltration rate and resistance to soil erosion.

Use of biosolids can be considered to contribute to the requirement to maintain the land in 'Good Agricultural and Environmental Condition' (GAEC) because it improves soil structure, builds soil organic matter and increases water infiltration rate and resistance to soil erosion.

Stakeholder agreement

In the latter part of the 1990s, ADAS facilitated discussions between the British Retail Consortium, Country Land and Business Association, Environment Agency, National Farmers’ Union, Department of Environment Transport and Regions, Ministry of Agriculture Fisheries and Food, Water UK and other stakeholders to review the Regulations and agree additional requirements that would enable the use of biosolids on farmland to be welcomed by all parties as part of sustainable development. The Agreement on the method of working came into force on 31st December 1998. All agreed that the existing Regulations dealt with chemical risks adequately. Measures were agreed to further control the risk of disease transmission compared with the Regulations. Sludge treatment was to be based on HACCP (Hazard Analysis and Critical Control Point) principles, with two classes of treated sludge and defined rules on how they could be used. With this additional control and with FACTS training for advisors, all parties agreed that using biosolids to complete nutrient cycles and conserve organic matter is part of sustainable development. Conservation and recycling of phosphate by using biosolids to complement mineral fertiliser is especially relevant to sustainability; the phosphate industry estimates that the world’s reserves will be exhausted in between 100 and 300 years at the current rate of exploitation, depending on price.

Reducing concentrations of heavy metals in biosolids from a large wastewater treatment works (1972-2000)
Reducing concentrations of heavy metals in biosolids from a large wastewater treatment works (1972-2000)

The agreement was published by ADAS as the ‘Safe Sludge Matrix’; it affects all applications of biosolids to agricultural land. Defra planned to revise the Sludge Regulations to embembody the provisions of the Agreement and published a consultation in 2001, but revision now looks unlikely.

The Agreement on sludge treatment is:

Untreated sludge

Not permitted on land for food crops since 1999, nor for non-food crops since 2005.

Conventionally treated biosolids

These are biosolids that have undergone biological, chemical or heat treatment to reduce the fermentability and possible health hazards. The commonest treatment is anaerobic digestion. Conventional treatment ensures that at least 99% of pathogens have been destroyed.

Enhanced treated biosolids

These are biosolids subjected to treatment that virtually eliminates any pathogens. These materials will be free from Salmonella and the treatments will destroy 99.9999% of all pathogens.

These guidelines summarise the current regulations on the use of biosolids but are subject to regular review and change – if in doubt check with your biosolids supplier, your buyer or the contacts in the Matrix cropping categories table.

The Safe Sludge Matrix

All applications must comply with Sludge (Use in Agriculture) Regulations and Defra Code of Practice for Agricultural use of Sewage Sludge.
Applications not allowed (except where stated conditions apply

Grass & Forage (deep injected or ploughed down only)3 week no grazing and no harvest interval applies 3 week no grazing and no harvest interval applies

Food Crops Conventionally treated biosolids Enhanced treated biosolids
Salads (30 month harvest interval applies) 10 month harvest interval applies
Vegetables (12 month harvest interval applies)
Combinable & Animal feed crops    
  – Grazed
Grass & Forage   (deep injected or ploughed down only) 3 week no grazing and no harvest interval applies 3 week no grazing and no harvest interval applies
– Harvested
  (No grazing in season of application)  
Industrial Crops  
Industrial oilseed rape  
High erucic acid rape (HEAR)  
Hemp for fibre  
Coppice and other biomass crops  
Other industrial crops  

Matrix cropping categories

Fruit Salad Vegetables Horticulture Combinable and animal feed crops Grassland and forage
(e.g. ready to eat crops) Harvested Grazed
Top fruit (apples pears etc.)


Stone fruit (plums, cherries etc.)


Soft fruit (currants & berries)







Beans (including runner, broad & dwarf French)
Vining peas
Mange tout
Red beet
Brussels sprouts
Soil based glasshouse and polythene tunnel productions (including tomatoes, cucumbers, peppers, etc.)


Nursery stock and bulbs for export

Basic nursery stock

Seed potatoes for export

Basic seed potatoes

Basic seed production

Field peas
Field beans
Oilseed rape
Sugar beet
Grass silage
Maize silage
Fodder mangolds
Fodder beet
Forage rye
Turf production
For further information on the Safe Sludge Matrix contact:-

ADAS Gleadthorpe. Tel 01623 844331
SEPA, Stirling. Tel 01786 452407
or download the Safe Sludge Matrix for Agricultural Land or the Safe Sludge Matrix for Industrial Crops from the ADLib website.

For any queries concerning sludge treatment or products contact your local Water Company

Best fertiliser practice


Best fertiliser practice requires nitrogen (N) to be applied to an economic optimum level with timing matched to plant growth so that risk of N loss to the environment is minimised. Excess nitrogen carry-over should be avoided.

The Code of Good Agricultural Practice for the Protection of Water (1998) recommends a limit on the amount of total nitrogen to be applied in biosolids and other organic manures to no more than 250 kg/ha/year. Sludge cake which contain little available N may be applied at rates up to 500 kg/ha of total N in one application every two years in catchments less sensitive to nitrate leaching.

More stringent limits apply in NVZs – at the field level it is not permitted to apply more than 250 kg/ha total nitrogen per year. Also, full allowance should be made crop-available nitrogen supply from biosolids when calculating how much inorganic fertiliser N (if any) a crop needs.

Phosphate and Potash

The principles are quite different to those for nitrogen.

Best practice requires the maintenance of acceptable levels of these nutrients in the soil on a long-term basis. Fertiliser applications should aim to raise soil phosphorus (P) and potassium (K) to such levels that no further economic response occurs, and then to maintain these levels in the soil by replacing the nutrients removed in harvested crops.

Target soil fertility levels Soil P Soil K
mg/l (Olsen) Index mg/l Index
Vegetables 26-45 3 181-240 upper 2+
Arable, forage crops and grassland 16-25 2 120-180 lower 2-

At soil P Index 3 phosphate additions (biosolids or fertilisers) should not exceed crop offtake over the rotation. For most crops no phosphate is recommended at Index 4 or above, but biosolids may be applied, taking care to minimise the risk of run-off, soil erosion or leaching.

Nitrogen and potash balance

graph 1 showing the relationship between N, K and yieldN and K are essential partners. If potash is limiting, the crop will not respond fully to nitrogen thus penalising yields and product quality. Biosolids can provide large quantities of nitrogen which must be balanced with adequate potash to ensure its efficient use.

Inefficiencies of N uptake and use in the plant may result in significant losses of N to the environment if potash levels are deficient.

Visual symptoms of P and K deficiency

Do not rely on visual symptoms in a crop to determine the adequacy of potash or phosphate supply. Deficiency symptoms are easily confused with other problems, frequently appear too late for damage to be prevented or may be totally absent despite yield/quality penalties.

The best way to ensure the maintenance of adequate soil fertility is to apply nutrients to replace those removed by cropping, and to monitor soil reserves by regular analysis every 4-5 years.

Potash and profit

Unlike many factors, potash fertility is within a farmer’s control and it is unwise to allow low reserves of this relatively inexpensive input to prejudice crop performance.

The penalty from inadequate potash will depend upon the soil type, the crop and the growing conditions. Even with so-called ‘less responsive’ crops such as cereals or grass the effects can be large.

Potash, profit and grass Potash profit and cereal

Nutrient uptake

Typical uptake for cereal crops
Typical uptake for cereal crops


For arable crops the typical uptake pattern of major nutrients is shown in the graph. Relatively small amounts of nutrient are required during the establishment period, but such supplies are vital to the development potential of the crop. Daily uptake increases dramatically during vegetative growth and restriction of nutrient supply at this stage can critically affect final yield and product quality such as grain sample, tuber size and numbers, sugar content etc. Peak uptake for potash is around flowering time for most combinable crops and at the end of the growing season for root crops. Peak uptake for nitrogen and phosphate coincides with the end of season.

Typical potash uptake and removal for high yielding crops

Plants must be able to take up nutrients when required at the rate to match potential growth. The total supply must provide for peak uptake rather than just the quantity removed at harvest. These can be very different especially for potash.

Peak uptake Removal
Cereals – grain only 310 45
Cereals – grain & straw 310 125
Oilseed rape 375 40
Potatoes 440 310
Sugar beet 400 100
Peas, beans 190 50

Grass and forage crops

With grazing, the livestock are continuously re-cycling nutrients back to the soil via dung and urine so replenishment needs are low. However hay and silage are similar to the arable situation – but more so!

Grassland conservation removes the whole crop, possibly several times a season, thus both uptake and removals of potash can be very large.

Typical K2O removal in cut grass forage crops

Hay 1 cut 100
Silage 1 cut 150
2 cuts 250
3 cuts 310
Maize 220
Fodder beet 250
Whole crop 200

Timing of application of potash

Grass – re-seeding Apply at or before re-seeding
– grazing only Any time except during March-May spring flush
– cutting Apply recommended rates before each cut up to a maximum of 90 kg/ha. Apply amounts above this in the previous autumn/winter. For single cut systems potash can be applied at anytime over winter or in early spring for the following season.

All spring sown crops except root crops
Under 100 mg/l Light soils – between January and sowing
Other soils – between autumn and sowing
101 – 150 mg/l At any time
Over 150 mg/l At any time (rotational manuring possible)

All winter sown crops
Under 100 mg/l
Light soils – up to 50% in seedbed rest in spring
Other soils – all in seedbed
101 – 150 mg/l At any time
Over 150 mg/l At any time (rotational manuring possible)
Note: recommendations in this leaflet relate to ammonium nitrate exchangeable potassium and the standard Index classification of results.
Index 0 = 0-60 mg K/l; Index 1 = 61-120; Index 2 = 121-240; Index 3 = 241-400; Index 4 = 401-600; Index 5 = 601-900; Index 6 = 901-1500

Apply up to 190 kg/ha in the seedbed
Apply the remainder earlier in the spring or in the previous autumn/winter (after year-end for very sandy soils)

Sugar Beet, Fodder Beet, Mangolds
Under 100 mg/l Apply in Jan/Feb after ploughing
100-120 mg/l Apply before ploughing, except for 40kg/ha (30 units/acre) of K2O – to be applied immediately after drilling
Over 120 mg/l Apply before ploughing
If more than 190 kg/ha of potash is required some should be applied in autumn or earlier in the rotation.
Sodium (salt) is also needed for these crops except where soil Na is more than 40 mg/l as in some organic soils and fen silts. Normal requirement is 200 kg/ha Na2O which is equivalent to 150 kg/ha Na. Salt should be applied at least 3 weeks pre-drilling.

Additional potash requirements for crops receiving biosolids

For average-yielding crops assuming no other manure is applied
Fertiliser policy should be tailored to individual situations taking into account: rotation, crop grown, yield, treatment of residues such as straw, soil type and soil fertility level. Refer to other PDA leaflets or to the MAFF RB209 recommendations for precise guidance for individual crops. Check soil nutrient status every 4-5 years by soil analysis.

soil K index
kg/ha K2O 0 1 2- 2+ 3 4
Straw returned
winter cereals (8 t/ha) 95 70 45 20 0 0
spring cereals and oats (7 t/ha) 90 65 40 20 0 0
Straw removed
-winter cereals (8 t/ha) 145 120 95 70 25 0
-spring cereals (7 t/ha) 145 120 95 70 25 0
-all oats (7 t/ha) 170 145 120 95 50 0
Oilseed rape 90 65 40 20 0 0
Linseed 75 50 25 0 0 0
Peas and Beans 90 65 40 20 0 0
Earlies, seeds (30t/ha) 220 195 170 145 50 0
Maincrop, 2nd earlies (50t/ha) 350 325 300 275 150 0
Sugar Beet (60t/ha) 150 125 100 75 0 0
(Assuming sodium is also applied)
Fodder Beet (65t/ha) 310 285 260 235 175 0
Forage Maize (40 t/ha) 230 205 180 155 110 0
Kale (40 t/ha) 250 225 200 175 130 0
Reseeding 120 80 60 40 0 0
Grazing only 60 30 20 0 0 0
1 cut + grazing 140 115 90 65 20 0
1 cut + grazing 200 170 140 120 30 0
2 cuts + grazing 320 270 230 180 70 0
3 cuts + grazing 370 320 280 190 90 0


The application of biosolids to agricultural land makes a useful contribution to the recycling of nitrogen and phosphate, and completes natural carbon cycles. However, biosolids are seriously imbalanced nutritionally because of their low potash content. Where they are used additional potash is required and there is frequently only a need to apply a straight potash fertiliser to balance the N and P in the biosolids. Serious soil fertility and crop yield problems will usually arise if this balancing potash is omitted. Potash is firmly held in most soils so there is considerable flexibility in timing the applications of this essential nutrient. (see Timing of Potash)