9 April 1999

Sewage sludge on land

– questions answered…

Recent changes to the way

water companies can

dispose of sludge mean

agriculture is being targeted

as a key outlet this spring.

But is sludge safe? And

what are the pros and cons

of using it? John Tearle

asks two experts

What is the risk from pathogens?

A: "Severn Trent Water has been applying sludge to farmland for the past 20 years, and illness has never been linked to sludge application," says Dr Chris Rowlands, sludge planning manager for the midlands company.

"Contractors and operators have had no higher incidence of disease than anybody else. The risk has been taken out of proportion. Its a problem of perception. Faecal aversion is a very British trait.

"Despite lack of evidence the British Retail Consortium states that untreated sludge is no longer acceptable. From December this year untreated sludge will no longer be used in agriculture."

See table (right) for guidance on retailer acceptance.

What is the risk from heavy metals?

A: "Sewage sludge contains a lot less heavy metal contamination than it has done in the past," says Dr Rowlands. "But heavy metals can still be a problem. In the Severn Trent region, sludge from Birmingham has a high heavy metal content. However, this sludge is incinerated and does not go into agriculture.

"To assess the risk of heavy metal exceeding maximum limits, soil samples are taken before the initial sludge application to find background levels and then a statutory sample has to be taken every 20 years to monitor changes. Severn Trent take samples every 5 years," says Dr Rowlands.

"In most situations large numbers of repeat applications have to be made to raise heavy metal levels from the naturally occurring background level to exceed maximum limits."

"We have to look at the issue of heavy metals in context," adds Dr Alan Keeling, senior lecturer at Harper Adams University College, Shropshire.

"Plants and animals need small amounts of heavy metals for biological function. Some soils can even be deficient in metals such as zinc. Indeed it is thought that a significant percentage of the human population may be deficient in zinc.

"Selenium levels have to be monitored in sludge and soil and yet it can be bought over the counter as a food supplement. Rock phosphate can contain as much cadmium as sludge when applied at the same rate of phosphate.

"At most agricultural soil pH, only a small fraction of the total metals is available to plants. The rest is chemically bound in the soil and is not taken up by the crop. On the other hand, it has been shown that heavy metals such as copper and cadmium can have an effect on soil ecology at relatively low available levels."

Are there any other contaminants in sewage sludge?

A: "There are any number of organic molecules, including PCBs, aromatic hydrocarbons and dioxin in sewage effluent," explains Dr Keeling.

"Though generally concentrations are falling as we pollute less, it is an area for concern. But again risks are low. Most organic molecules are quickly degraded in the soil, and the plant root acts as a good barrier to most organic compounds."

What precautions are taken to protect the environment?

A: "All the water companies are bound by legislation to protect the environment, and in addition there is a UK code of practice for sludge disposal which aims to follow good agricultural practice, ensure the long term viability of the soil, and to cause no public nuisance or health risk," says Dr Rowlands.

What plant nutrients are in sewage sludge?

A: "Sludge contains very useful levels of nitrogen, phosphorus, sulphur and magnesium," explains Dr Keeling of Harper Adams.

"If it is applied in the spring by injection there is very little nitrogen loss and up to 60% of the total nitrogen and half the total phosphate is available in the first year. The remainder will become available for crop uptake in subsequent years. There is sufficient sulphur for most crops at typical sludge application rates. Sludge can be used on most soil types, though it is not permitted in organic systems."

What does it cost?

A: "Recycling sludge to agriculture is not necessarily the cheapest option available to the water companies, but it is the best practical environmental option," says Dr Rowlands. "For this reason Severn Trent supply and apply sewage sludge free of charge to farmers."

"And I believe the other water companies have the same policy, although advanced treated sludge is being retailed as composted dried pellets in some areas."


Untreated sludge Digested sludge Advance treated sludge

Fruit No No No

Salad No No No

Vegetables No No Yes

Horticulture No No Yes

Combinable and Target end date Yes Yes

animal feed crops 31/12/1999

Grass

Silage Banned with effect Yes Yes

Grazing from 31/12/98 Yes Yes

Maize Silage Banned with effect Yes Yes

from 31/12/98

Produced by ADAS to reflect existing regulations and the aspirations of the

British Retail Consortium.


Sludge Type Application rate Crop Available nutrients (kg/ha)

N P205

Liquid digested 60cu m/ha Grazing 72 48

80cu m/ha Silage and 96 64

spring cereals

Cake digested 50t/ha Cereals 55 225

Grass reseed

150t/ha Restored ground 165 675


Element Typical background Heavy Metal Number of

heavy metal concentration application rate when applns required

in soil (mg/kg) sludge providing 250 kg/ha to reach advisory

nitrogen is applied (kg/ha) soil limit

Zinc 80 7.65 51

Copper 20 4.76 78

Nickel 25 0.54 332

Cadmium 05 0.04 179

Lead 50 1.67 491

Chromium 50 1.73 686

Mercury 0.1 0.03 98

Linseed for spring break crop

SPRING linseed is Lincolnshire grower Tim Cannons favoured break crop this year. Careful management delivers worthwhile yields and the crop helps cut the cost of establishing the following wheat.

"We have three break crops on this farm, spring beans, spring linseed and sugar beet, all grown for seed," explains Mr Cannon. "This year we have dropped the beans and increased our linseed because we only have to minimally cultivate afterwards.

"By contrast, land after spring beans has to be ploughed and cultivated, adding considerably to the costs of establishing the next wheat."

Careful management has delivered an average yield of 2.5t/ha (1 t/acre) over recent years, with variable costs of £186-235/ha (£75-95/acre), mainly for herbicides, fungicides and a desiccant.

Key to linseed success is good establishment, Mr Cannon maintains. "It pays to be patient. Dont panic if it isnt drilled yet – wait until the soil conditions are right."

His target at Cardyke Farm, Swaton, Sleaford, is April 5. "But linseed has to go into a warm seedbed without compaction, so were happy to drill right up to April 15 on heavy land."

Equally important, is weather at flowering. "Ideally, we need cool, bright days and no rain."

The 11ha (27 acres) destined for new variety Symphonia was winter ploughed then power harrowed. Winter weeds were sprayed off with 2l/ha of glyphosate.

Seed rate varies from 58-65kg/ha according to seed-bed. The seed is treated with Vitavax RS (carboxin + gamma-HCH + thiram) to combat flea beetle and seed-borne diseases and the field rolled after drilling if possible.

Provided soil indices are 2 or more no P or K is used. Nitrogen use matches previous cropping, soil type and winter weather. Usually 44-50kg/ha (35-40 units/acre) goes on when plants are about 2.5cm (1in) tall. "Once we can see the tramlines, we get it on."

An inherent brassica weed problem from growing mustard on the farm means herbicides target charlock, volunteer rape and brown mustard. Two doses of a cheap mix of Vindex (bromoxynil + clopyralid) and MCPA help hit repeated weed flushes.

"We never had any problems using MCPA on linseed, provided it was applied before the crop was 4in high. But we are now moving to Ally, Vindex or Eagle, although they will be more expensive."

Spraying in the evening minimises herbicide damage, Mr Cannon stresses.

Disease control at mid-flowering uses a mix of Sportak (prochloraz), Folicur (tebuconazole) and mbc. "It costs £25/ha, but we need to do it because we are growing for seed. On a commercial crop I might not bother."

Despite rotation benefits and good returns linseeds place at Cardyke Farm hinges on future seed demand. "Its place will be determined by Dalgetys seed requirement," Mr Cannon concludes.

LINSEED TIPS

&#8226 Compaction-free, warm seedbed essential.

&#8226 April 5 target, 15 on heavy land.

&#8226 Match seed rate to seedbed.

&#8226 Spray weeds in evening.

&#8226 Use N when plants 2.5cm.

&#8226 Compaction-free, warm seedbed essential.

&#8226 Match seed rate to seed-bed.

&#8226 Spray weeds in evening.

&#8226 Use N when plants 2.5cm.

Low-cost OSRestablishment thumbs-up

OILSEED rape establishment trials in Suffolk have overwintered well, with little visual difference between crops. Cost savings of up to £108/ha (£44/acre) could be possible.

HGCA funded plots examining low cost rape establishment have emerged from the wet winter with satisfactory plant counts and little pest or disease damage.

"Looking at the crops now, you would be hard pushed to tell the difference between the systems," says Morley Research Centres Ben Freer.

Plots will now be taken to yield to assess the final outcome. "That will be the deciding factor," says Mr Freer.

The Autocast system, which uses a broadcaster on the combine header, got crops off to a much quicker start, with seedlings drawn up through the stubble seeking light. "Plants on this system were much bigger earlier on."

By contrast the disced and drilled plot resulted in higher numbers of small plants showing less vigour. "We suspect the plant population on the disced plot was largely due to volunteer rape from earlier crops," he explains.

Slug activity was kept to a minimum by frequent monitoring, the non-tillage systems needing two Draza (methiocarb) applications to keep the pest at bay.

"We expected the earlier drilled non-tillage systems to require an additional fungicide application," says Mr Freer.

"As it was, phoma did not come until late in the season, so we treated the whole site in mid-November and were able to avoid this potential extra cost."

The mulch effect from chopped wheat straw kept herbicide costs low on non-tillage systems.

But consideration needs to be given to preceding crop and agronomy policy.

"Drilling is not recommended before Aug 10, so you need to think a year ahead about the preceding crop.

"Concerns have been voiced about the use in preceding crops of sulfonyl-ureas, diflufenican and pre-harvest Round-up. Residues from these chemicals may affect rape germination in dry years.

"Farmers using this system, should not rush into it blindly. They should think about it, feel their way and only sow a proportion of the crop in this way," he concludes.

OSR establishment trial

&#8226 Broadcast into standing crop.

&#8226 Autocast at harvest.

&#8226 Minimal cult – disc, press, drill.

&#8226 Conventional – plough, press, drill.

&#8226 3 or 6kg/ha seed rates.

HGCA – Autocast trial

Activity Autocast Broadcast Disc + drill Plough + drill

Seed sown 17.08.98 17.08.98 21.08.98 21.08.98

Rolled or pressed 17.08.98 17.08.98 21.08.98 21.08.98

Rolled – – 21.08.98 21.08.98

Draza 17.08.98 19.08.98 – –

Draza 14.09.98 14.09.98 14.09.98 14.09.98

Fusillade 250 + Cypermethrin 19.09.98 19.09.98 19.09.98 19.09.98

AN 34.5% – 30kg/N/ha 09.10.98 09.10.98 09.10.98 09.10.98

October plant populations 50/sqm 72/sqm 100/sqm 53/sqm

(mean of two seed rates)

Fungicide – Plover 07.11.98 07.11.98 07.11.98 07.11.98

Herbicide – Kerb – – 07.11.98 07.11.98

Fortrol – – 06.12.98 06.12.98

Provisional costs (£/ha) 32.00 32.00 116.00 140.00


Activity Autocast Broadcast Disc + drill Plough + drill

Seed sown 17.08.98 17.08.98 21.08.98 21.08.98

Rolled or pressed 17.08.98 17.08.98 21.08.98 21.08.98

Rolled – – 21.08.98 21.08.98

Draza 17.08.98 19.08.98 – –

Draza 14.09.98 14.09.98 14.09.98 14.09.98

Fusillade 250 + Cypermethrin 19.09.98 19.09.98 19.09.98 19.09.98

AN 34.5% – 30kg/N/ha 09.10.98 09.10.98 09.10.98 09.10.98

October plant population 50/sqm 72/sqm 100/sqm 53/sqm

(mean of two seed rates)

Fungicide – Plover 07.11.98 07.11.98 07.11.98 07.11.98

Herbicide – Kerb – – 07.11.98 07.11.98

Fortrol – – 06.12.98 06.12.98

Provisional costs (£/ha) 32.00 32.00 116.00 140.00