Legislation

Biosolids recycling to agricultural land must comply with numerous pieces of legislation and best practice guidance, including:

• The Sludge (Use in Agriculture) Regulations 1989
• The Sludge (Use in Agriculture) (Amendment) Regulations 1990
• Code of Practice for Agricultural Use of Sewage Sludge (1996)
• The ADAS Safe Sludge Matrix (2001)
• Cross Compliance
• The Nitrates Regulations
• Codes of Good Agricultural Practice
• AHDB Nutrient Management Guide (RB209);

Best fertiliser practice

Nitrogen

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.

Protecting our Water, Soil and Air: A Code of Good Agricultural Practice for farmers, growers and land managers recommends a limit on the amount of total nitrogen to be applied in sludges and other organic manures to no more than 250 kg/ha/year.

More stringent limits apply in NVZs – at the field level it is not permitted to apply more than 170 kg/ha total N/year. This limit is the loading limit and applies as an average across the holding. It’s separate from the field limit of 250 kg/ha from organic manures. Also, full allowance should be made for 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.

Soils should be analysed regularly, ideally every 3 to 5 years (at least every 5 years is a requirement of the Farming rules for Water 2018), to set and maintain a correct fertiliser policy. This can be done as part of a nutrient management plan.

At soil P Index 3 and above, 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

N and K are essential partners. If potash is limiting, the crop will not respond fully to nitrogen, 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 nitrogen if potash levels are deficient.

Visual symptoms of P and K deficiency

Visual symptoms in a crop cannot be relied upon 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 and 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 3-5 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 essential input to impact crop performance.

The penalty of 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.

Sulphur

Crops should be provided with a balanced supply of the major nutrients, including sulphur. As with readily-available nitrate, sulphate can be liable to loss through leaching. Therefore, as with nitrogen, economic optimum levels should be applied with timing matched to plant growth, ensuring applied nitrogen is utilised as efficiently as possible.

Nutrient uptake

Arable

For arable crops the typical uptake pattern of major nutrients is shown in figure 1. 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.

Grass and forage crops

With grazing, the livestock are continuously recycling 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, often several times a season, thus both uptake and removals of potash can be very large.

Typical K₂O removal in cut grass forage crops

Potash application timing

Grass

Winter & Spring sown crops

Potatoes

Sugar Beet, Fodder Beet, Mangolds*

Additional potash requirements for crops receiving biosolids

Typical applications of biosolids (digested cake) supply just 12 kg/ha available potash, which is only a small proportion of the total requirement for most crops, highlighting that potash applications following biosolid applications should not be overlooked.

18-38 kg/ha available SO₃ is also supplied following a typical application of biosolids, based on the assumed 10-20% availability seen in recent research. However, sulphur from autumn applications may be lost via overwinter leaching. The quantity leached will depend on soil type and overwinter rainfall and is likely to be higher on light textured soils in high rainfall areas.

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 AHDB Nutrient Management Guide (RB209) for precise guidance for individual crops. Check soil nutrient status every 3-5 years by soil analysis.

Conclusion

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.