Biosolids and the need for Potash

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.

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

N 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.