You can cut P and K use – but it hinges on soil indices and sticking to the rules

In recent years high fertiliser prices or poor prospects for grain prices gave rise to the expression and practice of ‘taking a holiday from purchasing Ps and Ks’. In cases where nutrient levels were already low, that resulted in a reduction in grain yield, but where levels were high it had little or no impact on yield.

Doing so again this year is an option for some. But it is only safe to do so when you know soil fertility levels are high.

Before considering reducing applications of P and K, it is important to consider what they do for a crop.

Phosphorus promotes root development and tillering. A crop that is short of P will have slow establishment and will have fewer ears per plant and therefore poorer yield potential.

Energy transfer within the plant will be poorer and nitrogen use efficiency impaired.

Cold, wet soils will aggravate crop deficiencies as will low (<6.5) or too high (>7) a soil pH.

The severity of the impact of high pH is easy to overlook but is accepted within our Nitrate Regulations, where 20kg/ha of P is permitted on fields for cereals with very high P levels.

Do not omit P applications on high pH soils. P is largely immobile in the soil so it is important to have it in close proximity to the plant roots. This is the major advantage of sowing fertiliser and seed together — combine drilling.

Excessive soil P levels will cause zinc and iron deficiencies in plants

Potassium is critical for straw strength, to prevent lodging and to promote disease resistance. K is particularly important on drought-prone soils, where it maintains leaf turgidity, reduces wilting and helps to hold moisture within the plant.

K is partly mobile in the soil so it is not as critical to have immediate contact between roots and K.

Some soils are naturally high in K and you can harvest several crops with little impact on soil nutrient status.

Soil analysis laboratories test the soil nutrient levels and classify them under an index system:

Index 1 — very deficient and a definite response from fertiliser application.

Index 2 — deficient and likely that you will get a response to fertiliser application.

Index 3 — satisfactory nutrient status and unlikely that you will get a response to fertiliser.

Index 4 — very high nutrient status and no response to fertiliser application.

If you are both cost- and yield-conscious, you should apply nutrients where soil nutrient status is 1 or 2 and omit fertiliser where soil nutrient status is 3 or 4.

However, you must remember that where you omit fertiliser application, you are depleting soil fertility.

Application of P to soils at P index 4 will result in penalties on your BISS payments unless the pH is 7 or over as outlined above.

Remember that on tillage soils you must have a currently valid soil sample proving that the P index is less than 4 or pH is high before P application is allowed.

Nutrient removal rates by crops are 3.8 P (kg/t) of all cereal crops; 10 K (kg/t) for winter wheat and barley, 11.4 K for spring barley and wheat and 14.4 K for winter and spring oats.

Subtraction of nutrient removal rates on a field-by-field basis from nutrient applications will give you a good indication of trend in soil fertility. Land from which heavy crops have been harvested is more likely to be depleted in nutrients unless high fertiliser rates have been applied.

Do not rely on soil samples that are three or four years old to decide on not applying nutrients this year.

While fertiliser is the largest variable cost, the rate should only be reduced where you are confident that the reduction will not impact yield.

‘Do not rely on soil samples that are three or four years old to decide on not applying nutrients’