P & K holidays: why they’re high risk without regular soil analysis

Author: Alli Arden

1st April 2025

NRM

Agriculture

All farm businesses must review their costs to ensure they remain profitable or, at the very least, break even. In today’s volatile agricultural environment, managing input costs is more critical than ever. With economic uncertainty and the withdrawal of the Sustainable Farming Incentive (SFI) placing pressure on farm incomes, many businesses are seeking ways to reduce expenditure. Phosphorus (P) and potassium (K) are often the first casualties in the race to cut input costs—known as P & K holidays.

While nobody is suggesting applying fertiliser unnecessarily, it is essential to pay close attention to the crops where applications have been reduced or omitted. Additionally, whilst P & K holidays seem like a reasonable short-term saving, they carry significant risks if not carefully managed. Without regular soil and crop analysis, you could be jeopardising long-term soil fertility, crop performance, and ultimately, farm profitability.

Why P & K holidays need monitoring

P and K are essential macronutrients required for optimal plant growth and yield. Skipping applications without understanding soil reserves can quickly lead to depleted indices.

The best way to mitigate the risks of P & K holidays is to monitor soil and crop health more frequently through regular analysis.

• Soil sampling should move from a four-year cycle to every two years—especially if your fields are below index 2.
• Performing In-season plant tissue testing at key growth stages to keep a close eye on crop growth by analysing the nutrient content of the crop. This is essential to determine whether crops are recovering all they need from the soil.

NRM’s CropCheck plant tissue analysis service is growth stage-specific and is a valuable tool to ensure crops are maintaining healthy growth. This will support them during the critical grain fill stage later in the season.

Understanding crop offtake during P & K holidays

High-yielding, productive crops can remove a considerable amount of P and K from the soil, especially if straw is removed at harvest. If these offtakes are not replaced for several seasons, soil indices can quickly become depleted.

According to the national guidance RB209, rebuilding soil P and K indices can take years. Substantial amounts are required to progress from one index to the next, making the process time consuming and expensive.

For example, raising P index by just 10mg/l can require between 200 and 300kg P₂O₅/ha (or 435 and 650kg/ha TSP). Currently, TSP costs in the region of £480/t. So, if you were attempting to build an index by 10mg/l over 50 ha of wheat-growing land, you would need to apply between 22t and 33t. That’s an investment of £10,560 to £15,840.

This is why you must be mindful of how much phosphate and potash crops remove in their grain and biomass to avoid running down indices.

Typical P & K offtake over 3 years

What to do (and what not to do)

Evidence collected over many years confirms that, if soil indices are allowed to fall due to P & K holidays, rebuilding them to maintenance indices can take a significant amount of time and cost. This is why RB209 suggests that following a “remove and replace” nutrient management strategy is the best way to avoid indices crashing and the inevitable cost of rebuilding them.

Economic cost isn’t the only reason to keep an eye on our soil indices. Many nutrients and their availability for plant uptake are interdependent, and inefficiencies can easily occur if one or more nutrients are lacking.

For instance:

• Phosphate recovery rates are improved when nitrogen levels are optimal, and vice versa
• A deficiency in one nutrient can limit the effectiveness of others
• Excess N or P can severely disrupt our natural systems, polluting our watercourses and ground water supplies
• There is also evidence to suggest that managing the soil P index at a lower level doesn’t always mean lower yields. In fact, the opposite may be true. However, this only works if soil structure is healthy, crop rooting has reached its maximum potential, and regular monitoring is in place.

Healthy, biologically active soils are more efficient at nutrient cycling and delivery. This means farmers can potentially manage at lower input levels, but only if they have confidence in their soils and closely monitor crop uptake throughout the season.

How NRM can help you manage P & K holidays

When farm finances are tight, don’t be tempted to cut inputs without regularly monitoring and knowing your soils and crops. At NRM, we provide the range of analytical services you need to make informed decisions, even when taking P & K holidays.

Contact us to learn about how analysis can help you manage your business.

References

GB fertiliser prices | AHDB

RB209 Section 1 Principles of nutrient management and fertiliser use | AHDB