Autumn or spring?

P and K can be applied in autumn or in combination with N in spring.

Which option is the best?

Young roots need nutrients

To be taken up, phosphorus needs to be found in the immediate vicinity of the roots. In spring, low temperatures and weak root systems limit access to soil phosphate. Autumn-applied phosphorus undergoes aging processes during winter and is less available in spring. Young plants therefore benefit from spring application of readily available phosphorus. Trials have shown that spring-applied P penetrates the soil up to a depth of 5 cm (fig. 3). Likewise, spring-applied potassium is immediately available for uptake by the shallow root system while soil potassium supply from the upper crust might be diminished.

Figure 3: Distribution of phosphate in the soil upon starter application of compound NPK-fertilizer.

Figure 4: NPK starter fertilization increases yield compared to CAN only. Results for different crops. Data from 250 one-year field trials between 1991 and 2003 in Germany.

Creating synergy

Farmers apply P and K according to one of the following application strategies:

  • As a base dressing in autumn, either combined or in a multiannual rotation of P and K 
  • As a NPK starter fertilization in spring in addition to the base dressing in autumn, which is reduced accordingly. 

Spring fertilization is most efficient in terms of nutrient uptake and yield. All nutrients are simultaneously available, offering optimum synergy between them. Spring NPK application acts as a starter after winter dormancy and for establishment of spring sown crops. Crops with poor early root growth like potato and sugar beet or barley/wheat after wheat are most responsive to this starter NPK application (fig. 4).