Improving potassium fertiliser recommendations

Soil fertility and effective nutrient management are crucial for the sustainability of Irish grasslands.

Phosphorus (P), Potassium (K) and Sulphur (S) are essential plant macronutrients typically applied to agricultural soils to increase plant growth, and are required in large quantities to support grassland productivity.

Recent research, led by Teagasc highlights the importance of tailoring potassium fertilisation advice according to soil characteristics to improve fertiliser K use efficiency, improve yields and reduce losses of valuable nutrient-K from the soil.

Potassium in grassland agronomy

Potassium plays an irreplaceable role in plant nutrition, in processes including photosynthesis, enzyme activation, and water regulation.

In grasslands, particularly under intensive systems, the application of artificial fertiliser K (AFK) can significantly improve dry matter yields (DMY) and forage feed quality.

Swards used for silage production exhibit particularly high levels of K removal from the field in harvested material which requires replenishment in chemical and/or organic fertilisers to avoid declines in soil fertility.

For instance, a typical four-cut silage system yielding 14 tonnes DM per hectare can remove over 350 kg of K per hectare annually.

The Irish Soil K Index system:

The Irish soil K Index system categorises soils into two indices or ‘bands’ based on their (a) K fertility levels; determined from standard agronomic soil testing using Morgan’s K extraction reagent, and (b) expected crop response to fertiliser K addition.

The current recommended agronomic optimum soil K index is three and corresponds to 100-150 mg K/L in the soil test.

The rationale is that at this K index three level, the soil will have sufficient K fertility to meet crop K demands over the growing season while increasing crop yields using additional fertiliser K will be ‘unlikely’ (see Table 1).

The limitation of these recommendations is they do not account for differences in background soil characteristics that may regulate soil K storage and supply across different Irish soils which will also affect the quantities for K fertiliser applications required.

A recent study carried out at Teagasc, Johnstown Castle Research Centre, Co Wexford has identified the importance of specific inherent soil properties in determining K requirements in different soils.

Twenty-two Irish soils, representing the major soil types distributed across Ireland were evaluated to determine how inherent characteristics such as cation exchange capacity (CEC), clay content, soil pH and organic matter regulate the availability and retention of K in soils for supplying crops.

Optimising Fertiliser K Inputs: The amount of fertiliser K required to achieve optimum yields of 19 tonnes DM/ha varied significantly across different soils, requiring 390, 280, and 200 kg K/ha for the low, medium and high CEC soils respectively.

Soils with lower CECs required greater levels of fertiliser K input despite not achieving the same levels of production as swards grown on higher CEC soils.

Alternatively, swards grown on higher CEC soils achieved higher yields, while requiring lower levels of fertiliser K input due to their ability to retain and re-supply K.

Implications for agronomic advice

These findings highlight the potential to refine Ireland’s K fertilisation guidelines by incorporating soil-specific characteristics into fertiliser requirements.

For example, higher CEC soils can maintain comparatively high soil K fertility levels over time. It may be more practical on such soils to ‘build up’ K fertility (i.e. above 100mg K/L) as per current recommendations.

On the other hand, lower CEC soils demonstrated a lower capacity to retain applied fertiliser K, reflected by the greater quantities of K leached.

This reinforces the idea that farms operating on such soils should use a ‘feed to yield’ fertiliser K approach; to supply adequate K to meet anticipated crop demand ahead of the next harvest/grazing in order to mitigate K losses via leaching.

While not directly linked to water quality degradation like nitrogen and phosphorus, potassium leaching represents an inefficient use of a finite resource, and a financial implication at farm level, particularly in an era of expensive K fertilisers.

Delivering tailored fertiliser K recommendations to match soil characteristics and requirements can help mitigate environmental K losses while maintaining grassland productivity.

The integration of these research findings with existing national soil mapping resources such as the Irish Soils Information System, can facilitate the development of soil-specific fertilisation tools.

Soil CEC can be incorporated as a factor in predicting K requirements in grassland soils, which in turn will enable farmers to make informed decisions on K requirements on a field-by-field basis, to maximise fertiliser K use efficiency.

Click here to for The soil potassium Index systems used in Ireland.