Potassium: essential for crop production and good nitrogen use

Many crops still fail to reach their full yield or quality potentials due to nutrient imbalances. A lack of adequate supply of one or more nutrients will prevent crop plants from achieving their yield potentials. Management of nitrogen application rates and timings typically dominates fertiliser planning at the expense of the other essential nutrients, which may be assumed to be available from soil reserves.

However, an imbalance or shortage in supply of other essential nutrients can lead to the application of more nitrogen (N) than the crop is able to use, leading to lower nitrogen use efficiency (NUE), and potential N losses.

Crop nutrient requirements

The proportions of the different essential nutrients required by crops and grass is illustrated in Figure 1, in which the full yield column represents the percentage contents of the dominant nutrients. The values are those from an oat crop at maximum biomass and will be similar for other cereals and grasses.

A less productive oat crop is represented by the half yield column, which contains the same nutrients in the same proportions as the full yield crop, but with smaller quantities. If the smaller yield had been caused, for example, by a limited availability of potassium (K) to the crop, then the requirements for the other nutrients are less, this being notably apparent for N.

Soil potassium reserves

Potassium is well buffered in most mineral soils and its management is less challenging compared to some other nutrients, which is possibly what led to it receiving less attention.

The recent trends in K levels from soils tested in Ireland (Teagasc data) indicate that about 50% of soils analysed for potassium are below the recommended optimum reserve status of K Index 3.

This implies that crops and grass grown on these deficient soils are unlikely to achieve their full yield potential and are at risk of sub-optimal nitrogen use efficiency.

The importance of potassium

Potassium has major effects on yield and quality, as well as on the general health and vigour of a crop. It is very important in the relationship between water and crop growth because it helps regulate the amount of water within the crop. Crops grown on soils with too little plant-available potash become deficient in potassium, resulting in reduced yields. Without sufficient potassium, crops fail to use water efficiently and consequently become more seriously affected by water stress in periods of drought.

Potassium has major effects on yield and quality as well as on the general health and vigour of a crop

Plants also use N less efficiently and are less able to handle stress caused by frost, heat, water-logging and wind. Thus, it is essential to bring soils to the Target Index for plant-available potassium (exchangeable K) and then maintain this level by replacing the amount of potash removed each year in the harvested crops. To ensure that this approach is maintaining the Target Index, soils should be sampled and analysed for potassium every four to five years.

Effect of potassium on nitrogen response

The relationship between nitrogen and potassium requires particularly careful management because of the quantities involved. An imbalance or shortfall in availability of one will alter the need for the other. If a sub-optimal rate of nitrogen is applied, then the requirement for potassium will also be lower than optimal and less potassium will be taken up from the soil reserve. Apart from a reduced level of production, this does not present a problem, as the ‘surplus’ of potassium is safely retained in most soils.

The relationship between nitrogen and potassium requires particularly careful management

However, if a rate of nitrogen is applied to achieve a full potential yield, but there is an insufficient supply of available potassium, then the full yield cannot be achieved and some of the nitrogen will be unable to be used by the crop, leading to a lower ‘nitrogen use efficiency’ (NUE).

This surplus nitrogen will not be held in the soil like the potassium, but will instead be lost to the environment.

Potassium removal in harvested crops

As shown, high tissue water content in a crop is directly associated with a high potassium content. This has implications when calculating the quantity of potassium that should be applied as a maintenance dressing to ensure that the soil does not become depleted. Any plant material that is green at harvest, such as grass silage or whole-crop for example, will remove potentially large amounts of potassium, as indicated in Table 1.

The wilting of silage to evaporate some of the tissue water will not alter the quantity of potassium removed from the field, which for three cuts of silage can be over 400kg K/ha. A cut of silage taken in late summer after grazing can remove approaching 100kg K/ha which, if not replaced by fertiliser or slurry/manure, will significantly deplete soil potassium reserves of these grazing fields.

The wilting of silage to evaporate some of the tissue water will not alter the quantity of potassium removed from the field

A relatively high removal of potassium is also found in harvested products, which have senesced after having been green tissue, such as straw. The quantity will be reduced by rainfall just prior to and post-harvest. Harvested grain has a relatively low potassium content, not having been green photosynthesising tissue with its high water and potassium content. However, this relatively small potassium offtake in grain is in contrast with the large uptake required for the biomass of the crop that produced the grain.

Table 2 provides a guide to the average potassium content per fresh-weight tonne of some harvested products, as a guide to offtakes.

Crops and grass have large requirements for both nitrogen and potassium and the efficient utilisation of nitrogen depends on the availability of enough potassium for the growing crop.

The risk of poor crop performance and a low NUE are generally avoided on soils maintained at K Index 3.