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Potassium for the soil and crop: the importance of getting it right

May 2019

Patrick J. Forrestal1, Mark Plunkett1, Cathal Redmond1 and Martin Bourke2
1Soils, Land Use and Environment Dept. Teagasc, Johnstown Castle, Co. Wexford.
2Teagasc Advisory Service, Tinnahealy, Co. Wicklow

Introduction

Crops including spring barley require high levels of potassium (K) to support yield, often at similar levels to nitrogen (N). Potassium has traditionally received much less focus compared with nitrogen. Nevertheless, K can play an important role in improving nitrogen use efficiency (NUE) bringing environmental benefits in addition to yield, plant vigour and lodging resistance benefits. A recent Teagasc trial showed that application of K fertiliser increased NUE from low of 31% where soil test K was low and no K was applied to 85% at the highest K application rate on the same soil (Forrestal et al., unpublished). This dramatic effect highlights the importance of getting soil and fertiliser K right and the impact which K can have on NUE. Yield improvement over the past decades has resulted in increased K off-takes. Without soil testing to monitor soil status and appropriate K fertilisation from manures, crop residues and/or mineral fertilisers the potential for potassium deficiency to place a drag on crop yields and NUE has increased.

Figure 1. Visual early season deficiency symptoms and response to K application on a K deficient soil.
Figure 1. Visual early season deficiency symptoms and response to K application on a K deficient soil.

Behaviour and role of Potassium in plants

An important distinction between many nutrients (for example N, P, S) and Potassium (K) is that K is not incorporated into the structures of organic compounds in the plant. As K is not organically bound it becomes available from manures and residues (e.g. chopped straw) relatively quickly compared to other nutrients.  Potassium remains in its ionic form (K+) within the plant regulating cell osmotic pressure, the process which controls water loss and uptake by the plants. It also activates more than 80 enzymes controlling processes including photosynthesis, nitrogen fixation in legumes and formation of starch. As a result, adequate K nutrition is a key factor in the formation of healthy and full tubers and grains.

The dangers of allowing soil K levels to become depleted

In Ireland Index 1 and 2 are considered deficient and at these soil test index levels a positive response to K fertiliser application is to be expected. Figure 2 shows that at zero K application the effect of soil index (1 vs 2) was more than 2 tonnes/ha. Even with application of K fertiliser a wide gap remained between the grain yields at the index 1 and index 2 sites highlighting the importance of maintaining soil K levels informed by regular soil testing.

Figure 2. Effect of soil test K level and K fertiliser application on spring barley yield

Figure 2. Effect of soil test K level and K fertiliser application on spring barley yield

At a Morgan’s soil test K level of 26 mg/kg (Index 1) the ability of the soil to supply K to the crop was severely limited. Figure 3 shows that at this soil test level the soil supplied only 10 K kg/ha to the crop. There was a dramatic response in K uptake to the first increment of 40 kg K/ha application with a high level of apparent K fertiliser recovery at 40 kg K/ha. The crop responded positively to each incremental application of K at this site and particularly to the final increment from 160 to 200 kg K/ha. The sharp increase in K uptake in response to the final K increment is also associated with the crop reaching maximum yield potential for the site and year (Figure 4) showing some evidence of luxury K consumption. Luxury consumption, where plants absorb K which is in excess of growth requirements is a recognised down side to having over availability of K. It is therefore the important to conduct regular soil testing and to implement appropriate fertiliser programme planning to ensure that available K levels are not increased above the optimum recommended level (Index 3 in the Irish system). Excessive soil or fertiliser K availability to plants will lead to suppression of uptake of other cations (positively charged ions), in particular calcium and magnesium.

Figure 3. Soil and fertiliser capacity to supply K to spring barley crop at low soil test K
Figure 3. Soil and fertiliser capacity to supply K to spring barley crop at low soil test K

A dramatic yield response to K fertiliser application was observed at this low soil test K site with grain yield increasing from 2.4 to 6.7 t/ha or a 4.3 t/ha response to the first 40 kg K/ha applied (Figure 4). Grain yield continued to increase up to 200 kg/ha at this site resulting in a maximum yield of 8.6 t/ha or almost a 2 t/ha increase in yield compared to the 40 kg K/ha application rate. This shows just how responsive some sites can be to K fertiliser application where soil test levels have been run to a very low level. 

Figure 4. Effect of K fertiliser application on spring barley yield at low soil test K
Figure 4. Effect of K fertiliser application on spring barley yield at low soil test K

In summary, these experiments highlight the importance which K has in underpinning crop production. While the responses are extreme, they highlight what can happen if K nutrition is neglected and soil test K levels decline. Increasing crop yields have placed a greater removal pressure on soil K supplies than ever before. Potassium is subject to a level of leaching loss from soils in humid high rainfall regions such as Ireland and the U.K. and cropping systems which do not chop and return straw remove higher levels of K than those that do not. These factors combine to increase the importance of regular soil test and the design of fertiliser programmes which adequately provide for soil K build up and losses in addition to plant requirements and off-takes.