Importance of potassium for efficient nitrogen use.

The importance of maintaining an adequate supply of readily plant-available potassium (K, exchangeable K) in soil was highlighted in a recent paper "Potassium and nitrogen interactions in crop production" presented at the International Fertiliser Society's December 2007 Conference by George Milford and Johnny Johnston. The increased yield potential of many recently introduced crop varieties can justify the use of more nitrogen (N). However, to optimise the benefits of applying more N to increase yield and profitability, it is essential to have sufficient exchangeable K in the soil as shown by data from a number of experiments. On the silty clay loam soil at Rothamsted, for instance, larger yields of potatoes and spring barley were achieved on soils with adequate reserves of P and K compared to those on soil with adequate P only (Table 1). The benefits of having an ample supply of K in the soil were even more striking in recent experiments on the sandy clay loam soil at Saxmundham (Table 2). A maximum grain yield of 11 t/ha of winter wheat was achieved with only 160 kg N/ha on soils that contained adequate amounts of exchangeable K. Applying more N, 240 kg/ha, on the soil with less K gave a smaller yield. Thus there is an opportunity to avoid wastefully high N inputs, and therefore reduce costs, by ensuring that shortage of K does not limit yield.

The accompanying chart (Figure 1, below) compares the response of sugar beet and spring barley to fertiliser N on soils containing two levels of exchangeable K. Applying more than 50 kg N/ha for spring barley was not justified on the soil with least exchangeable K and applying more N than this represents a serious cost penalty. On soils with more exchangeable K, maximum grain yields were obtained with 100 kg N/ha. The effect of extra exchangeable K did not seem to be as important for sugar beet as for barley except when a large amount of N was applied. However, this result needs to be qualified. Sugar beet are able to use sodium (Na) to replace some of the functions of K within the plant. In the experiment on sugar beet, there was sufficient Na in the soil to augment the limited availability of exchangeable K at the three smaller amounts of applied N but not enough to allow the beet to respond to the largest amount of applied N and equal the yield on the soil with more exchangeable K.

All these results indicate the need to maintain adequate levels of exchangeable K in the topsoil for each crop grown within the rotation on each field. These levels are at least K Index 2- for cereals and Index 2+ for potatoes, sugar beet and vegetables.

Figure 1. Interactions between exchangeable soil K (Kex) and N fertiliser on the yield of (a) sugar beet at Saxmundham and (b) spring barley on Hoosfield, Rothamsted.

There is a sound physiological basis for these important interactions between applied N fertiliser and exchangeable K in soil. A rapid development of the leaf canopy of an arable crop in spring is essential for the efficient capture and conversion of sunlight energy to dry matter and is a prerequisite for large yields. Nitrogen plays a major role in the expansion of the leaf canopy through increases in tiller number and the number and size of the leaves on each tiller in cereals; and through increases in leaf size - but not leaf number - in sugar beet. These effects of N on leaf development and shoot dry matter are brought about by increases in both the number and average volume of leaf cells. Because much of the volume of each cell is occupied by water, the increase in leaf size results in a large increase in its water content. Plant tissues need to adjust to these large N-induced increases in tissue water by taking up correspondingly larger quantities of osmotic solutes in order to maintain turgor for continued cell expansion and growth, to prevent wilting, and to allow leaves to continue to photosynthesise and produce sugars and dry matter. For most arable crop species, K is the major cellular osmotic solute, consequently soils need to contain sufficient available K to satisfy an increased demand for K to fulfil this role, for example as a result of an increase in N supply. This osmotic balancing role of K in shoot tissue water helps explain why the tissue water concentration of K remains largely constant throughout much of a crop's growth whereas that in dry matter declines continuously. Growth is impaired and yields decreased if the constant tissue water concentration of K cannot be maintained because of poor availability of K in the soil. Thus there are good physiological and agronomic reasons why it is so important to maintain the appropriate level of exchangeable K in soil.

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Potash Development Association

update: Mar 2008