When considering the appropriate soil potassium (K) index for a crop or rotation, it is important to take into account that different crops have root systems with different total lengths, depths and efficiencies.

Generally speaking, the longer an annual crop is growing, the longer the root system, which means that winter combinable crops and grass etc., should have good long root systems, which they do.

Short season vegetable crops and spring-sown combinable crops generally have much less total root length and do not explore the same depth of soil.

It is therefore reasonable to expect that winter cereals will tolerate a lower soil nutrient concentration (index) than vegetables.

This explains why the general recommendation for cereals is to maintain a K index in the lower half of index 2, whereas for vegetables, it is in the upper half, and for phosphorus at indices 2 and 3 respectively.

Crop root systems

Crops with a small root system need higher concentrations of nutrient in the soil to satisfy their peak uptake demand.

The effect of limited availability of K on crop uptake, has been confirmed through numerous research trials.

Moreover, the total requirement for K is related to the uptake of nitrogen (N) and is necessary to maintain the turgor of the tissue cells; it is not ‘luxury’ K uptake.

The rate of K uptake/day within a spring barley crop can reach 7.2kg K2O equivalent.

However, the size of a crop’s root system cannot be estimated reliably from the length of the growing period alone.

Some species have shorter root systems than would be expected, often because they have fewer root hairs or are unable to create symbiotic relationships with soil mycorrhizae, which both play an important role in taking up nutrients from the soil.

Potatoes and field beans

Two crops which fall into the category of having unexpectedly limited root systems are potatoes and field beans.

Thus, logic would suggest that, like short-season vegetables, they also need higher soil nutrient concentrations to achieve maximum yield potential, as found experimentally to be the case.

Meanwhile, winter wheat has a root system which is more than six times longer than potatoes.

It also shows the unbelievable length of roots, including the root hairs, which crops will produce in order to feed themselves – these values are in kilometres.

If grown in unrestricted soil, winter wheat root lengths can reach 31km/m2 of soil.

It is clear that a low soil K index will be potentially much more serious for a potato crop than a winter wheat crop, which has to be considered when planning rotations and soil fertility levels.

Rotations

Unexpected changes in rotation planning, as may occur this year, bring additional challenges. However, as P and K applications should be considered across a rotation, rather than as responses to individual crops in the year of application.

In this scenario, there can be advantages from the use of fertiliser placement for potatoes and spring cereals, where zones of high nutrient concentration are established for the limited root system to find and to feed on.

This is also of particular importance for phosphate uptake, where high rates of uptake only occur when root length densities exceed 5cm/cm3 of soil.