Crop root systems explain need to maintain K Index level

Figure 1: Cumulative uptake of K by spring barley in a field experiment, kg/ha, from IFS Proc 613 (2007)

An important factor to take into account when considering the appropriate soil K Index for a crop or rotation is that different crops have root systems with different total lengths and efficiencies. Generally speaking the longer an annual crop is growing, the longer the root system, which means that winter cereals, winter rape, grass etc, should have good long root systems, and indeed they do. Short season vegetable crops generally have much less total root length. Thus it is reasonable to expect that 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. Crops with a small root system need higher concentrations of nutrient in the soil to satisfy their uptake demand. The effect of limited availability of K on crop uptake is illustrated in Figure 1, which shows the cumulative uptake of potassium by two spring barley crops, one with an adequate K content growing in soil at K Index 3 (300 mg/kg) and the other deficient in K at Index 0 (50 mg/kg) at Rothamsted. The total requirement for K is related to the uptake of N and is necessary to maintain the turgor of the tissue cells; it is not 'luxury' K uptake. The rate of K uptake at the peak of the graph for this crop was about 7.2 kg K₂O/ha/day.

However the size of a crop's root system cannot be estimated reliably from the length of the growing period. Some species have shorter root systems than would be expected, often because they have fewer root hairs; these play an important role in taking up nutrients from the soil. 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.

The table shows the total length of root per square metre the top 20 cm of soil for a number of crops, and it can be seen that winter wheat has a root system which is more than 6 times longer than potatoes. It also shows the unbelievable length of root, including the root hairs, which crops will produce in order to feed themselves - these values are kilometres!

Thus it is clear that a low soil K Index will be potentially much more serious for a potato crop than a winter wheat crop, and this factor has to be considered when planning rotations and soil fertility levels. It explains why there can be advantages from the use of fertiliser placement for potatoes, whereby zones of high nutrient concentration are established for the limited root system to find and to feed on.

However the dryness of the soil must also be taken into account. In order to be able to provide a daily K requirement of 6 kg K₂O/ha (which can be less than the highest rate required) the chart on the right shows the soil nutrient concentration required by two different crops on a medium soil. Clearly it is easier for plants to obtain the nutrient they need from wet soils, but for the spring barley crop the nutrient level required remains in the K Index 2 range, even as soils get drier. However for potatoes, not only does the requirement start in the middle of Index 2, as mentioned earlier, but as the soil dries out the concentration required moves quickly into Index 3 and even to Index 4.

These factors are of course taken into consideration for the fertiliser recommendations for the different crops, but an understanding of the reasons behind the recommendations is always helpful; indeed it is essential for the sound management of risk. Much agronomic decision making is in fact to do with assessment and response to risk. It is fortunate that for phosphate and potash, the risk of deficiency and its consequent financial loss is relatively easily managed for most arable crops by maintaining soil P and K Indices in the middle of Index 2.

Updated PDA Leaflet 15 'Potash for Potatoes'.

A copy of the latest revised PDA leaflet is enclosed with this Newsletter. This Leaflet 15 on 'Potash for Potatoes' highlights the importance of maintaining adequate reserves of soil P and K for this poor-rooting crop. It also provides good data on the high rates of removal of potash in the harvested crop, with an average of 5.8 kg K₂O being removed per tonne of yield. For example, a 70 tonne/ha potato crop will remove over 400 kg potash per hectare, and this must be replaced if soil fertility is not to decline.

For further information please contact:
PDA info@pda.org.uk
Potash Development Association

update: Jan 2008