How much NPK for rapeseed?

How much potassium and phosphorus does rapeseed need?
Is the symbolic 200 kg/ha even justified?

More potassium than phosphorus

Generally, rapeseed has a much higher demand for potassium than for phosphorus. However, it is not true that phosphorus can be omitted from fertilization, because it is just as important in the initial stages as potassium. Phosphorus is responsible for, among other things, the proper development of the root system, which is why it directly affects, among other things, water uptake, but it also definitely has an impact on overwintering, because the quality of winter dormancy depends largely on the extent to which plants have managed to develop their root system. Phosphorus should be applied before sowing, any supplementary fertilization with this component is not very effective (due to the fact that this component moves quite poorly in the soil). Poor fertilization with phosphorus at the start causes rapeseed to develop its root system much worse.

In turn, when it comes to potassium, its autumn uptake can already oscillate around 60 to even 100 kg per hectare. This was the case last year, when intensive development of the plantation was associated not only with a huge appetite for nitrogen, but also potassium. Here, let us note that this component has a direct impact on the development of the rosette. It also rationalizes the plant's water management. At least half of the potassium should be given in the autumn period (from 1/2 to 3/4 of the total dose). Here, let us note that rapeseed mostly absorbs potassium from the subsoil, so the starting, pre-sowing dose is very important due to the fact that this component must have time to be moved deep into the soil profile.

There are cases where pre-sowing nitrogen fertilization can be omitted. This applies to plantations after plants that fix atmospheric nitrogen in the soil, but in some cases also when fertilizing with manure (however, if this fertilizer was applied just before sowing, we must remember that in the first year a maximum of 40% of nitrogen will be available). In the case of using manure, we will therefore more often talk about reducing mineral fertilization than giving it up.

How much phosphorus and potassium should I apply to rapeseed?

It is assumed that per ton of seeds (and the appropriate amount of straw) rape will take up 20-30 kg of phosphorus, 50-70 kg of potassium, 40-60 kg of nitrogen. Let us note that the greatest uptake concerns potassium, then we have nitrogen, which is generally most important in the early spring period (although it is also necessary in autumn). Of course, there are other macroelements, although in pre-sowing fertilization, those indicated earlier constitute the core. Here, we can also include sulfur, which is often a component of NPK fertilizers, although the burden of fertilization with this element is generally transferred to the period just before the resumption of vegetation. Returning to the nutritional requirement. With a yield assumed at 4 t/ha, rape should take up at least 80 kg of P2O5, 200 kg of K2O, 160 kg of N. We are talking about the minimum amounts taken up by the plant. It is good to know the fertility of the soil – then pre-sowing fertilization is more effective.

By applying NPK 8 – 20 -30 fertilizer in the amount of 200 kg/ha, we will give the plantation 16 kg N, 40 kg P2O5 and 60 kg K2O. By increasing the dose to 300 kg, we will provide 24 kg N, 60 kg P2O5, 90 kg K2O in such fertilizer. It should be noted that in general, autumn fertilization with potassium does not meet the nutritional demand, including potassium, therefore it is necessary to supplement this element in late autumn or at the latest before the resumption of vegetation. The situation is different in the case of phosphorus, where the entire demand of the plantation for the element is met in pre-sowing application.

200 kg NPK per hectare? Good, but not always justified

Many farms made some savings in pre-sowing fertilization last year. This is important because some sites may have smaller amounts of nutrients at the start of this season. Savings in fertilization last year were not only due to the lack of fertilization. Often, the "customary" 200 kg/ha of cheaper fertilizers, but offering fewer nutrients, were used. Let's see that the differences here are big. By applying, for example, 200 kg/ha of NPK 4 – 10 – 20, we are actually applying 8 kg of nitrogen, 20 kg of phosphorus, 40 kg of potassium. In turn, by applying NPK fertilizer with a composition of 6 – 18 – 34 in the same dose, we are providing the site with 12 kg of N, 36 kg of P2O5, 68 kg of K2O. Therefore, each time you need to pay attention to the fertilizer content, the amount of pure ingredient, because it determines how many nutrients you will provide. The solubility of fertilizers in water is also important (given on the product labels).

When it comes to the proportion of ingredients in rapeseed, there is a certain correlation between their uptake, according to the principle, where NPK accumulation is as follows: 1: 0.6: 1.6. Here we notice the proportions of the uptake of ingredients. So when selecting a fertilizer for rapeseed, it is worth using one that offers an appropriate advantage of, for example, potassium over phosphorus. NPK, in which the amount of phosphorus and potassium is similar, will not be an ideal solution, unless we supplement potassium separately in the fall.

When selecting the fertilizer dose for rapeseed, you need to consider all the elements, such as the expected yield or the richness of the site. Fertilization should be slightly higher on weaker soils. You also need to consider the history of fertilization so far, because any failure to apply P and K last year will result in less of the component in the soil this year.