Spring 2024

Test soils to identify spring fertiliser requirements

Thomas Perrott, Crop input specialist

Ensuring crops receive sufficient nutrients this spring need not mean relying on the traditional blends, explains Thomas Perrott, Agrii crop input specialist.

After one of the wettest autumns on record and a similarly wet start to winter, crops are likely to face a nutrient shortfall come the spring. Crops on lighter soils or those low in organic matter are likely to need more attention but those on heavier soils are likely to face a nutrient shortfall too.


Addressing this situation is likely to require a more detailed focus on applications than might be expected in a typical year, but it need not all come on the form of traditional product derived from a bag. Bulk product is still the best means of supplying nutrient in large quantity, but there is now a far greater range of products and blends to choose from than was the case just five or so years ago. The ‘Nutri-Match’ service from Origin Fertilisers means growers can have a prescription fertiliser containing up to 13 nutrients that is tailored to their exact needs. Bespoke blends have done much to overcome the limitations of the conventional N:P:K:S products. Similarly, the development of protective coatings and liquid inhibitors has also extended the application flexibility to the benefit of growers while the introduction of polyhalite (sold as Polysulphate) has made it easier to meet winter wheat’s seasonal requirement for potassium and sulphur. Polysulphate contains 48% SO3 as sulphate, 14% K2O as from sulphate of potash, 6% MgO as from magnesium sulphate and 17% CaO as from calcium sulphate.

Accepting the reality that there will be changes in nutrient availability and that addressing any deficits is likely to involve greater expense, it makes sense to test soils in the early new year. Only a soil test will give a true representation of the nutrient status and with this knowledge, we can avoid the expense of over-applying or the loss of yield that would be incurred from under-applying. In a season such as this year when prolonged wet weather will have affected a soil’s nutrient status, assessing the soil’s true status is likely to be highly worthwhile, even if it is not due to be tested.

Soil moisture is the principal determinant of nutrient availability, but the soil’s chemical properties also influence availability. Neither pH nor the soil’s cation exchange capacity should be overlooked. Fortunately, pH is easily managed and while there is little that can be done to affect a soil’s cation exchange capacity, well-maintained field drains will be vital to helping them recover their aerobic capacity.

Nutrients that are leached more readily (see Table 1) are usually those that are less strongly held in the soil on clay particles or organic matter. In general, anions (borate, sulphate or nitrate) leach more easily than cations (potassium, magnesium or calcium), although phosphate is an exception to this. Crop demand for phosphate peaks in the late spring to early summer weeks, but availability is affected by a combination of environmental factors and soil properties.

Historically, growers seeking to ensure adequate phosphate availability after a wet winter would have applied up to 80 kg K20 but Agrii trials performed at Stow Longa in the 2020/21 season, indicate that through the use of Polysulphate it is possible to maintain yields with lower-rate applications.

As Table 2 shows, application timing is crucial. Treatment 3 reflects a typical nitrogen programme for quality wheat with applications backloaded. Applying 60 kg/Ha at the first split and 90 kg/Ha on the final one boosted the yield to 8.9 t/Ha, the protein content to 13.7% and gross margin to £1,030/Ha.

The value of the Polysulphate to the programme can be seen in treatment 5 where it was replaced with a NKS blend. Despite providing much more potash than the equivalent backloaded N programme and 50 kg/Ha more N, the average yield was noticeably lower at 8.65 t/Ha, as was the NUE at 78% and margin at £901.

It is perhaps due to the additional magnesium and calcium in Polysulphate, the balance of the nutrients it contains, their sulphate form or a combination of all these factors that makes it a product that seems to deliver more than the sum of its parts. Given the environmental focus on ammonium nitrate this work is of great significance.

Other work has considered the value of a protected phosphate starter fertiliser applied to the seedbed at drilling. A yield response of 3.9 t/Ha over the untreated and 0.63 t/Ha over the ammonium nitrate demonstrated its value (see Figure 1). It also dispels the myth that starter fertilisers must be applied down the spout at drilling.

Managing crop nutrient requirements this season will undoubtedly be a more considered task than might be expected in a typical year, but there are steps that, if adopted, could save growers time, hassle and unnecessary expense. An N-Min test for example, will determine what amount of nitrogen remains available to the growing crop helping to ensure that only what is needed is applied. As such, don’t be afraid to have soils analysed even if it is not due. Similarly, the range of granular and liquid forms of macro and micronutrient products has made it easier than ever to ensure crops receive the nutrition required to realise their potential.

Field drainage too is equally easy to overlook. It often pays to walk fields during wet spells to identify areas where poor soil structure or blocked drains may need further attention.

None of this is rocket science, but it’s a classic case of ‘you can’t manage, what you don’t measure’, so it is well worth spending some time assessing the reality of your soil nutrient status.

Helped by the eighth wettest October since records began in 1836, a total of 410 mm fell across the UK in autumn 2023 making it the eleventh wettest autumn period on record, according to data from the Met Office.

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BoronNitrogen (Nitrate)
Nitrogen (Ammonium)
Table 1: Example comparison of nutrient mobility in soils

Fertiliser Treatments*N
1: Standard N/poly-s 214 8.47 12.067803
2: Reduced N/poly-s1648.2912.483840
3: Reduced N/back-loaded/poly-s1648.9013.7981030
4: Reduced N/no poly-s164 7.9810.970 643
5: Standard/back-loaded/NKS2148.65 13.778901
Table 2: Stow Longa Wheat Nutrition Trial 2020/21

* Standard N: polysulphate (100 kg/Ha); liquid N (150 kg/Ha); foliar N (14 kg/Ha); liquid N (50 kg/Ha)
Reduced N: polysulphate (100 kg/Ha); liquid N (150 kg/Ha; foliar N (14 kg/Ha)
Reduced N/back-loaded: polysulphate (100 kg/Ha); liquid N (60 kg/Ha); foliar N (14 kg/Ha); liquid N (90 kg/Ha)
Reduced N/no polysulphate: Liquid N (150 kg/Ha) Foliar N (14 kg/Ha)
Standard N/back-loaded/NKS: NKS (343 kg/Ha); liquid N (60 kg/Ha): foliar N (14 kg/Ha); liquid N (80 kg/Ha)

Figure 1: The value of a protected phosphate in a blend compared with ammonium nitrate alone