We need fertilisers to provide essential nutrients, but mineral fertilisers are salts which can be detrimental to crop establishment if concentrations are too high when placed too close to the seed.
High osmotic pressure, caused by high salt concentrations, can make it harder for seeds to imbibe water needed to germinate. Once germinated, the concentration of salts can make life harder for developing roots.
Affected crops have reduced plant emergence and/or are sluggish to get going.
The likelihood of fertiliser toxicity depends upon many factors. It is hard to define ‘safe’ rates because crop safety depends on many variables such as crop species, soil types, products used, seeding bar configurations, and how much rain we get after seeding.
All we can do is try to minimise the risk without compromising fertiliser performance.
Two of the most sensitive crops we grow are lupins and canola. With these crops, there is a high risk of toxicity if the fertiliser is placed too close to the seed.
The risk varies with soil type. Lighter textured soils (i.e. sands) are particularly susceptible because they dry out quicker increasing osmotic pressure.
We know that on responsive soils, banding phosphorus (P) at seeding gives better results than topdressing. But where there are good soil reserves, topdressing at least some of the P as Super before seeding can maintain soil reserves.
We know that in cereals there can be big efficiency gains from banding K at seeding. Early supply to responsive crops is critical and early rains may not be enough to wash topdressed muriate of potash (MoP) into the root zone.
In lupins, banding K is very risky and muriate of potash should be topdressed.
Banding some K in canola may be beneficial, but good seed fertiliser separation of NPK products is critical and MoP is best topdressed.
Like P and K, there are significant efficiency gains to be made from banding nitrogen – especially in cereals that grow into a high stubble load.
In cereals, the risk of some toxicity from banding high rates of N (up to 100 L/ha Flexi-N) should be acceptable if there are big increases in nitrogen use efficiency. Keeping it at least two to three centimetres away from the seed will minimise the risk of toxicity.
In canola, the risk of toxicity from banded N is much higher, with crops on lighter soils again being the most vulnerable. On sands, it is recommended to limit banded Flexi-N rates to no more than 50 L/ha and keep it all away from the seed. Banding higher rates may be safer on sandy loams/loams.
It is important to do the things we can to manage fertiliser toxicity, but it’s just as important to supply our crops with the nutrients they need – efficiently and profitably.
Effects of fertiliser product and placement on lupin establishment (blue bars) and grain yield (green line) at Binnu, 2017. TD = topdressed before seeding, Drl = drilled with the seed, Bnd = banded 2-3 cm below and to the side of the seed. The crop was sown on 9 inch row spacings.