In the North Island Hill Country, early autumn is typically the time for pasture renewal and seed sowing. There is adequate soil moisture and temperatures are not too cold. For many of the same reasons, it is also the time when soil fertility is boosted by applying phosphates.
Since these two events occur at similar times, why are the seeds and fertiliser not mixed together and applied at the same time? One reason that this practice is not followed is the risk of ‘seed burn’.
Application costs are reduced because it’s safe to mix dicalcic phosphates with seed.
What is seed burn?
The term ‘seed burn’, or fertiliser burn as it is sometimes called, is actually a misnomer. It suggests that the seeds are burned by the fertiliser, but this is not what really happens. Instead, it is not the seed itself, but the newly germinated seedling which suffers, and rather than being burned, it becomes dessicated.
This occurs when high concentrations of dissolved fertiliser salts are too close to the roots of the new plant. The high concentration of salts causes water to be drawn from the roots of the newly germinated seedling into the soil by osmosis, a natural process whereby water moves from an area of high concentration to an area of lower concentration.
Unfortunately, once the roots are short of water, the entire plant becomes dessicated. The leaves, which can no longer photosynthesise without water, collapse. They become brown and crusty, taking on the burned look with which we are familiar. Often, the entire plant will die.
Clover seed most at risk
It’s important to note that small seeds are particularly sensitive to seed burn. So clover, which is frequently sown on hill country pastures because of its nitrogen-fixing abilities and high nutritional value, is more at risk than pastoral grasses. Brassica seeds, which are also small, come into this category.
Soil type and dry weather increase risk
Seed burn can also be a problem for some soil types. Sandy soils, with lower organic matter content like those found in parts of the Manawatu and Northland are examples where this can occur. Dry conditions exacerbate the risk of seed burn too, since the fertiliser salts in the soil are more concentrated due to lack of moisture.
The solution is dicalcic phosphate
So seed burn is a problem to be avoided and it seems like the safest option is to sow seeds, then apply fertiliser at a different time, incurring the extra application costs of doing that.
However, there is a simple solution: use a dicalcic phosphate fertiliser instead of superphosphate.
Dicalcic phosphates are not soluble like other phosphates. While other phosphates release most of their nutrients in an initial burst, dicalcic phosphates like Fertco’s KingPhos, SeaPhos, and Replenish, release them in a much more controlled way.
So even if the dicalcic phosphate granule is close to the roots of the seedling, the plant will not suffer dessication because a build up of highly concentrated nutrient in the soil does not occur.
A build up of highly concentrated nutrient is also avoided in dry weather since the dicalcic phosphate granule retains most of its nutrients until sufficient moisture becomes available.
Phosphates are not very mobile nutrients. They don’t tend to travel more than a few centimeters through the soil. So direct drilling in separate furrows doesn’t work well. It’s far better to be able to deliver the fertiliser down the spout with the seed.
Wherever oversowing is preferred or necessary due to steep terrain, accurate placement of seed and fertiliser is obviously not possible. Dicalcic phosphates are a great choice in these situations because both can be applied together without the risk of seed burn.
No matter what application method is chosen, being able to sow seeds and apply fertiliser at the same time is always going to save money.
This article was published in the Coast & Country News.
