Before planting maize, take soil tests to analyse potentially available nitrogen.
Since the 1990s, pastoral farmers in NZ have been using maize silage to fill feed deficits in the drier months of summer, or in the colder, wetter months of winter.
With an expected yield of 18t-28t DM/ha in optimal conditions, that’s more dry matter than most other crops. While optimal conditions include warm soil temperatures, good moisture levels, and lots of energy from the sun, the highest growth rates will only be achieved when nutrient levels are sufficient.
But how much is ‘sufficient’? As always, when it comes to fertiliser, the best practice is to soil test to find out.
Don’t guess nutrient needs
Applying the wrong amount of fertiliser can be costly. Insufficient nutrients may mean a poor crop whereas too much fertiliser may mean money spent on nutrients that aren’t used up, and damage to the environment through leaching. In either case, there is wastage, avoidable when you know the nutrient status of your soil before planting.
Guessing nutrient needs is also risky since every region has a different climate, and each property has a different soil type and cropping history. Maize may be cropped on the same block, a dairy effluent block, or grown on a poor or problem paddock that needs regrassing. So nutrient needs will differ for each situation.
Maize is nutrient-hungry
Maize is a nutrient-hungry crop which isn’t so surprising when you watch how fast it grows. It needs nitrogen, potassium, phosphorus, sulphur, magnesium, calcium, copper, and zinc. In some situations, manganese and boron might also be required.
Nitrogen needed most
Of all these nutrients, nitrogen is needed the most. It plays a vital role in the major plant functions like photosynthesis and amino acid formation. When nitrogen is short, plants become spindly and stunted with chlorotic or yellow leaves.
Nowadays, providing sufficient N that lasts throughout the growing cycle isn’t a problem with controlled release fertilisers that are protected from leaching and available in a few weeks from sowing. With a soil test, you can also get the quantity correct as well.
Soil N tests
Soil tests recommended for maize include a Basic Soil Profile. But additional tests are required for nitrogen. Being a mobile nutrient, it moves easily through the soil profile, and is also affected by soil temperature, moisture, and organic matter levels at the sampling time.
The Mineral N test
A well-established test is the ‘Mineral N test’ which reports on the initial inorganic N content of the soil, so it tells us how much N is immediately available for plants to take up.
This test is also known as the ‘Deep N’ test because soil samples are sometimes taken at depths of 60-90cm, useful because maize is a deep-rooted plant, apparently capable of extending its roots to 1.8m. Being able to factor in the available N from these soil depths helps calculate the total N available for plants.
Disadvantages of the Mineral N test are its analytical costs, the difficulties of deep sampling, and the many contextual factors that can affect the results.
But the biggest limitation of the Mineral N test is that it can only tell us how much N is available to plants at the actual sampling time.
Available-N test
To overcome this issue, the ‘Available-N test’, or the ‘Anaerobically mineralisable N’ test (AMN), is used. This test provides an estimate of the nitrogen that is potentially mineralisable throughout the growing season, or in other words, how much nitrogen will become available in the soil for plants to use while they grow.
The results of this test are important because the pool of potentially mineralisable nitrogen can be significant.
While this test has its own shortcomings, it’s relatively fast to do and less costly than the Mineral N test.
But the best thing about it is that it allows us to calculate the future requirements of the crop as it grows and not just rely on results from a sample taken on a single day that is unlikely to apply to a whole growing season.
This article was published in the Coast & Country News.
