Finding the right nitrogen (N) test for your crop depends on what you are growing, as well as the form of N you want to measure.
Testing to find out how much N is in soil enables more sustainable use of N fertiliser and can improve financial returns.
However, there are various tests for measuring N in the soil on offer and the only thing they have in common is helping to refine N fertiliser use.
Beyond that, N tests vary in terms of what they measure, as N is present in soil in many forms (see box), and only a small percentage of N in the soil is able to be used by plants.
So, what are the various N tests available, and what exactly is each one designed for?
Total N - what's available now and in the longer term
As the name suggests, a Total Soil N test measures the total pool of N in the soil, which includes both mineral and organic forms of N. It is used only for pasture. Taken at a depth of 75 mm, it provides information on N available for pasture growth, as well as the potential long- term supply of N.
Total N test results change slowly over time, so, depending on changes in land management, this test may only be needed every five to seven years.
Available N - what's available this season
The Available N test measures organic matter that is going to be mineralised during the life of the crop, so it only measures the potential supply of available N for the growing season. Done to a depth of 150mm, it is used for refining N applications for forage crops such as brassica, fodder beet and maize. The Available N test is completed before the crop is sown, when regular crop soil testing is done.
Mineral N - what's available now
Mineral N tests (or Deep Soil Mineral N tests) measure the N (ammonium and nitrate) in soil that is immediately available for plant uptake. Results are typically used to inform N fertiliser recommendations for cereals and grass seed. It is important the test is taken before any N fertiliser being applied.
A Mineral N test is performed at a depth of 600mm for deep- rooting crops such as cereals, and to 300mm for grass seed.
For autumn- sown cereals, the test needs to be taken in early spring before N is applied, and before the crop has a high N demand (stem elongation). For grass seed, the test needs to be taken before closing, or as close to N application as possible.
Types and forms of N in soil
The two broad types of N in soil are:
Article supplied by Ballance Agri-Nutrients