Finding the right nitrogen test

Finding the right nitrogen test

12 May 2020

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:

  • Mineral N refers to 'readily available' forms plants can use, and includes nitrate and ammonium. Plants use nitrate and ammonium for amino acid and chlorophyll synthesis. This keeps the leaf green, driving plant growth and development. N is also used in protein development.
  • Organic N cannot be used by plants. Microbes convert organic N into mineral N, making it available for plant use. Most organic N is contained in organic matter in the soil.

For more information on soil testing, contact your local Ballance Nutrient Specialist or contact your Farm Source TSR or visit your local Farm Source store.

Article supplied by Ballance Agri-Nutrients