Managing nitrogen in sustainable cropping systems

Declining fertility of arable soils is a serious problem of national and international significance. In eastern Australia's northern grain belt, soil erosion and nutrient removal are the main factors causing soil fertility decline.

In most cropping regions of southern Queensland and northern New South Wales, soil fertility rundown means that maximum yields cannot be obtained in good seasons without adequate attention to nitrogen nutrition.

The duration of cropping for many Central Queensland soils means that similar practices and strategies to supplement nutrients should be considered.

The Nitrogen Books for central Queensland and southern Queensland/northern New South Wales project are a compilation of research and farmer knowledge on soil nitrogen management. The books outline the range of tools that are available to assist growers to manage nitrogen and climate risk for maximum yields and to address soil fertility decline.

Variable rainfall (and yields) means that nitrogen demand varies widely. In order to better manage risks associated with applying nitrogen fertiliser application it is recommended to:

  1. Measure or estimate soil water and soil nitrogen levels close to planting
  2. Use the CropArm program to view the full range of possible yield outcomes.

Deep soil testing is the best method for determination of soil nitrate nitrogen, but consider the number of cores needed to get the representative sample.

After determining the soil nitrate level, the nitrogen fertiliser rate required can be calculated by the difference between the expected crop nitrogen demand and the soil nitrogen supply (extra soil nitrogen may be available between soil sampling and planting).

Determining the range of possible yields

The CropAm program allows users to enter a range of different inputs (including nitrogen fertiliser rates) and look at a range of possible outcomes which result from the combination of soil water and possible nitrogen inputs. This range of outcomes, along with growers' experiences of paddock performance, provides a good basis for nitrogen decisions. Growers are able to see how different levels and combinations of inputs (and therefore risks) can affect potential yield and gross margins.

The Nitrogen Books discuss how to improve soil fertiliser and highlights that nitrogen fertiliser application in conjunction with reduced tillage practices can slow the decline of soil nitrogen.

The books discuss soil organisms and their role in fertility processes as well as gains and losses of nitrogen in the system. Nitrogen movement in the soil is complex but it is useful to understand the different pools and relative proportions of nutrients, including micro-organisms within them.

Soil's capacity to supply nitrogen

Soil organic carbon level is identified as one indicator of current nitrogen fertility status, reflecting the capacity of the soil to supply plant-available nitrogen. This is an indicator of how much response can be expected from applied nitrogen fertilisers.

The book outlines estimated monthly releases of nitrate nitrogen from soils with different organic carbon levels and shows how the CropARM and Howwet programs can be used to estimate soil nitrate levels after fallow periods. It shows how previous yield and protein values of past cereal crops can estimate soil nitrogen status.

Calculating crop nitrogen needs

Calculating crop nitrogen fertiliser requirements requires some consideration of climate and a range of possible yield outcomes. Ways to estimate yield and these risks are discussed in detail. A computerised soil nitrogen calculator is provided with the book.

Fertiliser application techniques and the nitrogen contribution of legume crops and ley pastures is discussed. 'Free' nitrogen can be sourced from ley legumes or pulse crops. The amount of nitrogen added will be in proportion to the quantity of vegetative material returned to the soil which in turn is influenced by the seasonal conditions. Pulse crops may contribute very little nitrogen if the biomass removed as grain is large in proportion to the vegetative material but may provide additional benefits apart from nitrogen.

The Nitrogen Books were compiled by Primary Industries and Fisheries (now Department of Agriculture and Fisheries) and the Central Queensland Sustainable Farming Systems and the South Queensland Sustainable Farming Systems projects. The Grains Research and Development Corporation provided investment for these projects.

Further information

Download a copy of The Nitrogen Book for central Queensland or southern Queensland/northern New South Wales.