2.11 Plant tests

Rapid Sap-Nitrate Test

Detailed, predictive calibrations of this test have been undertaken for wheat, barley, oats and canola, and diagnostic interpretation is available for maize, potatoes and tomatoes.

A number of tests are required to assess the trend in nitrogen levels. The test should be carried out:

Figure 1: 
rapid sap-nitrate test

Sap is extracted by garlic press from the lower stem of 30 - 40 plants, onto the sensor pad of the cardy meter. Nitrogen is displayed as Nitrate (ppm).

Figure 2: 
Rapid sap-nitrate test results

This value plus crop growth stage and target yield are used to calculate an appropriate nitrogen recommendation

Care should be taken to avoid sampling crops affected by stress, waterlogging, drought, frosts and within 10 days of herbicide applications.

Near Infra red Reflectance

Near infra-red reflectance spectroscopy (NIR) measures the total plant nitrogen and fructan (sugar) levels at early tillering to earl stem elongation stage of wheat. Currently available for wheat, barley oats and canola in the pipeline.

Samples are taken, shoot densities counted per square meter and growth stage identified. This information is then sent to the laboratory.

High fructan levels may indicate some stress factors affecting crop growth eg. phosphorus deficiency or disease. When considered in conjunction with nitrogen levels an assessment of the likely response to applied nitrogen can be made.

Figure 3
Near infra-red relectance

Below these levels wheat is likely to respond to nitrogen, past Z32 this will mainly affect grain protein level.

Shoot densities

The number of shoots per meter square can give a good indication of the likely response to applied nitrogen for an average year. The following figures asume there are no other limiting factors.

Likely response of wheat to 100kg/ha urea topdressed at Z30, pre-stem elongation.

Figure 4: 

Shoot-density test

Plant tissue testing

Useful for detecting hidden plant hunger before deficiency symptoms appear. Plant tissue testing is considered the most reliable method for assessing the level of plant available nutrients.

Plant parts are sampled and sent to the laboratory for analysis. Due to limited data on critical levels at different growth stages it is essential that samples are collected at stages where data is available, otherwise results may be misleading.

Figure 5: 
Plant tissue test

Figure  6: 

Marginal Plant Tissue Nutrient Levels (mg/kg)

Cereals -Youngest fully emerged blade (YEB)

Legumes and canola - Youngest open leaf blade (YOL)

Grain testing

Grain nutrient content affects price in terms of protein and germination and vigour for home saved seed. Testing can be carried out during or after harvest to identify macro and micro nutrient limitations.

Wheat and barley are the only grains than can currently be tested. Samples are sent to the laboratory.

Marginal levels in wheat and barley grain for vigorous seedlings

Figure 7: 
grain testing

A grain protein of less than 11% indicates the crop yield was limited by insufficient nitrogen

2.11 Plant tests

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