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Testing for nitrates

Any nutrient can be measured in the soil solution collected by the FullStop. Most of our work has involved the monitoring of soil nitrate.

Nitrogen is required in large amounts by plants. For plants that are not legumes, nitrogen must be taken up as dissolved nitrate or ammonium by the plant roots. Most of the available nitrogen is usually in the nitrate form.

Nitrate is a difficult nutrient to manage. Unlike most plant nutrients it is not attracted to soil particles, so tends to move with the water. Excess irrigation quickly leaches nitrate below the root zone.

Nitrate is easy to measure using colour test strips. A drop of water is placed on the strip and the colour changes from white, through light pink to purple, depending on the concentration. The measurement is inexpensive and takes about one minute to complete.

Testing for nitrates

It is not so easy to say what the nitrate levels should be. It is better to look at the trends in soil nitrate and evaluate those results against your usual fertilizer practice whilst observing the condition of the crop. In many cases this will help to reduce nitrogen inputs or alter the timing of side dressings.

Our experience shows that nitrate levels tend to be high at the start of the season when the demand for nutrients by young crops is low. Since young crops have shallow root systems they are frequently given too much water. This water can move nitrate below the root zone.

The cost of losing nitrate is usually many times greater than the costs of using too much water. For young crops we often irrigate so as not to activate either the shallow or the deep FullStop. Our strategy is to keep as much of the nutrients as possible in the upper layers of soil, so the plants can access them when rapid growth starts.

As plants enter the rapid growth stage, soil nitrate levels tend to drop sharply. Monitoring nitrate can show when side dressings are required.

If the irrigation water is relatively fresh, it is possible to get an indication of current nutritional status of the soil by watching trends in EC. However, monitoring of both nitrate and EC helps to show which salts are contributing to EC. Occasional measurement of other nutrients will complete the picture.

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