There are three main ways in which soils can be tested for likely gypsum response. These are:
1. On-farm tests, these are the quickest but least reliable means of assessing likely gspsum response. The simplest test is to drop about eight small (3-5 mm diameter) air-dry aggregates of soil into a container of rainwater or irrigation water. Leave undisturbed for approximately 2 hours, then look for clay dispersion, which is seen as murky cloud around the slaked aggregate. Observe again after 24 hours. Test topsoils and subsoils separately. Clay soils that disperse strongly, particularly after only 2 hours, usually respond to gypsum application.
If gypsum is available, add some to rainwater or irrigation water in a bottle and shake vigorously to partially dissolve it. Repeat the test using this solution. Dispersion should be prevented if gypsum is to be successful in improving soil structure.
Another sign of dispersion in the field is the separation on the soil surface of light coloured sand from clay after heavy rain or irrigation and subsequent drying.
2. Laboratory tests, this method is intermediate in speed and reliability, it is particularly useful to confirm or refute on-farm test results. It provides information about the nature of the structural instability problems.
3. Trial strips, the third method, application of trial strips of gypsum, is the slowest of the three, but is often the most reliable and realistic. For best results choose the most uniform and representative part of the paddock. Use several different rates of gypsum application, including nil, and replicate each rare two or three times. if possible. apply the gypsum well before sowing (for annual crops and pastures) or before the start of the irrigation season (for irrigated perennial crops and pastures).
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