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Sampling Pumice or Sawdust Bags

1. Collect a minimum sample of one core from each of 20 bags for either your own CF and pH checks or for analysis by the laboratory.

2. Use a length of 25 mm stainless steel pipe for collecting the samples. Pumice abrades other metals sufficiently to contaminate the sample and this gives erroneous results for trace elements.

3. Take sample cores from the full depth of the bag, midway between the centre and the side of the bag.

4. For our clients write your name and the sample name with a felt tip marker on the printed Hill Laboratory bag and put all the cores for the laboratory sample in the bag. Send a Hill Laboratory Request form with the sample. Complete the form with your name and address, and charge to my account. Write the sample name in the box headed others and write pumice or sawdust under sample type and N2 under Analysis Details.

Send the sample with the laboratory request form to Hill Laboratories Ltd, 1 Clyde Street, Hamilton. Couriers usually provide a overnight service from any where in the North Island, and prepaid courier bags are cheap. An E20 bag will hold 2 pumice or sawdust sample bags.

Complete our advice note with details of your sample and fax it or send by fast post or email to us on the same day that you send the sample. Do not send our advice note to the laboratory, they are not interested in the sample details.

5. For checking your own samples, collect the minimum of 20 cores in a clean plastic bucket and then make a 1 volume of media: 1.5 volumes of water extract. Thoroughly mix the sample and then sub sample any convenient volume, say 100 mls. Place the 100 ml sample in a suitable container of about 300-400 ml capacity and add 150 ml water. Shake the sample with the water for at least 2 minutes, and then allow to settle. Measure the CF of the water. The CF may continue to rise for some time after shaking so allow the sample to stand and check the CF again later. Some pumice samples may need to stand overnight before obtaining the maximum CF reading.

6. If a pH check is required the 1:1.5 volume sample extract must be made with distilled or deionised water. The pH and CF can then be measured on this extract. pH checks of extracts made with tap water only show the pH of the tap water.

7. Pumice and sawdust media used have a water holding capacity between about 32 and 45% by volume, with a mean of about 37.5% water holding capacity. Thus all the nutrients in 100 ml of sample were originally dissolved in 37.5 ml of solution. The CF measured is due to the nutrients dissolved in this 37.5 ml plus the added 150 ml of water or a total of 187.5 ml. Thus the nutrients have been diluted 5 times, and the CF of the media in the bags is 5 times the CF of the 1:1.5 water extract. A reading of 8 CF in the 1:1.5 water extract gives an estimated CF in the bags of 40.

8. Weekly CF checks are essential for bag crops. Keep records of your weekly checks. A steadily rising CF indicates that too much feed or too little drainage is being given, and if this is the case either the feed CF must be reduced or the amount of drainage must be increased. Since increased drainage results in waste of fertiliser and additional costs to the grower, it is usually more profitable to reduce feed CF in this case. Conversely a falling CF indicates either too much drainage or too low a feed CF.

9. All growers should be monitoring the volume and CF of feed applied every day, and should be monitoring the volume of drainage. Many growers also check the CF of the drain water. This can be a useful check, but it can vary considerably from day to day and from bag to bag, and generally more reliable management decisions can be made by weekly core sampling and measurement of CF of the 1:1.5 water extract.