Normally only a small proportion of the whole growing medium is submitted for analysis and therefore it must be a representative sample of the whole. This is not easy because of the variability of growing media, particularly soils. It is recommended that each sample submitted for testing should be taken from an area no greater than 4 hectares (see Figure 21.8). The material sampled must itself be uniform and so only areas with the same characteristics and past history should be put in the same sample. Irrespective of the area involved, from small plot to 4 hectare field, at least 25 sub-samples should be taken by walking a zigzag path, avoiding the atypical areas such as headlands, wet spots, old paths, hedge lines, old manure heaps, etc. The same amount of soil should be taken from each layer to a depth of 150 mm. This is most easily achieved with a soil auger or tubular corer.
Peat bags should be sampled with a cheese-type corer by taking a core at an angle through the planting hole on the opposite side of the plant to the drip nozzle from each of 30 bags chosen from an area up to a maximum of 0.5 hectares. Discard the top 20 mm of each core and if necessary take more than 30 cores to make up a one litre sample for analysis. Samples should be submitted to analytical laboratories in clean containers capable of completely retaining the contents. They
Figure 21.8 Sampling growing media. Suitable tools to remove small quantities of growing media are corers or augers which have the advantage of removing equal quantities from the top and bottom of the sampled zone. The material to be sampled must be clearly identified, then 25 cores should be removed in a zigzag that avoids anything abnormal
Figure 21.8 Sampling growing media. Suitable tools to remove small quantities of growing media are corers or augers which have the advantage of removing equal quantities from the top and bottom of the sampled zone. The material to be sampled must be clearly identified, then 25 cores should be removed in a zigzag that avoids anything abnormal should be accompanied by name and address of supplier, the date of sampling and any useful background information. All samples must be clearly identified. Further details of sampling methods in greenhouses or orchards, bags, pots, straw bales, water, etc., are obtainable from the advisory services used. Remember, the result of the analysis can be no better than the extent to which the sample is representative of the whole
The soil solution is normally a weaker solution than the plant cell contents. In these circumstances plants readily take up water through their roots by osmosis. As more salt, such as soluble fertilizer, is added to the soil solution, salt concentrations are increased and less water, on balance, is taken up by roots. When salt concentrations are balanced as much water passes out of the roots as into them. When salt concentrations are greater in the soil the roots are plasmolyzed. The root hairs, then the roots, are 'scorched', i.e. irreversibly damaged, and the plant dries up.
Symptoms of high salt concentration above ground are related to the water stress created. Plants wilt more often and go brown at the leaf margin. Prolonged exposure to these conditions produces hard, brittle plants, often with a blue tinge. Eventually severe cases become desiccated.
Salt concentration levels are measured indirectly using the fact that the solution becomes a better conductor of electricity as salt concentration is increased. The conductivity of soil solution is measured with a conductivity meter (see ionic compounds, p373).
Salt concentration problems are most common where fertilizer salts accumulate, as in climates with no rainfall period to leach the soil and in protected culture. Periods when rainfall exceeds evaporation, as in the British Isles during winter, ensure that salts are washed out of the ground. Any plant can be damaged by applications of excess fertilizer. Some plants, such as tomatoes and celery, are more tolerant than others, but seedlings are very sensitive. Young roots can be scorched by the close proximity of fertilizer granules in the seedbed (see band placement).
In protected culture large quantities of fertilizer are used and residues can accumulate, particularly if application is not well adjusted to plant use. Sensitive plants, such as lettuce, are particularly at risk when following heavily-fed, more tolerant plants, such as tomatoes or celery. Salt concentration levels should be carefully monitored and feeding adjusted accordingly, applying water alone if necessary. Soils can be flooded with water between plantings to leach excess salts. Large quantities of water are needed, but should be applied so that the soil surface is not damaged. Every effort should be made to minimize the effect on the environment and quantities of water needed to flush out the excess salts by reducing the nutrient levels as the crop comes to an end.
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