de julio de 1659 años D. Julián de Balmaseda, Teniente de Cura bautizó y puso

The use of an extract from a saturated paste is advantageous for characterizing saline soils since it closely approximates salinity in relation to plant growth. One can also obtain soluble cations and anions by this method and estimate other important parameters such as Sodium Adsorption Ratio (SAR) which, in turn, predicts Exchangeable Sodium Percentage (ESP). Criteria for B toxicity tolerance by various plant species have been also developed for such an extract (see Appendix 15).Thus; a saturation extract is routinely used where salinity is a concern. The cations analyzed in saturation extracts are Ca, Mg, K, and Na, while the anions are SO4 (sulfate), CO3 (carbonate), HCO3

(bicarbonate) and Cl (chloride). Boron (B) in saturation extracts is often measured where its toxicity is suspected.

Apparatus

Porcelain dishes

Spatulas or mixing spoons Vacuum filtration system

Procedure

1. Weigh 200-300 g air-dry soil (< 2-mm) into a porcelain dish.

2. Slowly add DI water, and mix with a spatula until the soil paste:

- Does not have free-standing water on the surface of the paste.

- Slides freely and cleanly off a spatula (does not apply to clay soils, > 40% clay).

- Flows slightly when the container is tipped to a 45 ^o from horizontal.

- Soil surface glistens as it reflects light.

- Consolidates easily by tapping after a trench is formed in the paste with the flat side of a spatula (does not apply to sandy soils, >70% sand).

3. Allow the paste to stand for 1 hour, and then re-check the criteria for saturation by adding more DI water or soil, as needed.

4. Leave the paste for 6 to 16 hours and then filter with a vacuum filtration system using a Buchner funnel fitted with Whatman No. 42 filter paper.

5. Collect filtrate in a small bottle and keep it for subsequent measurements. If the initial filtrate is turbid, re-filter.

Technical Remarks

1. Do not stir soil until the entire soil mass is wet.

2. After first wetting, the paste usually stiffens and loses its shine on standing. Adding water and re-mixing gives a mixture that retains the characteristic of a saturated paste.

3. Gypsum soil samples should not be oven-dried above 70 ^oC, prior to extracting for soluble salts, because heating to 105 °C partially converts gypsum (CaSO4.2H2O) to partially hydrated gypsum (CaSO4.½H2O), which is more soluble in water than the fully hydrated gypsum.

4. For organic and peat soils (>16 % OM) it is advisable to start with a 150 mL water and slowly add soil material and water. Soak overnight wetting to obtain a definite end point for the saturated paste. Some high organic matter soils swell considerably upon addition of water. In such cases, the method must be repeated until the saturated paste characteristics are stable.

5. Fine textured soils (> 40% clay) may puddle easily. To minimize puddle and obtain more definite endpointfor the saturated paste, water should be added with a minimum amount of stirring, especially in the early stages of wetting. The method must be repeated until the saturated paste characteristics are stable.

6. For some soil types (coarse textured soils, sandy loam and loamy sand with less than 15%

clay), may not exhibit saturated paste characteristics of finer textured soils. Therefore, the relative accuracy of the method declines and should be noted when making soil comparisons.

7. For salinity appraisal, the saturated paste can be extracted after 4 hours.

8. For sodic soil samples, it should stand 16 or more hours.

9. For the soluble B, it should stand 24 hours or more (saturated paste equilibration is required).

10. For CaCO3 if precipitates are noted in the extract, dilute paste extract 1:1 with DI water and note dilution in subsequent analysis. The paste extract may be refrigerated (4 ^oC) for storage (do not allow to freeze) more than 30 days by adding small quantities (2-mL) of toluene to minimize microbial activity.

11. Approximately one quarter to one third of the water added in making the saturated paste can be recovered as extract.

12. Because KCl from the pH electrode can contaminate the paste, it is recommended that a separate paste should be made for EC determination. If, however, there are not enough samples for two separate pastes, then the same paste can be used for both analyses; ensure that the pH electrode is not left in the paste unnecessarily.

5.1.1. Saturated Paste Percentage

Soil moisture, as determined on a separate sample, is taken into consideration by adding it to the amount of water used in preparing the saturation paste. Report the saturation percentage (SP) to the nearest 0.1 %. The SP is calculated by dividing the total amount of water added (mL) by the oven-dry weight of the soil (g) and multiplying by 100.

Procedure

1. Weigh a 30-50 g sub-sample of the paste into a dried and weighed metal can with lid.

2. Dry in an oven, with the lid unfitted, at 105 °C for 4 hours.

3. Remove from oven, fit the lid, and cool in a desiccator for at least 30 minutes.

4. Re-weigh and calculate SP.

Note: Oven-dry moisture values will be slightly higher than the direct method as air-dry soil will retain 3-5 % moisture, dependent on clay and salt content.

Calculations

Or

Technical Remarks

1. The SP is directly related to field moisture range (e.g., PWP is about one fourth of SP, and FC is about one half of SP).

2. Oven-dry moisture values will be slightly higher than the direct method as air-dry soil will retain 3-5 % moisture, dependent on clay and salt content.

3. For mineral soils, about twice of the FC is generally the amount of water required to obtain SP.

4. The SP is related to soil texture (based on OM contents less than 3%), as follows: sand or loamy sand (<20); sandy loam (20-35); loam or silt loam (35-50); clay loam (50-65); clay (65-135); and organic soils (> 81).

5. For fine-textured soils and those high in Na (SAR > 10), the SP cannot be used to estimate FC and PWP values.