SOIL SOLUBLE SALTS - DETERMINATION OF AND

Transcription

1 GERALDSON: SOIL SOLUBLE SALTS 121 SOIL SOLUBLE SALTS - DETERMINATION OF AND ASSOCIATION WITH PLANT GROWTH C. M. GERALDSON Gulf Coast Experiment Station Bradenton Soluble salts in the soil are easily and ac curately measured. Many growers associate the soluble salt test only with a possible ex cess that could cause poor plant growth; how ever, because nutrient as well as non-nutrient salts are measured, the test can also be used as an indicator of fertility level. In other words, the soluble salt content of the soil can be used as a measure of the intensity factor of nutrition. Although the actual measure of soluble salts is a simple process, the interpretation in regard to plant response is of a more complex nature. Interdependent factors such as the nature and quantity of the salts, variations in soils and soil moisture, variations in the ef fective root zones of plants and variations in salt tolerance of plant species, cause the major difficulty in interpretation. In order to appraise the association of soil salts with plant growth, the soluble salt read ings must be determined or calculated for actual field moisture content. The soluble salts concentration varies inversely with mois ture content. The soil solution can be dis placed from soils, but the procedure is difficult and lengthy. The more practical methods in volve the adding of water to the soil in excess of field capacity and the determination of the salt content of that water extract. The salt content of the soil solution at field moisture level is calculated from that contained in the extract. Methods Used to Extract and Measure. Soluble Salt Content of Soils 1. A 1:2 soil to water ratio is an easy and perhaps the most widely used method for ex tracting salts from the soil. One part of soil is mixed with two parts of water and a Wheatstone bridge or a Solu-bridge can be used to determine the conductivity of the unfiltered mixture. The conductivity reading is a direct Florida Agricultural Experiment Station Journal Series No indicator of the total soluble salts contained in a solution. The limitations of this method should be clearly understood and will be in cluded in a later discussion. 2. The conductivity of the saturation ex tract (obtained by vacuum filtration of a soil that has been saturated by addition of water while stirring) is recommended as a more ac curate method for measuring soil salts when appraising the effect of soil soluble salts on plant growth. The special advantage of the saturation-extract method lies in the fact that the saturation percentage is directly related to the field moisture range. Measurements of soil moistures indicate that over a wide textural range the saturation percentage is about twice that at field capacity and four times the wilt ing percentage. Therefore the soluble salt at field capacity moisture would be twice that contained in the saturation extract. As the soil moisture decreased, the salt content would proportionately increase to a maximum (field capacity to wilting point) of about 4 times that contained in the saturation extract, For this reason the conductivity of the saturation ex tract can be used directly (except for muck and certain sandy soils) for appraising the effect of soil salts on plant growth. Regardless of whether the 1:2 or saturation extract method is used, the calculated con ductivity for an actual or given field moisture should be used for most accurately evaluating effects of soluble salts on plant growth. Discussion of Methods The 1:2 Method: One part of soil on a dry weight basis mixed with 2 parts of de-ionized or distilled water has been specified for the 1:2 extract (2). The ratio of the 1:2 extract moisture level (%) to the field moisture level is used to calculate the conductance at the field moisture level from the measured conductance of the 1:2 extract. For example, if the 1:2 extract of a sandy soil was found to have a measured conductance of mhos x 105/cm.*, theoretically the same soil at a field *Electrical conductivity (EC) is expressed in mhos/cm. Because most solutions have an EC that is much less than 1 mho, a sub-unit such as EC x 103 is used to give a more convenient location of the decimal point.

2 122 FLORIDA STATE HORTICULTURAL SOCIETY, 1957 moisture level of 10% would have a calcu lated conductance of 1000 mhos x 105/cm. ( x 20). A comparison of a number of theo retical conductances is presented in Table 1. It is obvious that the conductance varies in versely with the moisture level. The con- Table 1. Conroarisoti of Theoretical Conducttvtties (mhos x 10^/cm) of the 1:2 Extract, Saturated Extract and Field Moisture Soil Sand Sand Sand 1:2 Extract* %H?0 Conductivity (15)** (30) (37.5) Saturated Extract % H90 Conductivity Field Moisture 7. H2O Conductivity loam Clay loam Marl muck Muck, (45) (75) (90) (300) (4) *Hypothetically the 1:2 (weighed dry soil to water) extract for each soil has a given conductivity of mhos x 105/cm. The specific conductance can be correlated with plant growth only after it has been calculated for the field moisture of the specific soil. A field moisture conductivity of 600 mhos x l05/cm is the maximum at which even the least salt tolerant crops will maintain good growth. **Theoretical conductivity of the 1:2 extract if the field moisture conductivity is 600 mhos x 10 05/cm. FIG. I ASSOCIATION OF PLANT GROWTH WITH CONDUCTIVITY AS IT VARIES WITH SOIL MOISTURE OSMOTIC PRESS PPM SALT ELECTRICAL (ATMQS) (AS KCL) CONDUCTIVITY _ MHOS X io5 X 0018 MHOS X I05 X 7 EC* MHOS X IO9/CM SALT TOLERANCE VEGETABLE CROPS POTENTIAL CROP RESPONSE OAROEN BEETS KALE ASPARAGUS SPINACH LETTUCE SWEET CORN POTATOES CARROTS PEAS WHEN THE CALCULATED CONDUCTIVITY (FIELD MOISTURE LEVEL) FALLS OPPOSITE A SPECIFIC CROP THE POTENTIAL YIELD OF THAT CROP MAY BE REDUCEO AS MUCH AS SO PER CENT SQUASH CUCUMBERS IOO 1 INADEQUATE NUTRIENT INTENSITY FOR MANY CROPS - PER CENT SOIL MOISTURE

3 GERALDSON: SOIL SOLUBLE SALTS 123 ductance of the 1:2 extract correlates with plant growth only at a moisture level that is prevalent in the field. Separate correlations would be necessary for each different mois ture level of a given soil which in turn varies with the different soils. However, the calcu lated conductances given to specific field mois ture levels can be correlated with plant growth regardless of the soil being tested. The asso ciation of plant growth, conductivity and soil moisture is presented in Fig. 1. The salt tol erance of the given crops was obtained from the Handbook of the U. S. Salinity Laboratory (2). The effect of nutrient intensity levels on tomatoes, celery, pole beans and potatoes was studied at the Gulf Coast Experiment Station (1) and the salt tolerance of these Florida grown crops was found to be approximately the same as the tolerance indicated in the Handbook. In actual practice the 1:2 extract will often contain 5 to 20% more salt than the satura tion extract when both are calculated for field capacity moisture. This distinct disadvantage of the 1:2 method is the result of the larger amount of water dissolving more of the cal cium sulfate and the calcium and magnesium bicarbonates and carbonates contained in a soil. If a soil contains mainly chloride salts, the variation between extract methods will be small. Frequently, for convenience, the ratio 1:2 has been made on a volume basis (one volume of soil to 2 volumes of water) A dry sandy soil measured on a volume basis approaches a 1.6:2 ratio by weight. Another potential error is introduced if the soils are not dried before measuring. In either case, the assumed 1:2 ratio no longer has a specific moisture level (%) and the ratio between it and the field moisture level cannot be established; thus the conductance of the soil solution at the field moisture level cannot be calculated. For example, dry sandy soil measured on a volume basis 1:2 has a conductivity of 60 mhos x lovcm. If that same soil were meas ured 1:2 on a weight basis the reading might be about 35 to 40. Such an error would wrongly classify this soil as containing excess salts tor the more sensitive crops. As another example, a muck soil (mucks are difficult to dry) that still contains 100% moisture is measured on a weight basis, 1:2. Actually the resultant ratio is 1:5 (dry basis) and a con ductivity reading of would have read 0 if the soil had been dry when weighed. Thus such an error would wrongly classify this soil as containing no excess salts. The important point is that the introduction of such errors prevents calculation of the field moisture con ductivity with any reasonable degree of ac curacy. Table 2«Comparison of Determined Conductivities (mhos x lo of the 1:2 Extract, Saturation Extract and the Resultant Calculated Conductivity at Field Capacity Moisture Extract conductivity x factor = Field Capacity Conductivity 1:2 Soil Weight 73 x Basis* = C ,319 Volume 118 x 7 = Basis* : Saturated 420 x 2 = Muck Muck 570 x 2 0x1.3 = 1140 = x 4 = 220 x 3 = x 1O5 490 x 1.3 = 1 = 640 *The conversion factor was established by dividing the saturation extract conductivity (calculated for field capacity moisture) by the conductivity of the 1:2 extract. The given factor is an average of a number of soils included in a specific moisture hold ing capacity category. The determined conductivity of the extract x the appropriate factor equals the calculated conductivity at field capacity moisture.

4 124 FLORIDA STATE HORTICULTURAL SOCIETY, 1957 Even though the 1:2 extract method is not recommended for the more accurate determi nations, it can serve as a good estimate of the nutrient intensity level. In order to partially eliminate errors and confusing calculations, conversion factors can be established whereby the 1:2 extract conductivity (weight or vol ume basis) multiplied by an established factor will approximately equal the conductivity of the saturation extract calculated for field ca pacity moisture. The factor is established by dividing the saturation extract conductivity (calculated for field capacity) by the 1:2 ex tract conductivity and obtaining the average of a number of soils included in the same moisture holding capacity category. Results of a number of such comparisons are presented in Table 2. Testing many sandy soils (some included in Table 2) indicates that the 1:2 extract conductivity (weight basis x 11 and volume basis x 7) will approximately equal the saturation extract conductivity calculated for field capacity. The correlation of plant growth to conductivity can be completed by referring to Fig. 1. If the soil type varies, the conversion factors become less as the moisture holding capacity of the soil increases, as indicated in Table 2. Thus for each variation in soil moisture (2-5%) a new conversion factor should be established and this tends to be a limiting factor in the use of the method. However, when soils are consistently of one type and con version factors have been established, samples can be conveniently and quickly run, keeping in mind that the results are for the most part a rough estimate. In many cases such esti mates can be used to establish whether the nutrient intensity level is excessive, deficient or satisfactory. The Saturation Method: The saturation ex tract is obtained by vacuum filtration of a soil that has been saturated by addition of water (de-ionized or distilled) while stirring (2). The soil need not be dry nor must the soil and water be weighed or measured. The saturation point is the only critical measurement in this method. Water is added while stirring to the point where the soil surface is shiny, but no free water moves across the surface when tilted. It has been stated previously that over a wide textural range the soluble salt at field capacity rsoisture would be twice that con tained in the saturation extract. This same re lationship exists for sands that are adequately irrigated and have hardpans 18 to 24 inches below the surface. For non irrigated sands and those containing no hardpan, the field moisture may vary from & to % that contained in the saturation extract with salts 2 to 6 times that contained in the saturation extract. At the other extreme, mucks which may contain 100 to % moisture at field ca pacity do not contain twice that moisture when saturated. It has been found that the salt content of the saturated extract of mucks can be multiplied by 1.3 to obtain the approxi mate salt content at field capacity moisture. With sandy mucks this factor has been found to be about 1.5 (Table 1, 2). When using the saturation extract method, the field capacity moisture is for practical purposes (especially in irrigated soils) con sidered to be the field moisture level. How ever, to be most accurate, the actual field moisture should be determined. Association of Soil Soluble Salts with Plant Growth Quantity and Nature of Salts The conductivity of the soil solution can be directly correlated with plant growth (Fig. 1)*; The conductivity increases as the soluble salts in the soil solution are increased. However, because different salts at equivalent concentra tions have different conductivities, the conduc tivity of a soil solution can only be used as an estimate of the total soluble salts contained in the soil solution. For example, single salt solu tions at 1000 ppm concentrations have the following conductivities in mhos x 10ft/cm. at C: calcium sulfate 120, sodium sulfate 155, magnesium sulfate 133, cal cium chloride, sodium chloride 197, magnesium chloride 240, potassium *Use of Fig. 1: If the 1:2 extract (dry weight basis) of a sandy soil has an EC of mhos x 105 cm. (lower right at % moisture) the conductivity at a moisture level of 12.5% (Follow "" curve from % to 12.5% moisture) would be 800 mhos x 105 cm. and yields of low salt tolerant crops could be expected to be reduced. A reading of (1:2 extract) for an organic soil that might have a field moisture level of 100% could be considered as an inadequate nutrient intensity level (100 mhos x 105 cm. at 100% moisture). The saturation extract of the sandy soil would have a conductivity of and at field capacity moisture (half the saturation percent) would be 800. The saturation extract of the organic soil (approximately 1% moisture) would have a conductivity of 67 and at field capacity (saturation extract conductivity x 1.5) would be 100 mhos x 10" cm.

5 GERALDSON: SOIL SOLUBLE SALTS 1 chloride 158, calcium nitrate 132 and so dium bicarbonate 103 (1, 2). Potassium chloride (KC1) is generally used as a standard to estimate the average concentration or soil salts from conductivity readings. The con ductivity reading multiplied by 7 approxi mately equals the concentration of soluble salts contained in the solution expressed as ppm of KC1 (Fig. 1). The effect of the relative insolubilities of certain calcium and magnesium salts on the choice of an extracting method has already been discussed. However, this effect must always be considered when correlating con ductivity of the soil solution with plant growth. As the moisture content of a soil de creases the above mentioned salts become less soluble and the balance between nutrients changes. A calcium deficiency in the plant can result from a low calcium ratio in the soil solution (1). This same effect (a deficiency of calcium) can occur when excess concen trations of salts result from decreasing mois ture levels, or when additions of salts produce the excess. In some cases it may be difficult to distinguish whether excess salt (osmotic ef fect) or a calcium deficiency is the dominant factor affecting plant growth. Excesses of so dium or chloride or other specific cations or anions also tend to complicate high intensity evaluations. At the other extreme deficiencies tend to complicate low intensity evaluations especially, but deficiencies can also occur within any intensity range. Application of Fertilizers and Irrigation Water In evaluating the methods of extraction the inverse relationship between soil moisture and salt content has been discussed (Fig. 1 and Table 1). The total quantity of soil moisture contained by a soil must also be considered. An acre of sandy soil (6" depth) weighs ap proximately 2,000,000 pounds (dry basis). If such a soil contained 12.5% moisture, the weight of the soil moisture would be 2,000 pounds. A similar amount of muck soil weighs approximately 0,000 pounds and if it con tained 1% moisture, the weight of that moisture would be 7,000 pounds. A ton of average fertilizer ( organics) applied to the acre 6 inches would supply approxi mately 0 pounds of soluble salts. This amount of salt added to the sandy soil mois ture would increase the soluble salt content about 0 ppm. The conductivity of the soil solution at field capacity would increase about 300 mhos x 105/cm. The same amount of fertilizer added to muck soil moisture would increase the salt content 667 ppm and the corresponding conductivity 100 mhos x 105/ cm. If the fertilizer were applied to a bed or in bands instead of broadcast as above, these same readings would be increased proportion ately in the area of application. Therefore the effect on plant growth is dependent on the quantity and nature of the fertilizer applied, the placement of that fertilizer and the type of soil (specifically the moisture holding ca pacity) receiving it. An acre inch of irrigation water weighs ap proximately 2,000 pounds. One inch of ir rigation water containing 1000 ppm salt would add approximately 230 pounds of salt to the soil solution or about half as much as the ton of fertilizer. Soluble salts are also released to the soil solution from organic matter depending on the amount and rate of break down of the or ganic material in the soil. Soluble salts are removed from the soil solution mainly by plant uptake, soil utiliza tion (microorganisms and fixation) and leach ing. The soluble salt content of the soil solu tion is an index of the current status of this ever changing entity. Effective Root Zone The ideal situation is to supply sufficient nutrients and moisture to the effective plant root zone without building up excesses or ab normal balances in that same root zone. It is very seldom that the salt concentration is uni form in a given soil even when limited to the effective root zone. Even equivalent readings at two depths would not have the same effect on plant growth because soil moisture usually increases with depth. During periods of dry weather, especially with seep or sub irrigation, the salts tend to move toward the soil surface and to accumu late in the top of the bed. This area also con tains the least moisture and the effective root zone is usually at a lower depth. If a soil sample were taken of the top 6 inches, it might give a false indication of the salt-plant

6 126 FLORIDA STATE HORTICULTURAL SOCIETY, 1957 relationship. A deeper soil sample might in dicate a deficiency in the effective root zone. During periods of wet weather, the salts are leached downward and soil samples might in dicate low salt concentrations at the higher levels and increasing with depth. If the ef fective root system is also deep, the nutrition may be adequate. There is also the possi bility of an upward movement of the leached salts during a dry period following the wet. Rain or overhead irrigation tends to move the nutrients through a greater soil profile depth and salt concentrations at all depths may be moderate to low. However, if the moisture is not excessive, more total nutrients are avail able to the plant and better growth and yields are obtained than when almost all of the fertilizer is found in the top 6 inches or even closer to the surface. In other words, the final appraisal must be based on maintenance of an adequate salt concentration (nutrient intensity) within the effective root zone. Also, within an intensity range, nutrient balances for optimum yields and quality must be maintained. Conductivity as an indicator of total salts in the soil solu tion does not measure nutrient balances. How ever, determination of specific cations and anions contained in the soil solution can be used to study the association* between nutrient balance and plant responses. Summary The total soluble salts contained in the soil solution can be used as indication of whether the nutrient intensity level is deficient, ade quate or excessive. The 1:2 and saturation extract methods for obtaining a salt extract that will best represent the actual soil solution salt are ex plained and discussed. The saturation extract method is recommended as a general method for appraising the effect of soil soluble salts on plant growth. The soluble salt concentrations as indicated by electrical conductivity values when calcu lated for field moisture levels, which may vary from 0 to %, are graphically associated with the growth of a number of vegetable crops. Difficulty in the interpretation because of the quantity and nature of the salts and vari ations in soil moisture, effective root zones of plants and salt tolerance of plant species are also discussed. The final appraisal is based on maintain ing an adequate nutrient intensity within the effective root zone. LITERATURE CITED 1. Geraldson, C. AA. Soil solution soluble salts as an indication of fertility level and nutrient balance. Soil Sci. Soc. Fla. Proc. 15: Richards, L. A. et. al. Diagnosis and inprovement of saline and alkali soils. Argi. Handbook No. 60. USDA IRRIGATION EXPERIMENTS WITH TOMATOES ON A ROCKDALE SOIL John L. Malcolm and Roy W. Harkness Sub-Tropical Experiment Station Homestead With the advent of large scale rockland farming, irrigation has become one of the im portant cultural practices in growing toma toes in Dade County. Although field response has been observed, the variation in practice indicates that there is insufficient information on the optimum use of water. The dissimilarity of growing conditions in Dade County and elsewhere in regard to season, climate, and Florida Agricultural Experiment Station Journal Series No soil makes it impossible to apply the con clusions reached elsewhere either to practical farming or to the research program. Review of Literature There has been no previous research on irrigation of vegetable crops on Rockdale soils. The relationship between soil moisture and the growth of plants has been the object of many studies in other areas. The interpre tation of the results of this research may be conveniently divided into two categories. One group of investigators concluded that water was equally available from the field capacity to the permanent wilting percentage. The other concluded that moisture availability