USER’S GUIDE
1. FIELD ASSESSMENT AND SAMPLING STRATEGIES 1 Soil Survey 1 Soil Survey
1.3 Field Assessment
1.3.1 Salinity, Sodicity, and pH
1.3.1.1 Saline Soils 1.3.1.2 Sodic Soils 1.3.1.3 High pH Soils
1.3.1.4 Interactions, Salinity, Sodicity, and High pH 1.3.1.5 Sampling for Salinity, Sodicity, and High pH
After Gupta and Arbol (1990) and Pearson and Waskom (2003)
Salinity, High pH, Specific Ion Effects, and Sodicity: Symptoms of salinity, high pH, specific ion effects, and sodicity are frequently confused (Pearson and Waskom, 2007). All these conditions can have adverse effects on plant growth, differing significantly in their cause and relative impact. Effective management of these problems varies considerably and requires proper diagnosis if the problem is to be successfully addressed (Pearson and Waskom, 2007). While field assessments can help diagnose these problems, the analyses of soil and water samples complement these assessments and are critical to the accurate
diagnosis and correction of the problems. The field assessment techniques described herein and the analytical procedures described in Section 4.6 of this manual that address questions of salinity are convention based and provide only
point data. Depending on the nature of the condition, soil salinity may be too variable and transient to be appraised using the number of samples that can be practically processed by conventional soil sampling and analysis procedures.
Alternative procedures include the use of more rapid field-measurement technology, e.g., electromagnetic induction (EMI) or ground penetrating radar (GPR), consisting of mobile instrumental techniques for measuring bulk electrical conductivity (EC) directly in the field as a function of spatial location on the
landscape (Rhoades et al., 1999). Refer to Corwin and Lesch (2005) and USDA (2007) for discussion of appropriate equipment and protocols in using these field- scale soil salinity measurement techniques. Refer to Section 4.6 of this manual for a more detailed discussion of the chemical properties and estimates (e.g., EC, sodium adsorption ratio, exchangeable sodium, and pH) related to these types of soils.
Saline Soils: Salinity is a measure of soluble salts in the soil. A saline soil has, at the soil surface and/or in the soil profile, an accumulation of free salts that affect plant growth and/or land use (Isbell, 2002). Salinity is generally attributed to changes in land use or natural changes in drainage or climate that affect the movement of water through the landscape. Field observations are also useful indicators of salinity. Saline soils and plants grown on these soils may exhibit one or more of the following visual symptoms (Gupta and Arbol, 1990; Pearson and Waskom, 2007):
• Inhibited seed germination and irregular seedling emergence
• Symptoms of water stress even when soil is wet
• Fluffy appearence of soil surface
• Visible whitish salt crusts on soil surface
• Plants with leaf-tip burn, especially on young foliage, under sprinkler irrigation with saline water
Sodic Soils: Sodicity is a measure of exchangeable sodium in relation to other exchangeable cations, expressed as exchangeable sodium percentage (ESP). A sodic soil contains sufficient exchangeable sodium to interfere with plant growth. Field observations are also useful indicators of sodicity. Sodic soils and plants grown on these soils may exhibit one or more of the following visual symptoms (Gupta and Arbol, 1990; Pearson and Waskom, 2007):
• Cultivation problems related to (1) optimum soil water not uniform across field, with some areas wet and other dry; and (2) surface left cloddy, resulting in poor germination and variable crop stands
• Poor seedling emergence related to soil dispersion and crusting
• Stunted plants, often showing scorching and leaf-margin burn progressing inward between veins
• Shallow rooting depth
• Symptoms of water stress after irrigation or rainfall
• Variations in plant height across the field or yield variations upon harvest
• Dark, powdery residue on soil surface related to dispersed organic matter
• Soapy feel to soil upon wetting for texturing
• Poor drainage, crusting, or hardsetting
• Low infiltration rates; runoff and erosion
• Periodic stagnated water with cloudy appearance in low microrelief
• Soil wetness associated with only upper limits of soil; lower limits almost dry and hard in wetting cycle
• Upon drying, extereme hardening of soils and development of cracks, which vary in width and depth and close upon wetting.
• Dense hard subsoil with variable color; lime nodules possibly present
• Subsoil exposed or near to surface due to leveling or erosion
• Coarse structure (<20 mm), prismatic or columnar subsoil structure High pH Soils: High pH soils may not necessarily appear any different from soils with neutral pH. Problems typically appear as nutrient deficiencies if pH
>7.8. Plant symptoms can be useful indicators of sensitivity to high pH soils.
Soils with high pH and plants grown on these soils may exhibit one or more of the following visual symptoms (Gupta and Arbol, 1990; Pearson and Waskom, 2007):
• Powdery substance on soil surface
• Evidence of plant nutrient deficiencies, e.g., reduced availability of Zn, Fe, P, and B as follows: (1) yellow stripes on middle to upper leaves (Zn and Fe deficiency); and (2) dark green or purple coloring of lower leaves and stems (P deficiency)
Interactions, Salinity, Sodicity, and High pH: In general, a soil with sodic and saline properties exhibits the same symptoms as a saline soil. A soil exposed to high sodium and high salinity can remain permeable because the clays are flocculated, whereas soils with high sodium and low salinity can be characterized by greater dispersion and less permeability (Graaff and Patterson, 2001). Clays with a given sodicity are more dispersible with a high pH than with a low pH (McBride, 1994).
Sampling for Salinity, Sodicity, and High pH: In general, there are two primary objectives of sampling for salinity or sodicity: (1) to establish an average salinity level of the active root zone upon which crop thresholds are based; and (2) to manage suspected problem zones. Some general rules of thumb are as follows:
• Because high pH, salt, and sodium levels are rarely uniformly distributed across the field, map and sample suspected problem areas separately to fully understand the nature and severity of problems (Pearson and
Waskom, 2007).
• Sampling depths may vary, depending on crop type and nature of condition. To obtain a comprehensive diagnosis and evaluation of both the surface soil and subsoil, sample sequentially in 25-cm increments to a depth of 150 cm.
• If soil dispersion or slaking tests are to be conducted, collect
representative undisturbed samples from a soil core or spade sample as opposed to an auger sample. If a spade is used, dig a V-shaped hole, then cut a thin slice of soil from one side of the hole. These samples can also be used to describe important soil physical properties, e.g., structure, color, and consistence.
1.3 Field Assessment
1.3.2 Soil Fertility and Plant Nutrition
1.3.2.1 Soil Sampling as Basis for Fertilizer Applications 1.3.2.2 Plant Analysis as Basis for Fertilizer Applications
1.3.2.3 Remote Sensing for Crop Nitrogen Status and Plant Biomass After Mathers (2001) and Ryan, Estefan, and Rashid (2001)
Soil Fertility: Soil fertility is the status of a soil with respect to the amount and availability to plants of elements necessary for plant growth and is
particularly important in irrigated soils when nutrients would otherwise be leached out of the root zone (Soil Science Society of America, 2008). In general, there are five methods to detect mineral deficiencies (Mathers, 2001), as follows:
• Visual symptoms
• Plant tissue analysis
• Soil analysis
• Biological testing fertilizer trials
• Irrigation water analysis
Plant tissue analysis can be used to diagnose suspected mineral deficiencies and as a check on a fertilizer program. Tissue and soil analyses should be conducted together and do not stand alone. Fertilizer trials are not covered in this manual. In general, when using visual symptoms to assess mineral deficiencies (Mathers, 2001) consider the following:
• Adjust pH to correct some micronutrient deficiencies (e.g., Fe, Zn, B, Cu).
Other deficiencies are inherent to the soil and require fertilizer applications.
• Mineral deficiencies most likely develop early in the plant growth cycle.
Mild deficiencies are often difficult to detect as effects are chronic and not catastrophic.
• Leaves and stems are particularly sensitive to deficiencies. Leaves tend to be small and are characterized by loss of green color and chlorosis and sometimes by dead areas at tips and margins and between veins.
• Other conditions (water stress, impermeable or hardsetting soils, high salts, plant genetic factors and diseases, excess fertilizer, etc.) complicate the use of visual symptoms to diagnose deficiencies.
• It is nearly impossible to detect a particular deficiency if multiple deficiencies exist.
• Use of visual symptoms to diagnose a particular deficiency is best suited when used in conjunction with other methods of detection.
Soil Sampling as Basis for Fertilizer Applications: The procedures for interpreting soil test indices are to use data from long-term experiments and to conduct field calibration studies by growing crops in fields with a predetermined soil test value (Iowa State University Extension, 2003). When soil tests have been conducted many times at numerous locations to account for climatic and soil variation, a basis exists for reasonable interpretation of these tests.
Interpretations account for profitability as well as probability and magnitude of agronomic responses (Iowa State University Extension, 2003). Refer to Peck et al. (1977) for detailed description of the methodology of soil testing and the correlation and interpretation of analytical results.
Soil tests as a basis for fertilizer recommendations normally assume a weight/area ratio of soil from a specified depth. In the U.S. this has been traditionally based on 2 million lb/acre from a depth of 0 to 6 inches. Typically, this weight per unit volume (bulk density) assumes a medium soil texture with some compaction routinely incurred from cropping and harvesting. Variations in bulk density can make a difference of 10 percent in soil test results (Franzen and Cihacek, 1998). Consistency in soil techniques is important because of
differences in temporal properties, such as bulk density, especially in surface materials. Some general soil sampling recommendations (Ryan et al., 2001) are as follows:
• Fewer samples may be needed when little or no fertilizer has been used.
• More samples are typically needed when fertility varies in relation to broadcasting of fertilizers and/or cropping-livestock systems.
• Fertilizer banding poses problems for reliable sampling. Sample from and between areas that have received band applications.
• Avoid sampling directly after fertilizer or amendment applications.
• Sample at same time each year for comparative purposes.
• Sampling during crop growth provides information on soil nutrient status.
• Sampling depth depends mainly on the nutrient of interest, the crop to be fertilized, and the management system (e.g., tillage, irrigation) (Franzen and Cihacek, 1998).
• Sample to a 20-cm depth as plant available P, NO3--N, and micronutrients in such samples are related to crop growth and nutrient uptake (Ryan et al., 2001).
• Sample to 60- to 100-cm depth if in irrigated areas and monitoring NO3--N leaching (Ryan et al., 2001). Deeper sampling for NO3--N may be
appropriate for some crops, e.g., sugar beets and sunflowers. Deeper sampling is not performed to improve quality but is related to potential cost saving on fertilizers. Values of soil nitrate-N can be highly variable
throughout a field.
• Collect depth-wise samples when B-toxicity is suspected.
Plant Analysis as Basis for Fertilizer Applications: Plant tissue analysis is a rapid, simple, semiquantitative estimate of nutrient concentration (N, P, K, and trace elements) of the plant cell sap and can be used as an indicator of nutrient supply at the time of testing while the plant is in the field. In general, the
conductive tissue of the latest mature leaf is a good indicator of tissue N
concentration. As the time of day affects this concentration, collecting samples in the morning can reduce variability. If a plant is discolored or stunted and plant tissue shows high N, P, or K content, some other factor is limiting growth and further diagnostic tests are needed to identify the factor(s). Fresh material should be collected from both the normal and abnormal plants for comparative purposes.
Plant nutrient status can also be assessed in a nondestructive manner using chlorophyll meters. The meter is placed on leaf surface, and the amount of light (650 nm) transmitted through the leaf is measured. Increasing chlorophyll content results in decreasing light transmittance. Chlorophyll readings from nutrient-deficient leaves are compared to readings from reference plants in which nutrients are not limiting. The primary advantage of this method is the detection of nutrient stress before deficiency symptoms are visible. Leaf chlorophyll content can be interpreted directly for N, S, and K deficiencies. Chlorophyll readings generally decrease with plant maturity.
Remote Sensing for Crop Nitrogen Status and Plant Biomass: A more sophisticated technique, and one not covered in this manual, is the use of remote sensing for crop-N status and plant biomass. Visible and near-infrared sensors are commonly used to detect plant stress related to nutrients, water, and pests.
When light energy (green, blue, red, and near-infrared wavelengths) strikes a leaf surface, the blue and red wavelengths are absorbed by chlorophyll, whereas the green and near-infrared wave lengths are reflected. Reflected light is monitored by an optical sensor. Contrast of light reflectance and absorption by leaves enables assessment of quantity and quality of vegetation. Chlorotic, nutrient- stressed leaves absorb less light energy.