«University of California Division of Agriculture and Natural Resources Committee of Experts on Dairy Manure Management September 2003 February 2004, ...»
The calcium and magnesium ions commonly found in the lagoon dairy wastewater are especially susceptible to precipitation reactions. The amount of net precipitation/dissolution and the chemical composition of the water leaching out of the root zone was estimated based on known chemical thermodynamics. To assess the geochemistry of soil water in manure applications, we used the chemical speciation computer program WATSUIT for Windows (http://envisci.ucr.edu/faculty/laowu/default.htm). The model estimates typical precipitation/dissolution rates and the chemical composition of root zone leachate given a typical composition of diluted manure water applied to a summer and also to a winter crop at the rates recommended in chapter 5. WATSUIT is a steady state model and considers only soil water constituents, but not soil chemical properties. Given those limitations, the program provides a good first approximation. In practice, transient conditions may play an important role and more complex, coupled models that simultaneously consider pore water flow dynamics, solute transport dynamics, and geochemical reactions may be needed to fully evaluate the dynamics of these geochemical processes.
Table 7-2 summarizes several generic examples of annual salt loading to groundwater for four typical irrigation scenarios in the San Joaquin Valley, adjusted for plant K uptake. These estimates were obtained from steady-state WATSUIT computations (see Appendix J for details).
The results suggest that the use of dairy wastewater, under proper nutrient management practices and with manure as the only source of fertilizer, increases the annual salt loading by 2500 to 3500 kg ha-1. The recharge water also is significantly higher in hardness and alkalinity than recharge water in commercially fertilized crops (without manure). Lime precipitation does not appear to play a significant role in reducing the salt loads to groundwater.
Table 7-2: Salt loading of dairy wastewater application fields after adjusting for K uptake.
7.3.3 Groundwater Loading Estimation using Groundwater Monitoring Data Harter et al. (2002) and Harter and Talozi (2004) measured groundwater salinity in shallow groundwater originating from recharge in corrals, manure storage lagoons, and fields receiving liquid manure. Soils on the tested dairies were sandy with low ion exchange capacity. Their results confirm that the average salinity downgradient from fields receiving liquid manure is approximately twice as high (1.6 mmho cm-1) as in groundwater upgradient from the investigated dairy facilities (0.8 mmho cm-1). Even higher salinity losses were found to occur underneath corrals and lagoons (2.3 mmho cm-1). However, a direct comparison of these study results to the above modeling estimates would not be appropriate. In the groundwater study, “upgradient” groundwater at the measured locations is recharged from commercially fertilized fields and orchards with unknown and variable crop production (and fertilization) rates. Also, few of the manured fields monitored for those studies had been subject to the type of sustainable nutrient management activities suggested in chapter 5. Actual loading rates on individual facilities and in individual fields will vary significantly depending on feed rations, manure and nutrient management, and irrigation practices. To some degree, they will also be a function of soil properties, particularly in soils with naturally higher salinity.
7.3.4 Estimating Groundwater Salt Loading from EC Measurements
The above methods provide estimates and case studies of average or actual salt loading to the field and to groundwater under conditions typical for dairies in the Central Valley of California.
For site-specific measurements of groundwater salt loading, it has been common practice in irrigation agriculture to measure the electrical conductivity (EC) of the irrigation water (ASCE, 1990). However, this simple method cannot be applied to estimate salt loading from dairy wastewater applications because the method does not account for nutrient uptake and has not been tested on liquid manures. There are currently no well-established methods (and little research is available) on how to measure or estimate the site-specific groundwater salt loading from applications of dairy manure. Based on our geochemical modeling results and other work reported above, we recommend that annual salt loading from individual fields be based on the mass of salts applied to the field minus the annual plant nutrient (N + K) uptake.
7.4 Agronomic Impacts of Salinity 7.4.1 Overview In addition to its cumulative effect on groundwater quality, salinity of water may impact its utility for irrigation through impacts on soil properties and on plant physiology. The major cations in typical irrigation waters are Ca2+, Mg2+ and Na+ and the major anions are Cl- and SO42Typically, the plants absorb only a small portion of the total dissolved mineral input from irrigation water, so the soil solution concentration of dissolved minerals increases as water is extracted from the root zone and transpired. If the salinity of the soil solution becomes too high, water uptake is reduced and plant growth is impaired as a consequence of the reduced osmotic potential. The negative impact of soil salinity on plant physiology is remedied by applying extra water to leach excessive salts from the root zone. The amount of leaching required depends on the crop tolerance to salinity and the salinity level in the irrigation water. Models are available to simulate the consequences of irrigation water salinity and irrigation water availability on plant growth and on the concentration of salts in the water moving below the root zone. In a long-term sustainable crop production system that does not use manure, it is commonly assumed that the total amount of salt leached below the root zone equals the total amount of salt applied in the irrigation water.
7.4.2 Crop Impacts
Dairy lagoon waters have high concentrations of NH4+ and K+. These carry electrical current as well as contribute to negative osmotic potential. However, unlike Ca2+, Mg2+, and Na+ that are only sparingly taken up by the crop, NH4+ and K+ are taken up in larger quantities over the course of the growing season. The uptake of these nutrients lowers the EC and reduces the effects of the negative osmotic potential. Indeed, if the application rate of NH4+ and K+ in lagoon water does not exceed the crop removal rate, they have essentially no salinity impact on crop growth or irrigation management by the end of the growing season.
The question remains as to the osmotic impact of NH4+ and K+ on plant response shortly after application and before much uptake by plants. The following reasoning is used to answer this question. The salinity of irrigation water without plant nutrients is typically used in experiments designed to establish the relationship between salinity and plant response. Nevertheless, plant nutrients are applied to the soil in these experiments to promote plant growth. In these experiments, the plant nutrients are often added in smaller increments relative to expected plant uptake so that the additional “salt effect” is minimized. Applications of fertilizer in the field cautiously consider placing fertilizer close to the seed for greatest early uptake but with low potential for “salt or high ammonia concentration” damage to the seedling. The “salt index” (WFH, 2002) is used along with the likelihood of ammonia toxicity from anhydrous ammonia (Colliver and Welch, 1970), di-ammonium or mono-ammonium phosphate (Alfred and Ohlrogge, 1964) or urea and other fertilizer salts (Cummins and Parks, 1961). Since the amount of starter fertilizer placed with or close to the seed is usually less than 200 lbs ac-1 (224 kg ha-1) and “safer fertilizers” are used, the possible deleterious salt effects are likely to be small. Higher rates of fertilizer are placed at deeper soil depths and farther away from the plant to avoid possible salt damage during early growth stages. The small additional osmotic contribution of the low rates of fertilizer placed at a safe distance from the seedling is not considered to significantly contribute in the experimental salinity evaluation. Because the NH4+ and K+ ions have both been demonstrated to be toxic to plant seedings, they can not be overlooked as part of the “salinity” of lagoon water and other manure applications. Yet, after these nutrients are taken up by the plant they no longer influence the salinity of the soil.
Analyses of dairy lagoon water sampled by Campbell-Mathews et al. (2001a) reveal that, on average, approximately 58% of the cation equivalents (measured in meq L-1) were NH4+ and K+.
Since both of these cations have been demonstrated to be among the more toxic to plants, particularly in the seedling and early growth stages, further research must be conducted to elucidate the relative salinity-specific ion effects. Also, a more complete evaluation of all the cations and anions making up the “salinity” in soil water following lagoon water applications is necessary. Certainly the more toxic chloride ion is often present in significant concentrations in lagoon water and other manure wastes but the other anion constituents may be considerably less toxic. Application of the undiluted lagoon water has been demonstrated to be injurious to many crops. For agronomic reasons, applications of undiluted lagoon water have therefore occurred either during fallow periods and incorporated into the soil after the soils have dried or by mixing and dilution with irrigation water. If lagoon water is diluted with irrigation water to match the N needs for crop nitrogen uptake, crop injuries are not likely to occur.
7.4.3 Salinity Impacts on Soil Physical Properties
Saline irrigation water and salt-affected soils may cause two undesirable effects on the receiving soils and growing plants. The total electrolyte concentration of the soil solution affects plant growth and the chemical composition potentially affects soil physical properties. Sodium and other monovalent cations cause dispersion of soil colloids; calcium, magnesium and other divalent cations tend to promote flocculation of soil colloids and stability of soil aggregates.
Therefore, the proportion of sodium (primary monovalent cations) vs. calcium and magnesium (primary divalent cations) in the soil solution will determine the physical state of soil particles.
Soil dispersion contributes to low water infiltration, soil compaction, and reduced aeration. The sodium adsorption ratio (SAR) of water is used as an index of the hazard of irrigation on
physical properties of the receiving soils. Mathematically, SAR is calculated as follows:
where [Na], [Ca], and [Mg] are equivalent concentrations of Na+, Ca2+ and Mg2+ expressed in meq l-1.
Although several factors can affect the relationship between SAR and soil physical properties, SAR = 15 is commonly considered as a reference point. Water with SAR values greater than 15 are considered hazardous and unsuitable for irrigation; water with SAR values less than 15 would be incrementally less hazardous in irrigation. Calculations on six waters analyzed by Mathews ranging in EC from 2.75 to 10.1 mmho cm-1 (dS m-1) resulted in SAR values ranging from 2.0 to 8.5 and the SAR tended to increase with an increase in EC. Use of these waters for irrigation would not be considered detrimental to soil physical conditions.
Establishing the hazard of lagoon waters on soil physical properties is non-trivial and does not follow established SAR guidelines. The NH4+ and K+ are monovalent cations prevalent in the dairy lagoon water. They will have less severe but notable adverse impacts on physical properties of the receiving soils. The usefulness of SAR values in the context of applying liquid dairy manure is therefore unknown. It may be argued that the impacts are transient in nature, because NH4+ and K+ are ultimately absorbed by plants and the adverse effect would disappear except for the acidification that occurs during the conversion of NH4+ to NO3- (Robbins et al., 1983, 1993, 1996; Robbins, 1984) However, clay dispersion and soil plugging are largely irreversible and can only be corrected by tillage to reform soil aggregates. The impacts of monovalent-dominated water on soil physical properties is not trivial, however transient, and further research is needed to address this issue with more confidence.
7.5 Production of Feed in the Central Valley: Regional Salt Balance Analysis From a regional salt cycling perspective, the production facilities of a dairy do not generate salts, they are merely part of a regional and national salt cycle. Salts are brought onto the dairy in form of imported feed stuff and exported via milk sales, manure exports, surface runoff, and groundwater leaching. What is the impact of dairies on the regional salt balance?
Feed rations for dairy animals consist of forage (legume and grass hays), plant by-products (almond hulls, beet pulp, citrus pulp, tomato pomace, etc.), grains and whole seeds (barley, corn, oats, wheat, cotton seeds, canola seeds, etc.), protein meals (canola meal, cottonseed meal, soybean meal, fish meal, blood meal, etc.), commercial supplements, mineral and vitamin ingredients, and miscellaneous ingredients. Forage, plant by-products, grains and whole seeds usually constitute 85% to 90% of the dry matter in the rations and contribute to the bulk of the dissolved minerals present in the excreted manure. For example, Ca2+, Mg2+, Na+, K+, Cl-, and SO42- typically are 0.85% to 1.45%, 0.27% to 0.42%, 0.15%, 1.8% to 3%, 0.34%, and 0.9% of the dry matter in alfalfa hays, respectively. More than 90% of the non-nitrogen related minerals in dairy feeds are excreted.