P. J. Shouse

Effect of high boron application on boron content and growth of melons

Management options for reducing drainage water volumes on the west side of the San Joaquin Valley of California, such as reuse of saline drainage water and water table control, have the potential to adversely impact crop yields due to a build up in soil solution boron concentration. An earlier experiment had shown that extrapolation of B soil tests to field conditions provided poor predictability of B content of melons despite statistically significant relationships. Consequently, three tests for extractable soil B were evaluated for their ability to predict conditions of potential B toxicity in melons grown under controlled conditions. Melons were grown for 95 days in two consecutive years in containers of Lillis soil (very-fine, smectitic, thermic Halic Haploxerert) that had been pretreated with solutions containing B concentrations as great as 5.3 mmol L−1. Extractable soil B was determined using ammonium acetate, DTPA-sorbitol, and a 1:1 aqueous soil extract at the beginning and end of the experiment. The B treatments caused various deleterious effects on melon growth and development. Fresh and dry plant matter decreased significantly with increasing B concentrations, while B concentration of plant leaves, stems, and fruits increased significantly with increasing B. The number of days to first flowering was significantly delayed from 35 days at B treatments < 2 mmol L−1 to 51 days at B treatments > 3 mmol L−1. Fruit set was completely inhibited at the highest B treatment of 5.3 mmol L−1. Plant analysis revealed a highly significant relationship between soil extract B obtained with all three extractants and leaf, stem, and fruit B content. Correlation coefficients for plant stems and fruits were much higher than for plant leaves. Correlation coefficients for all soil tests were almost equivalent, although the highest values were obtained for the DTPA-sorbitol extract indicating the greatest predictive capability. The soil tests were well able to predict B damage to melons in a container experiment.

Soil boron extractions as indicators of boron content of field-grown crops

Determining the relationship between soil B and crop B content can help predict when crops will respond to B fertilizer and when B toxicity may be expected. Such a relationship can then be used to make fertilizer recommendations or to flag conditions of potential B toxicity. Soil samples were obtained from 65 sites located in the Broadview Water District in the San Joaquin Valley of California. A diverse set of extractants was evaluated including: hot water-soluble, 1:1 soil:distilled water and 1:2 soil:distilled water, ammonium acetate, calcium chloride-mannitol, and DTPA-sorbitol extracts. Soil extract B values were correlated significantly with various B reactive soil constituents, including aluminum and iron oxide, clay, organic matter, and calcium carbonate content. The 1:1 water extract B was highly significantly correlated (99% level) with other measures of extractable B used in the study. Extractants were compared on soil samples collected from six depths at 65 field sites in the San Joaquin Valley of California that were cropped to alfalfa, melons, and cotton. Boron concentrations of whole plants and composites of 10 leaves were determined. Plant sampling occurred at the time of soil sampling for the alfalfa. Cotton and melons were sampled at flowering and prior to fruit set, the recommended growth stages, respectively, for tissue sampling, and 6 weeks thereafter. Five weeks later the cotton was sampled a third time. Significant correlations (95% level) between extractable soil B and plant B were found for melons and cotton but not for alfalfa. Correlation coefficients for the ammonium acetate, DTPA-sorbitol, and 1:1 water extract were not statistically significantly different (95% level). Although significant correlations (95% level) were obtained, the equations provided relatively poor predictive capability. These results illustrate the difficulty of predicting plant B content based on soil B analyses from a single soil sampling.

EFFECTS OF WATER STRESS AT VARIOUS GROWTH STAGES ON SEED YIELD AND NUTRIENT CONCENTRATIONS OF FIELD-GROWN COWPEAS

In 1976 and 1977, we conducted experiments at the University of California Agricultural Experiment Station at Riverside to evaluate drought effects on cowpeas during the vegetative, flowering, and pod-filling growth stages. Particular emphasis was given to drought effects on yield of seeds and nutrient concentrations in the seeds. Water stress during flowering and pod-filling reduced seed yield by 44 and 29 percent, respectively, when compared with yield from the control treatment. Water stress during the vegetative stage showed no significant effect on seed production by comparison with the control. The most severe seed yield reduction was found in treatment plots subjected to water stress during both flowering and pod-filling stages, which reduced seed yield by 67 percent. The differential growth-stage irrigation treatments significantly affected concentrations of nitrogen, calcium, magnesium, copper, boron, and iron in the seeds. The concentration of N in the seeds was inversely proportional to the dry-weight seed yield. There were several significant interactions between irrigation treatments and years on seed yields and concentrations of N, P, K, Ca, Mg, Zn, Mn, and Fe in the seeds.

Boron transport within an agricultural field: Uniform flow versus mobile-immobile water model simulations

The transport of boron in soil is important to agriculture because boron concentrations in soil water are beneficial to plants only over a limited range (0.37 to 1.39 mmol L−1 for tolerant crops). Irrigation water in the San Joaquin Valley, California, commonly has elevated B concentrations, and soil water B can reach phytotoxic levels as a result of the concentrating effects of evapotranspiration. Because the constant capacitance model was successful in computing B speciation in soil water and on mineral surfaces, it was incorporated into a multicomponent solute transport code, and a 2-year field test of the model was performed for 43 sites within a 65-ha field in the San Joaquin Valley. The model predicted the adsorbed B (XOB(OH)3−) concentration successfully with a median scaled root mean square error (SRMSE) of 11% for 43 sites. The median SRMSE was 36% for prediction of total B and 46% for solution B. The higher SRMSE for solution B may be caused by lack of detail in specifying the lower boundary condition. A steady increase in SRMSE from east to west in the field, the same trend as the seven tile drains, suggests an unknown E-W systematic variation in the lower boundary condition. A mobile-immobile water transport model failed to exhibit significant improvement over the standard uniform flow model (UFM) and, thus, the simpler UFM was preferred. The change in total B mass at all sites generated was accurately predicted with a relative error of only 4.1%. This work has potential practical application in the study of the effect of water management practices on soil B.

Vacuum method for field installation of pipes and casings in sandy soils

Soil moisture-monitoring equipment is difficult to install in poorly consolidated sand or sediments using hand tools because the loose material tends to collapse. The technique described herein uses a 5.5-hp wet/dry vacuum cleaner, powered by a portable gasoline generator, to remove the soil while an operator pushes a conductor pipe or casing into the profile. After initiating the hole using a hand bucket auger, an open-ended metal pipe or polyvinyl chloride (PVC) casing is inserted vertically into the shallow hole. A smaller tube, or stinger, attached to a wet/dry vacuum is inserted into the pipe to extract loose material while downward pressure is applied on the pipe. Once the casing is installed, instrumentation such as lysimeters, gypsum blocks, or tensiometers can be placed at the desired depth and backfilled with native soil. The casing is then raised and the soil allowed to collapse around the equipment, or the pipe can be left in place for neutron probe access. Measurements of soil water content after an infiltration experiment demonstrates uniform downward movement with minimal preferential flow or soil disturbance as a result of the vacuum installation of gypsum blocks and a neutron access tube.