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Salinity and crop nutrient use concerns in the southern San Joaquin Valley were discussed at a workshop hosted by the Kern County Cooperative Extension. These issues, said Tobias Oker, UCCE soils and irrigation advisor, must be addressed, and research is ongoing to sustain productivity of tree nuts in the region.
UC plant scientists Louise Ferguson and Patrick H. Brown have been studying the micronutrient boron in pistachio production and its effect on yields. Ferguson, a workshop speaker, said boron is essential for pollen viability and reproduction. She also noted there is a narrow range between boron deficiency and boron toxicity in pistachio trees.
Boron deficiency limits cell enlargement and cell division in the root meristem, limiting root growth. Deficiency affects cell wall synthesis, carbohydrate metabolism and reduces nitrate reductase activity, N and K levels.
Managing vegetative boron deficiency is easy, Ferguson said. Boron is generally not fixed in the soil, therefore available. In low B soils, it should be made available prior to key growth stages. Soluble B fertilizers are best. Foliar application effect is short term and localized due to immobility.
Excess B in pistachio trees causes a midseason leaf edge necrosis that increases in area through harvest. Unlike B deficiency, for which the cause has been determined and is easily corrected, the cause of B toxicity is unknown and less easily corrected.
Ferguson said the narrow range between deficiency and toxicity is a function of boron concentration in the soil. There is passive uptake in xylem. It is easily taken up if soil boron levels are high and soil moisture is high. Boron accumulates in leaf margins and cannot be redistributed by phloem. Boron toxicity coexists with sodium and chloride in arid regions but does not contribute to salinity.
How harmful boron toxicity is and how boron produces its harmful effects are questions. Ferguson said trials showed that as soil boron levels increased, yield declined. The trials also noted no significant correlation between leaf boron level and soil boron levels. Increases in leaf boron did not decrease yields significantly and only moderately increased percentage of leaf damage.
While soil boron levels decreased yields, it is not correlated with leaf boron and not correlated with percent of leaf damage. This suggests, Ferguson said, that leaf damage is not the reason boron decreases yield.
UC agriculture water management specialist Daniele Zaccaria noted effects of salinity on evapotranspiration in pistachio trees, with Kern County having 42% of California pistachio production and expansion into saline and saline-sodic grounds unsuitable for other crops.
Salinity Effects
In his research, he has determined that salinity decreases the soil osmotic potential, resulting in more metabolic energy being needed by trees to extract water and nutrients from the soil root zone. With lower stomatal conductance, trees transpire less water.
Salinity also reduces tree growth, Zaccaria said, resulting in less interception of light. This is due to specific ion toxicity to leaves causing lower leaf efficiency, and reduced uptake of water and nutrients and less assimilation of carbon. In salt-affected orchards, more sunlight reaches the soil surface, causing more soil evaporation if the soil is wet.
Acknowledging the standard 4Rs in fertilizer application for tree nuts, UC farm advisor Phoebe Gordon expanded on recent research. Nitrogen in irrigation water is an effective and free source of nitrogen for crop use, she said. Nitrate in irrigation water is the same as nitrate in fertilizer.
System maintenance is also an important part of delivering crop nutrients. Distribution uniformity can change over time due to lack of maintenance or pump disrepair. Periodic pressure checks can help.
Nitrogen Use Efficiency
Gordon noted a multiple-year nitrogen use efficiency trial where new N management was compared with traditional management. The new protocol involved estimating yield at bloom using 68 pounds of N per 1,000 pounds of kernels and a 70% fertilizer use efficiency. In-season April leaf sampling was used to predict July leaf tissue values, the N in the irrigation water was taken into account and N applications were made in March, April, May and August.
Nitrogen use efficiency was improved primarily due to including the N in irrigation water, Gordon said. Lower NUE in some years was the result of overestimating yield potential and not taking into account early leaf sampling. Yields were not negatively impacted by implementing the new methods.
With potassium, many of the 4 Rs still apply, Gordon said. Uptake of K depends on the root systemās exploitation of soil. Maximum uptake efficiency depends on getting K fertilizer in the wetted zone of drip emitters.
The key to potassium fertilizer application, Gordon noted, is applying a product that works for you in a way that is appropriate for your soil.
Orchard nutrition and irrigation can play a part in disease development. Stress from under or over irrigation or poor nutrition predisposes trees to canker diseases, Mohammad Yagmour, UCCE farm advisor in Kern County, said. Saline conditions and poor water infiltration open the door to disease development.
His advice to growers is to consider all aspects of planning and planting a new orchard including water quality, soil preparation and rootstock selection. History of the site needs to be understood, as in previous crops, microclimate and adjacent crops. Disease management should include careful selection of rootstocks, amendments or cover crops to increase water infiltration, nutrient management and chemical control.
Salinity Strategies
Irrigation and agronomy farm advisor emeritus Blake Sanden outlined strategies for dealing with saline conditions. The first is to understand normal salinity standards and toxicity. Sanden, known for helping to establish the salinity tolerance of pistachios, said as salinity increases above the plantās threshold, crop water use begins to decline. This is often combined, he added, with toxic levels of sodium, boron and chloride that accumulate in leaf tissue.
The next strategy is to know how to calculate potential impacts on yield. Without documented thresholds, soil texture, drainage, irrigation scheduling and the ratio of certain salts to others, along with rootstocks, the numbers in the published tolerance limits can shift. Sanden recommended comparing soil and water numbers with a neighbor. He said water penetration problems can result in rootzone salinity and tree stress due to lack of leaching even when water salinity appears to be acceptable.
He also warned growers to ācontinue to search out the most recent salt tolerance data for new varieties and revisions of field research before making a final decision.ā
Growers should use soil surveys, Google Earth images and backhoe pits to assess different soil textures and layers across a field.
Sanden said it is essential to sample different depths of orchard soils to ensure that salts are moving down out of the more sensitive upper part of the root zone.
Understanding distribution uniformity, irrigation efficiency and leaching fraction can be the most difficult strategy for growers. Quality system design, field slope and precise pressure regulation are critical.
Information on managing salinity, soil and water can be found at https://ucanr.edu/county/kern-county/MANAGING SALINITY, SOIL AND WATER AMENDMENTS.
http://cekern.ucanr.edu/IRRIGATION_MANAGEMENT/IMPROVING_WATER_PENETRATION
Cecilia Parsons | Associate Editor
Cecilia Parsons has lived in the Central Valley community of Ducor since 1976, covering agriculture for numerous agricultural publications over the years. She has found and nurtured many wonderful and helpful contacts in the ag community, including the UCCE advisors, allowing for news coverage that focuses on the basics of food production.
She is always on the search for new ag topics that can help growers and processors in the San Joaquin Valley improve their bottom line.
In her free time, Cecilia rides her horse, Holly in ranch versatility shows and raises registered Shetland sheep which she exhibits at county and state fairs during the summer.
