The amount of phosphorus (P) accumulated by vegetable crops is relatively small compared to nitrogen and potassium. However, large amounts of P fertilizer are often required for optimal yield and quality. Typically P added to soils is quickly converted to unavailable forms resulting in low crop utilization efficiencies. These low P uptake efficiencies have long been of economic concern and a major focus of agronomic and horticultural research. Additionally, in certain regions where crop production areas are hydrologically linked to wetland ecosystems, P fertilization is also of environmental concern. This presentation will summarize important P soil transformations, biotic and abiotic factors influencing the availability of P to crops, and P fertilizer management strategies for improved crop utilization efficiency and reduced environmental impact.
Lettuce produced in the desert typically shows large yield responses to N fertilization. However, concern about the potential threat of nitrate-N to ground-water has prompted additional studies aimed at developing improved N management practices. Field experiments were conducted between 1992 and 1995 to evaluate the response of crisphead lettuce to controlled-release N fertilizer (CRN). The use of CRN was compared to a soluble N fertilizer applied preplant (PP), and a soluble N fertilizer applied in split-sidedress applications (SD). Rates of N fertilizer application ranged from 0 to 300 kg·ha–1. Lettuce generally showed significant responses to N rate and N management practice. However, response to management practice varied by site-season. When conditions for N loss were high, SD and CRN management strategies were superior. However, in other site-seasons, SD management sometimes resulted in inferior head quality and marketable yield when compared to other management strategies. Data averaged over six site-seasons shoed improved yield and quality to CRN management strategies compared to PP and SD strategies.
Approximately 33% of all irrigated lands worldwide are affected by varying degrees of salinity and sodicity. Soils with an electrical conductivity (EC) of, the saturated extract greater than 4 dS/m are considered saline, but some horticultural crops are negatively impacted if salt concentrations in the rooting zone exceed 2 dS/m. Salinity effects on plant growth are generally considered osmotic in nature, but specific ion toxicities and nutritional imbalances are also known to occur. In addition to direct toxic affects from Na salts, Na can negatively impact soil structure. Soils with exchangeable sodium percentages (ESPs) or saturated extract sodium absorption ratios (SARs) exceeding 15 are considered sodic. Sodic soils tend to deflocculate, become impermeable to water and air, and have a strong tendency to puddle. Some soils are both saline and sodic. This workshop presentation will summarize various considerations in the management of saline and sodic soils for the production of horticultural crops.
The low desert region of Arizona is the major area of lettuce (Lactuca sativa L.) production during the winter. Most lettuce is grown on alluvial valley loam and clay loam soils. There is interest in moving some vegetable production onto sandy soils on the upper terraces (mesa) to partially relieve the intensive production pressure currently being placed on land in the valleys. Water and N management is a major concern in coarse-textured soils. Studies were conducted to evaluate the response of crisphead lettuce to sprinkler-applied water and N fertilizer on a coarse-textured soil (>95% sand). The experiments were irrigated using a modified lateral irrigation system that applied five levels of water and five levels of N in specified combinations. Nitrate-N concentrations were determined in samples collected in ceramic suction cups placed below the crop rooting zone. Leaching fraction was estimated by frequent neutron probe soil moisture measurements. Lettuce yield increased with water and N but rates required for maximum economic yield exceeded rates typically required on finer-textured valley soils. These data show the potential for large N leaching losses on this coarse-textured soil.
Imidacloprid is a new, chloronicotinyl insecticide currently being used to control sweetpotato whitefly [Bemisia tabaci Genn, also known as silverleaf whitefly (Bemisia argentifolii Bellows and Perring)]. Large growth and yield increases of muskmelon (Cucumis melo L.) following the use of imidacloprid have caused some to speculate that this compound may enhance growth and yield above that expected from insect control alone. Greenhouse and field studies were conducted to evaluate the growth and yield response of melons to imidacloprid in the presence and absence of whitefly pressure. In greenhouse cage studies, sweetpotato whiteflies developed very high densities of nymphs and eclosed pupal cases on plants not treated with imidacloprid, and significant increases in vegetative plant growth were inversely proportional to whitefly densities. Positive plant growth responses were absent when plants were treated with imidacloprid and insects were excluded. Results from a field study showed similar whitefly control and yield responses to imidacloprid and bifenthrin + endosulfan applications. Hence, we conclude that growth and yield response to imidacloprid is associated with control of whiteflies and the subsequent prevention of damage, rather than a compensatory physiological promotion of plant growth processes. Chemical names used: 1-[(6-chloro-3-pyridinyl)methyl]-4,5-dihydro-N-nitro-1-H-imidazol-2-amine (imidacloprid); [2 methyl(1,1′-biphenyl)-3yl)methyl 3-2-chloro-3,3,3-trifluoro-1-propenyl]-2,2-dimethylcyclopropane carboxylate (bifenthrin); 6,7,8,9,10,10-hexachloro-1,5,5a,6,9,9a-hexahydro-6,9-methano-2,4,3-benzodiaxathiepin 3-oxide (endosulfan).
Lettuce is planted in the southwestern U.S. desert from September through December and harvested from November through April each year. During this period mean soil temperatures range from 7 to 30C. Lettuce produced on desert soils shows a large yield response to P. Soil solution P is replenished by desorption from the labile soil P fraction and this process is temperature sensitive. A field study was conducted over 6 years to evaluate the response of lettuce to soil solution P levels under different ambient soil temperature regimes. The soil temperatures under which lettuce was grown were varied each year by altering planting dates. Soil solution P levels were established and maintained each season using P sorption isotherm methodology. Lettuce responded to P in all experiments. Phosphorus levels required for maximum yield varied with each experiment. Soil P levels required for optimal yield were best correlated to mean soil temperatures during the last 20 days before harvest. Lettuce accumulates over 70% of its P during the heading stage of development and it is likely that during this period of rapid growth and nutrient uptake, solution P becomes limiting when soil temperatures are cool.
Incorporation of specific vitamins such as thiamin to the rooting media has been reported to stimulate root and shoot growth. Thiamin is involved in the Kreps cycle decarboxylation of pyruvate to citrate as a coenzyme in the pyruvate decarboxylase enzyme complex. Axenic and soil glasshouse studies were conducted to determine the tissue nutrient concentrations (ICP analysis), especially Ca, in response to low application rates of thiamin. In a 50 d axenic “Grand Rapids” lettuce study, thiamin (5 mg mL-1 0.5 N Hoagland's) stimulated shoot length (25%), root length (23%), Ca (8%), K (14%), and P uptake (18%) compared with control values (no thiamin added). Soil glasshouse “Grand Rapids” lettuce studies showed that thiamin (6 mg kg-1 soil) stimulated N (72%), Ca (58%). K (12%), and P uptake (11%) compared with control values. Additional glasshouse-soil-thiamin form studies with “Black seeded Simpson” lettuce (20 mg each form kg-i soil) showed thiamin compounds increased Ca tissue levels from 3 to 10% and organic C content from 5 to 30%. The prospect of using these compounds to reduce tipburn in lettuce is being investigated in follow-up studies.
The economics of producing sweet corn (Zea mays L.) and head lettuce (Lactuca sativa L.) on Florida Histosols were analyzed with and without P application constraints based upon quadratic response functions derived from two experiments with each crop. At the lower end of the historical price range, production of both crops is unprofitable, especially when P is constrained. At higher prices, net returns for sweet corn under P constraints are relatively small compared with the capital invested; net returns are higher for lettuce. However, an analysis of historical monthly prices showed that those high prices rarely occur. Values for the marginal contribution of the last unit of P show that constraints greatly limit net returns. Many sweet corn and head lettuce producers may be forced out of business if P fertilization rates are arbitrarily lowered below the economic optimal rate.
Approximately 30,000 ha of iceberg lettuce (Lactuca sativa L.) are produced in the low desert region of the southwestern United States during the fall–winter–spring period each year. During this period, soil temperatures in lettuce beds range from 10 to 30°C. During the cooler part of the growing season, growers typically use nitrate-N sources because they believe they are generally more available for plant uptake. However, limited experimental evidence exists to support this practice. Three field studies were conducted during the 1994–1995 growing season to evaluate the response of iceberg lettuce to N rate and N source. The N sources urea, ammonium sulfate, ammonium nitrate, and calcium nitrate were applied at rates ranging from 0 to 300 kg N/ha. Although lettuce growth, N accumulation, and marketable yield significantly increased by N rate, there were generally no differences due to N source.
About 33% of all irrigated lands worldwide are affected by varying degrees of salinity and sodicity. Soil with an electrical conductivity (EC) of the saturated extract >4 dS·m−1 is considered saline, but some horticultural crops are negatively affected if salt concentrations in the rooting zone exceed 2 dS·m−1. Salinity effects on plant growth are generally osmotic in nature, but specific toxicities and nutritional balances are known to occur. In addition to the direct toxic effects of Na salts, Na can negatively impact soil structure. Soil with exchangeable sodium percentages (ESPs) or saturated extract sodium absorption ratios (SARs) > 15 are considered sodic. Sodic soils tend to deflocculate, become impermeable to water and air, and puddle. Many horticultural crops are sensitive to the deterioration of soil physical properties associated with Na in soil and irrigation water. This review summarizes important considerations in managing saline and sodic soils for producing horticultural crops. Economically viable management practices may simply involve a minor, inexpensive modification of cultural practices under conditions of low to moderate salinity or a more costly reclamation under conditions of high Na.