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  • Author or Editor: Charles A. Sanchez x
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The low desert region of Arizona is the major area of lettuce (Lactuca sativa L.) production in the United States during the winter. Lettuce is commonly grown on the loam, clay loam, and clay soils of the alluvial river valleys. There is some interest in moving a portion of the vegetable production onto the sandy soils of the terraces (mesa) above the alluvial river valley to partially relieve the intensive production pressure being placed on lands in the valley. Of major concern in these sandy soils is water and N management. Studies were conducted during two seasons to evaluate the response of crisphead lettuce to sprinkler irrigation and N fertilizer and to evaluate the potential for leaching of nitrate-N on a coarse-textured soil. Lettuce yields increased in response to water and N, and were maximized by 55 cm of water and 271 kg·ha–1 N in 1991–92 and 76 cm water and 270 kg·ha–1 N in 1992–93. These water and N rates exceeded those typically required on finer-textured alluvial valley soils. At N and water rates required for maximum yields, 88% and 77% of the applied N was not recovered in the aboveground portions of the plant during the 1991–92 and 1992–93 seasons, respectively. Overall, data for the amount of N fertilizer not recovered, estimates of nitrate-N leaching determined during one growing season, and analysis of soil samples collected after harvest indicate the potential for large N leaching losses on this coarse-textured soil. Alternative production methods that enhance water and N use efficiencies, such as drip irrigation and/or the use of controlled-release fertilizers, should be considered on this sandy soil.

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Nitrogen (N) in a soil that is not immediately taken up by a crop is subject to leaching, denitrification and other mechanisms of loss. Nitrogen uptake studies identify the total amount of N accumulated by the crop and the period of peak demand. This information can be used to devise management strategies aimed at supplying N preceding anticipated uptake. Split sidedress application, fertigation, and use of controlled release fertilizers (CRN) are all viable options for N management, depending on the crop production scenario and available infrastructure. Soil and plant tissue testing can be useful feedback tools for adjusting N applications for soil contributions of N and unexpected N losses. Efficient irrigation is of paramount importance in achieving efficient N fertilization regardless of management practice.

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Criteria for managing fertilizer nitrogen (N) applications for lettuce (Lactuca sativa L.) based on midrib nitrate-N analysis were developed 25 years ago. However, this test had not been recently evaluated for the newer cultivars of lettuce currently grown or the higher yield potential now obtained. More recently, quick sap nitrate-N tests have been correlated to the traditional dry midrib test and preliminary criteria for making diagnosis based on these sap tests have been proposed. Field experiments were conducted at 20 locations across the low desert region of the southwestern United States from 1996-1999 to evaluate the traditional dry midrib and sap nitrate-N tests. Tissue samples were collected before each sidedress N application and diagnostic accuracy was evaluated by determining lettuce growth and yield on both N-treated and untreated plots and comparing predicted to actual response. Overall, the variability associated with the quick sap test seemed to limit its application as a predictive N management tool in the low desert. Although less variable than the quick sap test, the high frequency with which the dry midrib test resulted in incorrect diagnosis suggests that either this test needs revision or that it is an unreliable N management tool.

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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.

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Vegetable and fruit crops produced in the desert southwestern United States generally do not respond to K fertilization. Even when pre-plant soil test K levels are low and crop K accumulations are high, responses are infrequent. We have performed a number of evaluations aimed at understanding why crops produced in this region fail to respond to K fertilization. First, data show the potential for substantial K inputs through irrigation. For example, Colorado River water, which is widely used for irrigation in this region, contains ≈5 ppm K, resulting in potential K inputs of 30 to 60 kg K/ha. Second, many of the soils used for crop production have a clay content and mineralogy making a response to K unlikely. Studies evaluating the kinetics of K release from the mineral fraction of soils in the region has shown that many soils used for crop production have a high capacity to replenish K to the soil solution and exchange sites following crop uptake. Finally, the observation that Na can partially substitute for the K requirement of many fast-growing leafy vegetables may also be a contributing factor for the infrequent K fertilizer responses for these commodities.

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Abstract

Studies were conducted during five winter cropping periods in the Everglades, near Belle Glade, Fla., to determine effects of shade applied at various times throughout the growing period on the growth and yield of lettuce (Lactuca sativa L.). Ancillary studies also were conducted in a greenhouse to determine effects of shade on the light response of lettuce with respect to net CO2 assimilation. The maximum net CO2 assimilation rate (Pn) for lettuce decreased as the irradiance at which the plants were grown decreased. Continuous shading from thinning to harvest reduced crop growth approximately in direct proportion to the reduction in irradiance. Lettuce was most sensitive to reductions in radiation when growth and development were most rapid. These data suggest that lettuce growth from planting through the eight-leaf stage is not affected by small reductions in radiation that might occur in nature, but appears to be largely influenced by temperature. This observation is consistent with data collected during greenhouse experiments that showed that Pn at this early growth stage was low regardless of the shade treatment. Lettuce growth from the eight-leaf through the preheading stage was reduced by low shade levels (75% of prevailing solar radiation). Lettuce yield, however, generally was not affected by low shade levels through the preheading stage. Shading, regardless of the degree, reduced growth and yield during the heading stage of development. Results from greenhouse experiments indicated that the light saturation point of lettuce for photosynthesis during this latter growth stage could reach 800 μmol·s−1·m−2. This light level is higher than prevailing light that often exists during fall and winter growing seasons in southern Florida.

Open Access

Abstract

The Diagnostic Recommendation and Integrated System (DRIS) approach was used to identify mineral deficiencies associated with mango decline (a disorder of unknown etiology) of ‘Tommy Atkins’ mango (Mangifera indica L.) trees in the field. Nutrient deficiencies associated with decline were related to the nutrition of entire orchards and not to the nutrient status of individual trees within an orchard. The nutrient imbalance index (NII) was higher for trees in the orchards with the largest percentage of declined trees compared with the healthy orchard. The most deficient elements in orchards with declining trees according to DRIS were Mn, Fe, or a combination of both elements. The concentration of these elements was below the critical value in two of the three declined orchards sampled. Magnesium concentration was generally higher in declined orchards than in healthy orchards. Phosphorus had the most negative DRIS index, but the concentration was still above the critical value in an orchard that contained no declined trees. DRIS determinations from potted trees showing no mineral deficiency symptoms in a previous study also showed P to have the most negative DRIS index. DRIS, when used along with sufficiency ranges, appears to be a useful approach for identifying nutritional deficiencies involved in a mango decline.

Open Access