Nutrient Removal Amounts by Major Vegetable Crops in Texas

Soil nutrient depletion through crop removal is a major limitation to sustainable production especially for horticultural crops which tend to have high fertilizer input requirements. In the long-term, a balance between nutrient inputs and crop removal is required. Although nutrient removal amounts for many field crops are available, such values for fruit and vegetable crops are rare. Knowledge of nutrient removal amounts is critical in developing fertilizer management practices to sustain yields and quality while maintaining soil fertility. This long-term project will quantify the nutrient accumulation and removal rates of diverse commercial vegetable crops in relation to different yield expectations and soil types.


Justification

Relatively high levels of fertilizer applications are required to ensure adequate yields and quality of many horticultural crops (including fruits and vegetables). During the course of the growing season, crops take up and accumulate various nutrients in biomass, some of which will eventually be removed from the site in harvested products. Factors such as crop species, cultivar, yield potential, and cultural practices influence the degree of nutrient uptake and removal. Among the essential mineral nutrients, potassium (K) is the element required in the largest amount (after nitrogen) especially in fruit crops (Marschner, 1995). Potassium plays a crucial role not only in boosting yields, but also in improving various quality traits (Usherwood, 1985; Jifon and Lester, 2008; Lester and Jifon, 2007). Nutrient imbalance, especially inadequate K supply, is often a major factor contributing to the decline in vegetable crop yields and quality even though most soil tests commonly indicate sufficient levels (>150ppm) of soil K (Jifon and Lester, 2008; Lester and Jifon, 2007). This is often the case in most calcareous soils in Texas and other major vegetable production regions where high levels of soil calcium (Ca) and magnesium (Mg) typically exacerbate the apparent K deficiency problem through competitive nutrient uptake inhibition interactions. Our previous research (Lester et al., 2005, 2006, 2007) has shown that supplementing soil-derived K with foliar applications can alleviate this apparent K deficiency and enhance quality traits of muskmelons such as sweetness, texture, color, vitamin C and beta-carotene contents (Lester et al., 2005). However, in order to develop foliar K recommendations for improving yield and quality, information regarding crop nutrient removal amounts is essential. Although nutrient removal amounts for many field crops are available, such values for fruit and vegetable crops are rare (Heckman et al., 2003). Furthermore, intensive cultivation, even in the face of improved soil fertility and management practices, tends to deplete soil nutrient pools through crop removal and leaching. In the long-term, a balance between nutrient inputs and crop removal is required. Knowledge of average nutrient removal amounts by different crops during a growing season is critical in determining the amounts that must be applied to sustain yields and quality while maintaining soil fertility.


Objectives

The proposed project will quantify the nutrient (N, P, K, S, Ca, Mg, Zn, Mn, Cl, Cu, B, Fe) accumulation and removal rates in relation to different yield expectations by fruiting and non-fruiting horticultural crops (melons, onions and spinach) grown in sites with contrasting soil types (light vs heavy) in S. Texas. This information will be useful in developing guidelines for nutrient application rates to assure productivity while maintaining soil fertility and in selecting crops for specific sites based on their nutrient accumulation/removal capacities.


Methodology

A popular commercial variety of a leafy vegetable crop, spinach (Spinacia oleracea L.), a root crop- onion (Allium cepa), and a fruiting crop - muskmelon (Cucumis melo L.) will be grown at several locations in s. Texas following commercial production practices (Dainello, 1996). Soil in this agriculturally important part of the State are moderately fine textured and comprise mainly of young alluvial sediments along the Rio Grande River delta. There are wide areas of low, clayey soils separated by raised pockets of sandy loam. In parts of Hidalgo County, there are older delta deposits rich in clay and gravel, often with a noticeable hardpan (caliche) layer. The study sites will be selected to represent the major soil types in this part of the State. At least ten soil cores (3-cm diam.) will be randomly collected from the 0-30 cm topsoil layers at each site, air dried, and analyzed (texture and minerals) before and after each study to established nutrient balances following production of these crops. All soil and tissue analyses will be conducted at Texas A&M’s Soil, Water and Forage Testing Laboratory, College Station, TX. Additional fertilizer nutrients will be supplied as recommended by the soil test analyses. Leaf, petiole, stem and fruit samples (when/where applicable) will be collected at regular intervals during the growing season for mineral nutrient analyses. Additional samples will be collected from commercial fields representing diverse soil types and cultural practices. All tissues will be rinsed with distilled water, dried (70°C for 48 h & weighed) and ground (<0.5mm) prior to nutrient analyses. At crop maturity, yield and quality data will be recorded and fruit (melons) samples will be further divided into peel, mesocarp and seed tissues for mineral analyses. Total nitrogen (N) concentration of tissues will be analyzed by the Kjeldahl method (Bremner, 1965). Mineral nutrient concentrations (P, K, Ca, Mg, S, Zn, Mn, Cu, Fe, B) will be analyzed by inductively coupled plasma (ICP) emission spectroscopy, following tissue digestion with nitric acid and hydrogen peroxide. Additionally, soil samples will be collected for controlled experiments in the greenhouse to further determine nutrient uptake dynamics among the contrasting soil types (light-sandy to heavy-clay).

Since crop nutrient removal amounts can vary with soil type, cultural practices, yield levels and weather conditions, this project will be conducted over multiple years and at several locations in Texas representing contrasting production conditions (soils, weather etc). This will help establish a large enough and representative data set for development of reliable nutrient removal values and replacement guidelines for vegetable production in this region. During this initial year of investigations, experiments will be conducted in the Lower Rio Grande Valley, the region with the most intense horticultural production in the State. In subsequent years, the project will be expanded to cover the other four regions namely – the Winter Garden, High Plains, Eastern region, and the Far West regions. Although production intensity may vary with region, the crops under investigation (melons, onions and spinach) are grown in all the production regions (Smith and Anciso, 2005).

Statistical analysis: All data will be analyzed using the general linear model procedures of SAS (SAS, Cary, N.C.). Appropriate mean separation tests (P ≤ 0.05) will be performed when necessary to further evaluate the magnitude of differences between treatments (soil type, growth stage, species).

Expected outcomes: Improved estimates of nutrient removal amounts for use in developing guidelines for (1) nutrient application rates to assure productivity while maintaining soil fertility and (2) site-specific fertilizer management strategies in selecting crops for a given site based on their nutrient accumulation/removal capacities.