Do we need micronutrient applications to the rice crop if we reduce water use?

Authors: TARPLEY, LEE - Texas A&M Agrilife; MOHAMMAD, ADBUL - Texas A&M Agrilife; Pinson, Shannon; RATNA-RAJ, RATNAPRABHA - Texas A&M Agrilife

Submitted to: Meeting Abstract
Publication Type: Experiment Station
Publication Acceptance Date: 6/19/2020
Publication Date: 6/29/2020
Citation: Tarpley, L., Mohammad, A.R., Pinson, S.R., Ratna-Raj, R. 2020. Do we need micronutrient applications to the rice crop if we reduce water use?. Meeting Abstract. Texas Rice, special section, Highlighting Research in 2020. pp. 30-32. https://beaumont.tamu.edu/eLibrary/

Interpretive Summary: Rice producers are interested in growing their rice crop in new ways that use less water than traditional paddy rice production. Some rice in the U.S. is being produced in furrow-irrigated rows, while other producers are using an Alternate Wetting and Drying (AWD) system wherein the rice is flooded, then the water is allowed to evaporate until the soil surface becomes dry before a new flood is applied. It is well known that the release of soil nutrients into pore water, making them available for plant uptake, increases significantly when paddies are flooded. While farmers can monitor the greenness of their plants to guide rates and timing of nitrogen fertilizer applications under these new reduced-water production schemes, very little knowledge exists to guide farmers in adjusting fertilizer rates for other nutrients. In this study, we evaluated effects on six essential micronutrients: calcium, copper, iron, magnesium, manganese, and zinc. We started by collecting a typical southern U.S. heavy clay, rice-production soil from the Beaumont, TX area. Soil samples were sent to a standard soil analysis lab to determine their elemental contents and fertilizer recommendations, and in-house we measured the speed and degree to which soil pH and oxygen levels changed as samples of this soil were allowed to dry after being completely saturated. We then made availability predictions for calcium, copper, iron, magnesium, manganese, and zinc under flooded and reduced-water production systems by combining the measured soil data with information available in the literature that model nutrient availability at different pH and oxygenation levels. Results indicated that our typical rice soil contains sufficient amounts of these six elements that even though the availability of some elements decreases in unflooded soil, enough of each element remains available to support rice crop production.

Technical Abstract: There is increasing interest among southern U.S. farmers in growing rice in new manners that use less water than traditional flooded paddy production. The goal is to reduce water and energy use due to irrigation pumping costs while preserving yield. Most of the new reduced-water rice production systems leave the soil unflooded for a period of time during the cropping season. Exposure to air causes two major soil changes. It increases the oxygen levels in the soil during much of the rice growing season, and it causes the soil pH of the clay soils, typical of rice growing areas in southeast Texas and southern Louisiana, to remain somewhat acidic rather than the near neutral pH typical for flooded paddies. Both oxygen and pH affect the availability of many mineral elements for uptake by rice plants. This study asks if micronutrients need to be supplied to boost rice productivity under reduced-water management conditions. To address this question, several sources of information were weighed. These sources included: (1) results from actual soil testing; (2) predictions from the scientific literature concerning the effects of oxidized (aerobic) vs low-oxygen (flooded) soil conditions on micronutrient availability in the soil; (3) predictions from the scientific literature concerning the effects of soil pH on micronutrient availability in the soil solution; (4) typical plant micronutrient deficiency levels (also obtained from the scientific literature); and (5) results obtained from previous studies conducted at Beaumont, TX in which the micronutrient levels in grains of diverse rice varieties grown under aerobic and prolonged flooded conditions were observed. This study focused on six of the micronutrients of most likely interest (calcium, copper, iron, magnesium, manganese and zinc) and for various reasons in each case, as detailed below, concluded that no micronutrient fertilization is warranted for reduced-water management, at least for rice grown on the clay soils in and around the Beaumont, TX area. CALCIUM levels in plants do not change measurably due to change in water management as long as the plants receive adequate water. If a soil does not require calcium fertilization for conventional flood rice management, then none should be added for growing rice under reduced-water management. COPPER levels in the soil solution are expected to be the same to slightly higher under reduced-water management compared to conventional flood. Furthermore, grain levels from the previous studies indicated that plant levels were likely adequate under reduced water-management as well as under conventional flood-management. IRON levels determined in the grain suggested that iron concentrations in the plants were similar under the two water management practices, and were not considered deficient. In plants, physiological processes exist that help maintain steady amounts available in plant cells for plant growth and photosynthetic needs. As long as the levels in the plants are considered adequate, there is little advantage to supplementing the iron supply. MAGNESIUM levels in the soil solution were considered in the optimum range according to the soil tests; this should hold true under both flooded and reduced-water management practices. The levels in the plants based on the previous studies were not as high as might be expected based on the amounts measured in the soil samples, but are still considered to be adequate. Although the reduced-water management would be expected to slightly decrease magnesium levels in the plant, given the good supply of magnesium in a typical rice soil, the data did not suggest need to supplement magnesium supply with fertilizer application. MANGANESE availability increases somewhat under reduced-water management and levels were considered adequate based on the pore water and plant tissue levels observed in previ