Nitrogen (N) fertilizer, more than any other nutrient amendment, has increased commercial rice yields. Managing N in rice fields is challenging because flooded soils have several pathways for N loss. The greatest amount of N loss in rice fields is usually by bacteria converting nitrate to atmospheric N gas.
The goal of rice farmers should be to apply enough N to avoid deficiency without applying excessive N fertilizer. Overfertilization of N can lead to increased insect and disease occurrence, maturity delays, and lodging in rice crops. In a 5-year test at the Missouri Rice Farm at Glennonville, rice yields were greatest when the recommended N fertilizer rate was applied. Results showed that yields were reduced when N fertilizer was applied at 150% of the recommended amount. Therefore, assessing N status in a quick and reliable fashion is a critical aspect in rice production.
Can I Avoid Midseason Fertilization?
To combat the N loss pathways in rice production, urea fertilizer applications are typically split to spread the risk for N loss. Rice plants need N early in the season to stimulate tillering, and later as the plant begins the reproductive stages. For drill seeded rice, the recommended method is to apply 2/3 of the total N each season immediately before permanent flood. The remaining 1/3 is broadcast by airplane in two midseason applications.
Mid-season N applications are not always necessary. Factors such a rice variety, preflood N fertilizer rate, and weather conditions influence soil N availability to rice. The use of instruments such as chlorophyll meters have shown potential for predicting when midseason N is needed.
Chlorophyll is a green pigment present in plants that captures the sunlight that is used in photosynthesis. Nitrogen is a key element found in chlorophyll molecules. A lightweight, portable instrument was developed by the Soil-Plant Analyses Development (SPAD) unit of Minolta Camera Company to estimate chlorophyll levels in plants.
The meter approximates the amount of chlorophyll present by measuring the amount of light that is transmitted through a leaf. In essence, it determines "how green is this plant".
Chlorophyll meters can provide instantaneous, on-site information in a nondestructive manner. Fields can be monitored easily throughout the year. The chlorophyll meter is user friendly and can compute the average value of several samples.
How to use Chlorophyll Meters
Calibration of a chlorophyll meter is necessary each time the meter is used. Before inserting plant tissue, calibrate by pressing on the finger rest to close the head. After the meter beeps, a display is shown indicating the meter is ready for the first sample.
Sample rice leaves with the chlorophyll meter before scheduling aerial applications for mid-season nitrogen. Normally, this is done shortly after the internode begins to elongate in the rice stems. Insert the newest fully expanded leaf from each plant while avoiding old or immature leaves. Keep track of how many readings have been made by monitoring the 'N= ' displayed at the top of the screen on the chlorophyll meter.
We recommend collecting a minimum of 20 chlorophyll readings on small fields (less than 40 acres). On large fields take one reading per two acres. (e.g. an 80 acre field would require 40 samples.) Be sure to collect readings from each bay of the field. Try to avoid areas in fields that are not representative of fieldwide conditions. After all the readings are made, press the average button to compute the average reading for the field.
Interpreting Chlorophyll Meter Results
Chlorophyll meter readings can be influenced by many factors other than N alone. Anything that can alter the color of plants (i.e. diseases, other nutrient deficiencies, variety differences) can influence chlorophyll meter readings. Do not take chlorophyll meter readings on leaves with lesions or leaves with bronzing around the margins. Lesions on leaves are usually caused by blast, sheath blight, or brown spot diseases. Plants with leaves with bronze leaf margins may be deficient in potassium.
Differences in plant color between rice varieties is an important factor. When the same amount of N is applied on Cypress and Kaybonnet rice varieties, Kaybonnet will usually have a greener color. Therefore, we recommend that growers use a calibration curve specific for each rice variety to determine whether a rice field needs a mid-season nitrogen application.
Research was conducted at the Missouri Rice Farm in 1997 and 1998 to develop calibration curves for Kaybonnet and Cypress (Figure 1 and Figure 2). As leaf chlorophyll readings increased above 40 (for Cypress) and 41 (for Kaybonnet) the chance of obtaining a economically favorable yield response to midseason N decreased. In Texas, researchers determined the critical chlorophyll value near the panicle differentiation growth stage for the Lemont variety was also 40.
Growers should understand that the chlorophyll meter does not indicate how much N to apply, but only when N may be needed. Chlorophyll meters, when used in combination with other available diagnostic tools, provide improved opportunity to manage N in rice.
1 Crop Production Specialist, University of Missouri-Delta Center; Natural Resources Conservation Service; Rice Farm Manager, University of Missouri-Delta Center.
Figure 1. Relationship between yield increase and Minolta SPAD 502 chlorophyll readings at ½-inch internode elongation for Cypress (Missouri Rice Farm 1997-98).
Figure 2. Relationship between yield increase and Minolta SPAD 502 chlorophyll readings at ½-inch internode elongation for Kaybonnet (Missouri Rice Farm 1997-98.)Missouri Rice Research Reports