This study was designed to determine the interval of sensitivity of maize (Zea mays L.) yield components to moisture stress, and to evaluate that interval using estimates of plant available water (PAW). Individual maize plants were grown in containers in a glasshouse. For each treatment, water was withheld until the accumulated water use in well-watered control containers was 20 L, approximately twice the PAW in each container. Containers were well watered at all other times. Containers were weighed to determine water use rates and to estimate PAW. Moisture stress was assumed initiated when water use rates declined below the average for well-watered containers. The interval when kernel number was sensitive to moisture stress began 2 to 7 dafter silking and ended 16 to 22 dafter silking. Stress initiated prior to silking but relieved within 2 d after silking did not reduce kernel number, kernel weight, or plant yield. The fewest number of kernels, 45% of the control, occurred for stress during the 7-d period after silking. Kernel weight was reduced by stress during the grain filling period, and the lowest weight, 51% of the control, occurred for stress 12 to 16 d after silking. Water use rates in treatment plant containers were compared to estimates of the soil moisture stress index (SMI) determined as the percentage of PAW in the containers. Water use rates declined when SMI declined below thresholds of between 0.20 and 0.30. These thresholds were similar to those reported for other crops. Thus, this analysis demonstrated that parameters based on PAW can be useful for evaluating the timing of moisture stress on maize yield components.

A system is needed to measure ammonia volatilization from N fertilizer applied under field conditions. This study was undertaken to develop a device for making these measurements and still maintain a field environment. The basic system developed consists of a vacuum pump, a chemical trap to capture ammonia, and the volatilization chamber.The volatilization chamber consists of a steel cylinder 21.8‐cm I.D. by 15‐cm long that is forced into the soil with its top flush with the soil surface to provide a microplot. The lid assembly attached to the cylinder includes a hinge and reversible electric motor that allows the lid to be automatically rotated from the microplot. The volatilization chamber was designed with the removable lid so that the lid is closed only for short intervals during the day while ammonia loss is measured. The total ammonia loss is calculated by integrating the rates of loss over time. Between measurements, the lid is open to allow normal environmental conditions of temperature, wind movement, etc., on the micro‐plot. The volatilization chamber has five air intake ports and one exhaust port. Ammonia loss was unaffected by air flow rate if air movement through the chamber exceeded 15 exchange volumes/min.Our complete system consisted of eight volatilization chambers with a system of multiple chemical traps that can operate automatically without attention for 24 hours. When tested in the field, duplicate measurements of ammonia loss from ammonium sulfate agreed quite closely, both in the rates of loss and the total ammonia loss. In another test, ammonia loss from ammonium chloride agreed reasonably well with other laboratory results.

Reductions of leaf development and transpiration are closely related to soil water deficits. Few studies have analyzed the effects of water deficits on both processes during different growth stages. A study was conducted to analyze and quantify the effects of water deficits during different growth stages on leaf development (number, extension, and senescence) and transpiration rates of sorghum [Sorghum bicolor (L.) Moench] and cotton (Gossypium hirsutum L.). The study was conducted at the Blackland Research Center at Temple, TX, in a glasshouse using covered pots and in the field using covered lysimeters. In the glasshouse, the sorghum and cotton pre‐flowering treatments were irrigated at 60, 35, 15, and 0% of water used in the control pots. In the lysimeters, water deficit treatments of 50, 30, and 0% plant available water (PAW) were imposed on sorghum during the vegetative period (before panicle initiation and between panicle initiation and anthesis) and after anthesis. Leaf length and transpiration rates were measured two to three times per week. Leaf extension was reduced to 0% of well‐watered sorghum and cotton when the PAW decreased from 50 to 0%. Transpiration per unit leaf area decreased from 100 to 0% of well‐watered sorghum and cotton when PAW decreased from 28 to 0% for each stressed period. Sorghum leaf senescence was enhanced and leaf number increased in the 0% PAW treatments compared to the well‐watered and 30% treatments. These relationships of leaf development, transpiration, and PAW compare favorably with other published results. The PAW threshold values when each process is affected would be useful in developing criteria for scheduling irrigation and in improving the accuracy of crop growth models in estimating leaf development and transpiration.

Ground cover determined by light interception is the percentage of the soil surface shaded by the plant canopy; i.e., it is a measure of the shadow projected by the plant canopy. The best time to measure ground cover is near solar noon when changes in solar angle result in the least change in ground cover. Measurements of ground covered by canopy foliage are used to evaluate the effectiveness of sunlight interception in photosynthesis and evapotranspiration studies. Ground cover measurements are also useful indicators of the ability of various row crops and cropping systems to intercept rainfall and reduce runoff and erosion.Ground cover of grain sorghum [Sorghum bicolor (L.) Moench], cotton [Gossipium hirsutum L.], sunflower [Helianthus annus (L.)], soybean [Glycine max (L.) Merr.], and corn [Zea mays (L.)] were determined using a meter stick, overhead photography, and photosensitive light sensors.The meter‐stick method is as accurate, faster, simpler, and more economical than any of the other methods used to determine ground cover. There were no significant differences in ground cover determinations using the meter‐stick method, overhead photographs, spatial quantum sensor, or traversing quantum cell. Ground cover was not linearly related to Leaf area index (LAI). A single measurement of ground cover across the plant row (perpendicular) was as accurate as the average of 21 inter‐row measurements parallel to the plant rows.

Evaporation during first stage drying was measured with evaporation plates on the soil surface in grain sorghum plots with 25‐, 50‐, and 100‐cm row spacings, during several stages of grain sorghum [Sorghum bicolor (L.) Moench] development in 1972 and 1973. Evaporation during first stage drying was affected by row spacing, leaf area index (LAI), and soil shading. Both LAI and soil shading were related to row spacing and plant population. The more complete plant canopy of grain sorghum with narrow‐row spacing decreased penetration of radiant energy to the soil surface and reduced the evaporation rate of soil water during first stage drying. Evaporation was greatest during the early part of the season when shading was least and lowest during boot stage and pollination, when all leaves were fully expanded and provided maximum plant canopy and soil shading. Narrow‐row spacing established an earlier, and more complete plant canopy than conventional 100‐cm row spacing and shaded more of the soil surface from early vegetative growth to maturity. Soil surface shaded near solar noon was a better indicator of first stage evaporation beneath a canopy than LAI.The combined effect of a mulched surface and a plant canopy on reducing evaporation was the product of the fractional reduction in potential evaporation resulting from the plant canopy and the evaporation resulting from the mulched surface with no canopy cover. An empirical equation was developed from the evaporation data for use in calculating first stage evaporation as related to potential evaporation, fraction of the soil surface shaded, and mulch rate.