More than 0.5 million ha of irrigated cotton (Gossypium hirsutum L.) are grown in the Southern High Plains of Texas. Conservation tillage cotton in terminated wheat (Triticum aestivum L.) has been shown to improve water use efficiency and reduce wind erosion. However, limited N fertilizer response research has been done in this system. The objective of this 3‐yr field study at Lubbock, TX was to characterize the response to N fertilizer (0, 28, 56, 84, or 112 kg N ha−1) at varying irrigation levels [0, 25, 50, or 75 % Evapotranspiration (ET) replacement] for conventional and conservation tillage cotton in an Acuff loam (fine loamy, mixed, superactive, thermic, Aridic Paleustoll). Additionally, we tested the chlorophyll meter as an indicator of in‐season N status of cotton and compared it to petiole NO3–N analysis. Cotton lint yields showed a quadratic response to irrigation level in 1996 and 1997, and a linear response in the drought year of 1998. Maximum lint yield varied from 71 to 97 % ET replacement. In 1997 and 1998, cotton lint yields responded to N at the 50 and 75% estimated ET replacement irrigation levels, but not at the 0 or 25% ET levels. Quadratic‐plateau models indicated that 19 to 38 kg N additional fertilizer ha−1 was needed to produce economically optimum lint yields near 1100 kg N ha−1 with conservation tillage than with conventional tillage. Chlorophyll meter and petiole NO3–N readings were positively related to N rate but were not affected by tillage system.

Pearl millet [Pennisetum glaucum (L.) R. Br.] production in the West African Sahel is constrained by low, erratic rainfall and low soil nutrient (particularly P) availability. Outdoor pot and growth chamber experiments tested the hypothesis that increasing soil P supply increases transpirational water‐use efficiency (WUET), under waterstressed and non‐water‐stressed conditions. Pearl millet was grown outdoors under semiarid conditions in covered pots containing 85 kg of acid, P‐deficient Betis sand (sandy, siliceous, thermic Psammentic Paleustalf). Plants were treated with four P levels and two water treatments, and harvested at 14‐d intervals. Significant main and interactive effects on WUET due to P level, water treatment, and time of harvest were found. The slope of the curve relating DM to cumulative transpiration (Tcum) increased with P level and water stress when data from all harvests were pooled. In the growth chamber, WUET of nonwater‐stressed plants ranged with increasing P level from 3.22 to 9.12 g kg−1 at 29 days after sowing (DAS) in pots containing 6 kg soil, and from 0.84 to 9.24 g kg−1 at 49 DAS in pots containing 18 kg soil. The ratio of leaf net photosynthetic rate to transpiration 0,WUEGMS,) at 500 μmol m−2 s−1 photosynthetic photon flux density (PPFD) ranged from 1.88μg mg−1 for plants receiving no P to 10.25μg mg−1 for those receiving 0.310 g P 6 kg−1 soil. Between PPFD levels orS00 and 2000 μmol m−2 s−1, plants receiving no P increased WUEGAS to only 3.60 μg mg−1, whereas those receiving higher levels of P increased WUEGAS to as much as 18.2μg mg−1. Our finding that increasing soil P avail° ability increases WUET under water‐stressed and non‐water‐stressed conditions reinforces previous conclusions that water supply in the Sahel and similar semiarid environments cannot be effectively managed for improved crop production without addressing soil fertility constraints.

Depending on soil and rainfall characteristics, pearl millet [Penniseturn glaucum (L.) R. Br.] production in the Sahel can be limited by inefficient use of nutrients, especially N and P, or by inefficient use of water. This study measured pearl millet N and P uptake and compared the efficiency with which N, P, and water are used for growth under varied soil P and water availability. Millet was grown outdoors in semiarid West Texas using rain-sheltered pots of low pH, P-deficient sandy soil. Treatments consisted of four P levels (0-56 g-z) and two w ater treatments (stressed and not). Plant P concentration decreased strongly with plant age; added P and water stress increased stem and leaf P concentration. Plant N concentration also decreased with age and increased with water stress, but decreased with added P. Because of the effects of age, water availability, and P level on organ nutrient concentration, P-use efficiency (PUE) increased with age, decreased with water stress, and decreased with added P. Nitrogen-use efficiency (NUE) also increased with age and decreased with water stress, but tended to increase with added P. Shoot transpiration efficiency (WUF~) increased with water stress and added P, and so varied inversely with PUE throughout the growth cycle. Phosphate root uptake efficiency (PRE) was less sensitive than PUE to age, P availability, and water stress, because of the compensating effect of root growth; PRE was also positively correlated with WUEx and yield. For crop improvement programs interested in increasing both P-and water-use efficiency, PRE is probably a better selection index than PUE.

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