Corn and alfalfa production as influenced by limited irrigation

Summary Several corn and alfalfa varieties were subjected to varying levels of water deficits under field conditions over a two-year period at the Utah State University Evans farm at Logan, Utah. Dry matter yields of corn and alfalfa varieties showed a linear relationship to evapotranspiration (E _t . In most cases grain yields also showed a linear response to E _t . When relative yields of dry matter and grain of corn varieties were related to relative E _t it was found that the slopes of the linear regression lines changed more between the two years than between the varieties within a given year. It was found that yields predicted by Hanks' (1974) model were highly correlated with measured yields for both corn and alfalfa.in coperation with Agric. Exp. StnGraduate student and professor, respectively     

Response of soybean genotypes to different irrigation regimes in a humid region of the southeastern USA

Studies on irrigation scheduling for soybean have demonstrated that avoiding irrigation during the vegetative growth stages could result in yields as high as those obtained if the crop was fully irrigated during the entire growing season. This could ultimately also lead to an improvement of the irrigation water use efficiency. The objective of this study was to determine the effect of different irrigation regimes (IRs) on growth and yield of four soybean genotypes and to determine their irrigation water use efficiency. A field experiment consisting of three IR using a lateral move sprinkler system and four soybean genotypes was conducted at the Bledsoe Research Farm of The University of Georgia, USA. The irrigation treatments consisted of full season irrigated (FSI), start irrigation at flowering (SIF), and rainfed (RFD); the soybean genotypes represented maturity groups (MGs) V, VI, VII, and VIII. A completely randomized block design in a split-plot array with four replicates was used with IR as the main treatment and the soybean MGs as the sub-treatment. Weather variables and soil moisture were recorded with an automatic weather station located nearby, while rainfall and irrigation amounts were recorded with rain gauges located in the experimental field. Samplings for growth analysis of the plant and its components as well as leaf area index (LAI) and canopy height were obtained every 12 days. The irrigation water use efficiency (I_WUE) or ratio of the difference between irrigated and rainfed yield to the amount of irrigation water applied was estimated. The results showed significant differences (P < 0.05) between IR for dry matter of the plant and its components, canopy height, and maximum leaf area index as well as significant differences (P < 0.05) between MGs due to IR. Differences for the interaction between IR and MG were significant (P < 0.05) only for dry matter of pods and seed yield. In general, seed yield increased at a rate of 7.20 kg for each mm of total water received (rainfall + irrigation) by the crop. Within IR, significant differences (P < 0.05) on I_WUE were found between maturity groups with values as low as 0.55 kg m⁻³ for MG V and as high as 1.14 kg m⁻³ for MG VI for the FSI treatment and values as low as 0.48 kg m⁻³ for maturity group V and as high as 1.02 kg m⁻³ for maturity group VI for the SIF treatment. We also found that there were genotypic differences with respect to their efficiency to use water, stressing the importance of cultivar selection as a key strategy for achieving optimum yields with reduced use of water in supplemental irrigation.     

Sugar beet production as influenced by limited irrigation

Summary Sugar beets (Beta vulgaris L.) were grown on a Millville silt loam soil at Logan, Utah to study the relationships between yield (total dry matter, fresh root, and sucrose) and various levels of irrigation simulating different types of limited irrigation under drought conditions. There were four harvest dates. A model, PLANTGRO, was tested for yield prediction under the imposed conditions. A line source sprinkler irrigation system which applied irrigation water from an excess to a zero amount, was used to impose the various levels of irrigation. Irrigation was continued throughout the season on half of the area and terminated at mid-season on the other half. For both irrigation treatments, yield responses to irrigation levels were large. Unlike continuous irrigation throughout the season, when irrigation was terminated in mid-season, there was no increase in yield (total dry matter, fresh root, or sucrose) from harvest 1 to harvest 4. The relation of yield to termination of irrigation depended on the amount of stored soil water at the time of termination. Yield and relative yield exhibited a strong linear relationship with ET. Percent sucrose was not significantly affected by irrigation regimes or harvest date, but tended to increase as amount of applied irrigation water increased. The model PLANTGRO gave good predictions for relative yields of fresh roots, sucrose, and total dry matter under full-season irrigation. The relative yield relations of fresh roots, sucrose and total dry matter were similar. Where irrigation was terminated in mid-season the model slightly under-predicted yield at high irrigation levels.Contribution from Utah Agricultural Experiment Station, Utah State University, UT 84322, USA     

Effects of different irrigation regimes applied during the final stage of rapid growth on an early maturing peach cultivar

Different irrigation regimes were performed on container-grown early-season peach trees (cv. Alexandra) during stage III of fruit growth. In the first experiment, three water treatments were applied: T1, control irrigation; T2, light water restriction; T3, high water restriction. In the second experiment, T4, a light water restriction, was compared to T5, the same total amount of water as T4 but with alternating periods of water withholding and subsequent re-irrigation. Compared to T1 and T2, leaf photosynthesis was limited under T3. Fruit yield and quality did not differ between T1 and T2, while fruit yield, average weight and percentage in the higher commercial grade decreased and total soluble solids (TSS) increased under T3, compared to T1 and T2. Comparing T5 to T4, yield, fruit firmness and average weight did not vary, but heterogeneousness of fruit diameter and TSS at the lower fruit grade tended to be higher. Peach sensitivity to brown rot was likely to decrease under T3 compared to T1 and T2. Peach water loss and brown rot incidence after contamination in conidial suspensions were enhanced under T5 compared to T4, implying that re-irrigation after water withholding should be avoided in order to limit brown rot incidence.     

Corn production as influenced by irrigation and salinity — Utah studies

Summary This paper reports the results of a two-year field study at Logan, Utah which was one of a series of similar experiments carried out at Ft. Collins, Colorado, Davis, California and Yuma, Arizona. A range of water application rates were imposed using the line-source system (Hanks et al., 1976) and in some treatments water was withheld during certain growth stages. Salinity variables were imposed by presalinization before planting and by the use of saline irrigation water. Regardless of irrigation or salinity regime, corn grain and total dry matter production were linearly related to evapotranspiration, which was measured as the sum of irrigation, rainfall and soil water depletion minus drainage. Presalinization of the soil decreased yields in proportion to the salinity imposed, the decrease being associated with reductions in evapotranspiration caused by reduced soil water depletion as compared to the nonsalinized treatments.Contribution from Utah State University, Department of Soil Science and Biometeorology, in cooperation with Utah Agr. Exp. Sta., Logan, Utah 84 322. Journal Paper No. 2188. Supported in parts by Grants No. C-5189, Consortium for International Development and No. B-121-UT, Utah Water Res. Lab. and USDI, Office of Water Res. and TechnologyProfessor, Associate Professor, and Research Assistants, respectively     

Potato evapotranspiration and yield under different drip irrigation regimes

A field experiment comparing different irrigation frequencies and soil matric potential thresholds on potato evapotranspiration (ET), yield (Y) and water-use efficiency (WUE) was carried out in a loam soil. The experiment included five treatments for soil matric potential: F1 (-15 kPa), F2 (-25 kPa), F3 (-35 kPa), F4 (-45 kPa) and F5 (-55 kPa) and six treatments for irrigation frequency: N1 (once every day), N2 (once every 2 days), N3 (once every 3 days), N4 (once every 4 days), N6 (once every 6 days) and N8 (once every 8 days). Results indicate that both soil matric potential and drip irrigation frequency influenced potato ET, Y and WUE. Potato ET increased as irrigation frequency and soil matric potential increased. Comparing soil water potential, the highest ET was 63.4 mm (32.1%) more than the lowest value. Based on irrigation frequency treatments, the highest ET was 36.7 mm (19.2%) more than the lowest value. Potato Y and WUE were also found to increase as irrigation frequency increased. Potato Y increased with an increase in soil water potential then started to decrease. The highest Y and WUE values were achieved with a soil matric potential threshold of -25 kPa and an irrigation frequency of once a day.Communicated by J. Ayars     

Sustainability of regulated deficit irrigation in a mid-maturing peach cultivar

The level of irrigation restriction to apply in a deficit irrigation (DI) programme for sustainable peach (‘Baby gold 6’) production was investigated. The experiment involved four irrigation treatments over five consecutive seasons (2007–2011). They were full irrigation (control), reducing irrigation by 20 % during the first half of stage III (DI-80 %), withholding irrigation until reaching a light stress level (DI-L) and withholding irrigation until reaching a moderate stress level (DI-M). The withholding of irrigation in both DI-L and DI-M was applied only during stage II and postharvest periods and was based on midday stem water potential thresholds (Ψ _stem). For the DI-L treatment ?1.5 MPa was used in both periods, and for DI-M ?1.8 and ?2.0 MPa were used during stage II and postharvest, respectively. Average Ψ _stem values during DI periods were approximately ?1.4 and ?1.2 MPa for DI-M and DI-L, respectively. The pre-defined thresholds required to trigger irrigation were rarely reached. No significant differences between treatments were found in terms of yield in any experimental year. However, DI-M and to a lesser extent DI-L had lower final fruit fresh mass at harvest related to lower Ψ _stem after three consecutive years of the experiment (during 2010 and 2011). Therefore, in terms of fruit size, DI was not sustainable. Rather than lowering Ψ _stem thresholds, we recommend discontinuing DI after 3-year application.     

Yield and water relations of wheat as influenced by irrigation and depth of tillage

Summary Development of a ploughpan has been reported in Bangladesh for almost all ploughed soils which are puddled for transplanted rice cultivation. Field information on the water requirement of dryland crops such as wheat and the effects of loosening the dense layer on crop yield and water use efficiency are very limited. Field experiments were, therefore, conducted in the grey floodplain soil of Sonatala series (Aeric Haplaquept) to study the irrigation and tillage effects on the yield and water relations of wheat (Triticum aestivum L. cv. Sonalika). The split plot design experiment comprised four irrigation treatments in the mainplots viz. W₀ = no irrigation, W₁ = irrigation of 5 cm at 4 weeks after planting, W₂-W₁ + irrigation(s) of 5 cm each at irrigation water to cummulative pan evaporation (IW/CPE) ratio of 0.75 and W₃- W₁ + irrigation(s) of 5 cm eacht at IW/CPE ratio of 0.50. The sub-plot tillage depth treatments were: A-7.5 cm (traditional), B-15 cm, C-22.5 cm, D-22.5 cm practised in alternate wheat seasons. Measurements were made of grain and straw yield, soil water depletion and water expense efficiency.Irrigation had no effect on grain or straw yield. Tillage to 15 cm increased wheat yield by about 15% over traditional depth to ploughing. In general, deep tillage coupled with one irrigation at four weeks after planting produced the largest wheat yield.Soil water depletion (SWD) in the 0–90 cm profile was greatest in the treatment receiving two irrigations, one at 4 weeks and again at IW/CPE ratio of 0.50. The average SWD in this treatment was 113 in 1982–83 and 82 mm in 1983–84. Plots receiving traditional tillage (7.5 cm) had the greatest SWD. Total water expense were the greatest in treatments receiving three irrigations. The maximum water expense efficiency (WEE) of wheat was observed in the non-irrigated plots in 1982–83 and 1983–84, respectively. Deep tillage treatments, in general, had significantly greater WEE than those under traditional ploughing. Intensive irrigation and efficient soil and water management are important factors in enhancing crop productivity. The former not only permits judicious water use but also better utilization of other production factors thereby leading to increased crop yield which, in turn, helps stabilize the farming economy. The best way to meet increasing demand for water is to adopt efficient water management practices to increase water use efficiency.Irrigation should aim at restoring the soil water in the root zone to a level at which the crop can fully meet its evapo-transpiration (ET) requirement. The amount of water to be applied at each irrigation and how often a soil should be irrigated depend, however, on several factors such as the degree of soil water deficit before irrigation, soil types, crops, and climatic conditions (Chaudhury and Gupta 1980).Knowledge of movement of water through the soil is imperative to efficient water management and utilization. The presence of a dense pan impedes water movement into the sub-soil. As a result, the top soil becomes saturated by irrigation and sensitive dryland crops can fail as this plough layer impedes the penetration of roots into deeper soil layers and decreases water extraction. Crops growing in these soils often undergo severe water stress within 5–8 days after rainfall or irrigation (Lowry et al. 1970). Due to decrease rates of water flow, the lower soil layer may remain unsaturated and as a result, the recharge and soil water storage in the profile are considerably decreased (Sur et al. 1981).In Bangladesh, ploughpans develop to varying degree in almost all ploughed soils (Brammer 1980). They are particularly marked in soils which are puddled for transplanted rice cultivation where the pan is usually only 8–10 cm below the soil surface and 3–5 cm thick. Its presence is generally regarded as advantageous for cultivation of transplanted rice in that it prevents excessive deep percolation losses of water. But in the same soil this cultivation for a subsequent dryland crop would adversely affect yield. A slight modification of the plough layer could enable good yields of both rice and a dryland crop to be obtained in the same soil in different seasons (Brammer 1980). The sub soils have a good bearing capacity, both when wet and dry and the pan can easily be reformed, if desired, for cultivating transplanted rice after a dryland crop like wheat.Professor of Soil Science, Dhaka University, Dhaka, Bangladesh     

Soil microbial response, water and nitrogen use by tomato under different irrigation regimes

The objectives of this study were to investigate the effects of full irrigation (FI), deficit irrigation (DI) and partial root-zone drying (PRD) on plant biomass, irrigation water productivity (IWP), nitrogen use efficiency (NUE) of tomato, and soil microbial C/N ratio. The plants were grown in pots with roots split equally between two soil compartments in a climate-controlled glasshouse. During early fruiting stage, plants were exposed to FI, DI, and PRD treatments. In FI, both soil compartments were irrigated daily to a volumetric soil water content of 18%; in PRD, only one soil compartment was irrigated to 18% while the other was allowed to dry to ca. 7-8%, then the irrigation was shifted; in DI, the same amount of water used for the PRD plants was equally split to the two soil compartments. The results showed that, the FI treatment produced significantly higher dry biomasses of leaves, stems, and fresh weight of fruit and water productivity of aboveground dry biomass production than either DI or PRD, however, fruit IWP in DI was 25% higher than that of FI, and harvest index in DI and PRD were 50% and 22% higher than FI, respectively, for the 26% and 23% less water used in the DI and PRD, respectively, than the FI treatment. The DI treatment caused the smallest losses of N and highest N use efficiency by fruit. Both DI and PRD caused a significant increase in the soil microbial C/N ratio, meaning ratio of fungal biomass was high at low soil water contents. The result indicates that more work is needed to link the aboveground N uptake and the underground microbially mediated N transformation under different water-saving irrigation regimes.     

Carbon retention in the soil-plant system under different irrigation regimes

Carbon (C) sequestration through irrigation management is a potential strategy to reduce C emissions from agriculture. Two experiments (Exps. I and II) were conducted to investigate the effects of different irrigation strategies on C retention in the soil-plant system in order to evaluate their environmental impacts. Tomato plants (Lycopersicon esculentum L., var. Cedrico) were grown in split-root pots in a climate-controlled glasshouse and were subjected to full irrigation (FI), deficit irrigation (DI) and alternate partial root-zone irrigation (PRI) at early fruiting stage. In Exp. I, each plant received 2.0 g chemical nitrogen (N), while in Exp. II, 1.6 g chemical N and maize residue containing 0.4 g organic N were applied into the pot. The results showed that, in both experiments, the concentration and the amount of total C in the soil were lower in FI and PRI as compared to DI, presumably due to a greater microbial activity in the two treatments; particularly the PRI induced drying and wetting cycles of the soils may cause an increase of microbial activities and respiration rate, which could lead to more C losses from the soil. However, in both experiments the total C concentration in the PRI plants was the highest as compared with the FI and DI plants, and this was seemingly due to improved plant N nutrition under the PRI treatment. Consequently, the total amount of C retained in the soil-plant system was highest in the FI and was similar, but lower, for the PRI and DI. The different N input in the two experiments might have affected the C retention in the soil and in the plant biomass. Nevertheless, with a same degree of water saving, PRI was superior to DI in terms of enhancing C concentration in the plant biomass, which might have contributed to a better fruit quality in tomatoes as reported by [Zegbe et al., 2004] and [Zegbe et al., 2006].     

Water use and wheat yields in northern India under different irrigation regimes

Field studies were conducted during a 3-year period to determine wheat (Triticum aestivum L.) yield in response to irrigation scheduling and variable fertilization.Irrigation scheduling was done on the basis of cumulative pan evaporation. Irrigations were given at 25, 50 and 75% available water in the top 60 cm soil profile. The amount of irrigation water applied at each irrigation was equivalent to 75% of the cumulative open pan evaporation. The crop was fertilized at the rate of 0, 60, and 120 kg/ha nitrogen.The yield of wheat was significantly affected by irrigation water and nitrogen treatments. Maximum yield was obtained with irrigation at 50% available soil water and 120 kg/ha nitrogen addition (5092 kg/ha). Consumptive use of water was maximum when irrigation was applied at 75% available soil water. The irrigation at 50% available soil water, however, resulted in greatest yield per cm water use by the crop.     

Modelling uptake of Na and Cl by tomato in closed-cycle cultivation systems as influenced by irrigation water salinity

The aim of the present investigation was to simulate the uptake concentrations (weights of ion per volume of water absorbed) of Na⁺ and Cl⁻ in hydroponic tomato crops as a function of the NaCl concentration in the root zone. An empirical model was calibrated and validated, which can be incorporated into on-line operating decision support systems aimed at optimizing the nutrient supply and minimizing the discharge of drainage solution in tomato crops grown in closed-cycle hydroponic systems. Three experiments were conducted, of which one was carried out to calibrate the model using irrigation water with NaCl concentration ranging from 0 to 14.7 mol m⁻³ while the other two experiments were commissioned to validate the model within either a low (0.5-2 mol m⁻³) or a high (1.2-12 mol m⁻³) concentration range. The model could successfully predict the uptake concentration of Na⁺, but Cl⁻ could not be simulated by this model at external Cl⁻ concentrations lower than 10 mol m⁻³. The results indicate that Na⁺ is excluded actively and effectively by the tested tomato cultivar even at low external Na⁺ concentrations, while Cl⁻ is readily taken up at low concentrations, particularly during the initial growing stages. Due to the efficient exclusion of Na⁺ by tomato, the Na⁺ concentration in the root environment increased rapidly to extremely high levels even when the Na⁺ concentration in the irrigation water was relatively low. These results indicate that tomato genotypes characterized by high salt-exclusion efficiency, require irrigation water with a very low NaCl concentration, if they are grown in closed hydroponic systems and the drainage water is not flushed periodically. To maintain Na⁺ at levels lower than 19 mol m⁻³ in the root zone of the tomato hybrid ‘Formula’ in closed hydroponics, a maximum acceptable Na⁺ concentration of 0.53 mol m⁻³ was estimated for the irrigation water.     

Distribution of water and salt in soil under trickle and pot irrigation regimes

An ellipitical clay pot was buried vertically in the centre of a lysimeter as a means of supplying water to the soil. The distribution of water and salt in the soil emerging from the pot source was compared with that under trickle irrigation. Five hundred milliequivalents of calcium chloride was applied to the soil by both methods. Calcium chloride was subsequently leached by applying 50 l of tap water. The soil solution was sampled periodically using suction cups. Soil samples were also taken for measurements of water content and chloride ion concentration. Water applied at the rate of 130 ml/h by the pot moved the salt to a radial distance of 41.5 cm in 390 h, but applying water by trickle at the rate of one l/h moved the salt 42 cm in 52.5 h. For an equal amount of water applied, salt moved deeper in the profile at the lower application rate. More salt spreading was observed from the trickle source with higher application rate. After 72 h of redistribution, the wetted volumes were approximately equal for trickle and pot irrigation regimes.     

Water pillow irrigation compared to furrow irrigation for soybean production in a semi-arid area

Field studies were done in 2003 and 2006 to evaluate the performance of water pillow (WP) irrigation as an alternative to furrow irrigation (FI) for soybean growth in semi-arid climatic conditions. There were four irrigation treatments: two of which (FI and WP_1.0) were full irrigation, in that the water deficit in the soil profile (0.9 m) was brought to field capacity in 10-day intervals. The other two treatments (WP_0.75 and WP_0.50) were deficit irrigation treatments, and received 75% and 50% of WP_1.0 irrigation amount. The highest seed yield was achieved with the WP_1.0 treatment. Irrigation water use efficiency (IWUE) and water use efficiency (WUE) were influenced significantly by the irrigation methods and levels (P ≤ 0.05). The highest values of WUE and IWUE were obtained by the WP_0.75 and WP_0.50 treatment, respectively, in both study years. However, the smallest irrigation amount resulted in lower total yield for the WP_0.50 treatment, and is not recommended. In conclusion, the WP_0.75 treatment is recommended for soybean production in order to attain higher values of IWUE and WUE, and to conserve water and maximize yield with the same volume of water.     

Soil water storage and surface runoff as influenced by irrigation method in arid soils with surface crust

The effects of irrigation methods, application rates and initial moisture content on soil water storage and surface runoff were studied in soils liable to surface crust formation during 1995–1996 at the University of Jordan Research Station near Al-Muwaqqar village. Four irrigation methods were tested (sprinkler, furrow, basin and trickle) and four application rates (6.2, 14.4, 24.4 and 28.4 mm/h). Two runs were performed (soil initially dry and soil initially wet). Basin irrigation provided the highest application efficiency followed by trickle, sprinkler and furrow irrigation methods. Entrapping water by the basin borders increased soil water storage by allowing more water to infiltrate through the surface crust. Decreasing the application rate from 28.4 to 6.2 mm/h increased soil water storage significantly in all 150 mm layers to a depth of 600 mm. If the soil was already wet, soil moisture storage decreased owing to siltation during the prewetting and formation of a surface crust and low soil water storage capacity. A sedimentary crust formed at the bottom of the furrows in the furrow irrigation treatment, which reduced soil water storage and increased surface runoff significantly owing to the reduction in infiltration. Increasing the application rate from 6.2 to 28.4 mm/h in the furrow surface irrigation treatment increased the runoff discharge 10-fold. Even with the lowest application rate the runoff coefficient under sprinkler irrigation was 20.3% indicating high susceptibility of Al-Muwaqqar soils to surface crust formation.     

Peach tree response to single and combined deficit irrigation regimes in deep soils

The response of mature ‘Andross’ cling peach (Prunus persica L. Batch) trees to regulated deficit irrigation in deep soils was studied for 3 years. Trees were either fully irrigated or subjected to deficit irrigation during Stage II of fruit development and/or during post-harvest. Single regulated deficit irrigation regimes reduced irrigation by 13–24%, while combined regime reduced it by 23–35%. Deficit irrigation during Stage II and/or post-harvest significantly reduced vegetative growth of the trees. Fruit production was not affected by any irrigation regime until the fourth year when fruit set decreased slightly with combined deficit irrigation. Overall, the results indicate that regulated deficit irrigation can be used successfully on peach trees grown in deep soils.     

Effects of irrigation regimes on the yield and water use of strawberry

Summary Strawberry plants (Fragaria x annanasa D. cv Chandler) were grown in field plots and in drainage lysimeters under controlled soil moisture regimes. Four irrigation treatments were established by watering the plants when soil water potential reached -0.01, -0.03,-0.05 and -0.07 MPa. The maximum yield was attained at -0.01 MPa soil water potential. Differences in yield were caused by both changes in the number of fruits per plant and in the fresh weight per fruit. Yield reductions were associated with reductions in total assimilation rate resulting from the decreased assimilatory surface area in plants irrigated at lower soil water potentials. The crop water production function calculated on a fruit fresh weight basis resulted in a yield response factor (K _y) of 1.01.     

Long-term growth and yield responses of olive trees to different irrigation regimes

The aim of this work was to evaluate long-term effects of different irrigation regimes on mature olive trees growing under field conditions. A 9-year experiment was carried out. Three irrigation treatments were applied: no irrigation, water application considering soil water content (short irrigation), or irrigation without considering soil water reserves and applying a 20% of extra water as a leaching fraction (long irrigation). Leaf water content, leaf area, vegetative growth, yield and fruit characteristics (fruit size, pulp:stone ratio and oil content) were determined yearly. Results showed that growth parameters did not show significant differences as a consequence of applied water. Yield was increased in irrigated trees compared to non-irrigated ones, but little differences between short and long irrigation were observed, only when accumulated yield from 1998 to 2006 was considered. Irrigation did not cause significant differences in fruit size or pulp:stone ratio either. Irrigation regimes similar to those applied in this experiment, under environmental conditions with relatively high mean annual precipitation, does not increase growth, yield or fruit characteristics when compared to rain-fed treatment, and consequently, the installation of a irrigation system could be not financially profitable.     

Wheat root distribution,water extraction pattern and grain yield as influenced by time and rate of irrigation

The effect of first irrigation (26, 40 and 54 days after seeding) and the rate of irrigation (5.5, 7.5 and 9.5 cm) applied subsequently at $\text{IW}\text{E}{}_{\mathrm{<mtext>pan</mtext>}}$ ratio of 0.9 on wheat root distribution, water extraction pattern and grain yield was studied on a barrier-free, sandy loam soil. The crop developed a more extensive root system when the first irrigation was applied after 26 days than after 40 and 54 days. With the first irrigation on the 26th day, the crop, receiving subsequent irrigations less frequently but at a heavier rate, developed a deeper root system than the crop receiving frequent, light irrigations. The water extraction pattern corresponded with the root distribution pattern. A relatively small difference in root density in the deeper layers caused a greater difference in soil water content than in the upper layers. Light and frequent irrigations produced maximum grain yields. However, for developing an extensive root system and enhancing water utilization in the subsoil, an early, light irrigation with subsequent irrigations applied less frequently at a relatively heavier rate seems desirable.     

Yield and growth characteristics for cotton under various irrigation regimes on sandy soil

Over-watering cotton wastes a valuable and scarce resource; it can also lead to rank growth, nutrient leaching, and contaminated groundwater. Since under-watering can decrease yields, the question becomes one of finding the optimum application regime. An irrigation experiment was set up to apply water at six different application rates, ranging from 33% to 144% of normal, with hopes of identifying the regime that produces maximum yield. Two cultivars, Acala Maxxa and Acala PhytoGen-72 (Gossypium hirsutum L.), were planted on sandy soil and irrigated daily with a highly efficient subsurface drip irrigation system for four seasons. The results showed that on the average there was no significant difference in the yield of the two cultivars and there was no significant difference in the yield for the three wettest treatments. The driest of the three wettest treatments, treatment 4, was a critical point on the water production function curve. It represented the least amount of water applied that still produced essentially maximum yield, and it had the highest water use efficiency. This critical level of water application during mid-season was found to be, on the average, 95% of Class A pan evaporation; it corresponded to a total seasonal application of 654 mm of water. Any application less than this critical level decreased yields. Reducing the water application by 5% below the critical level caused about a 4.6% reduction in yield. The critical level produced a soil moisture level that remained nearly constant throughout the season. The final plant height was closely related to the depth of water applied, with the wettest treatment producing plant heights of 2.0 m, and the driest treatment producing plant heights of 0.6 m. At the extremes of the water application rates there were some small differences in the early-season growth rate of the plants, but the main cause of differences in final plant height was the date of cutout (cessation of main stem node production). The length of season for the driest treatment was about 4 weeks shorter than for the wettest treatment on both cultivars. Results showed that deficit irrigation of cotton on sandy soil can greatly reduce yield, and the practice should probably be avoided.