Fertilizer ammonium : nitrate ratios determine phosphorus uptake by young maize plants

We investigated the interacting effects of inorganic nitrogen and the main inorganic phosphorus form in dairy manure (dicalcium phosphate, CaHPO₄) on growth, nutrient uptake, and rhizosphere pH of young maize plants. In a pot experiment, three levels of CaHPO₄ (0, 167, and 500 mg P pot^?1) were combined with nitrogen (637 mg N pot^?1) applied at five NH₄‐N : NO₃‐N ratios (0 : 100, 25 : 75, 50 : 50, 75 : 25, and 100 : 0) and a nitrification inhibitor in a concentrated layer of a typical acid sandy soil from Denmark. ¹⁵N‐labeled NH₄‐N was applied to differentiate the role of nitrification and to partition nitrogen uptake derived from NH₄‐N. Among treatments including nitrogen, shoot biomass, rooting and phosphorus uptake were significantly higher at the five‐leaf stage when CaHPO₄ was applied with NH₄‐N : NO₃‐N ratios of 50 : 50 and 75 : 25. In these treatments, rhizosphere pH dropped significantly in direct proportion with NH₄‐N uptake. The fertilizers in the concentrated layer had a root‐inhibiting effect in treatments without phosphorus supply and in treatments with pure NO₃‐N or NH₄‐N supply. Increased nitrogen uptake as NH₄‐N instead of NO₃‐N reduced rhizosphere pH and enhanced acquisition of applied CaHPO₄ by young maize plants, which may have positive implications for the enhanced utilization of manure phosphorus.     

Weed abundance and soil seedbank responses to tillage systems in continuous maize crops

Weed abundance in crops undergoes frequent changes, often due to changes in tillage practices. Annual species, with quick germination, a short vegetative stage, profuse seed production and long-lived seeds become problematic under zero-tillage systems. Portulaca oleracea L. and Amaranthus blitoides L. are widespread weeds in the Mediterranean area, prominent in irrigated crops. We studied the total weed abundance in the field, and specifically these two species (Portulaca oleracea and Amaranthus blitoides) with high frequency of occurrence in monoculture maize, from 2012 to 2014, in the field and soil seedbank. Results showed significant differences between zero-tillage (ZT) and conventional tillage (CT) systems on total weed abundance and relative abundance of Portulaca oleracea. Total weed abundance decreased in ZT plots (from 136 plants m^?2 to 25 and 46 plants m^?2, in 2013 and 2014 respectively). The same trend was observed in Portulaca oleracea recorded in ZT plots, but the abundance of Amaranthus blitoides did not vary in this system. Weed seedling germination and weed seed numbers both of total weed seedbank and Portulaca oleracea, were greater in ZT plots compared to CT, regarding Amaranthus blitoides seedling germination and seed count, the values did not increase with ZT, in continuous maize crops.     

Fate of phosphorus in soil during a long‐term fertilization experiment in Finland

Accumulation and depletion of soil phosphorus (P) was studied in a long‐term (37 y) field experiment in Southern Finland. The loam soil had a high pH (7.5–7.7) due to an earlier liming. Spring barley, spring wheat, oat, and ryegrass, grown in rotation, were annually fertilized with 0, 32, or 67 kg P ha^?1 y^?1 (P₀, P₁, and P₂K) and sufficient N. The average dry matter grain yield 2,600 kg ha^?1 of the P₀ plots increased by about 500 kg ha^?1 at P₁ treatment and another 600 kg ha^?1 by P₂K. Soil samples were collected in 1978 (beginning), 1995, 2005, and 2015. According to the Chang and Jackson sequential extraction, the P₂K and P₁ treatments increased the inorganic soil P by 732 and 32 kg P ha ^?1 in 37 years, respectively, while the P₀ plots were depleted by –459 kg P ha ^?1. The P₂K treatment increased all four P fractions, extracted with NH₄Cl (easily soluble), NH₄F (Al‐P), NaOH (Fe‐P), and H₂SO₄ (Ca‐P). Continuous depletion (P₀) decreased the NH₄Cl‐P and NH₄F‐P pools, NaOH‐P and H₂SO₄‐P pools remaining stable. None of the P pools changed significantly at P₁. The remarkable gap between the measured change and the balance for the P₂K and P₁ treatments cannot be explained solely by lateral soil movement, meaning that a significant proportion of the applied P was lost either in surface runoff or transported below the investigated depth of 40 cm. Despite large P applications, the degree of P saturation reached only 20% in the P₂K topsoil, assuming a 50% reactivity of Fe and Al oxides. As derived from sorption isotherms, a high EPC₀ (i.e., equilibrium P concentration at zero net P sorption or desorption) of 1.30 mg L^?1 had been built up in the P₂K treatment, while in the P₁ treatment EPC₀ (0.33 mg L^?1) had remained unchanged and P depletion (P₀) had caused a decrease to 0.12 mg L^?1. These results demonstrate that P sorption and desorption properties respond strongly to both P fertilization and null fertilization treatments and that in a long‐term field experiment only a low proportion of the residual fertilizer P can be recovered from soil.     

Irrigation scheduling of plastic greenhouse vegetable crops based on historical weather data

Irrigation scheduling based on the daily historical crop evapotranspiration (ET_h) data was theoretically and experimentally assessed for the major soil-grown greenhouse horticultural crops on the Almería coast in order to improve irrigation efficiency. Overall, the simulated seasonal ET_h values for different crop cycles from 41 greenhouses were not significantly different from the corresponding values of real-time crop evapotranspiration (ET_c). Additionally, for the main greenhouse crops on the Almería coast, the simulated values of the maximum cumulative soil water deficit in each of the 15 consecutive growth cycles (1988–2002) were determined using simple soil-water balances comparing daily ET_h and ET_c values to schedule irrigation. In most cases, no soil-water deficits affecting greenhouse crop productivity were detected, but the few cases found led us to also assess experimentally the use of ET_h for irrigation scheduling of greenhouse horticultural crops. The response of five greenhouse crops to water applications scheduled with daily estimates of ET_h and ET_c was evaluated in a typical enarenado soil. In tomato, fruit yield did not differ statistically between irrigation treatments, but the spring green bean irrigated using the ET_h data presented lower yield than that irrigated using the ET_c data. In the remaining experiments, the irrigation-management method based on ET_h data was modified to consider the standard deviation of the inter-annual greenhouse reference ET. No differences between irrigation treatments were found for productivity of pepper, zucchini and melon crops.     

Use of stem diameter variations to detect plant water stress in tomato

The sensitivity of stem diameter variations (SDV) measured with linear variable transducer (LVDT) sensors as indicators of plant water status in tomato was evaluated. Two tomato crops were grown sequentially in a sandy loam soil in an unheated plastic greenhouse. These were an autumn–winter tomato crop (autumn crop) and a spring–summer tomato crop (spring crop). One drying cycle of 61 days was imposed to the autumn crop in winter at 92 days after transplanting (DAT). Two drying cycles, each of 29 days, were applied to the spring crop, to young (58 DAT) and mature plants (121 DAT). For each drying cycle, four replicate plots did not receive irrigation, and four were well watered. During each drying cycle, LVDT sensors continuously measured SDV, and daily measurements were made of leaf (Ψ _leaf) and stem water potential (Ψ _stem). SDV data was interpreted using the SDV-derived indices, maximum daily shrinkage (MDS) and stem growth rate (SGR). The response of SDV-derived indices to water deficit differed with (1) climatic conditions during stress imposition and (2) crop age. In the winter drying cycle of the autumn crop, the responses of the SDV-derived indices to soil drying were relatively small and slower than Ψ _leaf and Ψ _stem. Under warmer conditions, the SDV-derived indices were much more responsive to soil drying. In rapidly growing young plants, where SDV was characterized by high SGR and small MDS, SGR was the most sensitive SDV-derived index. In more mature plants with little stem growth, MDS was the most sensitive SDV-derived index. In mature plants grown in warm to hot conditions, MDS (1) responded at a similar time or earlier than Ψ _leaf and Ψ _stem and (2) had larger “signal” values (ratio of values from unwatered to control plants) than Ψ _leaf and Ψ _stem. However, there was appreciably more “noise” (coefficient of variation, CV) associated with the SDV-derived indices, giving lower “sensitivity” values, determined from “signal” to “noise” ratios, than for Ψ _leaf and Ψ _stem. Regression analysis between MDS of well-watered plants and climatic variables gave best results for a linear relationship between MDS and daily maximum vapor pressure deficit. There were strong linear relationships between MDS and Ψ _leaf for each drying cycle. The slopes of these relationships differed with crop age indicating that there was no constant relationship between MDS and Ψ _leaf for a whole season. Overall these results demonstrated that MDS and SGR can be sensitive indicators of the water status of tomato crops under conditions of moderate to high evaporative demand. However, the variability associated with the SDV-derived indices and the changing MDS–Ψ _leaf relationship with crop age represent major issues regarding the development of irrigation scheduling protocols for tomato.     

Subsurface drip irrigation and reclaimed water quality effects on phosphorus and salinity distribution and forage production

In the Canary Islands, water scarcity is one of the constraints for agricultural activity. Non-conventional water resources generally represent more water volume than conventional ones. The distribution of these resources frequently permits the possibility of a conjunctive use of desalinated (DW) water and reclaimed municipal wastewater (RW). Field testing with both water qualities and different irrigation systems is necessary for optimal site-specific management. The objective of this work was to evaluate soil salinity and phosphorus distribution, and alfalfa yield in a 20 month field experiment carried out in the island of Gran Canaria, using municipal RW and freshwater (FW) under subsurface drip irrigation (SDI). Phosphorus speciation was performed both in irrigation waters and in soils (Olsen's inorganic, organic, and microbial). RW had large EC values (2.4 dS m⁻¹) with a remarkable nutrient load contribution and an average total P around 3 mg L⁻¹, predominantly hydrolysable forms, while FW had very low salinity and negligible amounts of P. For the RW treatment a salt gradient was established, causing plant mortality between the irrigation lines. The study of P speciation allows describing P distribution and plant uptake in terms of P forms. Large values of microbial P were produced for the two irrigation waters around the emitters, especially for FW.A faster P-cycling could have contributed to the significantly larger inorganic P contents observed in FW irrigated soils, in spite no external sources were added by the irrigation water.     

Tillage, cover crops, and nitrogen fertilization effects on soil nitrogen and cotton and sorghum yields

Sustainable soil and crop management practices that reduce soil erosion and nitrogen (N) leaching, conserve soil organic matter, and optimize cotton and sorghum yields still remain a challenge. We examined the influence of three tillage practices (no-till, strip till and chisel till), four cover crops {legume [hairy vetch (Vicia villosa Roth)], nonlegume [rye (Secaele cereale L.)], vetch/rye biculture and winter weeds or no cover crop}, and three N fertilization rates (0, 60–65 and 120–130 kg N ha⁻¹) on soil inorganic N content at the 0–30 cm depth and yields and N uptake of cotton (Gossypium hirsutum L.) and sorghum [Sorghum bicolor (L.) Moench]. A field experiment was conducted on Dothan sandy loam (fine-loamy, siliceous, thermic, Plinthic Paleudults) from 1999 to 2002 in Georgia, USA. Nitrogen supplied by cover crops was greater with vetch and vetch/rye biculture than with rye and weeds. Soil inorganic N at the 0–10 and 10–30 cm depths increased with increasing N rate and were greater with vetch than with rye and weeds in April 2000 and 2002. Inorganic N at 0–10 cm was also greater with vetch than with rye in no-till, greater with vetch/rye than with rye and weeds in strip till, and greater with vetch than with rye and weeds in chisel till. In 2000, cotton lint yield and N uptake were greater in no-till with rye or 60 kg N ha⁻¹ than in other treatments, but biomass (stems + leaves) yield and N uptake were greater with vetch and vetch/rye than with rye or weeds, and greater with 60 and 120 than with 0 kg N ha⁻¹. In 2001, sorghum grain yield, biomass yield, and N uptake were greater in strip till and chisel till than in no-till, and greater in vetch and vetch/rye with or without N than in rye and weeds with 0 or 65 kg N ha⁻¹. In 2002, cotton lint yield and N uptake were greater in chisel till, rye and weeds with 0 or 60 kg N ha⁻¹ than in other treatments, but biomass N uptake was greater in vetch/rye with 60 kg N ha⁻¹ than in rye and weeds with 0 or 60 kg N ha⁻¹. Increased N supplied by hairy vetch or 120–130 kg N ha⁻¹ increased soil N availability, sorghum grain yield, cotton and sorghum biomass yields, and N uptake but decreased cotton lint yield and lint N uptake compared with rye, weeds or 0 kg N ha⁻¹. Cotton and sorghum yields and N uptake can be optimized and potentials for soil erosion and N leaching can be reduced by using conservation tillage, such as no-till or strip till, with vetch/rye biculture cover crop and 60–65 kg N ha⁻¹. The results can be applied in regions where cover crops can be grown in the winter to reduce soil erosion and N leaching and where tillage intensity and N fertilization rates can be minimized to reduce the costs of energy requirement for tillage and N fertilization while optimizing crop production.     

Inheritance of Aluminum Tolerance and its Effects on Grain Yield and Grain Quality in Oats (Avena Sativa L.)

Summary Aluminum toxicity due to the cation Al⁺³ is a major factor limiting yields in acid soils. Wide genetic variability to aluminum tolerance is found in oat genotypes. The objectives of this study were to determine the number of genes controlling aluminum tolerance in oats and to verify if any detrimental effects were present of the aluminum tolerance genes on grain yield and grain quality in Al⁺³free soils. Aluminum tolerance was estimated as the average regrowth of the main root after exposure to toxic levels of Al⁺³ in a hydroponic solution under controlled conditions. The number of genes controlling that trait was estimated from the distribution of the average root regrowth frequencies in a population of 333 recombinant inbred lines (RIL's) in generations F_5:6 and F_5:7. The effects on grain yield and grain quality were assessed in a subpopulation of 162 RIL's chosen based on their aluminum tolerance response. Aluminum tolerance in the evaluated population was controlled by one dominant major gene with the tolerant genotypes carying Al _a Al _a and the sensitive ones al _a al _a alleles. No detrimental effects of the Al _a allele on grain yield or grain quality were detected.Part of the Master of Science dissertation of the first author     

Screening for low grain cadmium phenotypes in sunflower, durum wheat and flax

Cadmium (Cd) level in nonoilseed sunflower (Helianthus annuus L.), flax (Linum usitatissimum L.), and durum wheat (Triticum turgidum L. var. durum) grown on uncontaminated, alkaline soils has exceeded limits established in Northern Europe. Separate field experiments were conducted to investigate variability of grain Cd levels among sunflower, durum wheat and flax germplasm, and to seek an efficient screening method for future breeding. There were large variations in leaf Cd concentration among 200 sunflower lines. These lines performed more consistently for Cd uptake at the R5 stage than at the V8 stage across 4 locations with markedly differing soils. Cd concentration in V8 leaves was not related to Cd in grain. The positive correlation between R5 leaf Cd and kernel Cd level was obtained from nonoilseed hybrid (Sigco 954) (R²; = 0.74**), and 200 lines (R² = 0.44**) tested over 4 locations in 2 field trials, respectively. This indicates that an efficient and low cost screening method can be developed for genotype selection, but plants must be grown to the R5 stage. A preliminary evaluation of 30 durum wheat and 74 flax lines indicated large variations in grain Cd level of durum wheat and flax. Grain Cd concentration ranged from 0.11 to 0.34 mg Cd kg^-1 DW for durum wheat, and 0.14 to 1.37 mg Cd kg^-1 DW for flax, respectively. This variability indicates that breeding for low grain Cd in durum wheat and flax should be feasible. This revised version was published online in July 2006 with corrections to the Cover Date.     

Heading date and resistance to septoria tritici blotch in wheat not genetically associated

Nineteen cultivars, with large differences in heading date, were evaluated for their response to septoria tritici blotch in two experimental setups in Njoro, Kenya. Due to the more or less constant temperatures during the growing season and the overhead irrigation applied the epidemic conditions were similar over the whole observation period for the early and late cultivars. In experiment 1 the cultivars were assessed for disease severity at the same moment irrespective of the developmental stage, while in experiment 2 the cultivars were assessed at the same developmental stage. Measured at the same time, the disease severity was highest in the early maturing cultivars and lowest in the late maturing cultivars (r = –0.78). When assessed at the same development stage the disease build up was independent of heading date (r = –0.10) but strongly dependent on resistance level. There were no indications that early heading cultivars were more susceptible than late heading cultivars. This revised version was published online in July 2006 with corrections to the Cover Date.