水磷一体化对磷素有效性与磷肥利用率的影响

水肥一体化是发挥水肥耦合效应提高养分效率的重要途径,然水磷一体化研究较少。本文在模拟滴灌条件下研究了液体磷肥和固体颗粒磷肥(TSP)及其不同施用方法对土壤磷移动性、各形态无机磷含量动态变化的影响,比较了玉米磷素营养与磷肥利用率对不同磷源及其施用方式的响应,旨在提出滴灌条件下磷肥高效利用的最优策略。研究结果表明:1)与TSP肥料分次施用相比,液体磷肥分次施用更能提高土壤磷素有效性,在各土层Ca2-P与树脂磷(resin-P)平均含量分别提高12.4%与21.6%,且可显著提高磷在土壤中的移动性(P0.05),resin-P含量的垂直下降幅度降低56.5%;2)与TSP分次施用相比,液体磷肥分次施用的土壤中高活性无机磷含量(Ca2-P、resin-P及Na HCO3-P之和)占无机磷总量的比例提高21.0%,而低活性无机磷含量(Ca10-P与residue-P之和)占无机磷总量的比例则下降10.1%,说明液体磷肥分次施用可减小磷肥在土壤中的固定转化;3)玉米地上部干物质、叶片吸磷量和植株磷素累积吸收量均对不同磷源与施用方式有明显响应(P0.05),液体磷肥分次处理的玉米生物量、吸磷量及肥料利用率分别比TSP肥料分次处理提高27.1%、34.6%及61.4%。水磷一体化施用可提高磷在土壤中的移动性和有效性,减少磷的固定转化,显著改善玉米磷素营养,并明显提高磷肥利用率。     

A stable‐isotope methodology for measurement of soil‐applied zinc‐fertilizer recovery in durum wheat (Triticum durum)

A growth‐chamber study was used to develop a zinc stable‐isotope (⁶⁷Zn) tracing technique to directly measure the amount of soil‐applied zinc (Zn) granular fertilizer taken up by durum wheat (Triticum durum L.) in four different soil types. The ⁶⁷Zn‐tracer technique was then applied under field conditions at one site to test the ability of the method to measure the crop recovery of soil‐applied Zn granular fertilizer (⁶⁷Zn). The technique was developed by comparing plants treated with nil Zn fertilizer to natural‐abundance‐Zn‐coated fertilizer and ⁶⁷Zn‐coated fertilizer with plant parts analyzed for Zn isotopic composition using inductively coupled plasma–mass spectrometry. Zinc‐fertilizer recovery under growth‐chamber conditions was inversely related to the concentration of labile Zn in soils, with the plants on the most Zn‐deficient soils having the greatest amount of Zn derived from the added fertilizer. Zinc derived from fertilizer ranged from 0.3% (Luvisol) to 0.6% (Solonetz) to 13% (Calcisol) and 18% (Lixisol) for soils with DTPA‐extractable Zn of 3.5–0.21 mg kg^–1. Across the experiments, fertilizer recovery was measurable but very low (< 1% of added fertilizer being recovered by durum wheat). The recovery of Zn added to the Luvisol was slightly higher in the field than in the glasshouse (but all < 0.1%). Using this stable‐isotope technique, it was possible to directly assess the supply of soil‐applied Zn fertilizers to crop plants.     

Phosphorus accumulation, leaching and residual effects on crop yields from long-term applications in the subtropics

The effects of 25 years of annual applications of P fertilizer on the accumulation and migration of soil Olsen‐P, and the effects of soil residual P on crop yields by withholding P application for the following 5 years, were evaluated in a subtropical region. Annual application of P fertilizer for 25 years to crops in summer (groundnut), winter (wheat, mustard or rapeseed) or in both seasons raised the Olsen‐P status of the plough layer (0–15 cm) from initially very low (12 kg P ha^?1) to medium (18 kg P ha^?1) and very high levels (40–59 kg P ha^?1), depending on the amount of P surplus (amount of fertilizer applied in excess of removal by crops) (r = 0.86, P ≥ 0.01). However, only 4–9% of the applied P fertilizer accumulated as Olsen‐P to a depth of 15 cm (an increase of 2 mg kg^?1per 100 kg ha^?1 surplus P) in the sandy loam soil. In the following 5 years, the raising of 10 crops without P fertilizer applications decreased the accumulated Olsen‐P by only 20–30% depending upon the amount of accumulated P and crop requirements. After 29 years, 45–256 kg of residual P fertilizer had accumulated as Olsen‐P ha^?1 in the uppermost 150 cm with 43–58% below 60 cm depth; this indicates enormous movement of applied P to deeper layers in this coarse textured soil with low P retention capacity for nutrients. Groundnut was more efficient in utilizing residual P than rapeseed; however, for both crops the yield advantage of residual P could be compensated for by fresh P applications. These results demonstrated little agronomic advantage above approximately 20 mg kg^?1 Olsen‐P build‐up and suggested that further elevation of soil P status would only increase the risk of environmental problems associated with the loss of P from agricultural soils in this region.     

Effects of 15N-labelled ammonium nitrate and urea on soil nitrogen during growth of wheat (Triticum aestivum L.) under field conditions

We studied the effects of ¹⁵N-labelled ammonium nitrate and urea on the yield and uptake of labelled and unlabelled N by wheat (Triticum aestivum L., cv. Mexi-Pak-65) in a field experiment. The dry matter and N yields were significantly increased with fertilizer N application compared to those from unfertilized soil. The wheat crop used 33.6–51.5 and 30.5–40.9% of the N from ammonium nitrate and urea, respectively. Splitting the fertilizer N application had a significant effect on the uptake of fertilizer N by the wheat. The fertilizer N uptake showed that ammonium nitrate was a more available source of N for wheat than urea. The effective use of fertilizer N (ratio of fertilizer N in grain to fertilizer N in whole plant) was statistically similar for the two N fertilizers. The application of fertilizer N increased the uptake of unlabelled soil N by wheat, a result attributed to a positive added N interaction, which varied according to the fertilizer N split; six split applications gave the highest added N interaction compared to a single application or two split applications for both fertilizers. Ammonium nitrate gave 90.5, 33.5, and 48.5% more added N interaction than urea with one, two, and six split N applications. A values were not significantly correlated with the added N interaction (r=0.557). The observed added N interaction may have been the result of pool substitution, whereby added labelled fertilizer N replaced unlabelled soil N.     

Root renewal of sugar beet as a mechanism of P uptake efficiency

Sugar beet (Beta vulgaris L.) was grown in two different long‐term P fertilization experiments on a sandy and a loamy soil. The P supply levels of the soils were ”︁low”, ”︁sufficient”, and ”︁high”, according to the German recommendation scheme. The low P level decreased shoot and storage root yield only on the loam soil, where the recovery of the P‐deficient plants after a drought period was slower than at a sufficient P supply. The size of the living root system, as determined by a conventional auger sampling method, peaked at early July and decreased until harvest on the sandy soil without any influence of the P level. On loam, the living root systems were more constant and larger at P shortage. Total root production, as determined by the ingrowth core method, was about 120 km m^—2 in the well P supplied loam treatments and 200 km m^—2 at P deficiency, which was 3—4 times and 5 times higher than the average size of the living root systems, respectively. Hence, a rapid root renewal took place. On sand, where no P deficiency occurred, total root production was not different between the P supply levels but higher than in the well‐supplied loam treatments. Modelling P uptake revealed that this root turnover and the concomitant better exploitation of the soil facilitates P uptake at a low P level in soil, but is of no advantage at a sufficient P supply. The increase of root production at P shortage increased calculated P uptake by 25% compared to a calculation with the ”︁usual” root production at a sufficient supply.     

Assessing the Effect of Phosphorus Fertilizer Levels on Soil Phosphorus Fractionation in Rhizosphere and Non-Rhizosphere Soils of Wheat

This study evaluated the effects of phosphorus (P) fertilizer levels on inorganic P fractions. Wheat cultivars (Azadi and Marvdasht) were grown in the soils amended with the four rates of P fertilizer levels (no fertilizer, 10, 15, and 25 mg available P kg^?1 soil). Soils were sampled from rhizosphere and non-rhizosphere areas after 6 weeks. The mean of all P fractions was significantly different in various P fertilizer levels. The smallest and the largest amounts of all P fractions were observed in the soil with no P and in 25 mg kg^?1 soil P level, respectively. The Azadi cultivar, as P-efficient, showed the smallest increase in soil P fractions with increasing soil P levels. The means of all P fractions except Al-phosphates (Al-P) were significantly higher in non-rhizosphere soil. There were differences between these cultivars associated with the more inaccessible fractions at the 15 mg P kg^?1 soil level.     

The determination of total phosphorus improves the accuracy of the bicarbonate extraction as an availability index

The efficient use of phosphorus (P) in agriculture should rely on accurate soil P tests (SPT). Organic P contributes to P supply to plants; however, it is not usually taken into account in assessing P fertilizer requirements. We hypothesized that there would be an increased accuracy of bicarbonate extraction as SPT in predicting P uptake by plants if total P (TP) in this soil extract is taken into account. We conducted a soil P depletion experiment with 36 soils involving four consecutive crops in pots. Molybdate‐reactive P (MRP) and total P were determined in extracts centrifuged at 19,000 g (Bic‐MRP_C and Bic‐TP_C) or not (Bic‐MRP and Bic‐TP). MRP in extracts explained <47% of the variance in the cumulative P uptake, while total P (centrifuged at 19,000 g or not) provided the most accurate estimation of P uptake (59% with Bic‐TP) and threshold values for fertilizer response (R² = 0.58 with Bic‐TPc). When soils were separated in two groups according to their Ca carbonate equivalent to clay ratio, the variance in the cumulative P uptake explained by Bic‐MRP was above 63%, and that explained by Bic‐TP was above 73%. This separation also enabled more realistic estimation of the threshold values for fertilizer response. It can be concluded that the use of total P instead of MRP in bicarbonate extraction was promising in terms of improving its accuracy in assessing P fertilizer requirements.     

Rice plant growth and nitrogen accumulation from a midseason application 1

Nitrogen (N) fertilization in rice (Oryza sativa L.) is extensive throughout the world, but fertilizer N recovery is generally low. Split fertilizer applications that coincide with plant demand have been suggested as a method of improving fertilizer N efficiency. However, the effectiveness of split applications has not been established. Furthermore, there is little information available on plant N accumulation after a midseason application. The purpose of this study was to measure plant dry matter, root growth, and N accumulation after a midseason N application and to determine the length of time during which midseason N is accumulated by the plant. ‘Cypress’ rice was drill‐seeded in a Crowley silt loam soil (fine, montmorillonitic, thermic Typic Albaqualf) and urea‐N was broadcast at 101 kg N ha^‐1 preflood. Microplots enclosed by retainers were established prior to panicle initiation (PI), and ^l5N‐labeled urea was topdressed at PI into the floodwater within each microplot at 67 kg N ha^‐1. Microplots were harvested at 1 day after topdress (DAT), 3 DAT.7DAT, 14 DAT, and at 90% heading (35 DAT). Dry matter production was not affected by the midseason N application and increased linearly from the time of midseason application until 90% heading. Root growth at the time of the midseason application was extensive and roots could be seen at the soil surface. Root length density was greatest in the top 7.5 cm of the soil profile and decreased with depth. Most accumulation of midseason N occurred within 7 DAT. Both midseason N and native N in the plant increased during this period. About half of the midseason N was accumulated by the crop, probably because of the extent of the root system. This approximates N recovery from preplant or preflood N applications. Nitrogen loss was probably due to ammonia (NH₃) volatilization. Nitrogen accumulation by the plants continued throughout the duration of the experiment. This study shows that N broadcast into the floodwater at PI is quickly and efficiently utilized.     

Enhancing phosphorus availability,soil organic carbon,maize productivity and farm profitability through biochar and organic–inorganic fertilizers in an irrigated maize agroecosystem under semi‐arid climate

Biochar amendments offer promising potential to improve soil fertility, soil organic carbon (SOC) and crop yields; however, a limited research has explored these benefits of biochar in the arid and semi‐arid regions. This two‐year field study investigated the effects of Acacia tree biomass‐derived biochar, applied at 0 and 10 t ha^?1 rates with farmyard manure (FYM) or poultry manure (PM) and mineral phosphorus (P) fertilizer combinations (100 kg P ha^‐1), on maize (Zea mays L.) productivity, P use efficiency (PUE) and farm profitability. The application of biochar with organic–inorganic P fertilizers significantly increased soil P and SOC contents than the sole organic or inorganic P fertilizers. Addition of biochar and PM as 100% P source resulted in the highest soil P (104% increase over control) and SOC contents (203% higher than control). However, maize productivity and PUE were significantly higher under balanced P fertilizer (50% organic + 50% mineral fertilizer) with biochar and the increase was 110%, 94% and 170% than 100%‐FYM, 100%‐PM and 100% mineral fertilizer, respectively. Maize productivity and yield correlated significantly positively with soil P and SOC contents These positive effects were possibly due to the ability of biochar to improve soil properties, P availability from organic–inorganic fertilizers and SOC which resulted in higher PUE and maize productivity. Despite the significant positive relationship of PUE with net economic returns, biochar incorporation with PM and mineral fertilizer combination was economically profitable, whereas FYM along biochar was not profitable due to short duration of the field experiments.     

Nitrification and utilization of fertilizer nitrogen by highbush blueberry

Nitrification rates and nitrogen (N) recovery by 3 year‐old highbush blueberry ( Vaccinium corymbosum L. cv. Bluecrop) were compared following applications of ammonium sulfate with or without the nitrification inhibitor dicyandiamide (DCD) on a sandy loam soil with pH 4.8. Ammonium sulfate solutions containing 7.9 grams N (10.2 atom % ¹⁵N), with or without 0.6 g DCD, were applied to the soil surface beneath bushes. Concentrations of fertilizer derived nitrate were significantly lower in DCD treated soils 2 weeks following application, but DCD had no effect on total nitrate levels or fertilizer derived nitrate later in the season. Uptake of fertilizer‐N by blueberry plants was observed by collecting fruit during the growing season and assessing N partitioning within whole plants at the end of the season using ¹⁵N as a label for fertilizer N. The DCD had no effect on fertilizer derived or total N levels in plants. Plants recovered an average of 3% of applied N by the end of the season.     

Effect of potassium,sulfur, boron,and molybdenum fertilization on alfalfa production and herbage macronutrient contents

An established two‐year‐old stand of ‘Apollo’ alfalfa (Medicago saliva L.) was used to determine the alfalfa yield and macronutrient contents response to potassium (K), sulfur (S), boron (B), and molybdenum (Mo) fertilization under a high yield environment. A split, split block field design was used with nine micronutrient treatments (0, 50, and 100 g Mo ha^‐1 and 0, 1, and 2 kg B ha^‐1) in a factorial arrangement (3²) as the subplots and three K levels (150, 300, and 600 kg K ha^‐1) as the main plot in three replications. Two levels of S (0 and 240 kg S ha^‐1) fertilization were applied in strips across the main plots (K levels) resulting in the split, split block design. Alfalfa yield and macronutrient contests were determined. Increased in K or S rate increased K contents of the plants, however, the differences between the K or S rates were not significant and B or Mo application did not have a marked effect on alfalfa K levels. Alfalfa calcium (Ca), magnesium (Mg), or phosphorus (P) content was not significantly affected by K, S, B, or Mo fertilization. Potassium, S, B, or Mo fertilizer application also did not have a marked effect on alfalfa yield during this study. Combinations of K, S, B, and Mo fertilizer had variable effects and the effects were dependent on the combination of fertilizer, sources, and levels. With a few exceptions, there was lack of alfalfa yield and nutrient contents response to K, S, B, and Mo applications which was due to the effect of low available soil moisture as a result of low incident rainfall during the study on these nutrients availability, uptake, and alfalfa growth.     

Fertilizer‐use efficiency of different inorganic polyphosphate sources: effects on soil P availability and plant P acquisition during early growth of corn

Polyphosphate‐based fertilizers are worldwide in use, and their effect on crop yield is often reported to be similar to orthophosphate products, although some studies showed higher yields with polyphosphate applications. However, information on how these fertilizers may influence plant P acquisition is very limited. A pot experiment was carried out under controlled conditions with corn (Zea mays L.) growing on a sandy soil (pH 4.9) and a silty‐loam soil (pH 6.9) differing in P‐sorption properties. The objective was to evaluate phosphorus fertilizer–use efficiency (PFUE) of several polyphosphate (poly‐P) compounds (pyrophosphate [PP], tripolyphosphate [TP], and trimetaphosphate [TMP]) using orthophosphate (OP) as a reference. Focus was put on evaluating plant parameters involved in plant P acquisition, i.e., root length and P uptake per unit of root length. Furthermore, soil P availability was characterized by measuring ortho‐P and poly‐P concentrations in soil solution as well as in CAL (calcium‐acetate‐lactate) extracts. The P availability was differentially influenced by the different P sources and the different soils. In the silty‐loam soil, the application of poly‐P resulted in higher ortho‐P concentrations in soil solution. In the same soil, CAL‐extractable ortho‐P was similar for all P sources, whereas in the sandy soil, this parameter was higher after OP application. In the silty‐loam soil, poly‐P concentrations were very low in soil solution or in CAL extracts, whereas in the sandy soil, poly‐P concentrations were significantly higher. Phosphorus fertilizer–use efficiency was significantly higher for poly‐P treatments in the silty‐loam soil and were related to a higher root length since no differences in the P uptake per unit of root length among poly‐P and OP treatments were found. However, in the sandy soil, no differences in PFUE between OP and poly‐P treatments were observed. Therefore, PFUE of poly‐P compounds could be explained by better root growth, thereby improving plant P acquisition.     

Availability of P from 32P-labelled endogenous soil P and 33P-labelled fertilizer in an alkaline soil producing cotton in Australia

Yield responses of irrigated, field‐grown cotton to phosphorus fertilizer application in Australia have been variable. In an attempt to understand better this variability, the distribution of fertilizer P within soil P fractions was identified using ³²P and ³³P radioisotopes. The soil chosen, an alkaline, grey, cracking clay (Vertosol), was representative of those used for growing cotton in Australia. Chang and Jackson fractionation of soil P from samples collected within 1 h of application indicated that 49, 7 and 13% of the P fertilizer was present as 0.5 m NH₄F, 0.1 m NaOH and 1 m H₂SO₄ extractable P, respectively. Over 89% of the P fertilizer was recovered as Colwell extractable P in these samples, suggesting that the majority of these reaction products was in a highly plant‐available form. Fertilizer‐P remained in an available form within the band 51 days after application, and 68% of the applied fertilizer‐P was recovered as Colwell‐P (1071 mg kg^?1). The Colwell‐P concentration in the band was 35 times that in the unfertilized soil. Thus, the variability in crop response to P fertilizer application in these soils is not a consequence of fertilizer‐P becoming unavailable to plants. These results confirm the suitability of the Colwell (1963) sodium bicarbonate extraction method for measuring available P in these soils.     

长期施肥和耕作管理对华北平原土壤肥力的影响

In the North China Plain, fertilizer management and tillage practices have been changing rapidly during the last three decades; however, the influences of long-term fertilizer applications and tillage systems on fertility of salt-affected soils have not been well understood under a winter wheat (Triticum aestivum L.)-maize (Zea mays L.) annual double cropping system. A field experiment was established in 1985 on a Cambosol at the Quzhou Experimental Station, China Agricultural University, to investigate the responses of soil fertility to fertilizer and tillage practices. The experiment was established as an orthogonal design with nine treatments of different tillage methods and/or fertilizer applications. In October 2001, composite soil samples were collected from the 0–20 and 20–40 cm layers and analyzed for soil fertility indices. The results showed that after 17 years of nitrogen (N) and phosphorous (P) fertilizer and straw applications, soil organic matter (SOM) in the top layer was increased significantly from 7.00 to 9.30–13.14 g kg-1 in the 0–20 cm layer and from 4.00 to 5.48–7.75 g kg-1 in the 20–40 cm layer. Soil total N (TN) was increased significantly from 0.37 and 0.22 to 0.79–1.11 and 0.61–0.73 g N kg-1 in the 0–20 and 20–40 cm layers, respectively, with N fertilizer application; however, there was no apparent effect of straw application on TN content. The amounts of soil total P (TP) and rapidly available P (RP) were increased significantly from 0.60 to 0.67–1.31 g kg-1 in the 0–20 cm layer and from 0.52 to 0.60–0.73 g kg-1 in the 20–40 cm layer with P fertilizer application, but were decreased with combined N and P fertilizer applications. The applications of N and P fertilizers significantly increased the crop yields, but decreased the rapidly available potassium (RK) in the soil. Straw return could only meet part of the crop potassium requirements. Our results also suggested that though some soil fertility parameters were maintained or enhanced under the long-term fertilizer and straw applications, careful soil quality monitoring was necessary as other nutrients could be depleted. Spreading straw on soil surface before tillage and leaving straw at soil surface without tillage were two advantageous practices to increase SOM accumulation in the surface layer. Plowing the soil broke aggregates and increased aeration of the soil, which led to enhanced organic matter mineralization.     

Soil phosphorus dynamics in a Vertisol as affected by cattle manure and nitrogen fertilization in soybean‐wheat system

Repeated application of phosphorus (P) as superphosphate either alone or in conjunction with cattle manure and fertilizer N may affect the P balance and the forms and distribution of P in soil. During 7 years, we monitored 0.5 M NaHCO₃ extractable P (Olsen‐P) and determined the changes in soil inorganic P (P_i) and organic P (P_o) caused by a yearly dose of 52 kg P ha^—1 as superphosphate and different levels of cattle manure and fertilizer N application in a soybean‐wheat system on Vertisol. In general, the contents of Olsen‐P increased with conjunctive use of cattle manure. However, increasing rate of fertilizer nitrogen (N) reduced the Olsen‐P due to larger P exploitation by crops. The average amount of fertilizer P required to increase Olsen‐P by 1 mg kg^—1 was 10.5 kg ha^—1 without manure and application of 8 t manure reduced it to 8.3 kg ha^—1. Fertilizer P in excess of crop removal accumulated in labile (NaHCO₃‐P_i and P_o) and moderately labile (NaOH‐P_i and P_o) fractions linearly and manure application enhanced accumulation of P_o. The P recovered as sum of different fractions varied from 91.5 to 98.7% of total P (acid digested, P_t). Excess fertilizer P application in presence of manure led to increased levels of Olsen‐P in both topsoil and subsoil. In accordance, the recovery of P_t from the 0—15 cm layer was slightly less than the theoretical P (P added + change in soil P — P removed by crops) confirming that some of the topsoil P may have migrated to the subsoil. The P fractions were significantly correlated with apparent P balance and acted as sink for fertilizer P.     

Phosphorus acquisition and wheat growth are influenced by shoot phosphorus status and soil phosphorus distribution in a split‐root system

Root proliferation and greater uptake per unit of root in the nutrient‐rich zones are often considered to be compensatory responses. This study aimed to examine the influence of plant phosphorus (P) status and P distribution in the root zone on root P acquisition and root and shoot growth of wheat (Triticum aestivum L.) in a split‐root soil culture. One compartment (A) was supplied with either 4 or 14 mg P (kg soil)^–1, whereas the adjoining compartment (B) had 4 mg P kg^–1 with a vertical high‐P strip (44 mg kg^–1) at 90–110 mm from the plant. Three weeks after growing in the split‐root system, plants with 4 mg P kg^–1 (low‐P plants) started to show stimulatory root growth in the high‐P strip. Two weeks later, root dry weight and length density in the high‐P strip were significantly greater for the low‐P plants than for the plants with 14 mg P (kg soil)^–1. However, after 8 weeks of growth in the split‐root system, the two P treatments of compartment A had similar root growth in the high‐P strip of compartment B. The study also showed that shoot P concentrations in the low‐P plants were 0.6–0.8 mg g^–1 compared with 1.7–1.9 mg g^–1 in the 14 mg P kg^–1 plants after 3 and 5 weeks of growth, but were similar (1.1–1.4 mg g^–1) between the two plants by week 8. The low‐P plants had lower root P concentration in both compartments than those with 14 mg P kg^–1 throughout the three harvests. The findings may indicate that root proliferation and P acquisition under heterogeneous conditions are influenced by shoot P status (internal) and soil P distribution (external). There were no differences in the total root and shoot dry weight between the two P treatments at weeks 3 and 5 because enhanced root growth and P uptake in the high‐P strip by the low‐P plants were compensated by reduced root growth elsewhere. In contrast, total plant growth and total root and shoot P contents were greater in the 14 mg P kg^–1 soil than in the low‐P soil at week 8. The two P treatments did not affect the ratio of root to shoot dry weight with time. The results suggest that root proliferation and greater P uptake in the P‐enriched zone may meet the demand for P by P‐deficient plants only for a limited period of time.     

Bone char as phosphorus fertilizer involved in cadmium immobilization in lettuce,wheat, and potato cropping

Bone char is a potential clean and renewable P fertilizer with Cd‐immobilization capabilities, but the P–Cd interactions in cropping of vegetable, grain, and tuber crops are unknown. In the present pot experiment bone char was evaluated on its effect on the growth and P supply of various crops (lettuce, wheat, potatoes) as well as its capability to reduce the uptake of Cd from a moderately Cd‐contaminated and P‐deficient soil (soil 1) and a highly Cd‐contaminated soil with sufficient P supply (soil 2). When averaging the dry‐matter yield over the treatments for each crop for the P‐sufficient soil 1, the following order was obtained: triple superphosphate (TSP) > diammonium phosphate (DAP) > BC, whereas for the soil 2 with sufficient P supply it was inverted with BC > DAP > TSP. The P‐deficiency resulted in a more pronounced effect of TSP and DAP on the plant growth, whereas P sufficiency in the soil promoted a crop‐quality‐enhancing effect of bone char. The Cd concentration in the consumption‐relevant plant parts was mostly insignificantly affected by treatments; however, the total Cd concentration in the whole plants tended to decrease with fertilizer addition for all crops in soil 1 even at very low bone‐char application levels. Similar results were obtained for soil 2 with an exception for the Cd concentration in potatoes, as the total Cd concentration was significantly increased in the TSP and DAP treatments. This most likely results from the introduction of Cd with TSP and DAP as they contained ≈ 27–28 mg Cd kg^–1. Thus, this study demonstrated the potential of bone char as a clean P fertilizer, which can efficiently decrease the Cd contamination of potato on contaminated soils.     

Dinitrogen fixation and soil n uptake by soybean as affected by phosphorus availability

The effects of phosphorus supply (0, 30, and 90 mg P kg^‐1) on growth, N₂ fixation, and soil N uptake by soybean (Glycine max (L.) Merr.) were studied in a pot experiment using the ¹⁵N isotope technique. Phosphorus supply increased the top dry matter production at flowering and the dry matter production of seeds, straw, pod shells, and roots at late pod filling of inoculated soybeans. Phosphorus supply reduced the N concentration of plant tops at flowering, but increased the amount of N accumulated at both flowering and late pod filling. In inoculated soybeans total N accumulation paralleled the dry matter production. The P concentration in above‐ground plant parts of nodulated soybeans was not affected by P application. At flowering only 18 to 34% of total N was derived from N₂ fixation, whereas as much as 74% was derived from N₂ fixation at late pod filling. Only the addition of 90 mg P kg^‐1 soil significantly increased the amount of N₂ fixed at the late pod filling stage. Phosphorus supply did not influence the uptake of fertilizer or soil N in soybeans, even if the root mass was increased up to 60% by the P supply.     

Phosphorus balances for arable soils in Southern England 1986–1999

The behaviour of P in a range of English arable soils was examined by plotting the change in resin P in the topsoil (ΔPres) at the end of a 3‐ to 5‐year period, against the P balance over the same period (fertilizer P applied minus offtake in crops, estimated from farmers’ reported yields and straw removal). Based on the assumption that values for offtake per tonne of crop yield used for UK arable crops are valid averages, 20–60% of ΔPres was explained by the balance. Applying excess P fertilizer increased Pres, and reducing P fertilizer use decreased it; typically 3–4 kg P ha^?1 was required for each mg L^?1ΔPres (6–8 kg ha^?1 for each mg L^?1 of Olsen P). About half the P balance seems to be resin extractable and this differed little between soil groups, except in cases of very low P (index 0) in which the P buffering was stronger, and on very high P soils (index 4/5) when buffering was less. However, on calcareous soils and red soils, when fertilizer was applied in accord with offtake, Pres fell by up to 4 mg L^?1 year^?1 (2 mg L^?1 yr^?1 olsen P) and to prevent this an extra 3–10 kg P ha^?1 year^?1 fertilizer was required. But on most non‐calcareous soils, replacing offtake maintained Pres, with perhaps slight rises on soils of low clay content or greater organic matter content. In soils under arable rotations, the apparent recovery of P from fertilizer was often around 100%, falling to 85% on Chalk soils and 75% on medium–heavy soils on limestone or Lower Chalk. The fate of the ‘missing’ P needs clarification. The case for corrections to current P fertilizer recommendations in the UK on certain soil types is discussed.