Impact of residue characteristics on phosphorus availability in West African moist savanna soils

The role of residue characteristics in enhancing the availability of P was investigated in a greenhouse study using two soils from the northern Guinea savanna (NGS) and four from the derived savanna (DS) zones of the West African moist savanna. Eight organic residues of varying C-to-P ratio were used and maize ( Zea mays) was grown for 7 weeks. The effect of the organic residues on P availability (measured as resin P and maize P accumulation) differed among the soils. On average, the increase in resin P, calculated as {[(soil+residue)–control]/(control)×100}, was between 8% (Davié, DS) and 355% (Danayamaka, NGS). Maize P accumulation was increased by ca. 11% in Davié and Niaouli (DS) soils and 600% in Danayamaka soil. The increase in maize total dry matter yield (DMY) ranged from 2% to 649%. Residues with C-to-P ratio >200 produced lower DMY than those with lower ratios. Residue organic P (Po) extractable with 0.2 N H₂SO₄ (acid-Po) accounted for 92% ( P =0.0001) of the variation in DMY in a step-wise regression with residue parameters as independent variables and mean DMY as the dependent variable. The residue Po extractable with 0.5 M NaHCO₃ (HCO₃-Po) correlated significantly with DMY in Danayamaka and Davié soils, and with P accumulation in Danayamaka soil. The relationships between the residue Po and DMY might imply that Po fractions in decomposing residues contribute to P availability. However, the suitability of using the Po content of organic residues to predict their agronomic value with respect to P nutrition needs further evaluation.     

长期不同施肥条件下黑土水稳性团聚体中磷的分布及其有效性

利用中国科学院海伦农业生态实验站的长期定位试验,研究了长期不同施肥条件下[对照(CK)、施用化学氮磷肥(NP)、化学氮磷肥配施有机肥(NP+OM)]农田黑土不同粒径水稳性团聚体中磷的分布及其有效性。结果表明,施肥增加了黑土各粒级水稳性团聚体中全磷含量,其中,NP处理比对照(CK)全磷含量增加28.9%~37.8%,NP+OM处理比NP处理增加44.0%~63.9%。施肥增加了黑土各粒级水稳性团聚体中有效磷含量,NP处理比CK处理有效磷含量增加146%~183%,NP+OM处理有效磷含量是NP处理的3.4~5.3倍。各处理水稳性团聚体全磷和有效磷均表现为较均匀地分布在0.053mm的各粒级水稳性团聚体中,而0.053mm粒级水稳性团聚体内全磷和有效磷均显著降低。施磷可显著增加土壤磷的有效率,且以化肥配施有机肥处理表现更为明显。NP处理不同粒径水稳性团聚体中磷的有效率是CK处理的1.94~2.32倍,NP+OM处理是NP处理的2.13~2.83倍。     

Effect of phosphorus status of the soil on selenium availability

Due to its importance for human and animal health, low bioavailability of selenium (Se) is of concern in large parts of the world. Among the factors determining Se availability is competition for binding sites by other anions. In order to evaluate the effect of different soil P status on Se availability from fertilizer, adsorption studies were conducted with soils ranging from low to very high available P as measured in ammonium lactate (P‐AL) and addition of Se as either selenate or selenite. Generally selenite, and to some extent also selenate, adsorption decreased with increasing P‐AL status of the soil. However, in a silt loam, the increase in P‐AL from 140 (high) to 210 mg P kg^–1 (very high) did not result in a corresponding decrease in Se adsorption. Phosphorus saturation, on the other hand, was found to be lower in the sample that was very high in available P, suggesting that both the total amount of P on binding sites and the amount of plant‐available P influence Se availability. Selenate addition caused an increase in P availability, especially when added together with phosphate to a silt loam with very high P‐AL status.     

Effect of Nanoclinoptilolite/Nanohydroxyapatite mixtures on phosphorus solubility in soil

Phosphorus (P) is a vital element for plant growth and maturity, yet most soils have low P availability. The aim of the present study was to synthesize a mixture of nanohydroxyapatite (nHA) and saturated nanoclinoptilolite (nCp) and compare its ability to improve the P solubility with that of a natural system (Cp/rock phosphate). Consequently, as nCp were saturated with different salt solutions, the P solution concentration decreased in the following order: ((NH₄)₂SO₄) NH₄–nCp > (NH₄Cl) NH₄–nCp > (KCl) K–nCp > (K₂SO₄) K–nCp. Higher P availability was achieved at higher ratios of nCp/nHA. The nCp/nHA appear twice more effective in P released than Cp/PR (42.73% vs 19.23%). The greatest increase (5 mg/L) in the quantity of P released was at a nCp/nHA ratio of 15. Parabolic diffusion and power function equations fitted best to the data. This work demonstrates that nCp/nHA can act as an effective P fertilizer in calcareous soils.     

Low-molecular weight organic acids improve plant availability of phosphorus in different textured calcareous soils

Understanding the role of organic acids on phosphorus (P) sorption capacity of soils is very important for its economic and friendly management. Combining P application with low-molecular weight organic acids could result in its higher plant availability for prolonged time. Therefore, citric and oxalic acid (at the rate of 1.0 mM kg^?1 soil) were evaluated for their effect on P sorption capacity and its plant availability in two different textured calcareous soils. Organic acids decreased P sorption capacity and organic carbon partition coefficient (K_oc) whereas increased Gibbs free energy (ΔG) of P. Organic-acid-treated soils required lesser quantity of P fertilizer to produce soil solution P concentration optimum for plant growth (external P requirement [EPR_0.2]), that is, 0.2 mg L^?1. Citric acid was efficient than oxalic acid in the above effects. P sorption parameters of Freundlich model were negatively correlated with lime potential and ΔG whereas had positive correlation (P < 0.05) with EPR_0.2 and K_oc. Incubation with oxalic acid increased available P in loamy sand and loam soil by 20% and 30%, respectively. Thus, organic acids could help reduce application rate of P fertilizer through lowering its adsorption in highly P-fixing soils without compromise on yield.     

选用解磷菌剂改善缺磷土壤磷素的有效性

山西省大部分土壤为石灰性土壤,耕地土壤缺磷较严重,施入土壤的化学磷肥极易被固定,为了提高缺磷土壤磷素的有效性,分离筛选的B2和B67菌株研制的解磷菌剂(后称菌剂),首先接种到以磷酸三钙为唯一磷源的培养基中,液体速效磷含量比CK提高12.92倍和9.18倍,然后又接种到其它典型缺磷土壤中,可使土壤速效磷含量较CK增加1.35~3.04倍,且发现其溶磷效果和液体的pH值有关,也与土壤磷酸酶活性和有效活菌数相关,而且菌剂在提高土壤速效磷含量的同时,也提高了土壤速效钾的含量。另外菌剂在缺磷的盆栽和大田试验中取得相同效果,除显著提高土壤速效磷含量,培肥土壤外,同时还有改善作物农艺性状,提高作物产量的功效。     

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。     

Magnesium-fortified phosphate fertilizers improve nutrient uptake and plant growth without reducing phosphorus availability

Magnesium (Mg) deficiency can significantly limit crop yield and quality. Separate application of straight Mg fertilizer is unattractive because of additional labor costs. Meanwhile, bulk blending Mg with other macronutrient fertilizers is also a suboptimal solution because bulk blended fertilizers often yield poor nutrient distributions. One rapid and economical alternative to alleviating Mg deficiency is to co-granulate macronutrient fertilizers with Mg. However, few commercial products have implemented this approach. One of the barriers hindering the production of Mg-fortified phosphorus (P) fertilizers is the assumption that precipitation of P with Mg will reduce P solubility. In this study, four Mg compounds, anhydrous magnesium sulfate (MgSO₄), magnesium oxide (MgO), anhydrous magnesium chloride (MgCl₂), and dolomite (CaMg(CO₃)₂), were co-granulated with mono-ammonium phosphate (MAP), and their granule strength, Mg and P availabilities, and agronomic effectiveness were evaluated. Results showed that there were no significant differences in P solubility between Mg-fortified MAP and MAP treatments. X-ray diffraction (XRD) indicated that the Mg species after co-granulation were boussingaultite (Mg(NH₄)₂(SO₄)₂·6H₂O), schertelite (Mg(NH₄)₂H₂(PO₄)₂·4H₂O), magnesium hydrogen phosphate (Mg(H₂PO₄)₂), and dolomite (CaMg(CO₃)₂). A pot experiment using an acidic soil demonstrated an average 9.6-fold increase in shoot Mg uptake, 3.0-fold increase in shoot P uptake, and 3.2-fold increase in soybean shoot dry matter in Mg-fortified MAP treatments, compared to those in MAP treatment. The current study provides a simple, effective, and low-cost approach for the addition of Mg to macronutrient fertilizers, to minimize Mg deficiency.     

栽培模式及施肥对玉米和大豆根际土壤磷素有效性的影响

栽培模式及施肥管理对作物吸收利用土壤磷素的影响较大,本研究为探明玉米/大豆套作系统作物根系交互作用下根际土壤无机磷组分动态变化特征,利用盆栽试验测定了玉米/大豆套作(M/S)、玉米单作(MM)和大豆单作(SS)3种栽培模式以及不施肥(CK)、施氮钾肥(NK)和施氮磷钾肥(NPK)3种施肥处理下玉米和大豆地上部生物量及吸磷量和根际与非根际土壤速效磷、无机磷组分含量,以期为优化玉米/大豆套作系统磷素管理提供理论依据。研究结果表明同一施肥水平下,套作玉米的籽粒产量显著高于单作玉米;施磷显著提高了单作玉米籽粒产量,而对套作玉米籽粒产量影响不大。无论施肥与否,套作大豆秸秆及籽粒产量均高于单作大豆。所有施肥处理均表现为套作模式下单株作物地上部磷积累量显著高于单作模式。玉米成熟期,CK、NK处理下套作玉米根际土壤速效磷含量分别比单作玉米高54.2%和71.8%;大豆始花期,NPK处理下套作大豆根际土壤速效磷含量比单作大豆高19.8%。大豆成熟期,NK、NPK处理下套作大豆根际土壤速效磷含量分别比单作大豆高23.8%和108.0%。无论是单作还是套作模式,玉米根际土壤Al-P含量在3个施肥处理下均低于非根际土壤。CK和NK处理下单作玉米根际土壤Al-P含量分别是套作玉米的1.19倍和1.22倍;NPK处理下单作玉米根际土壤Fe-P含量是套作玉米的1.21倍。在CK、NK和NPK施肥处理下,单作大豆根际土Al-P含量分别是套作大豆1.12倍、1.30倍和1.25倍,单作大豆非根际土Al-P含量分别是套作大豆的1.22倍、1.30倍和1.06倍。CK、NK处理下单作大豆根际土壤Fe-P含量分别是套作大豆的1.47倍和1.12倍。研究得出结论,低磷条件下,与单作相比,玉米/大豆套作更有利于作物对土壤Al-P、Fe-P的活化吸收。     

Effects of carbon additions on iron reduction and phosphorus availability in a humid tropical forest soil

In the highly weathered soils of humid tropical forests, iron (Fe) plays a key role in ecosystem biogeochemical cycling through its interactions with carbon (C) and phosphorus (P). We used a laboratory study to explore the role of C quantity and quality in Fe reduction and associated P mobilization in tropical forest soils. Soils were incubated under an ambient atmosphere headspace (room air) with multiple levels of leaf litter leachate or acetate additions. Net Fe reduction occurred in all the treatments and at every time point. The more complex mixture of organic compounds in leaf litter leachate stimulated Fe reduction as much acetate, an easily fermentable C source. At the end of the experiment, Fe reduction was generally greater with higher C additions than in the low C additions and controls. The microbial biomass P had increased significantly suggesting rapid microbial uptake of P liberated from Fe. This occurred without increases in the available (NaHCO₃) P pool. The immobilization of P by microbes during the incubation provides a P conservation mechanism in these soils with fluctuating redox potential, and may ultimately stimulate more C cycling in these highly productive ecosystems. Iron cycling appears to be an important source of P for the biota and can contribute significantly to C oxidation in upland tropical forest soils.     

Modifications of the phosphorus cycle with development of lignite mine soils

Various aspects of the P cycle in four and seven year old soils from the Meirama lignite mine (northwest Spain) were studied. With increasing soil age, The organic P (P_o) content increased in parallel with the organic matter content. the observed increase in secondary inorganic P (Bic—P_i + NaOH-P_i + us-NaOH P_i) is related to the increase in the retention capacity for P with soil age. the observed increase in the HCl-P fraction can be attributed to a gradual dissolution of the non-extractable forms of P produced from fertilizers added in the course of spoil management. Both the C:P_o ratio and phosphatase activity also increased with soil age, suggesting that biochemical mineralization processes are taking place and indicating a need for the hydrolysis of phosphate esters to ensure an adequate phosphorus supply to plants. All these findings suggest that in the long term plants May, suffer a pronounced P deficiency.     

超微活化条件对磷矿粉磷素形态及肥效的影响

采用化学分析、红外光谱分析以及盆栽试验研究了活化剂添加量及研磨时间对超微活化磷矿粉磷素形态及生物有效性的影响.结果表明,与普通磷矿粉相比,磷矿粉经超微活化处理后有效磷提高了45.1％～58.7％,活性磷提高了169.4％～203.6％,水溶性磷含量随活化剂添加量的增加也明显提高;当活化剂添加量达到5％时,3次水溶性磷的累积释放量较相同条件下制得的超微细磷矿粉提高84倍以上.适当延长研磨时间有利于磷矿粉磷素的释放,但效果不显著.红外光谱分析结果表明:超微细活化磷矿粉中与PO3-、HPO2-相关的特征吸收峰强度较普通磷矿粉显著增强,且生成了与PO43-、HPO42-相关的新特征吸收峰,使磷矿粉中的磷向有效状态转变.盆栽试验结果进一步表明,超微活化磷矿粉处理的玉米生物量、磷素利用率均显著高于磷矿粉、普通超微细磷矿粉以及过磷酸钙和磷酸二铵等常规水溶性磷肥,可增强抗固定能力而显著提高土壤有效磷含量.超微活化磷矿粉较高的活性磷总量和水溶性磷持续适度的供磷强度是其生物有效性高的原因.     

The use of biologically based phosphorus fractions to evaluate soil P availability in reduced P‐input paddy soils

Chemical soil phosphorus (P) extraction has been widely used to characterize and understand changes in soil P fractions; however, it does not adequately capture rhizosphere processes. In this study, we used the biologically based phosphorus (BBP ) grading method to evaluate the availability and influencing factors of soil P under four P fertilizer regimes in a typical rice–wheat cropping rotation paddy field. Soil P was assessed after seven rice‐growth seasons at multiple growth stages: the seedling, the booting and the harvest stage. Soil CaCl₂‐P, citrate‐P and HC l‐P (inorganic P, Pi) as well as enzyme‐P (organic P, Po) were not significantly different between soil treated with P fertilizer during the wheat season only (PW ) and during the rice season only (PR ) compared with soil treated during both the rice and the wheat seasons (PR +W) at all three rice‐growth stages. No P fertilizer application during either season (Pzero) significantly reduced the concentration of soil citrate‐P and HC l‐P at the rice‐seedling and harvest stages. Significant correlations were observed between the HC l extraction and Olsen‐P (R ² = 0.823, P  <  0.001), followed by enzyme‐P (R ² = 0.712, P  <  0.001), citrate‐P (R ² = 0.591, P  <  0.001) and CaCl₂‐P (R ² = 0.133, P  <  0.05). Further redundancy analysis (RDA ) suggested that soil alkaline phosphatase (S‐ALP ) activity played a role in soil P speciation changes and was significantly correlated with enzyme‐P, citrate‐P and HC l‐P. These results may improve our ability to characterize and understand changes in soil P status while minimizing the overapplication of P fertilizer.     

Effect of the exotic invasive plant Solidago gigantea on soil phosphorus status

Invasions by exotic plant species can modify biogeochemical cycles and soil properties. We tested whether invasion by early goldenrod (Solidago gigantea, Asteraceae) modifies soil phosphorus pools at three sites in Belgium. Aboveground phytomass and soil samples (0–10 cm) were collected in early goldenrod patches and in adjacent, uninvaded, grassland vegetation. Soil P fractions varied between the three sites in line with corresponding differences in organic matter, carbonate and clay contents. In addition to site-specific impacts, plots invaded by goldenrods generally had higher concentrations of labile P [i.e. resin-extractable inorganic P (Pi) and bicarbonate-extractable Pi and organic P]. Soil CO₂ release and alkaline and acid phosphomonoesterase activities were also higher in invaded plots, suggesting that the increase in labile Pi was due to enhanced mineralization. Phosphorus uptake by vegetation was 1.7–2.1 times higher in invaded plots, mostly due to the higher annual yield of S. gigantea. Altogether, the results indicate that S. gigantea enhances P turnover rates in invaded ecosystems.     

Assessment of phosphorus availability in soil by introducing P-solubilizing novel bacterial and fungal strains of Lower Himalaya

ABSTRACT

Phosphorus (P) is an essential element for soil quality and plant growth. But little is known about P fractions consequent to microbial inoculation in soils amended with inorganic phosphorus. In this regard, the overall soil P fractions after microbial treatments varied as hydrochloric acid – P (HCl-P) > Sodium hydroxide – P (NaOH-P) > Sodium bicarbonate – P (NaHCO₃-P) > Water – P (H₂O-P) determined through sequential fractionation. Inorganic P amendment of soil increased residual P fractions indicating non-availability of chemical P fertilizers added in soil. Bacterial–fungal co-inoculation, significantly increased NaHCO₃-P and H₂O-P fractions, as compared to mono inoculations and un-inoculated controls. Inoculated microbial populations increased in numbers contributing to the modification of the soil pH. In soil without TriCalcium Phosphate (TCP), microbial treatments showed a reduced pH compared to the control, with a maximum pH decrease in mono bacterial inoculations. In conclusion, tested bacterial and fungal strains exhibited efficient P solubilization in soil, and thus, have potential to be used as biofertilizers, subject to other necessary trials.     

外源磷输入对农区湿地土壤碳库有效性及周转特性的影响

通过添加土壤原总磷(TP)0~60%的过磷酸钙和室内培养的技术手段,研究了外源磷素输入对农区湿地土壤碳库有效性及其周转特性的动态影响。结果表明,随外源磷素输入水平的增加,土壤可溶性有机碳(DOC)含量和微生物量碳(MBC)含量增加;土壤总有机碳(TOC)含量下降,下降率最高达23%。活性有机碳成分中,外源磷输入对高活性有机碳(HLOC)影响最显著,60%磷素输入处理HLOC含量比未施加外源磷素高54%;易氧化有机碳(ROC)含量随外源磷输入水平的增加而下降,最大下降率为22%;中活性有机碳(MLOC)含量、活性有机碳(LOC)含量无明显改变。涉碳循环生物酶中,β-葡聚糖苷酶(βG)和纤维二糖水解酶(CBH)活性均随外源磷素输入水平的增加而显著提升,但脱氢酶(DH)活性并无显著变化。此外,土壤碳矿化强度和潜在矿化势也与外源磷素输入后的土壤TP呈正相关。总体来看,外源磷素输入显著增加了湿地土壤碳库的有效性及周转速率。     

A review of phosphate oxygen isotope values in global bedrocks: Characterising a critical endmember to the soil phosphorus system

Understanding the phosphate oxygen isotope (δ¹⁸O‐PO₄) composition of bedrock phosphate sources is becoming ever more important, especially in areas of soil research which use this isotope signature as a proxy for biological cycling of phosphorus (P). For many of these studies, obtaining a sample of the source bedrock or applied mineral fertiliser for isotope analysis is impossible; meaning there is now a demand for a comprehensive characterisation of global bedrock δ¹⁸O‐PO₄ to support this work. Here we compile δ¹⁸O‐PO₄ data from a wide range of global bedrocks, including 56 new values produced as part of this study and a comprehensive overview of those within the previously existing literature. We present δ¹⁸O‐PO₄ data from the range of major phosphatic lithologies alongside as much metadata for the samples as could be gathered. Much of the data comes from bedrocks of marine sedimentary origin (< 1 Ma = > +22‰, > 540 Ma = ≈ +12‰), but we also present data from bedrocks associated with guano (range: +19.5 to +15‰) and igneous deposits (range: +12 to –0.8‰), both of which have distinct δ¹⁸O‐PO₄ signatures due to their formation mechanisms. We show that where repeat measurements of the same formation have been undertaken, regardless of method or exact sample location, there is an average within formation error of ± 1.25‰. This is important, as is constitutes a reasonable level of uncertainty for phosphorus cycling studies which need to estimate bedrock δ¹⁸O‐PO₄ composition based on the literature. In combination, this data set presents 284 δ¹⁸O‐PO₄ values from 56 countries; a comprehensive starting point for researchers interested in understanding bedrock end member δ¹⁸O‐PO₄.     

不同灌溉和施肥模式对稻田磷形态转化和有效性的影响

为阐明不同灌溉和施肥模式对水稻磷吸收和利用效率、稻田土壤磷形态转化特征的影响及其对土壤磷素有效性的贡献,该研究以杂交籼稻中浙优1号为供试材料,设常规淹灌（Conventional Flooding,CF）、干湿交替（Alternate Wet and Dry irrigation,AWD）2种灌溉模式,以及不施肥（CK）、常规尿素（Ureal,100%PU）、常规尿素减氮20%（80% of Urea,80%PU）、缓控释复合肥减氮20%+生物炭（80% of Control-Released Fertilizer+Biochar,80%CRF+BC）和稳定性复合肥减氮20%+生物碳（80% of Stable Fertilizer+Biochar,80%SF+BC）5种施肥模式,对比分析了不同灌溉和施肥模式下水稻产量、磷吸收效率、稻田土壤磷有效性及土壤磷形态变化特征。1）与CF相比,AWD灌溉模式下80%CRF+BC和80%SF+BC处理水稻产量显著高于100%PU和80%PU处理（P<0.05）;2）AWD灌溉显著增加了成熟期80%SF+BC处理水稻穗部磷累积量,且80%CRF+BC与80%SF+BC处理水稻各器官磷累积量、磷吸收效率与磷肥偏生产力均显著高于80%PU处理;3）AWD灌溉显著提高80%CRF+BC和80%SF+BC处理土壤有效磷、无机磷、有机磷含量与磷活化系数,以及土壤各形态无机磷和0～15 cm 土壤中活性有机磷（Moderately Labile Organic Phosphorus,MLOP）、活性有机磷（Labile Organic Phosphorus,LOP）含量,且其含量均显著高于两组尿素处理;4）相关分析表明,土壤中稳态有机磷（Moderately Resistant Organic Phosphorus,MROP）、LOP、MLOP和Al-P是土壤有效磷的主要决策因子,O-P（闭蓄态磷）和Ca-P是有效磷的主要限制因子。通过适宜的水肥管理提高MROP、LOP、MLOP含量可能是提高土壤有效磷的潜在有效途径。AWD灌溉模式下,生物炭配施稳定性复合肥/缓控释肥能通过调控土壤磷形态转化和磷素活化提高稻田磷有效性,进而提高水稻磷吸收累积和磷素利用效率。研究结果可为通过不同水肥管理模式提高水稻磷利用效率提供理论依据。     

Techniques for observing phosphorus mobilization in the rhizosphere

Summary The techniques described here were developed to visualize the dissolution of sparingly soluble calcium phosphate and the presence of root-borne phosphatase in the rhizosphere. Newly formed calcium phosphate precipitate was suspended in agar containing other essential nutrients. The agar was poured into Petri dishes and acrylglass boxes and was used as a growth medium for seedlings of wheat, rape, buckwheat, and rice. With NH ₄ ⁺ applied as the N source, the precipitate dissolved in the root vicinity and this was attributed to acidification. No dissolution occurred with NO ₃ ^– as the N source. The release of a neutral phosphatase from roots was verified by embedding the roots of young seedlings in agar at pH 7 containing phenolphthalein phosphate. After pH was raised to the alkaline range by adding sodium hydroxide, the agar around the roots turned purple, especially around the roots of P-deprived plants. The most intensive phosphatase activity was found in apical root regions.