The Effect of Humic Substances and Phosphate Fertilizer on Growth and Nutrient Uptake of the Potato

ABSTRACT

The use of applied phosphorus (P) and the uptake of nutrients from the soil by plants can be improved when the fertilizer is combined with the application of humic substances (HS). However, these beneficial effects are inconsistent and can depend on the type of soil. This study was performed to evaluate the effects of the application of HS (0, 1.25, and 7.50 mL pot^–1), as Humic HF®, and fertilizer-P (10, 50, 100, and 200 mg P dm^–3), as triple superphosphate, on root morphological characteristics, dry matter accumulation, nutrient uptake, and tuber yield of potatoes grown in sandy and clayey soils. Only under low P supply in the sandy soil did the supply of HS, at the rate of 1.25 mL pot^–1, increase the plant growth, yield of tubers, and uptake of macronutrients by the plants, without affecting the efficiency of the P fertilization. In the clayey soil, which had a higher organic matter content, the application of HS did not affect plant growth, tuber yield or nutrient uptake. In both soils, P fertilization increased plant growth, tuber yield, and nutrient uptake. The combined application of HS and P increased the root length of potatoes in sandy soil.     

增值磷肥对潮土无机磷形态及其变化的影响

在实验室条件下制备了海藻酸磷肥、腐植酸磷肥和氨基酸磷肥3种增值磷肥,利用室内土壤培养试验研究增值磷肥对潮土无机磷组分及其变化的影响。结果表明, 1）培养180 d后,普通磷肥（磷酸一铵,下同）和增值磷肥均显著提高了土壤速效磷含量,并降低了土壤pH; 施用增值磷肥提高土壤速效磷的幅度为34.6～41.92 mg/kg,高于普通磷肥; 施用增值磷肥降低土壤pH的幅度为0.23～0.36个单位,高于普通磷肥。2) 与普通磷肥相比,增值磷肥明显降低土壤对磷的固定,腐植酸、海藻酸和谷氨酸增值磷肥处理的固定率分别比普通磷肥降低7.32%、7.13%和11.99%。3）培养180 d后,与普通磷肥处理相比,增值磷肥均提高土壤Ca2-P、 Ca8-P 和 Al-P含量,减缓Al-P 向 Fe-P 的转化。     

Organic complexed superphosphates (CSP): physicochemical characterization and agronomical properties

A new type of superphosphate (organic complexed superphosphate (CSP)) has been developed by the introduction of organic chelating agents, preferably a humic acid (HA), into the chemical reaction of single superphosphate (SSP) production. This modification yielded a product containing monocalcium phosphate complexed by the chelating organic agent through Ca bridges. Theoretically, the presence of these monocalcium-phosphate-humic complexes (MPHC) inhibits phosphate fixation in soil, thus increasing P fertilizer efficiency. This study investigateed the structural and functional features of CSP fertilizers produced employing diverse HA with different structural features. To this end were used complementary analytical techniques: solid-phase 31P NMR, 13C NMR, laser-confocal microscopy, X-ray diffraction, and molecular modeling. Finally, the agronomical efficiency of four CSP have been compared with that of SSP as P sources for wheat plants grown in both alkaline and acidic soils in greenhouse pot trials under controlled conditions. The results obtained from the diverse analytical studies showed the formation of MPHC in CSP. Plant-soil studies showed that CSP products were more efficient than SSP in providing available phosphate for wheat plants cultivated in various soils with different physicochemical features. This fact is probably associated with the ability of CSP complexes to inhibit phosphate fixation in soil.     

磷肥中腐植酸添加比例对玉米产量、磷素吸收及土壤速效磷含量的影响

【目的】 腐植酸可提高磷肥的肥效,对于其在磷肥中适宜添加量的研究可为我国磷肥的增效减量提供依据。【方法】 将腐植酸增效剂按1%、5%、10%和20%的比例添加到磷酸一铵中,制成四种腐植酸磷肥试验产品（HP1、HP2、HP3和HP4）,利用土柱栽培试验研究在等磷量（施P2O5量0.1g/kg干土）投入及等肥料重量（施磷肥实物量0.16g/kg干土,即施P2O5量分别减少1%、5%、10%、20%）投入情况下,腐植酸磷肥对玉米产量、磷素吸收利用及土壤速效磷含量的影响。【结果】 1）在等磷量施用情况下,与普通磷肥（P）相比,四种腐植酸磷肥处理玉米籽粒产量增加4.5%～13.6%,且腐植酸添加量越大产量越高,均显著高于普通磷肥处理;在等肥料重量施用下,随着腐植酸磷肥施入P2O5量的减少,玉米籽粒产量逐渐降低,当P2O5施用量减少20%时籽粒产量与普通磷肥处理相比仍未显著降低。2）腐植酸磷肥处理在等磷量施用下较普通磷肥处理可显著提高玉米籽粒磷吸收量和地上部吸磷总量,分别增加6.0%～15.4%和6.3%～14.0%,但秸秆磷吸收量无显著变化;当腐植酸磷肥施入P2O5量减少20%时籽粒磷吸收量和地上部磷吸收总量会显著低于普通磷肥处理。3）与普通磷肥处理相比,在等磷量施用下,腐植酸磷肥的表观利用率提高5.9～13.1个百分点,农学利用率、偏生产力分别提高26.5%～79.1%、4.5%～13.5%,且均达到显著水平。4）施入腐植酸后主要影响050cm土层的土壤速效磷含量,其中1530cm土层速效磷含量增加最为显著,与普通磷肥处理相比增加18.1%～36.6%。【结论】 腐植酸增效剂在1%～20%的添加比例范围内对磷肥均具有较好的增效作用,可提高玉米产量、磷素吸收量及磷肥利用效率,并可提高土壤中的速效磷含量,且腐植酸添加量越大效果越好;利用腐植酸的增效作用来减少磷肥施用量是可行的,在当前磷肥施用量的基础上可减少磷肥用量20%左右而保证玉米不减产。     

腐殖酸对农田土壤磷素吸附行为的影响研究

土壤有机腐殖酸类物质具有巨大的比表面积和表面活化基团,在土壤污染物和养分的吸附、利用和转化过程中起着重要作用。该研究选取5种中国典型农区土壤,采用增加和减少有机腐殖酸类物质含量的方法,设置不同含量梯度,利用振荡平衡法来定量研究不同腐殖酸含量对磷素吸附特性的影响,并对其利用Freundlich、Langumuir和Temkin方程来进行定量化拟合分析研究,试图定量探讨有机腐殖酸类物质对磷素吸附行为的影响。研究结果表明:Freundlich、Langumuir和Temkin这3种方程均可对磷素吸附特征进行拟合,但拟合相关性以Freundlich最好,对于不同土壤而言,对磷素吸附拟合方程的适应性,以Freundlich方程对黑土和红土最好,塿土和黑垆土次之,潮土最差。加入腐殖酸明显地减少土壤对磷素的吸附量和吸附速率。腐殖酸对于土壤磷素有一定的活化激发作用,无论是增加还是减少腐殖酸时,黑土和红土对磷素的最大吸附量变化比例不是很大,而塿土和黑垆土变化最大,可以减少50%～60%左右,而潮土次之。腐殖酸可作为土壤P素增效剂,改善肥料物理性状,提高其稳定性和有效性,尤其是在黄土高原石灰性土更为有效。     

腐植酸促进植物生长的机理研究进展

【目的】腐植酸在我国农业生产中发挥了重要作用,许多研究证实,腐植酸具有促进植物生长的功能,本文从腐植酸刺激植物根系生长、调控土壤与肥料养分转化及肥料利用率和影响土壤微生物和酶活性方面,系统总结了国内外施用腐植酸促进植物生长的途径,阐述了腐植酸对植物生长促进作用的机理,旨在梳理腐植酸促进植物生长机理的研究现状,为腐植酸的进一步研究和应用提供参考依据。[主要进展]1）腐植酸能够对植物产生类似生物刺激素的效应。它能够提高植物根系H+-ATP酶等的活性、刺激植物根伸长和侧根生长点的增加,从而增加根系活力及植物根系与土壤养分的接触面积,增加植物对养分的吸收;2）逆境胁迫下,腐植酸能够通过调节植物体内的新陈代谢并改善植物生长环境,缓解甚至消除逆境胁迫对植物的伤害,从而促进植物生长;3）腐植酸能够通过与氮素、磷素和钾素发生结合效应,与磷酸盐产生竞争效应和对钾离子的吸附作用固持与活化土壤与肥料中的养分,提高土壤肥料有效性和缓释性能,提高肥料利用率,从而促进植物生长;4）腐植酸还能够影响土壤中与养分转化相关的酶活性和微生物群落结构及数量,在活化养分的同时,保蓄养分,降低养分的损失,为植物生长保障持久的养分供应;5）腐植酸对植物生长的促进效应受腐植酸结构特征、添加量和供试植物种类等因素的影响。[建议与展望]由于技术手段的限制和研究技术的差异,人们对腐植酸促进植物生长机理的认识还不够系统和深入,因此,腐植酸的基本特征、影响腐植酸作用的主控因子、土壤-植物系统中腐植酸促进植物生长的主要途径和腐植酸对土壤功能性微生物等的影响都将成为未来研究的重要方向。     

Effect of humic acid on growth and crop nutrient status of wheat on two different soils

The effect of plant-derived humic acid (PDHA) and coal-derived humic acid (CDHA) on wheat growth was tested on two alkaline calcareous soils in pots. Humic acid derived from plant and coal materials was applied at the rate 0 (control), 50 and 100 kg/ha to wheat in pots carrying two soils viz. clayey loam soil and sandy loam soil separately. Data was collected on plant growth parameters such as spike weight, grain and straw weight, and plant nutrients (macronutrients and micronutrients). Results showed that spike weight increased by 19%, 15%, and 26%, and 11% with application of PDHA at the rate of 50 and 100 mg/kg in clayey loam and sandy loam soil, respectively. Grain yield show an increase of 21% and 11% over control with application of PDHA and CDHA at the rate of 50 mg/kg on both soils, respectively, and 10% and 22% with application of PDHA and CDHA at the rate of 100 mg/kg on both soils.     

改性磷矿粉在石灰性土壤上的生物有效性及其机理研究

为缓解磷资源危机,充分利用不适于化学加工的磷矿资源,对磷矿采用活化技术开发利用,由此生产的改性磷肥已被证明在酸性土壤上有很高的生物有效性,但在石灰性土壤上的研究还比较少。本文选择4种不同产地(云南昆明、四川德阳、四川绵竹和贵州开阳)的磷矿粉,分别用有机活化剂和无机活化剂进行活化处理,制备改性磷矿粉,并就其在石灰性土壤对苗期春小麦的生物有效性进行了盆栽试验。结果表明,4种磷矿粉经活化处理后有效磷和水溶性磷含量均明显增加,无机活化剂提高了有效磷的含量,而有机活化剂对水溶性磷的提高幅度较大。改性磷矿粉处理均不同程度提高了石灰性土壤上春小麦的生物量干重、植株吸磷量和磷利用效率,有机活化剂处理制备的改性磷矿粉对春小麦生长的促进作用更为明显。运用红外光谱谱学技术对4种磷矿粉及相应的8种改性磷矿粉结构分析结果表明,磷矿粉经改性后其结构发生了明显变化,H2PO4和HPO42的特征谱带明显增加,增加的程度因磷矿粉的产地和活化剂种类不同而有差异。对磷矿粉化学成分分析结果表明,不同产地的磷矿粉其磷以及钙、铁、铝、镁等化学成分的含量差异较大,活化剂对磷矿粉的活化效果与磷矿粉本身的氧化物含量有关。在本试验条件下,磷矿粉Ⅲ(四川绵竹)的活化效果相对最好,与其氧化镁含量最高、总氧化物含量最低有关。不同类型活化剂对磷矿粉的活化效果不同,红外光谱分析结果表明,无机活化剂活化效果较好,而土培试验结果表明有机活化剂的活化效果较好,这一结果有待进一步的研究。     

腐植酸在土壤中迁移的模拟实验研究

浅层地下水中的腐植酸与人体健康密切相关,腐植酸在土壤中的迁移影响土壤和地下水中腐植酸含量。采用土柱模拟实验,研究了不同条件下腐植酸的迁移规律。结果表明,腐植酸的出水浓度随时间逐渐升高,最后趋于稳定;同一时间段内,初始浓度为15mg·L-1时的出水浓度比10mg·L-1时高,且早24h达到稳定浓度;淋滤液pH为9时的出水浓度比pH为4时高,且早16h达到稳定浓度;粉质粘土柱对腐植酸的吸附和阻滞作用大于砂壤土柱,出水浓度较低,晚32h达到稳定浓度;土层厚为5cm的土柱出水浓度高于15cm土柱,早32h达到稳定浓度。分析认为,土壤对腐植酸的吸附作用是出水浓度变化的主要原因,腐植酸和土壤的理化性质是影响腐植酸在土壤中迁移的主要因素。     

Crystallization of single and binary iron‐ and aluminum hydroxides affect phosphorus desorption

In acidic soils, phosphorus availability is affected by its strong affinity for mineral surfaces, especially Fe‐ and Al‐hydroxides. Plant roots have developed adaptive strategies to enhance the availability of phosphorus, including producing and exuding low molecular weight organic acids with a high affinity for phosphorus that competes with high molecular weight organic ligands formed during humification and mineralization. The aim of this study was to characterize the kinetics and mechanism of phosphorus desorption from Fe‐ and Al‐hydroxides of variable crystallinity, as well as binary Fe:Al‐hydroxide mixtures. Long‐term desorption experiments (56 days) were conducted with CaCl₂, CaSO₄, citric acid, and humic acid as competitive sorptives. CaCl₂ and CaSO₄ were selected as general inorganic sorptives and citric and humic acids were selected as organic ligands produced by organisms in the rhizosphere or following humification. The cumulative phosphorus desorption increased following the order CaCl₂ < CaSO₄ < humic acid < citric acid. Amorphous ferrihydrite and Fe‐rich Fe:Al‐hydroxides exhibited much less desorption when exposed to inorganic solutions than the crystalline and Al‐rich Fe:Al‐hydroxide mixtures. Models of the desorption data suggest phosphorus desorption with citric acid is diffusion‐controlled for ferrihydrite and Fe‐rich amorphous Fe:Al‐hydroxides. When humic acid was the sorptive, metal‐organic complexes accumulated in the solution. The results suggest organic compounds, especially citric acid, are more important for liberating phosphorus from Fe‐ and Al‐minerals than inorganic ions present in the soil solution.     

水培法磷矿石对小麦生长的影响

Screening cultivars to grow under conditions of low phosphorus (P) availability and utilize P efficiently from compounds of low solubility in soils may be beneficial to overcome poor plant growth in P-deficient soils.The growth behavior and P utilization efficiency of seven wheat cultivars grown in hydroponics were studied,using rock phosphate as P source.The wheat cultivars grown for 30 days were significantly different in biomass accumulation,P uptake and P utilization efficiency.The dry matter production of all the cultivars was significantly correlated with P uptake,which in turn correlated to the drop in the root medium pH.The ranking of wheat cultivars on the basis of dry matter yield,P uptake and P utilization efficiency was Zamindar 80 ＞ Yccora ＞ C 271 ＞ WL 711 ＞ Barani 83 ＞ PARI 73 ＞ Rohtas.The cultivar Zamindar 80 appeared to possess the best growth potential in P-deficient soils.     

Identification and performance of stress‐tolerant phosphate‐solubilizing bacterial isolates on Tagetes minuta grown in sodic soil

Tagetes minuta is moderately adapted to a wide range of climate and due to its tolerance to larger salt, pH and exchangeable sodium percentage (ESP) in soil it is considered to be a potential crop for salt‐affected soil. Its tolerance to adverse condition and association with halophilic microbes can combine to play a greater role in crop production and improvement in soil health. After screening, the potential phosphate‐solubilizing bacteria (PSB) RS‐1, RS‐2 and RS‐3 were isolated from sodic soils and tested in pot experiment using a naturally occurring sodic soil of pH 9.3 and an ESP of about 45. Under optimum conditions in the laboratory, these bacteria showed phosphorus solubilization potential in liquid medium containing tricalcium phosphate (TCP). Inoculation of PSB significantly increased plant growth in terms of height, number of branches, dry matter accumulation and nutrient uptake. Significant changes were also found in content and quality of essential oil. It was observed that PSB also improved the physical, chemical and biological properties of soil. The bacterial strains tested in this study have the potential for use as a biofertilizer in sustaining the growth of Tagetes minuta in salt stress soil and mitigating soil stress problems.     

Dissolution of aluminum and iron phosphate by humic acids

Abstract

An investigation was conducted to study the effect of humic (HA) and fulvic acid (FA) on the dissolution of aluminum phosphate (AlPO₄) and iron phosphate (FePO₄), to analyze the dissolution products, and assess their availability to plants. The rate of dissolution was determined by shaking 10 mg of Al‐ or FePO₄ with 0 to 800 mg L^‐1 of HA or FA solutions at pH 7.0 for 0 to 192 hours. The phosphorus (P) concentration was measured in the extracts by spectrophotometry, whereas the nature of P‐humic acid complexes was determined by ³¹P NMR analysis. Availability of dissolution products was studied by growing corn plants in aerated hydroponic solutions receiving treatments of 50 mg Al‐ or FePO₄ and 0 to 800 mg L^‐1 of HA or FA at pH 5.0. The results indicated that the amount of P released by HA or FA increased with time. Humic acid was more effective than FA in dissolving the metal phosphates. The ³¹P NMR analysis showed that the dissolution products contained free orthophosphates and minor amounts of P‐humic acid complexes. This confirms the role of HA as a powerful chelator of Al and Fe, liberating in this way the orthophosphate anions. Corn plants grown in hydroponics, with AlPO₄ or FePO₄ as the source of P, exhibited better growth performance when HA or FA are present.     

31P NMR characterization and efficiency of new types of water-insoluble phosphate fertilizers to supply plant-available phosphorus in diverse soil types

Hydroponic plant experiments demonstrated the efficiency of a type of humic acid-based water-insoluble phosphate fertilizers, named rhizosphere controlled fertilizers (RCF), to supply available phosphorus (P) to different plant species. This effect was well correlated to the root release of specific organic acids. In this context, the aims of this study are (i) to study the chemical nature of RCF using solid-state (31)P NMR and (ii) to evaluate the real efficiency of RCF matrix as a source of P for wheat plants cultivated in an alkaline and acid soil in comparison with traditional water-soluble (simple superphosphate, SSP) and water-insoluble (dicalcium phosphate, DCP) P fertilizers. The (31)P NMR study revealed the formation of multimetal (double and triple, MgZn and/or MgZnCa) phosphates associated with chelating groups of the humic acid through the formation of metal bridges. With regard to P fertilizer efficiency, the results obtained show that the RCF matrix produced higher plant yields than SSP in both types of soil, with DCP and the water-insoluble fraction from the RCF matrix (WI) exhibiting the best results in the alkaline soil. By contrast, in the acid soil, DCP showed very low efficiency, WI performed on a par with SSP, and RCF exhibited the highest efficiency, thus suggesting a protector effect of humic acid from soil fixation.     

Corn biomass,uptake and fractionation of soil phosphorus in five soils amended with organic wastes as P fertilizers

Abstract

Sustainable food production includes mitigating environmental pollution and avoiding unnecessary use of non-renewable mineral phosphate resources. Efficient phosphorus (P) utilization from organic wastes is crucial for alternative P sources to be adopted as fertilizers. There must be predictable plant responses in terms of P uptake and plant growth. An 18-week pot experiment was conducted to assess corn (Zea mays L.) plant growth, P uptake, soil test P and P fractionation in response to application of organic P fertilizer versus inorganic P fertilizer in five soils. Fertilizers were applied at a single P rate using: mono-ammonium phosphate, anaerobically digested dairy manure, composted chicken manure, vegetable compost and a no-P control. Five soils used varied in soil texture and pH. Corn biomass and tissue P concentrations were different among P fertilizers in two soils (Warden and Quincy), with greater shoot biomass for composted chicken manure and higher tissue P concentration for MAP. Plant dry biomass ranged from highest to lowest with fertilizer treatment as follows: composted chicken manure?>?AD dairy?=?MAP?=?no-P control?=?vegetable compost. Soil test P was higher in soils with any P fertilizer treatment versus the no-P control. The loosely bound and soluble P (2.7?mg P kg^?1) accounted for the smallest pool of inorganic P fractions, followed by iron bound P (13.7?mg P kg^?1), aluminum bound P (43.4?mg P kg^?1) and reductant soluble P (67.9?mg P kg^?1) while calcium bound P (584.6?mg P kg^?1) represented the largest pool of inorganic P.     

The involvement of iron and aluminum in the bonding of phosphorus to soil humic acid

Abstract

With a peat soil similar amounts of phosphorus (P) were coprecipitated with humic acid from alkali extracts over a limited range of strongly acidic pH, whereas with a mineral soil the amount was pH dependent. The difference between the two soils relates to the much greater total amounts of inorganic P and aluminum (Al) present in the extract of the mineral soil. In this acid mineral soil, Al rather than iron (Fe) may be involved in the formation of metal bridges in humic acid‐metal‐inorganic P complexes. Neither Al or Fe were implicated in binding of organic P to humic acid. The P species observed in humic acids was dependent on the pH at which they were precipitated from the alkali extracts. In the peat soil the inorganic P was an order of magnitude lower than the organic P.     

Genotypic variation for phosphorus uptake dinitrogen fixation in cowpea on low‐phosphorus soils of southern Cameroon

In cowpea, efficient N₂‐fixing genotypes are being selected to promote sustainable cropping systems in southern Cameroon (SC). However, N₂ fixation and growth of these genotypes are largely hampered by low levels of soil plant‐available P. To evaluate the genotypic variation in N₂ fixation and P uptake among cowpea (Vigna unguiculata L.) genotypes, field experiments were conducted over two years on two acid soils low in available P. The experiments were laid out in a split‐block design with four replications on typic (TK) and rhodic (RK) Kandiudult soils with seven cowpea genotypes. Phosphorus (P) fertilizers were applied on the main plots with 0 kg P, 30 kg P ha^–1 as triple superphosphate (TSP) and 90 kg P ha^–1 as Togo phosphate rock (PR). Nodule dry matter (DM), shoot DM, grain yield, and P uptake of cowpea significantly varied with site, P application, and genotype (p < 0.05). The N₂ fixation of the cowpea genotypes ranged from 29 to 51 kg N ha^–1 on both TK and RK soils and was significantly increased with P application. Significant genotypic variations in N₂ fixation were observed with superior ability of the genotypes IT89KD‐391 and IT90K‐59 to fix N₂. The harvest index (HI) did not significantly differ between soils and P application levels (p > 0.05). Four genotypes were selected to investigate root mechanisms responsible for efficient P acquisition in pot experiments. The results suggest that a better root infection by arbuscular mycorrhizal fungi (AMF) in genotype IT90K‐59 and root morphological and physiological characteristics in IT89KD‐391 were the most important factors for increasing P uptake.     

Phosphoraustrag aus Niedermoorböden - Ergebnisse eines Lysimeterversuches ohne Pflanzenbewuchs

Leaching of phosphorus out of fen soils - Results from pot trials without growing plants In contrast to acid bog peat soils, fen soils with high content of iron, calcium und partially lime may fix phosphates in the same way as mineral soils. In model experiments without growing plants the quantity of leached phosphorus is determined. 0,4–0,5 kg P/ha are leached out of a slightly acid fen soil (pH 5,5) with 550 mm percolated water. The phosphate fertilizers (300 kg P/ha) Tripelphosphat, Novaphos, Hyperphos and Thomasphosphat have no influence on the amount of leached phosphorus. The phosphate is fixed in the depth of 0–10 cm, into which the fertilizers were mixed. However, from a very strongly acid fen soil (pH 3,0; limed in 0–10 cm to pH 4,5) with 1000 mm percolated water 25,8 kg P/ha are leached. The fertilization with Tripelphosphat and Novaphos increases the amount of leached phosphorus whereas the water insoluble phosphate fertilizers (Thomasphosphat and Hyperphos) have no influence. In a fen soil with high content of iron water soluble phosphate will be sorbed and fixed very rapidly, therefore only low parts of the water soluble phosphate fertilizers can be extracted with lactate solution (DL), in comparison to the water insoluble phosphates.     

Orthophosphate and organic phosphate in the soil solution of four sandy soils in relation to pH‐evidence for humic‐FE‐(AL‐) phosphate complexes

Abstract

Soil from the Ap‐horizon of four acid sandy soils differing mainly in Corg content was adjusted to pH values between 3 and 7.5 with NaOH and HCl respectively and incubated for two weeks. Afterwards, displaced soil solution was obtained and analyzed.

The concentrations of Fe, Al, and P showed a broad minimum in the pH range from 4 to 6. The concentration of these elements strongly increased with the increase of pH to 7.5. Acidification below pH values of 4 led to a slight increase.

Separation of dissolved organic carbon by ultrafiltration before the photometric orthophosphate determination decreased measured concentrations in comparison to direct determination in two of the four soils. This decrease was more pronounced for soil solutions with higher concentrations of organic carbon. The effect of acid hydrolysis of organic phosphorus during orthophosphate determination can be explained by existence of humic‐Fe‐(Al phosphate complexes in the soil solution. These complexes can account for more than 50% of the total organic P in solution.