Changes In Soil Phosphorus Research Articles

Changes in soil phosphorus in different forest residue management

We analyze the short term effect of the harvest residues management on the Phosphorus (P) fractions of the soil. Four trials were installed in Northeast Argentina on Vertisols (S1), Mollisols (S2) and Entisols (S3 and S4). In S1 and S2 harvest residues and forest fl oor were either (1) removed (R), (2) retained (RR), 3) burnt (BR), or (4) retained at double the normal quantity (DR). In S3 and S4 harvest residues where (1) removed (WTH), 2) retained (RR) or 3) burnt (BR). Soil samples were obtained at 0-5 cm and 5-10 cm depth layers at 18 (S1), 22(S2, S3) and 10 (S4) months after burnt treatments. P fractions were determined according to Hedley et al. (1982) methods. Soil orders showed different total P: in Mollisols P concentration (S2: 314,3 mg kg-1) tripled those in Entisols (S3 y S4: 118,4 mg P kg-1). Differences were even higher for Organic P (Po): the highest concentration was found in Mollisols (171,4 mg Po kg-1) related to its higher organic matter, and the lowest concentration was in Entisols (44,0 mg Po kg-1). Available P was between 14 mg kg-1 and 25 mg kg-1, enough to satisfy annual demands of forest plantations (Eucalyptus and Pinus). RR and DR did not show changes in the P fractions, although an increase in the P reserves (Po y Pt) could be expected for the middle term, due to the decomposition of the retained/added residues. R or WTH did not affect neither inorganic nor organic P fractions. The most important change was found in BR, where the available P and other mean and highly stable inorganic P fractions increased, due to the ashes; the Po was not modified. The fi re was an instant mechanism to transfer the nutrients from the biomass to the soil.

The influence of time, soil characteristics, and land‐use history on soil phosphorus legacies: a global meta‐analysis

AbstractAgriculturally driven changes in soil phosphorus (P) are known to have persistent effects on local ecosystem structure and function, but regional patterns of soil P recovery following cessation of agriculture are less well understood. We synthesized data from 94 published studies to assess evidence of these land‐use legacies throughout the world by comparing soil labile and total P content in abandoned agricultural areas to that of reference ecosystems or sites remaining in agriculture. Our meta‐analysis shows that soil P content was typically elevated after abandonment compared to reference levels, but reduced compared to soils that remained under agriculture. There were more pronounced differences in the legacies of past agriculture on soil P across regions than between the types of land use practiced prior to abandonment (cropland, pasture, or forage grassland). However, consistent patterns of soil P enrichment or depletion according to soil order and types of post‐agricultural vegetation suggest that these factors may mediate agricultural legacies on soil P. We also used mixed effects models to examine the role of multiple variables on soil P recovery following agriculture. Time since cessation of agriculture was highly influential on soil P legacies, with clear reductions in the degree of labile and total P enrichment relative to reference ecosystems over time. Soil characteristics (clay content and pH) were strongly related to changes in labile P compared to reference sites, but these were relatively unimportant for total P. The duration of past agricultural use and climate were weakly related to changes in total P only. Our finding of reductions in the degree of soil P alteration over time relative to reference conditions reveals the potential to mitigate these land‐use legacies in some soils. Better ability to predict dynamics of soil nutrient recovery after termination of agricultural use is essential to ecosystem management following land‐use change.

Phosphatase activities in soil after repeated untreated and alum-treated poultry litter applications

Repeated additions of untreated and aluminum sulfate (alum)-treated poultry litter to soil affect ecology and consequent nutrient dynamics. The objective of this study was to determine how repeated annual poultry litter additions affected phosphatase activities in concert with changes in soil phosphorus (P). Field plots were amended annually since 1995 with either 2.24 or 8.96 Mg ha−1 alum-treated (AL-1 or AL-4, respectively) or untreated poultry litter (PL-1 or PL-4, respectively) or equivalent rates by N content of ammonium nitrate (AN-1 or AN-4, respectively). Soil pH, total C (CTot), microbial biomass C, double-stranded deoxyribonucleic acid (dsDNA), Mehlich-III P (M3P), water-soluble P (WSP), and acid and alkaline phosphatase activities were measured before and 10 days, 1 month, and 6 months after fertilizer applications in 2003 and 2004. M3P and acid phosphatase activities were higher in AL-4 soil than in all other treatments. Higher WSP in the untreated compared to the alum-treated litter resulted in higher WSP in the soil amended with untreated litter. At the same time, alkaline phosphatase activities were significantly higher in PL-4 compared to AL-4. In contrast, alkaline phosphatase activities were significantly lower in AN-4, the treatment with the lowest pH. Additionally, alkaline phosphatase activities expressed per unit CTot or dsDNA remained significantly greater in PL-4 and significantly lower in AN-4, than all other treatments. Thus, some factor beyond an increase in soil C or microbial biomass contributed to elevated alkaline phosphatase activities in PL-4, despite increased WSP in the treatment receiving the high rate of untreated litter.

Phosphorus deficiency inhibits growth in parallel with photosynthesis in a C3 (Panicum laxum) but not two C4 (P. coloratum and Cenchrus ciliaris) grasses.

This study compared the growth and photosynthetic responses of one C3 (Panicum laxum L.) and two C4 grasses (Panicum coloratum L. and Cenchrus ciliaris L.) to changes in soil phosphorus (P) nutrition. Plants were grown in potted soil amended with six different concentrations of P. One week before harvest, leaf elongation and photosynthetic rates and the contents of carbohydrate, P and inorganic phosphate (Pi) were measured. Five weeks after germination, plants were harvested to estimate biomass accumulation. At each soil P supply, leaf P contents were lower in the C3 (0.6-2.6 mmol P m-2) than in the two C4 grasses (0.8-4.1 mmol P m-2), and Pi constituted ~40-65% of total leaf P. The P deficiency reduced leaf growth, tillering and plant dry mass to a similar extent in all three grasses. In contrast, P deficiency suppressed photosynthetic rates to a greater extent in the C3 (50%) than the C4 grasses (25%). The foliar contents of non-structural carbohydrates were affected only slightly by soil P supply in all three species. Leaf mass per area decreased at low P in the two C4 grasses only, and biomass partitioning changed little with soil P supply. The percentage changes in assimilation rates and plant dry mass were linearly related in the C3 but not the C4 plants. Thus, P deficiency reduced growth in parallel with reductions of photosynthesis in the C3 grass, and independently of photosynthesis in the two C4 grasses. We propose that this may be related to a greater Pi requirement of C4 relative to C3 photosynthesis. Photosynthetic P use efficiency was greater and increased more with P deficiency in the C4 relative to the C3 species. The opposite was observed for whole-plant P-use efficiency. Hence, the greater P-use efficiency of C4 photosynthesis was not transferred to the whole-plant level, mainly as a result of the larger and constant leaf P fraction in the two C4 grasses.

Changes in Soil Phosphorus from Manure Application

Manure and mineral fertilizer P sources vary in their contributions to soil P pools. We conducted incubation experiments to (i) assess temporal changes in soil P concentration for 84 d following application of KH2PO4 or manure (beef, dairy, poultry, or swine), and (ii) to evaluate interactive effects of P rate, P source, and background P level on soil P pools 90 d after P application. Changes in soil P over time were evaluated following the amendment of a coarse‐loamy, mixed, frigid Typic Haplorthod soil (pH 5.8) at a rate of 100 mg total P kg−1 Water soluble P (WSP), 0.01 M CaCl2–extractable P (CaCl2–P), and modified Morgan P (MMP) declined to <3 mg P kg−1 soil within 21 d of application, following an exponential decay function; P extracted by anion‐exchange membrane (AEMP) and Mehlich‐III (M3‐P) also declined rapidly and are attributed to the high level of exchangeable Al and Fe in this soil. Ninety days after application, all soil P pools exhibited linear increases in concentration for application rates up to 800 mg P kg−1 soil, regardless of P source. Phosphorus applied as KH2PO4 was more efficient at increasing CaCl2–P and M‐3P. Manure P sources generally had a greater effect on MMP, and poultry manure was more efficient than all other sources. Efficiency of P sources at increasing soil P concentration (b, slope of linear regression) varied from <1% for rapidly available P pools (CaCl2–P) to nearly 50% for more recalcitrant P (M3‐P). Efficiency also increased as background M3‐P increased from 150 to 750 mg kg−1 soil. The amount of CaCl2–extractable P increased rapidly when soil P saturation ([Mehlich III‐P]/[Mehlich‐III Al + Fe]) exceeded 0.25 mol mol−1 Manure and KH2PO4 contribute to different soil P pools, and these differences are magnified at high application rates and high background soil P levels.

Changes in Soil Phosphorus from Manure Application

Manure and mineral fertilizer P sources vary in their contributions to soil P pools. We conducted incubation experiments to (i) assess temporal changes in soil P concentration for 84 d following application of KH2PO4 or manure (beef, dairy, poultry, or swine), and (ii) to evaluate interactive effects of P rate, P source, and background P level on soil P pools 90 d after P application. Changes in soil P over time were evaluated following the amendment of a coarse-loamy, mixed, frigid Typic Haplorthod soil (pH 5.8) at a rate of 100 mg total P kg−1 Water soluble P (WSP), 0.01 M CaCl2–extractable P (CaCl2–P), and modified Morgan P (MMP) declined to <3 mg P kg−1 soil within 21 d of application, following an exponential decay function; P extracted by anion-exchange membrane (AEMP) and Mehlich-III (M3-P) also declined rapidly and are attributed to the high level of exchangeable Al and Fe in this soil. Ninety days after application, all soil P pools exhibited linear increases in concentration for application rates up to 800 mg P kg−1 soil, regardless of P source. Phosphorus applied as KH2PO4 was more efficient at increasing CaCl2–P and M-3P. Manure P sources generally had a greater effect on MMP, and poultry manure was more efficient than all other sources. Efficiency of P sources at increasing soil P concentration (b, slope of linear regression) varied from <1% for rapidly available P pools (CaCl2–P) to nearly 50% for more recalcitrant P (M3-P). Efficiency also increased as background M3-P increased from 150 to 750 mg kg−1 soil. The amount of CaCl2–extractable P increased rapidly when soil P saturation ([Mehlich III-P]/[Mehlich-III Al + Fe]) exceeded 0.25 mol mol−1 Manure and KH2PO4 contribute to different soil P pools, and these differences are magnified at high application rates and high background soil P levels.

Seasonal changes in soil phosphorus and associated microbial properties under adjacent grassland and forest in New Zealand

Land-use change from grassland to short rotation plantation forest can have significant impacts on soil nutrient dynamics and microbial processes. Seasonal dynamics of soil phosphorus (P) and associated microbial properties were investigated in upper (0–5 cm) soils under adjacent unimproved grassland and a 19-year-old forest stand (mixture of Pinus ponderosa and Pinus nigra). Afforestation of grassland ameliorated upper soil moisture and temperature regimes, resulting in reduced but less variable soil moisture beneath forest, and reduced temperature extremes, with soil being cooler under forest in summer, but warmer in winter. Results from this study showed that levels of soil organic carbon (C), total nitrogen (N) and organic P fractions under grassland were consistently higher, but levels of inorganic P fractions (bicarbonate extractable Pi and total Pi), microbial biomass C and P, and phosphatase enzyme activities were lower compared with forest over all seasons. Similar seasonal patterns of soil P fractions under grassland and forest demonstrated that labile organic P was mineralized by increasing microbial activity to meet increasing plant demand in spring and summer, but accumulated as a result of increased organic inputs, slower plant growth and low microbial activity in late autumn and winter. It was concluded that P recycling was mainly driven by plant P demand and sustained by root litter inputs in the grassland ecosystem and leaf litter inputs in the forest ecosystem. Seasonal changes in environmental conditions (rainfall, soil moisture and temperature) influenced microbial processes involved in P cycling. Microbial biomass plays a pivotal role in P cycling. Annual release of P through microbial biomass was higher in the forest soil (16.1 kg ha −1) than in the grassland soil (13.9 kg ha −1). The turnover rate of biomass P was also higher in the forest soil (1.28 per year) than in the grassland soil (0.80 per year). In addition, abundant C and P (particularly labile forms) and high microbial and enzyme activities found in the forest floor highlight the importance of the forest floor in P cycling.

Changes in Soil Phosphorus during 200 Years of Shifting Cultivation in Indonesia

We investigated the long-term effects of shifting cultivation on soil phosphorus pools to understand the effects of repeated intense disturbance on nutrient cycling in rain forests and to assess the sustainability of a widespread tropical agricultural system. We studied 24 sites that formed a gradient of cultivation history near a village in West Kalimantan, Indonesia, where long-fallow shifting cultivation has been practiced for at least 200 yr. The sites, including primary forest, had undergone 0–10 prior cycles (each ∼20 yr) of cultivation and forest regrowth. The number of swidden–fallow cycles experienced at a given site was determined through interviews. Using sequential chemical extraction of surface soils, we inferred changes in the distribution and availability of soil phosphorus during 200 yr of shifting cultivation. We examined the relative importance of fire vs. secondary forest biota in mediating the changes observed by heating soils in the laboratory to simulate the conditions of a slash fire and assessing data on root presence by depth. Over the first four cycles (∼80 yr), total phosphorus in the top 30 cm of soil increased by a mean of 73 kg·ha−1·cycle−1 (3.7 kg·ha−1·yr−1), much of it in the organic NaOH-extractable and the inorganic, concentrated HCl-extractable, fractions. Accounting for differences in original site fertility, total P did not change significantly over 2–10 cycles (∼200 yr), but the distribution shifted from both available and highly recalcitrant inorganic forms to available organic and occluded inorganic forms. NaOH-Pi decreased from 28% to 13%, and residual P decreased from 30% to 10% of total P. In contrast, NaOH-Po increased from 13% to 26%, and cHCl-Pi increased from 15% to 40%. The significant increase in soil organic phosphorus in combination with greater concentrations of fine roots deep in the soil profile suggest that deep-rooting fallow vegetation, rather than fire, is responsible for increasing, and maintaining, total phosphorus. The magnitude of changes induced by the fire are not great enough to account for the increases observed over several cycles, especially considering coincidental losses. Despite losses of nonoccluded Pi, we believe the availability of phosphorus may have been enhanced over 200 yr due to an increase in the stable organic phosphorus fraction. Long-fallow shifting cultivation, as practiced in West Kalimantan, can continue to support rural populations for some time.

Changes in Soil Phosphorus and Nitrogen During Slash‐and‐Burn Clearing of a Dry Tropical Forest

Slash‐and‐burn clearing of forest typically results in an increase in soil nutrient availability. Throughout the tropics, ash from consumed vegetation has been accepted as the primary nutrient source for this increase. In contrast, soil heating has been viewed as a secondarily important mechanism of nutrient release. Through the use of multiple burn plots and intensive pre‐burn and post‐burn sampling of mineral soil, this study quantified changes in total P and N, P fractions, and KCl‐extractable N in soil during the slash‐and‐burn conversion of a Mexican dry forest to agriculture. Slash burning resulted in large transformations of non‐plant‐available P and N in soil into mineral forms readily available to plants. Anion‐exchange resin, NaHCO3‐extractable P, and KCl‐extractable N in soil increased by 37 kg P ha−1 and 82 kg N ha−1 Organic and occluded P (sequentially extracted with NaOH, sonication + NaOH, and NaOH fusion) and organic N (total N minus KCl‐extractable N) decreased after burning by 25 kg P ha−1 and 150 kg N ha−1 Immediately after burning, ash from consumed aboveground biomass contained 11 kg P ha−1 and 27 kg N ha−1, of which 55 and 74%, respectively, were quickly transported off the site by wind. At this dry forest site, soil heating had a much larger influence on soil P and N availability than inputs of ash.

Twenty‐five years of monitoring pH and nutrient status of soils in England and Wales

Abstract.The Representative Soil Sampling Scheme of England and Wales was started in 1969. During the 25 year period 1969–1993 mean soil pH values under arable and ley‐arable cropping changed little, but under grassland they fell by 0.3 units. Under arable cropping, mean available phosphorus declined whilst available potassium increased fairly steeply for the first eight years then declined again to near the start value by 1993. No significant changes in soil phosphorus and potassium levels were found under ley‐arable cropping, but the percentage of fields under ley‐arable rotations declined from 33% to 14%. Under grassland, phosphorus and potassium levels both declined, although potassium levels rose initially before falling.For this report England and Wales were also divided into five regions. Wales and the West Country had the lowest pH values; these regions together with the Northern region were also found to have declining pH values. Phosphorus and potassium concentrations have been falling in the Northern region which stands out as having the lowest current potassium status with 20% of all fields deficient at index 0.Changes in pH and nutrients are discussed for soils sampled following the main arable crops. The largest changes found were the reductions in phosphorus and potassium in soils after potatoes. In 1993 almost one in ten sugarbeet crop soils had an excessive phosphorus level (index 5+) giving an increased risk of water pollution by this element.Under arable cropping the proportion of soils with low (&lt; 1.8%) organic matter declined. Average soil organic matter levels for all crops and grass have remained static during the period. It is concluded that agricultural soils in England and Wales have not contributed to an increase in the greenhouse gas carbon dioxide.

Changes in some soil phosphorus availability parameters as induced by phosphorus addition and soil sorption properties

Changes in agronomic and environmental soil phosphorus (P) availability parameters, i.e., Mehlich‐ and Olsen‐extractable P, reversibly‐adsorbed P, soil‐solution P, and equilibrium‐P concentration were determined following equilibration of 13 Italian soils with five rates of P application (0, 12.5, 25, 50, and 100 mg P kg‐1 soil). Soil P availability as determined by each parameter increased with added P. The relative change in soil P availability with added P was a function of soil sorption index silicon (SI), according to the equation DP=(Padded)a*exp(b+g*SI). This equation accounted for 94 to 98% of the variance in soil‐P availability. The inclusion of SI in a soil testing program may increase the reliability in assessing both soil‐P fertilizer requirements and the vulnerability of a soil to P loss in runoff following land application of fertilizer or manure P.

Changes in soil phosphorus and acid phosphatase activity immediately following forest fires

Three soils affected by wildfires and a soil that had been subjected to a controlled fire were sampled at two depths (0–5 and 5–10 cm), between 1 day and 1 month after burning. In all cases an unburnt soil was used as a control. The controlled fire did not affect acid phosphatase activity and had only very slight effects on P fraction distribution, in good agreement with the low soil temperatures recorded during the fire (never ⪢ 50°C in the upper 5 cm of soil). On the other hand, wildfires strongly reduced acid phosphatase activity and had an intense mineralizing effect on organic P; consequently inorganic P greatly increased and P fractions distribution was profoundly altered. The reductions in acid phosphatase activity and the extent of organic P oxidation in the various soil samples were used to estimate the soil temperatures reached during each fire. We show that there is a close relationship between soil temperatures estimated in this way and changes in P distribution within the various inorganic and organic fractions.

Seasonal and fertilizer effects on the organic cycle and microbial biomass in a hill country soil under pasture

A two year investigation of soil phosphorus and the soil organic cycle was carried out on a typical hill country site in the North Island, New Zealand. This included investigation of changes in soil phosphorus, as well as seasonal and fertilizer (superphosphate) effects on soil microbial phosphorus and sulfur, sodium bicarbonate extractable phosphorus and calcium chloride extractable sulfur. No net utilization of soil organic phosphorus occurred when application of phosphate fertilizer was withheld. On the contrary, accumulation of organic phosphorus was found in both fertilized and unfertilized plots. Immobilization of inorganic phosphorus into organic forms appeared to be a significant factor in fertilizer phosphorus requirements at this site. It was also a significant cause of the decline in the soil phosphorus status when no fertilizer was applied. Despite declining pasture production, there were no effects of withholding superphosphate on the soil biological cycle as measured by soil microbial phosphorus and sulfur, total organic phosphorus and sodium bicarbonate extractable organic phosphorus. However, seasonal variations occurred indicating storage and release of phosphorus by the soil organic matter and microbial biomass. Release of phosphorus occurred during periods of rapid pasture growth and could account for phosphorus uptake by the pasture at those times.

Isotopic Dilution and Extraction Tests in Assessing Chemical-induced Changes in Soil Phosphorus

Abstract Soil samples incubated for 3 months with equivalent quantities of K2CO3 or CaCO3 or with KCl were analyzed for isotopically exchangeable P (E t ) and for P soluble into acid NH4OAc (PAAc) and into acid fluoride solution (Bray-P). In most samples, the chemical-induced changes in soil P obtained by the isotopic dilution were opposite to those revealed by the extraction tests. The isotopically exchangeable P was affected by CaCO3 only slightly, and it was markedly reduced by K2CO3. Bray-P, in contrast, tended to be increased by K2CO3 but lowered by CaCO3, obviously owing to the precipitation of CaF2. Both carbonates increased PAAc in two soils and decreased it in one. Furthermore, KCl evoked an increase in the E t values and a decrease in the extraction test indices. The response of the isotopically exchangeable P to the chemical treatments seemed to be arte facta derived from the type of equation used in the calculation of the E t value. Examination of the measurement data suggested that the K2CO3 and KCl treatments affected the distribution of added 32P between the solid and solution phase more than the equilibrium P concentration. This reaction pattern, in turn, had a pronounced effect on the calculated E t value, and explained the diverging behaviour of the isotopically exchangeable P.

Implications of long-term superphosphate applications on the accumulation and plant availability of soil phosphorus under irrigated pastures

Changes in soil phosphorus (P) associated with the establishment and maintenance of improved ryegrass-clover pasture under different superphosphate fertiliser treatments were examined over a 20-year period (1957-77). Results showed that soil organic P increased with increasing applications of P fertiliser. This represents a dynamic balance between rates of organic P addition and breakdown in the soil. This balance is reached slowly and may be significantly altered only by drastic changes in land use. In annually fertilised soils, amounts of inorganic P increased with time. However, the potential utilisation of this residual inorganic P is limited by its apparent stability in the soil. Keywords grazed pasture, irrigation, fertiliser P, soil inorganic P, soil organic P, soil P fractionation

INFLUENCE OF TEXTURE AND MANAGEMENT PRACTICES ON THE FORMS AND DISTRIBUTION OF SOIL PHOSPHORUS

Changes in soil phosphorus (P) forms, as determined by a sequential fractionation procedure, were used to assess the influence of soil texture and management practices on the forms and distribution of soil P in a Brown Chernozemic loam soil at Swift Current, Saskatchewan. Significant proportions of the variability of all P fractions except residual-P could be attributed to changes in sand content. Changes in the forms and distribution of soil P with decreasing sand content followed patterns similar to those associated with a weathering sequence. The proportion of total soil P in inorganic and organic extractable forms that were extractable sequentially with anion exchange resin (resin-Pi), sodium bicarbonate (bicarb-Pi and -Po), and sodium hydroxide (NaOH-Pi and -Po) increased with decreasing sand content. Acid-extractable inorganic P (HCl-Pi) was the only P fraction positively correlated with sand content. The presence of a crop increased the proportion of soil P present as the more labile organic-P fractions (bicarb-Po and NaOH-Po) but not as total soil organic P (soil-Po). The presence of a crop also increased the proportion of soil P present as the labile inorganic fractions (resin-Pi and bicarb-Pi), possibly due to a decrease in soil pH. Application of inorganic-P fertilizer caused significant increases in the proportion of soil P as these labile inorganic-P fractions (resin-Pi and bicarb-Pi) and as total soil organic-P (soil-Po), but did not affect the more labile organic-P fractions. Key words: P fractionation, labile P, organic P, inorganic P, texture, management practices

Use of phosphorus vectors to monitor changes in soil phosphorus

Abstract A study was conducted to evaluate the use of P vectors to predict the amount of P required on a yearly basis to maintain a constant solution‐solid phase P relationship in an irrigated calcareous and a dryland acid soil. Irrigated potato‐spring barley and dryland spring pea spring‐barley crop rotations established at the two locations. Mono‐calcium phosphate (0–45–0) was applied annually at five levels, ranging from 0–4 times estimated crop removal. Phosphorus vectors were determined on soil samples by equilibration with standard P solutions. Yields tended to increase with added P on the calcareous soil; however, significant responses were not recorded at either location. Consequently, critical P vectors were not established. A constant solution‐solid phase P relationship was maintained by addition of P equal to that removed by the crop on the calcareous soil. A constant solution‐solid phase P relationship was not maintained on the acid soil.

Changes in soil phosphorus and pH in a red earth soil during build-up and residual phases of a wheat clover ley farming system

In a red earth soil at Wagga eight soil phosphorus pools, and pH, were measured. Positive linear relationships were found between the amount of fertilizer phosphorus applied prior to the start of the experiment (1972) and the inorganic (Pi) pools. Despite a range in total phosphorus of 130 ppm the plots had similar organic (Po) pools. Mehlich phosphorus (0.05 N hydrochloric acid + 0.025N sulphuric acid) and its major component, aluminium phosphorus, increased more rapidly with each unit of fertilizer phosphorus than other Pi pools. During 1972-1974 the soil carried grazed clover pasture but no phosphate fertilizer was applied. There were linear declines in soil total phosphorus, aluminium phosphorus and Mehlich phosphorus pools. The rates of decline were higher on plots which had higher phosphorus levels at the beginning of the residual phase. The Mehlich phosphorus pool declined by 1.8 to 9.3 ppm. 'Available' phosphorus (Bray and Kurtz 1) declined at a similar rate, but 'available' phosphorus (Colwell) declined more rapidly. Po increased at a mean rate of 5.4 ppm year-1. This was similar to the estimated return of Po in plant and faecal material on low phosphorus plots, but only about 30% of the return of Po and high phosphorus plots, indicating that a substantial quantity of Po can be mineralized in a pasture ecosystem. Some movement of phosphorus to the subsoil was noted during and immediately following phosphate application. Soil pH declined by 0.1-0.2 units during the residual phase.