Phosphate adsorption characteristics of two soils responding differently to P fertilization

The soil samples of the present study originated from two field experiments in which five rates of P (annually 0, 13 or 16, 26 or 32, 47 or 56, 60 or 72 kg P/ha) had been applied for 11 or 12 years. The fields were silty clay soils (Cryochrepts) not differing markedly in pH, contents of clay, organic C, or poorly crystalline A 1 and Fe oxides. Before the field experiment, the quantities ofP extracted with an ammonium acetate solution (pH 4.65) were approximately 6 mg/dm in both fields. However, the fields differed considerably in the response of the crop to P fertilization. Phosphorus adsorption by the soil samples was studied by shaking the samples in solutions of different P concentrations (0 —0.5 mg/1). Soil I, showing greater response to P fertilization in the field, adsorbed P considerably more effectively than did soil 11. Also the quantities of reversibly adsorbed P were smaller in the subsamples of soil I as compared to those of soil II receiving the same fertilization. Fertilizer P applied during the field experiments had been adsorbed and converted to forms unavailable to plants to a larger extent in soil I, resulting in greater response to P fertilization in this soil. The difference in response to applied P or in residual effect of P fertilization could not be predicted from soil characteristics other than P sorption. Index words: adsorption isotherms, mineral soils, reversibly adsorbed P


Introduction
Single extractions with e.g.water or an am- monium acetate solution have rather successfully been used for predicting yield responses to P fertilization of cereal crops in Finland (e.g.Sippola and Saarela 1986).The results obtained with these methods give an index for the reserves of plant-available P at the time of sampling and for P fertilization requirement in that year.The residual effect of P fer- tilization cannot, however, be predicted by or- dinary soil testing methods prior to fertilizer application, because individual soils differ greatly in the rate of P fixation.For exam- ple, in 18 mineral soils of England, a large ap- plication of superphosphate lost half of its fer- tilizer value over a period of one to six years (Larsen et al. 1965).This phenomenon results at least to some extent in an unpredictable duration of fertilization effect of added P.
This study deals with soil samples taken from two long-term field experiments in which, despite the initially same fairly low lev- el of easily soluble P, a different response by cereal crops to P fertilization was observed (Yli-Halla 1989 b).In one experiment, no statistically significant response was observed until the eighth year, while in the other one, substantial response was measured already in the first experimental year.Further, more P was extracted with water or an ammonium acetate solution at the end of the field experi- ments from soil samples taken from the field less responsive to P fertilization.Yet, the ex- perimental fields did not differ considerably in the contents of clay, organic matter, poorly crystalline Al and Fe oxides or pH.
In the present study, the soils of these two experiments were examined in more detail in order to find out an explanation for the differ- ent yield responses to P fertilization in the ex- perimental fields.Adsorption-desorption iso- therms for phosphate were determined and the soils were analyzed for reversibly adsorbed P.

Materials and methods
The soil material consisted of samples tak- en from the unlimed plots of two field experiments in which five rates of P fertilization (0, 13 or 16, 26 or 32, 47 or 56, and 60 or 72 kg P/ha) had been applied annually for 12 years (soil I; four blocks, 20 samples) or for 11 years (soil II; three blocks, 15 samples).The ex- perimental fields, located in Vihti, Southern Finland, were silty clay soils and, according to the U.S. Soil Taxonomy, tentatively clas- sified as Cryochrepts.The experimental design, soil characteristics as well as the results of the field experiments, in which mainly cereal crops were cultivated, have been reported earlier (Yli-Halla 1989 b).In that previous paper, soils I and II were referred to as ex- periments A and B, respectively.Some properties of the experimental soils are summarized in Table 1.
Isotherms for phosphate adsorption and desorption were determined according to the method by Hartikainen (1982b).Duplicate soil samples (1.0 g) were equilibrated for 23 hours in 50 ml of solution containing differ- ent concentrations (0.01, 0.02, 0.05, 0.10, 0.20, 0.30, 0.40, or 0.50 mg P/l) of P as KH 2 P0 4 in centrifuge tubes.No supporting electrolyte was used.After adding the solu- tion, the suspensions were shaken for 1 h and allowed to stand for 22 h and then shaken again for 10 min.The suspensions were cen- trifuged, and the supernatant solutions were decantedand filtered through a membrane fil- ter (pore size 0.2 um) and analyzed for P by a molybdate blue method using ascorbic acid as the reducing agent (Anon. 1969).The quantities of P adsorbed or desorbed (y) were plotted against the initial P concentration (x) of the added solution (isotherm A) or against the P concentration of the supernatant solu- tion (isotherm B).The quantities of P ad- sorbed or desorbed were expressed as milligrams per kilogram of soil and the solu- •) extracted with 0.05 M ammonium oxalate at pH 3.3.separately.Means with a common letter are not dif- ferent at the 95 % level of statistical probability.tion concentrations as milligrams per litre.The isotherms had the general formula of y = bx + a.The details of the interpretation of the isotherms have been presented by Har-  tikainen (1982a).

Results
In the isotherm A, the quantities of P ad- sorbed or desorbed are plotted against the initial concentration of P of the added solution.Based on this plotting, observations can be made about the effect of a given P addition, e.g.fertilization, on the quantities of P ad- sorbed or desorbed.The type A isotherms (Fig. 1) showed that the subsamples of soil I adsorbed P somewhat more effectively than did the ones of soil 11.This can also be con- cluded from the larger coefficient b in the equations of the A-type isotherms (Table 3).
As an example, when the solution of 0.5 mg P/l was added (P addition of 25 mg P/kg), the subsamples of soil II not fertilized with P adsorbed 14.8 mg/kg (59 %) while as much as 20.2 mg/kg (81 %) was adsorbed by the corrensponding subsample of soil I. From the same addition, 61 % and 27 °7o was adsorbed by the subsamples fertilized with the highest P rate in soils I and 11, respectively.The sub- sample of soil I fertilized even with the highest  rate of P for 12 years adsorbed approximate- ly equal quantities of P as did the subsample of soil II not fertilized with P for 11 years.
In each soil, phosphate adsorption decreased with increasing P fertilization.Isotherm B (Fig. 2) allows conclusions to be drawn about P sorption and desorption when the P concentration of soil solution is changed.The slope reflects the P buffer power of the soil.Soil I seemed to be more buffered against changes in equilibrium concentration than was soil 11.In soil I there was a slight tendency of the buffer power to decrease with increasing P fertilization, while the change in soil II was inconsistent.
From isotherm B, the quantity of P adsorption required for the elevation of solution P concentration to 0.2 mg/1 was calculated (Table 4).This P concentration has been regarded as being sufficient for maximum yield for most cultivated crops.The calculation demonstrated the greater tendency for P sorption of soil I. Inversely, the subsamples of soil II receiving the three highest rates of P main- tained a solution concentration even higher than 0.2 mg/1.Extrapolation to the lower con- centrations is questionable because the isotherm is supposed to converge the y axis (Hartikainen 1982 b).
The equilibrium phosphate concentration (EPC) expresses the concentration at which no net exchange occurs between the soil and the surrounding solution.In soil I, EPC ranged from 0.05 mg/1 to 0.11 mg/1 (Table 5), while in soil 11, the range was wider, i.e. from 0.11 to 0.31 mg/1.According to the paired t-statis- tics, the subsamples of soil I maintained statistically significantly lower EPC values than did the ones of soil II at any level of P fertilization (t = 4.856**).The smaller changes in EPCs due to P fertilization in soil I, compared to the range of changes in soil 11, were in agreement with the higher buffer power of soil I.However, it should be observed that the EPC values were consistently elevated upon increased P fertilization in both soils.
In order to find out the size of labile pool of P, the quantity of reversibly adsorbed P (Pi) was determined.In that method, P con- centration is maintained at a very low level during the extraction due to removal of phosphate from the solution to the surface of a separate solid iron hydroxide phase, which promotes desorption.The quantity of P; (Table 5) was somewhat greater in the subsamples of soil 11, as compared with the paired t-statistics with the corresponding samples of soil I (t = 3.557*).
Table 4. Quantities of P adsorbed ( + ) or desorbed (-)   at equilibrium solution concentration of 0.2 mg P/l in subsamples of two soils receiving different rates of P fertilization.

Discussion
The current measurements showed that soil I adsorbed P more effectively and had a higher P buffer power than soil 11.Soil I was very much buffered, while soil II represented the average as compared with the clay soils of Hartikainen (1982c).During the last few years of the field experiments from which the soil samples originated, the highest yields in both soils were obtained at the fertilization level of 32 kg P/ha (Yli-Halla 1989 b).The EPC values of the subsamples taken from these plots were approximately 0.1 mg/1 and 0.2 mg/1 in soils I and 11, respectively, sug- gesting that a lower solution P concentration was sufficient for maximum yield in soil I.This is in accordance with the conclusion by Olsen and Watanabe (1970) that in soils of higher buffer power (soil 1), a lower P con- centration in the soil solution is needed for a given quantity of P uptake by plants.Thus, the lower EPC values in soil I do not as such imply poorer P supply to the plants in this soil.
However, in addition to lower P intensities, reflected by the EPC values of the soil sam- ples, the reserve of plant-available P, as meas- ured by the quantity of reversibly adsorbed P, Fig. 2. Type B isotherms.Relationship between P adsorp- tion or desorption (y) and the P concentration in the equi- librium solution (x) after adding P to subsamples of two soils (I and II) into which different rates of P had been applied annually.
was lower in the subsamples of soil I.As com- bined, these two results suggest that a more rapid conversion of added P into forms not available to plants had occurred in soil I.This conclusion is in accordance with the results of the field experiments in which much larger responses to P fertilization were obtained in soil I.
There were a few differences in the proper- ties of soil I and II which contribute to higher P adsorption in soil I: clay content (37% vs. 31%), pH (5.4 vs. 6.0) and oxalate-extractable AI (84 mmol/kg vs. 64 mmol/kg).On the con- trary, the content of oxalate-extractable Fe was greater in soil II (71 mmol/kg vs. 85 mmol/kg).It is not likely that the differences in P adsorption characteristics as well as longterm effect of P fertilization can completely be attributed to these minor differences.
The residual effect of P fertilization is de- pendent on the rate of slow long-term fixa- tion processes occurring in the soil, and the importance of these processes cannot be directly predicted by the determination of phosphate adsorption isotherms.Short-term P adsorption by 24 soils of Australia have been observed to be in close relationship with P sorption also in the long run and in positive correlation with the yield response by clo- ver to P fertilization in a pot experiment (Barrow 1972).In Sweden, StAhlberg (1982)   even suggested a method based on the deter- mination of P adsorption for the basis for P fertilization recommendations.Holford (1982) further showed that therecovery of fertilizer P over a period of four years in a pot experiment was in close correlation with the results of short-term P adsorption measurements.Also in the present study, there seemed to be a relationship between the yield response to P fertilization and P adsorption by the soil.Even though the current material consisted of samples taken only from two field experiments, the observed tendency is likely not a coincidence, because it is well in agreement with the results of other studies mentioned above.The results of the present study thus confirm that short-term P adsorption mea- surements are indeed able to serve as a valuable tool in practical soil fertility research in elucidating the tendency of adsorption of ad- ded P as well as the probable long-term yield response to P fertilization.
Previous results (Yli-Halla 1989 b) showed that repeated P fertilization for 11 or 12 years had a different effect on the quantities of eas- ily soluble P in the two soils studied despite the originally same P status of soils.It should be recognized that water extraction was in the present soils more accurate in reflecting the residual effect of P fertilization.The present results further showed that the changes in equilibrium phosphate concentration (EPC), attributable to P fertilization, differed wide- ly from soil to soil.The difference between the two soils could not be predicted from soil properties other than P sorption.Even though it is not possible by the results of ordinary ace- tate or water extraction to predict the residu- al effect of P fertilization, regular soil analy- sis is of utmost importance in monitoring the cumulative effect of P status of the soil.By knowing the quantities of P fertilization given and the consequent changes in soil testing results, the farmer is gradually able to get a sound idea of the residual effect of P fertilization in the field concerned.sa oli aluksi samat maarat happamaan ammoniumasetaat- tiliuokseen uuttuvaa fosforia (n. 6mg/1), ja niiden ominaisuudet olivat muutenkin lahes samanlaiset.Kuitenkin toisessa maassa (maa I) P-lannoitus lisasi satoa paljon enemman kuin toisessa (maa II).Kun kenttakokeiden paattyessa otettujen maanaytteiden fosforinpidatyskykya tutkittiin ravistelemalla maanaytteita (1 g) eri vahvuisissa (0 -0.5 mg/1) fosforiliuoksissa, havaittiin, etta maan I naytteet pidattivat fosforia huomattavasti tehokkaam- min kuin maan II naytteet.Myos labiilin fosforin koko- naismaara oli maan I naytteissa pienempi kuin maan II vastaavan lannoituksen saaneissa naytteissa.Selitys maalla 1 P-lannoituksella kenttakokeissa saatuihin suuriin sadonlisayksiin oli siis se, etta tassa maassa lannoitefosfori pidattyi nopeasti kasveille kayttokelvottomaan muotoon, kun taas maassa II lannoitefosfori sailyi suuremmassa maarin liukoisena.Koska nykyisin maa-analyysimene- telmin ei pystyta luotettavasti arvioimaan P-lannoituksen jalkivaikutuksen suuruutta maan ominaisuuksien perusteella, on viljavuusanalyysi tehtava kyllin usein maan Ptilan kehityksen seuraamiseksi.
, the results of which have been published earli- er (Yli-Halla 1989 b).

Fig
Fig. I. Type A isotherms.Adsorption or desorption of phosphate (y) as a function of the concentration of added P solution (x) by subsamples of two soils (I and II) into which different rates of P had been applied annually.

Table 1 .
Some chemical and physical characteristics of the experimental soils.

Table 2 .
Contents of phosphorus extracted with water (P w )or with an acid ammonium acetate solution (Paaac) from subsamples of two soils fertilized with different rates of P.*)*) The results of each soil and extraction have been tested

Table 3 .
The equations of phosphate adsorption isotherms of subsamples of two soils receiving different rates of P fertilization.