Mineral element content of oats ( Avena sutiva L . ) in an acid sulphate soil area of Tupos village , northern Finland

The mineral element content of oat grains grown in acid sulphate soils (n = 35) and other soils (n = 19) in Tupos was studied. In terms of P, K, Ca, Mg, S, Cu and Zn concentrations, Tupos oats did not differ from the averages and ranges presented for Finnish oats in other studies. Instead, Tupos oats were in general rich in Fe and Co and low in Se. High concentrations of Mn and Ni occurred in samples originating from acid sulphate soils. The lower the soil pH, the higher were the concentrations of Ni and Co in oats. Na and Ni were the only elements displaying a clear relationship between the concentration of the element in oats and the amounts of the element easily soluble in the soil. Tupos oats deviated less than Tupos timothy from the mineral contents ofboth these plant species usually reported


Introduction
The fields of Tupos village, situated on a coastal plane 20 km south of Oulu, consist of an even mixture of acid sulphate soils and other soils.Recently, a soil survey was made in Tupos in which samples were taken from timothy and oat fields (Erviö and Palko   1984).The analyses revealed that due to abun- dant sulphide material in the subsoil, the plough layer pH was low in most samples and, consequently, soils were rich in easily soluble Al, Fe, Mn, Cr, Co, Mg, Zn and S.This was reflected in the chemical composition of timothy grown in the area: the plant material contained more Co, Cr and also Ni than is usual in Finland (Palko 1986).The contents of macronutrients Ca, Mg, K and P as well as micronutrient Zn were, in turn, lower than the Finnish averages, being as much an indi- cator of soil properties as of insufficient fer- tilization.
The purpose of the present study was to investigate the chemical composition of oat grains grown in the Tupos area.It was hoped that the study would indicate possible exces- ses and deficiences of mineral elements in Tupos oats and would thus be of use in as- sessing the fertilization requirements of the acid sulphate soils and the mineral intake of cattle fed with the local oats.In addition, the relationship between some soil properties and mineral element content of oats was studied.

Materials and methods
The oat grains were harvested from 51 fields in 1983 in Tupos village (lat.64 50', long.25 30') when the grains were fully ripe.The grain samples were air-dried, ground and analysed for K, Ca, Mg, P, S, Cu, Zn, Fe, Mn, Na, Co, Ni and Se at Kemira Oy, Oulu Research Laboratory, according to the methods de- scribed by Saari and Paaso (1980).
Tupos soils have previously been character- ized in detail by Erviö and Palko (1984).Soil samples corresponding to the present oat fields were included in that material.K, Ca, Mg, P and S were extracted with 0.5 M acetic acid-0.5 M ammonium acetate (AAAc) (pH 4.65) (Vuorinen and Mäkitie 1955) and other elements with AAAc-0.02M Na 2 EDTA (Pa-  kanen and Erviö 1971).The soils were iden- tified according to Palko and Räsänen (1987)  as acid sulphate soils.For identification pur- poses the soils were classified according to the properties of the subsoil, not the plough layer, thus avoiding the effect of farming practices.The chemical properties of the present soil material are listed in Table 1.

Results and discussion
Macronutrients (P, K, Ca, Mg, S) The average content of various macronut- rients in Tupos oats (Table 2) was similar to the contents reported for oat grains in other Finnish studies (Pessi et ai. 1974,Jaakkola  and Vogt 1978, Syvälahti and Korkman   1978, Varo et ai. 1980,Jansson et ai.1985).
The present results also fell within the range reported in them.The oats from acid sulphate soils were poorer in Ca than oats grown in other soils (t = 2.96**).Although the soils were very rich in soluble S, oat grains did not contain S more than usual in Finland.P and K are usually added yearly into the field soil in fertilizers, and Ca and Mg are given in liming materials.The influence of the native macronutrient reserves on oat grains is likely to be masked by these farming practices.

Copper and Zinc
In the present study, the micronutrients Cu and Zn were also within the concentration range presented earlier.Only Jaakkola and Vogt(1978) have reported considerably lower Cu contents.In the present material, two samples had a very high Cu content, making the ranges wide.However, the bulk of Cu con- centrations were within the normal range.Iron and manganese The samples of Tupos oats proved to be rich in Fe.Even the lowest concentrations were higher than the averages in other Finnish studies in which the usual Fe content falls within the range of 40 -100 mg/kg.High con- centrations of Mn also occurred in Tupos oats, most of the results, however, falling within the normal range reported elsewhere, 40-80 mg/kg.The oats grown in acid sulphate soils exhibited a higher mean Mn content than the rest of the samples (t = 3.65**), even if the ranges were equal in both sample groups.

Sodium
The recent marine history of the Tupos soils was reflected in the rather high Na content found in some oat samples, although most re- sults were in accordance with the values re- ported by Jaakkola and Vogt (1978), 20 - 30 mg/kg.In 80 % of the oat samples the Na content was below_sCf mg/kg.The Na content of cereal grains may t however, be of minor importance in animal nutrition due to the high Na requirement of animals.

Nickel and cobalt
The heavy metals Co and Ni are not, ac- cording to the present knowledge, needed by oats.However, especially the Ni content in the plants was rather high, being usually of the same magnitude as the contenfof Cu.The Ni content of Tupos oats grown in soils not clas- sified as acid sulphate soils was of the same order as the Ni contents of oats reported elsewhere (Pessi et ai.1974, Syvälahti and   Korkman 1978, Varocl ai.1980).Oats' grown in acid sulphate soils, on the other hand, had an average Ni content two to three times higher.In the present material there were five samples of oats, all from acid sulphate soils, having Ni content even greater than 10 mg/kg.
The Co content of cereals generally range only in micrograms per kilogram of plant ma- terial.The present means for Co were even 10 times higher than those mentioned in other studies.Syvälahti and Korkman (1978) and Varo et ai.( 1980) have published results where the Co content of oats has been within the range of 0.05-0.10mg/kg.The highest concentration in the study be Varocl al. was 0.20 mg/kg, which was in fact the lowest value found in the present material.Eaviöand Pal- ko (1984) have indeed reported unusually high contents of easily soluble Co for Tupos soils, and according to Palko (1986), the Co concentrations for timothy in the Tupos area average five times higher than those reported elsewhere.

Selenium
The Se content of oat grains in Tupos was very low.Only three samples, none of which were growing in acid sulphate soil, exceeded thereliable level of detection, 0.01 mg/kg.In addition, five samples had an approximate Se concentration of 0.005-0.01mg/kg.The rest, including all 35 samples collected from acid sulphate soils, had a Se content lower than 0.005 mg/kg.The average Se content of Finnish oats has also been reported as being near the detection limit: mean 0.010 mg/kg, range 0.005 -0.021 mg/kg (VARoet al. 1980).In the study by Palko (1986), none of the 45 timothy samples had a Se content higher than 0.01 mg/kg.Significantly, both plant mate- rials were taken before the addition of Se in compound fertilizers in Finland.

Soil properties and plant analyses
As the distribution of soil and plant analysis data was not normal, the Spearman rank correlation coefficients were calculated between soil properties (soil pH and AAAc- or AAAc-EDTA-extractable amounts of elements) and the mineral element content in oats.The statistically significant (P < 0.05) correlation coefficients were as follows: The correlation between soil pH and Mn concentration in oats was low compared with the previous study on the mineral element con- tent of timothy grown in the same village (Palko 1986).In that study the content of Co, Ni and Mn in timothy increased very significantly as the soil pH decreased.As for Zn, its solubility in Tupos soils proved to be dependent on soil pH (Palko and Yli-Halla  1987), but the variation in the Zn content of the oats seemed not to be affected by this re- lationship.The varying concentrations of Ni and Na and, to a lesser extent, that of Mn were reflected in the composition of plants.
The Ni content of Tupos timothy was also higher when the AAAc-EDTA-extractable Ni in the soil was higher (Palko 1986).

Conclusions
In terms of macronutrient concentration Tupos oats did not differ considerably from oats grown in other parts of Finland.The higher Fe, Mn, Ni and Co content of oats grown in acid sulphate soils is obviously con- nected with soil acidity, but the apparently high concentrations of Co warrant further investigation.The present results suggest that oats grown in Tupos fields are able to main- tain their normal level of plant nutrients despite the somewhat exceptionally high con- centrations of certain elements in the soil.It was, however, noticed that the elements which are not plant nutrients (i.e.Na, Co and Ni) exhibited greater variation of concentration in the grain than especially macronutrients.The composition of the Tupos oats deviated less than that of the Tupos timothy from the mineral contents usually reported for these plant species in Finland.However, it should be recognized that the timothy samples have consisted mainly of the vegetative parts of the plant, while in the present study only grains were analysed.Jaakkola et al. (1982) noticed that the composition of grains varies less than that of the vegetative parts of cereals, i.e. straw.The results by YrÄRANTAand Sillan- pää (1984) also show that the concentration ranges of most elements in cereal grain are narrower than the ones in cereal straw and timothy.From the scientific point of view, it is possible that leaf and straw analysis would more probably reflect the concentrations of easily soluble mineral elements in the soil.Yet, as far as cereals are concenred, only the com- position of grains is important since straw is seldom used as fodder.

Table 1 .
Chemical characteristics of oat field soils inTupos village.

Table 2 .
Mineral element content of oats (g or mg/kg dry matter) grown in acid sulphate soils and other soils in