Effects of different ammonium nitrate levels on the amounts of exchangeable soil magnesium and applied magnesium in eight mineral soils

Eight mineral soils (pH(CaCl 2) 4.6—6.1, clay 4—65 %, org. C 1.9—5.7 <7o) were treated with ammonium nitrate and magnesium sulphate solutions adding 0, 20 or 40 mg mineral N and 0 or 4 rag Mg per 100 g soil. The soils were incubated for seven weeks at a constant temperature of 20 °C and a 25 % moisture level. After incubation, the exchangeable Mg was extracted with 1 M neutral ammonium acetate. The exchangeable magnesium content seemed to increase in some soils and to decrease in other soils with increasing ammonium nitrate amounts. The applied magnesium was fixed in a non-exchangeable form, especially at the highest ammonium nitrate level, in two clay soils taken from the rapakivi area of south-eastern Finland. In the other soils all applied magnesium was exchangeable irrespective of the amount of ammonium nitrate.


Introduction
A great part of the mineral N in compound N-P-K fertilizers is in the form of NH 4 -N.According to several studies (e.g.Nommik  1957, Schachtschabel 1961, Kaila 1962), the mineral soils have the ability to fix applied NH 4 + in a non-exchangeable form.
The cations Ca 2+ or Mg 2+ have no effect on this fixation (Nommik 1957).
Only few studies on the effects of different mineral N levels on the exchangeable cation content of the soil are available (Sippola et  ai.1973).In laboratory studies on exchangeable cations a common practice is to treat the soil with NH 4 + acetate or chloride.The NH 4 + -N containing fertilizers may have the same effect, even though the NH 4 + concentration in the soil remains lower than in laboratory studies.
In a pot experiment, the apparent recovery of fertilizer Mg was low in some clay soils (Jokinen 1981 a).The antagonism between NH 4 + and Mg 2+ in the cation uptake by Index words: Ammonium nitrate, magnesium sulphate, exchangeable Mg, non-exchangeable Mg, fine sand, finer fine sand, silty clay, sandy clay, heavy clay.

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JOURNAL OF AGRICULTURAL SCIENCE IN FINLAND plants was assumed to be the main reason for this.The effects of ammonium nitrate on soil Mg and applied Mg were not studied.
The aim of the present incubation experiment was to study the effects of different ammonium nitrate levels on the exchangeable Mg content of eight mineral soils and on the amounts of exchangeable Mg applied with magnesium sulphate.

Materials and methods
Eight mineral soils were incubated at 20 °C for seven weeks.The soil samples, three non-clay and five clay soils, represented the plough layer of cultivated soil from southern Finland.The same soils were used in an earlier pot experiment (Jokinen  1981 a) and incubation experiment (Jokinen 1981 b).The numbers and characteristics of the soils are given in the latter report.One of the soils, muddy silt (4), was not included in this study because of the too small amount of soil available.
The soils were air-dried and crushed to pass a 2-mm sieve.For the experiment, 100 g soil was weighed into 0.5 litre plastic pots and treated with ammonium nitrate and magnesium sulphate adding the following amounts of N and Mg: The treatments without N fertilization (N O MgO and N OMg,) were common with the incubation experiment on liming and Mg fer- tilization (Jokinen 1981 b).The fertilizer solutions were thoroughly mixed with the soil.Four replicates were made.Both experiments were incubated at the same time and in the same place.The moisture of the soils was maintained at 25 °7o of the soil weight, ad- ding de-ionized water as necessary.The pots were covered with perforated plastic film.
After incubation, the soils were air-dried at room temperature and repassed through a 2-mm sieve.The exchangeable Mg was ex- tracted by 1 M neutral ammonium acetate and exchange acidity (AI + H) by 1 M KCI (Kaila 1971).The amount of applied Mg found exchangeable in the soil was calculated as the difference Mg,-Mg 0 .The exchangeable NF1 4 + was extracted with 0.25 M K 2 S0 4 (soil : solution = 1 : 10, w/v, 2 h) and deter- mined by destination.The N0 3 ~was deter- mined from the same aliquot of extract after reduction with Devarda's alloy.The amounts of applied mineral N found in the soils were calculated as the differences N,-N 0 and N 2-N 0 .

Results and discussion
After seven weeks of incubation, almost all the mineral N (NH 4 + -N + N0 3 ~-N) ap- plied was found in the soils extractable in 0.25 M K 2 S0 4 at the N, level.In finer fine sand (3), sandy clay (6) and silty clay (8), the nitri- fication of NH 4 + -N seemed to be complete, since the amount of NOf-N increased in the same proportion.At the N 2 level, the nitrifi- cation of applied NH 4 + was observed in sil- ty clay (8) only, possibly because of the high amount of mineral N applied.
In finer fine sand (3) and in clays (6-9) without ammonium nitrate, the exchangeable Mg content seemed to be somewhat higher than with N (Table 1).Increased activ- ity of micro-organisms in soils 3, 6 and 8 by N fertilization was concluded on the basis of increased N0 3 --N content during incuba- tion.Some of the exchangeable Mg may be involved in the biological fixation.In fine sand (1) and silty clay (5), the exchangeable Mg content seemed to increase with increas- ing ammonium nitrate amounts.Some of the non-exchangeable Mg in these soils may be- come exchangeable without difficulty, e.g. by chemical weathering.This may explain the ability of ryegrass in the pot experiment to take up non-exchangeable Mg from fine sand (1).

Mgo
Mg rMg0 N" N, N 2 N 0 Results of an individual soil with the same letter do not deviate significantly (P = 5 %).The datas of Mg 0 and Mg|-Mg" were studied separately by Duncan's new multiple range test.
Without ammonium nitrate the applied Mg (4 mg/100 g soil) was found exchangeable in all soils after incubation (Table 1).Considerable amounts of Mg were released from heavy clay (9) in the exchangeable form during incubation.
With ammonium nitrate the applied Mg was partly tied up by fine sand (2) and sandy clay (6) at both N levels and by finer fine sand (3) and silty clay (8) at the N 2 level.
The soils 6 and 8 originated from the rapakivi area of south-eastern Finland where, ac- cording to Sippola (1974), K-feldspar is more common than elsewhere in Finland.In these soils the fixation of NFI 4 + -N into a non-exchangeable form seemed to be low be- cause of the high K content (Scherer 1982).Hence it is possible that applied NH 4 + -N contributed to the formation of nonexchangeable Mg compounds.In finer fine sand (3), the fixation of applied Mg in the non-exchangeable form may be a conse- quence of the formation of insoluble Al-Mg   compounds (HuNSAKERand Pratt 1970), since the 1 M KCI extractable Al 3+ content of this soil decreased with increasing amounts of ammonium nitrate.The content of H + remained constant.
With increasing amounts of ammonium nitrate the reactions against applied Mg deviated in two clays (6 and 7) as well as in two silty clays (5 and 8).From soils 5 and 7 Mg was released in the exchangeable form and in soils 6 and 8 the fixation of Mg in non-exchangeable form occured during incubation.
The apparent recovery of fertilizer Mg by ryegrass (total of 8 cuts) was for the »rapakivi» soils (6 and 8) very low at the N, level (1.6 % and 0.4 %), but somewhat higher values were obtained at the N 2 level (13.6 % and 39.7 %) in the pot experiment (Jokinen  1981 a).Ryegrass seemed to be able to take up fixed Mg from these soils during the two growing seasons studied.
From the agricultural point of view the high amount of ammonium nitrate may have positive effects on the exchangeable Mg con- tent of some soils.The applied Mg seemed to be fixed in the non-exchangeable form in some soils and this may contribute to the low recovery of fertilizer Mg.The antagonism between Mg 2+ and NH 4 + or K + in the ca- tion uptake by plants is the main but not the only reason for the restricted Mg uptake.