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White clover as a source of nitrogen on Estonian grasslands on acid soils poor in humus

Malle Kärner and Enn Kärner

Estonian Agricultural University, Tartu, Estonia

Materials and methods
Results and discussion


The herbage yield of a sward mainly depends on its supply of N. It is widely established that high N fertilizer rates increase grass yields (Toomre, 1960; Burg, 1970; Reidolf, 1983). It is often suggested to cultivate legumes as a source of N in grasslands, particulary on cultivated pastures (Sau, 1970; Older et al., 1994; Raave, 1994; Viiralt et al., 1994). Under the Estonian climatic conditions white clover is able to fix on average 173 kg N ha-1 (Sau, 1970). There is little experimental evidence on the efficiency and N-fixing ability of white clover grown with various grasses on acid soils low in humus.

Materials and methods

The experiment was conducted on a moderately podzolic loamy sand with an initial pHKCl value of 4.4, with humus and N contents of 1.56 -1.89% and 0.08 - 0.10%, respectively. The soil had a low level of nutrients: P2O5 and K2O, 1 and 6 mg per 100 g of soil respectively; and Cu, 0.33, Mo, 0.03 and B, 0.08 mg per kg of soil. The soil had high hydrolytic acidity (3.4 meq 100 g-1) and a low (50.0%) saturation. Lime should be spread over such soils and for that reason 10 t of oilshale ash (60% CaO) per ha was applied to the soil as well as leaving on area unlimed.

The experiment included the following species of grasses: timothy (Phleum pratense L.) Jõgeva 54, meadow fescue (Festuca pratensis L.) Jõgeva 47, cocksfoot (Dactylis glomerata L.) Jõgeva 220, red fescue (Festuca rubra L.) Jõgeva 70, smooth-stalked meadow grass (Poa pratensis L.) Jõgeva 1 and white clover (Trifolium repens L.) Jõgeva 4. These grasses were grown as pure species and in mixture with 9 kg ha-1 white clover. The experiment was conducted over a 7-year period. Paper is concerned with results of 2 to 7 (six) growth years because the yields of first (establish) year were very low.

Results and discussion

Grasses are low yielding herbage plants on acid soils poor in humus. The pure sown species yielded on average 1.85 to 1.99 t DM ha-1 and 0.210 to 0.248 t CP ha-1 with the highest yields comming from the limed soil. Over the experimental years there was a narrow fluctuation (1.4 times) in the yield of grasses. The highest yield of grasses was obtained in the fourth year of growth, but it declined in the fifth and sixth years and in the 7th year it again increased, achieving the level of the fourth harvest year. On the basis of 6-year average yields obtained from both the acid and limed soils the grasses ranked as follows: red fescue, meadow fescue, smooth-stalked meadow grass, cocksfoot and timothy.

The effect of white clover on yield level

The yields of the sown mixtures mainly depended on the associated grass species. Varying with the year of growth on the acid soil the species yielded on average 2.68 to 6.73 t DM ha-1 and 0.303 to 1.402 t CP ha-1 and on the limed soil 4.05 to 7.89 t DM ha-1 0.591 to 1.662 t CP ha-1. The influence of white clover on yield on the acid soil was an increase of 2.28 to 2.89 (mean 2.59) t DM ha-1 and 0.455 to 0.626 (mean 0.538) t CP ha-1 and on the limed soil 2.84 to 3.84 (mean 3.25) t DM ha-1 and 0.540 to 0.812 (mean 0.680) t CP ha-1 respectively. On the basis of mean annual yields, the ranking of the grass swards was as follows: smooth-stalked meadow grass, red fescue, meadow fescue, timothy and cocksfoot.

With liming the average yield of the mixed sowings was increased by 0.801 DM ha-1 and 0.142 t CP ha-1. In the second and third harvest years the influence of liming was twice that of the following years.

The effect of white clover varyied with the grass species but the average DM yield of the grasses over six years on the acid soil was increased by 0.25 to 0.58 t DM ha'1 and on the limed soil by 0.41 to 1.07 t DM ha-1. The mixed sowings ranked as follows: cocksfoot, smooth-stalked meadow grass, red fescue, meadow fescue and timothy.

The growth rate of white clover was best when grown with smooth-stalked meadow grass and least with cocksfoot. Depending on the grass species, the DM yield of clover on the acid soil amounted to 1.57 - 2.24 t ha-1 and on the limed soil, 1.82 - 2.77 t ha-1. With liming the average yield of clover was increased by 0.52 t ha-1

Effect of clover on CP content in the DM

The CP content in grass DM on the acid soil poor in humus depending on the species and year ranged from 6.0 to 14.2% (mean 11.3%) and with lime it increased on average by 0.8%. The effect of white clover on the grass CP content in mixed sowings varyed with the species and increased by 3.8 to 5.3% on the acid soil, and by 3.6 to 5.9% on the limed soil. The CP content in mixed sowings was highest when white clover was grown with smooth-stalked meadow grass and timoty, and lowest with cocksfoot. The CP content in the mixed sowings was higher by 1.2% on the limed soil than on the acid soil.

The CP content also depended on the year of growth. The maximum CP content was obtained in the third year. From the fourth year the CP content began to decline and in the fourth to seventh year on the acid soil it was less by 0.9 to 7.2%, and on the limed soil by 0.9 to 6.7% compared with values for the third year. The highest decline in the CP content of grass was in the case of the cocksfoot white clover sward.

Fixation of N by white clover and N offtakes in grass

Under climatic conditions favourable for plant growth, e.g. in New Zealand, depending on the mean temperature and rainfall, white clover fixes 224 to 600 kg N ha-1 (Sears, 1950). Under the Estonian climatic conditions white clover fixes 50 to 220 kg N ha-1 annually (Toomre, 1965; Sau, 1966).

Depending on grass species the N yield pure sowings on the acid soil was 30.2 to 37.9 kg ha-1 and on the limed soil 37.2 to 42.7 kg ha-1, while in the case of mixed sowings the offtakes were 104.5 to 134.8 and 123.7 to 168.2 kg ha-1, respectively. On both the acid and limed soils the N yields of mixed sowings ranked as follows: smooth-stalked meadow grass, red fescue, timothy, meadow fescue and cocksfoot.

The N yield of mixed sowings was increased due to an increase in the N yield of grasses in addition to the N yield of white clover. In the grass/clover mixtures the N yields on the acid and limed soils, respectively, were increased as follows: timothy by 13.0 and 10.6, meadow fescue by 11.7 and 13.5, cocksfoot by 11.5 and 21.4, red fescue by 11.9 and 15.9 and smooth-stalked meadow grass by 19.8 and 25.5 kg ha-1. The N yield of white clover averaged over the companion grasses amounted 75.7 to 97.6 (mean 88.7) kg ha-1 on the acid soil and 115.9 to 126.1 kg ha-1 (mean 110.9) on the limed soil.

The N quantity, accumulated in the herbage yield, roots and soil on the grass swards on acid soil accounted for 79.6 to 116.5 kg ha-1 (mean 96.2) and on the limed soil 89.4 to 148 kg ha-1 (mean 113.5), and in the case of mixed sowings, 187.3 to 292.7 kg ha-1 (mean 234.5) and 216.4 to 362.8 kg ha-1 (mean 283.3), respectively. Thus, depending on the companion grass white clover accumulated on the acid soil 100.1 to 176.2 kg ha-1 (mean 138.3) and on the limed soil, 127.0 to 214.4 kg ha-1 (mean 168.8).


Grasses are low yielding on an acid soil poor in humus. The DM yield and its CP content are increased by the sowing of grasses with white clover. The efficiency of white clover growth and N fixation depends on the grass grown with it. Liming of an acid soil raises the efficiency of white clover growth and development.


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