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Availability of mineral nutrients in the case of grasslands and of arable crops

Paru en 1980 dans Fourrages n°83 (page 55 à 78)

Auteurs : Chevalier H.

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Résumé :

The aim of this work is to compare the evolution of available mineral nutrients, phosphorus, potassium, calcium, magnesium, in soils supporting a succession of annual crops, or leys and permanent pastures.
The distribution of nutrients is very different under arable crops and under grass: in the latter case they are concentrated in a shallow layer of soil, whereas in the first case they are well distributed throughout the depth of the ploughed layer.
The requirements of herbage grasses are much higher than those of arable crops, especially as regards potassium. The importance of nitrogen fertilization is great, since by increasing the yields, it enhances the uptakes (Exp. Station at Aspach).
In the case of natural grasslands (Normandy), the uptake of mineral nutrients, although smaller than in the case of a pure grass ley, is at least as large as that of a succession of intensive crops.
In grasslands managed solely by grazing, an important part of potassium brought back by urine
migrates outside the reach of the roots. This fraction, inasmuch it does not get into deep layers of the soils nor is very strongly fixed by the absorbing complex, may be partly recovered when the pasture is ploughed up.
It is often observed that grassland soils have smaller nutrient contents than arable soils in the same farm, a reflection of the fertilizer dressings received by grasslands. This is the reason why, generally, a corrective fertilization should take place when grasslands are ploughed up for arable crops.
As a conclusion, it may be considered that the utilization by arable crops of a grassland soil alter ploughing up has no adverse effect on its mineral fertility, provided there is an adequate fertilization. The satisfactory supply of nutrients to crops in the first years alter ploughing corresponds to the utilization of minerals stored in the soil organic matter, and possibly also in the deeper layers. These nutrient reserves cannot be considered as an irreplaceable capital.

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