EFFECTS OF SOIL TEXTURE AND SOIL SALINITY ON THE PLANT WATER RELATIONSHIP, GROWTH, YIELD AND WATER USE EFFICIENCY OF THE SOYBEAN CROP - Soybean and Wheat Crops: Growth, Fertilization, and Yield - page 35

Figure 7 illustrates the relationship between the reduction in relative grain yield
(relationship between the yield observed for the saline treatment and the yield observed for
the control treatment) and the soil salinity (ECe). With the same level of soil salinity, the
relative yield observed in the clay soil is significantly lower than that observed in the loam
soil.
According to the observations of Van Hoorn et al. (2001), salinity and soil texture did not
affect the nitrogen content (mg N/ dry matter) of soybean. Moreover, the quantity of organic
nitrogen varies with the soil salinity and texture, according to the dry matter accumulation.
These quantities are 20 and 9.4 g/m 2 respectively for the 15 and 30 meq Cl/l treatments in
loam soil. In the case of clay soil, these quantities are inferior to those of the same treatments
in loam soil: 28% and 64% for the 15 and 30 meq Cl/l treatments, respectively. Therefore, the
organic nitrogen in loam soil ensures between 67% (for the 15 meq/l treatment) and 50% (for
the 30 meq Cl/l treatment) of the plants' nitrogen uptake. Instead, in the case of the same
treatments in clay soil, the organic nitrogen fixation ensures only 59% and 26% of the plants'
nitrogen uptake.

IV- D ISCUSSION AND C ONCLUSION

The observations obtained in this study provide the answers to the three questions posed
in the introduction to this chapter.
In the absence of salt stress, soil texture does not have a significant influence on the plant
water relationships, growth and yield for soybean. It can simply be noted that the plant water
status, yield and water use efficiency have a slight advantage in loam soil. Differently from
other species like sugar-beet, potato and sunflower (see the review by Katerji and Mastrorilli,
2009), soybean is not strongly influenced by soil texture.
In the presence of salt stress, soybean's tolerance to salinity changes depending on the
soil texture. The same soybean cultivar tolerated growing in loam soil better than in clay soil
(see table 5 and Fig. 7).
Two hypotheses can be formulated to explain soybean's greater tolerance of salinity in
loam soil:

1. Soybean is a species that has high nitrogen requirements (Bouniols et al., 1990). The

quantities of nitrogen provided by fertilisation in this study were lower than the
plants' requirements (van Hoorn et al., 2001). The higher organic nitrogen in loam
soil allows the plant to produce a greater yield and better tolerate the soil salinity.
Instead, the lesser nitrogen fixation in clay soil limits the growth and productivity of
the plant. The greater nitrogen fixation in loam soil can be explained by two possible
causes:

9 The texture of clay having a negative effect on nitrogen fixation. This effect is not
demonstrated in a comparison between the two control treatments. Actually,
according to the estimates by van Hoorn et al. (2001), the quantities of nitrogen fixed
by the two control treatments are very similar.