The m ain aim of the experim ents described in this Section was to select a Ca so u rce w hich w o u ld raise C a levels w ith o u t affecting o th er param eters significantly. A second aim was to select an A1 treatm ent which w ould raise soil A1 status to the desired levels of monomeric A1 in soil solution. Since it is obvious that addition of more Ca salts will raise soil Ca status to higher values, em phasis was given to which Ca source w o u ld have the m inim um effect on other characteristics (rather than w hich treatm ent raised Ca, and by how much). Therefore, treatm ents containing the sam e am ount of Ca were com pared (Figures 3.3, 3.4 and 3.5).
It is difficult to com pare the results of the present experim ent w ith those in the literature. A ttem pts such as this to raise soil Ca levels and subsequently exam ine the im pact on other chemical characteristics are not very common. G enerally lime is added to an acid soil by a level w hich is p re d e te rm in e d by a 'lim e req u irem en t' assessm ent (e.g., Shoem eker et al, 1961). In screening studies for acid or A1 tolerance, usually acid soil or Al toxic soil is lim ed at different rates to obtain a range of acidity (Carvalho et al, 1980; Smit et al, 1987 b) or Al toxicity (Krizek and Foy, 1988; Joslin and Wolfe, 1988) and the plants are grow n on for tolerance tests. This entanglem ent of factors leads to a great deal of confusion. Further there are also reports of the effects of lim ing on
74
plant grow th which include sim ultaneous nutrient availability evaluation during (Martini and Mutters, 1985 a; b) or after plant growth (Anandan et al, 1985). Reports are also available describing specifically the raising of soil Ca to predetermined levels by adding CaCOß, Ca(OH)2 etc (Simpson et al, 1977; Howard and Adams, 1965), but again the side effects of such treatments are not considered.
Anandan et al (1985) analyzed the liming of an acid sandy loam which then had peanuts grown for 105 days. They reported a rise in pH from 4.3 to 6.9 and a rise in exchangeable Ca from 4.3 to 24.3 me/kg. Although these changes were reported under field conditions as well as in pot cultures where plants were grown and irrigated, these results may be compared to the present data in terms of variation in pH, Ca and Al. Their effects (pH, exchangeable Ca and Al) are all smaller compared to the present experiment. For example, in the present experiment, pH w and exchangeable Ca increased by 0.52 units and 47.57 m e/kg respectively under a lime treatment of 4990 k g /h a compared to 2.6 units and 20 m e/kg respectively under a lime treatment of 2250 kg/ha (Anandan et al, 1985). The differences in post treatment Ca levels were quite high. However, their soil was initially more acid (by about one unit of pH) and the soils were subjected to plant growth, irrigation and possible leaching. A substantial part of the Ca (added as lime) may have been absorbed by the growing plants or may have been leached out [(Jarvis (1987) reported loss of Ca from soil columns after irrigating limed acid soil)]. A treatment of 1723 kg CaCC^/ha reduced the exchangeable Al to a trace (Ananadan et al, 1985) whereas in the present experiment exchangeable Al was present even at 9880 kg CaCC^/ha treatment. However, the effect of a Ca addition would have been different had this study been conducted under field conditions where downward movement of Ca would have occurred (Pearson et al, 1961; Juo and Ballaux, 1977).
The stability of pH , exchangeable Ca and A1 during the present experim ent may be com pared w ith that of M artini and M utters (1985 a;
b). In their experim ent both pH and exchangeable A1 became stable
earlier than in the p resen t experim ent. In their experim ent Ca levels increased up to 16 weeks for the high lime treatm ent soils w hereas lime rates w ere less in the present experim ent and Ca here rem ained fairly steady up to 12 weeks and then increased slightly until 16 weeks.
W hen all the analyses for Ca treated soils in the present experim ent are review ed together, the combined 3 treatm ent, (c) did not alter the Mn level at all. The pH s under the CaSC>4 treatm ent was closer to the control b u t this treatm ent increased the Mn status in the soil w hich is an im portant acid soil factor. The CaCOß treatm ent raised the
p H s and low ered M n an d A1 levels m ore than treatm en t (c). This
treatm ent, although it low ered the exchangeable A1 level, had A1 which was still higher than the CaCC>3 treatm ent. In conclusion, soil Ca levels in low Ca acid soils such as the present one can be raised by adding Ca from a com bined CaCC>3 and CaSC>4 source at a ratio of 2 : 1. This will raise Ca levels b u t w ill not alter other characteristics to a significant extent.
In order to bring small am ounts of monomeric A1 into the soil solution AICI3 6H 20 was added to soil. Higher A1 treatm ents resulted in greater quantities of other cations (including Al) in soil solution. This effect m ay be due to the displacem ent of other cations by Al in the
exchange complex. Since the addition of Al causes a change in soil
chemical characteristics, including the pH and concentrations of Mg, K and N a in soil solution, em phasis m ust be given to the selection of an Al treatm en t w hich brings m onom eric Al into soil solution w hile other chemical characteristics are not significantly affected.
CHAPTER 4