Sodium, cation exchange and crop response in acid KwaZulu-Natal soils.

The reason for the responses to Na was probably improved rooting in the subsoil due to a changed composition of the soil solution in the presence of Na. John Phipson, John Cunningham and Rick Phipson and numerous other staff from the KwaZulu-Natal Department of Agriculture and the Agricultural Research Council Soil Fertility Work Team for their assistance with fieldwork;

LITERATURE REVIEW

PLANT RESPONSE TO APPLIED SODIUM

• Sodium as an essential plant nutrient
• Sodium in plant physiology
• Pasture quality

Other work confirms the positive response of sugar beet to Na (El-Sheikh & Ulrich, 1970; Hamid & Talibudeen, 1976) and shows that both red table beet and fodder beet (also Beta vulgaris varieties) respond in a similar way (Harmer & Benne , 1941; Lehr, 1951). Response of sugar beet sugar yield (Mg ha") to Na in 101 field experiments conducted on mineral soils (after Durrant et al., 1974).

Table 2.1. Sodium requirements of a variety of plant species (after Brownell, 1979).

I DIGE ST IBILITY OF FEED

SOIL EFFECTS OF SODIUM

• Sodium and cation exchange
• Solubility of soil phosphate

As a result, sodium is often the dominant cation in the soil solution, even when soil exchangeable Na levels are relatively low. Although soil solution K levels are less affected by Na than Al, Ca, and Mg levels, lower solution K levels caused by the presence of Na can negatively affect plant K uptake.

Figure 2.5 . Desorption of phosphate after 96 h, and at seven solution:soil ratios, by 0.03 MNaCl, 0.01 MMgCl 2 or 0.01 M CaCI 2

CATION TYPE AND IONIC STRENGTH EFFECTS ON THE SOLUTION COMPOSITION OF AN ACIDIC SUBSOIL

• INTRODUCTION
• MATERIALS AND METHODS
• RESULTS AND DISCUSSION

A larger part of the added Na' ions therefore remains in the soil solution than is the case for the other cations (Figure 3.2). While K, Ca and Mg move similar amounts of the most exchangeable cations into the soil solution, in the case of 'Nll' one can see that K+ displaces more NH4+ than even the divalent Ca2+ or Mg2+ ions.

Table 3.1. Selected properties of the Pinedene subsoil used.

Ion ic strength

0 III NH 4

Treatment

CaClz

H 20 NoCI KCI MgCI 2

Although the increase in Al solubility associated with salt addition in Figure 3.1b can be explained by cation exchange, the near constancy of the aluminum hydroxide potential (Figure 3.4) suggests a possible equilibrium of the soil solution with gibbsite present in these soils ( table 3.1b). 3.1). However, because the clay fraction is dominated by kaolinite (Table 3.1), the argument that Al levels in the solution may be controlled by dissolution of solid mineral phases would be incomplete without considering the composition of the solution in relation to the solubility of kaolinite. In Figure 3.5 we see that the solution composition points are close together around a point that would correspond to the intersection of the solubility line of gibbsite with that of kaolin with a solubility closer to that of halloysite than to low defect kaolinite. quartz is also suggested).

The pH of the soil solution can drop to a value close to that of the soil pH of 1MKCI (1:2.5 soil:solution), which in this case is 4.1, and AJ3+ activity can increase to potentially toxic levels greater than 10- 5M. Plot of pAl (-log[Al3+]activity) versus pH for 34 soil solutions resulting from treatments with water and various salts. The solid line represents the equilibrium solubility of gibbsite (after Lindsay, 1979, p. 60). However, neither the water pH value at a wide soil:solution ratio (e.g. 1:2.5) nor that obtained only in molar salt solutions (e.g. KCl) is likely to be satisfactory, as both treatments result in conditions far from those in the field.

Another problem is that interpretations of acidity measurements at low ionic strength (whether pH or AI measurements) are that their usefulness depends on the predictability of possible changes in the composition of the soil solution between the time of sampling and the period of root growth . Data for one soil (Shannon, IS00331) was excluded from this analysis because the pH of the soil solution was 0.64 units higher (rather than lower) than the pH of the water. This effect of Na is obviously of interest to anyone who uses the pH of water as a measure of the acidity of soil solutions.

Figure 3.4. A plot of pAl (-log[Al 3 + ] activity) against pH for the 34 soil solutions resulting from treatment with water and different salts

Na fraction in soil solution

THE RESPONSE OF MAIZE TO SODIUM AND GYPSUM IN THE PRESENCE OF SUBSOIL ACIDITY

• INTRODUCTION
• MATERIALS AND METHODS
• RESULTS AND DISCUSSION
• Exchangeable cations
• Crop yield
• CONCLUSIONS

Plaster treatments resulted in leaching of Ca to a depth of more than 450 mm and also reduced exchangeable acidity at this depth, with the net result being a significant decrease in acid saturation in the 450-600 mm depth zone. Both gypsum and Na had a significant effect on grain yield in the first three seasons (Table 4.8). Attempts were made to measure treatment effects on root growth in the subsoil, but in all cases there was some root development in the subsoil of the control plots.

The above interpretation of the response to gypsum is supported by the data discussed above (Tables 4.5 and 4.6), which show that acid saturation (which is generally associated with improved root growth) was reduced by the gypsum treatment in the 300-450 mm depth range. Analysis of soil solutions extracted from 300-450 mm depth samples showed a trend of increasing pH with increasing Na (as would be expected from the data presented in Chapter 3), but the treatment effect was not statistically significant (using analysis of variance). due to variation in the ionic strength of the extracted solutions and variation in the acid saturation of the ECEC. The analysis showed that soil solution electrical conductivity (an approximation of ionic strength), soil solution Na and acid saturation can together explain 85% of the variation in soil solution pH and 79% of the variation in Al concentration (Figures 4.1 and 4.2).

This effect was also observed in the other maize trial in Geluksburg, where it was more pronounced (chapter 5). This may be due to the exchange of sulfate-phosphate ligands in the soil or to the promotion of P uptake by Ca. However, in this experiment, the relatively high amounts of N used (120 kg N ha" yr") may have overshadowed the Na carrier effects on ionic strength in the zero gypsum plots.

Table 4.2. Cultivars, and planting, sampling and harvesting dates for the Na level x carrier x gypsum trial.

THE RESPONSE OF MAIZE TO SODWM AND GYPSUM AT DIFFERENT LIME RATES

• MATERIALS AND METHODS
• CONCLUSIONS

Despite higher acid saturation levels (on average) in the Na treated plots, Na had a significant positive effect on water pH in the 1994/95 season (Table 5.4). In the third season, gypsum again significantly increased the number of leaves, but the effect of lime was not significant. Na treatment significantly reduced leaf Ca in all seasons, as shown in a previous maize experiment (Chapter 4).

Leaf Mg was reduced by gypsum in the first two seasons, but the gypsum effect was not significant in the third season (Table 5.7). The effect of lime on the leaf concentrations of Cu in the first two seasons was markedly different. Leaf Cu was significantly increased by lime in the wet 1993/94 season but was significantly reduced in the dry 1994/95 season (Table 5.9).

Gypsum had a positive effect on leaf Cu at all lime levels in the first two seasons, possibly as a result of increased Cu solubility due to the formation of soluble CuS040ion. Sodium chloride increased leaf Mn in the first maize trial (Chapter 4), but not in this trial, where NaCl was used as a Na treatment in 1993/94. Gypsum had no significant effect on leaf Mn in this trial, in contrast to the effect of gypsum observed in the first maize trial.

Table 5.1. Cultivars, and planting, sampling and harvesting dates for the second maize trial.

THE RESPONSE OF ITALIAN RYEGRASS TO SODIUM, LIME AND POTASSIUM ON AN ACIDIC SOIL

• INTRODUCTION
• MATERIALS AND METHODS
• RESULTS AND DISCUSSION
• GENERAL DISCUSSION

The experiment was replanted in the fall of 1990, but with Na treatments in the form of N~SO4 in stages. The topsoil (0-100 mm) was sampled after the first, third and sixth cut in the first season and after the first, third, fifth and seventh cut in the second season. The relationship between the total dry matter yield of ryegrass and the acid saturation of the soil for both seasons. The soil was sampled after the first mowing in each season.

This is probably why there was less response to lime in the second season (Figure 6.1 indicates only a 6% response to lime, with no significant response beyond the first lime level). This may have been due to better P availability (discussed in more detail below) or a higher soil solution pH in the NazSO 4 -treated plots, both of which may have reduced the response to lime. As in the second maize trial (Chapter 5), lime applications had a significant negative effect on Ambic-2 extractable soil P (Table 6.3).

The return response to K was significant only at the second (May 29), third (July 6), and fourth (August 23) cuts; a linear response to K was evident at the second and third cuts, after significant K removal at the first cut; at the fourth cut there was only a yield response to the first and second increases in K (Table 6.4). In this context, the effects of different treatments on the concentrations of Ca, Mg, K and Na in the grass are important (Mundy, 1983; Miles et al., 1986). A similar trend is visible in the case of Ca (Figure 6.7a), although in this case the interaction is not statistically significant.

Table 6.1. Topsoil (0-150 mm) data for the site used in die ryegrass trial. Soil P and K were determined in an Ambic-2 extract, and Ca, Mg, Na and titratable acidity in a 1 MKCI extract (Appendix 1).

DISCUSSION

• SODIDM AND SOIL ACIDITY
• SODIUM AND POTASSIUM IN PLANT NUTRITION
• SODIUM EFFECTS ON PHOSPHORUS-USE EFFICIENCY
• FORMS OF SODIUM FERTILIZER
• POSSmLE FUTURE WORK

Can simple cation-exchange equations (eg, the Gapon equation) be used to reliably predict these relationships, or are more complex models required. Effect of sodium in the nutrient medium on the occurrence of potassium deficiency symptoms in tomato plants. Plant and Soil. Brownell, P.F. & Crossland, C.l. 1972. The requirement for sodium as a micronutrient by species possessing the C4 dicarboxylic pathway of photosynthesis. Plant Physiology.

Interactions of rubidium, sodium and potassium in sugar beet plant nutrition. Physiology of plants. Jayawardane, N.S. & Chan, KY. 1994. Management of physical soil properties limiting crop production in Australian sodic soils - a review. Australian Journal of Soil Research,32,13-44. Effect of sodium and calcium on 14C-sucrose translocation in cut cotton roots.

Influence of sodium on the calcium nutrition of excised cotton roots. Plant and soil. The influence of neutral salts on the solubility of soil phosphate with special reference to the action of nitrates of sodium and calcium. 1974. The effect of sodium on potassium nutrition and ionic relations in Rhodes grass. Australian Journal of Agricultural Research.

ANALYTICAL METHODS

Samples are air dried with air forced to flow over the samples laid out in drying trays. When air-dried, the samples are ground (using a mill where the soil is crushed between rubber bands) and passed through a 1 mm sieve; material coarser than 1 mm is discarded. Samples are poured into trays each containing 11 PVC cups (capacity 70mL); a tray is used for nine unknown samples, one standard soil sample (for quality control) and one blank.

For operations such as dispensing and stirring and for quality control, batches of three trays (27 samples, three unknowns and three blanks) are used. Multiple dispensers and diluent/dispensers are used to dispense aliquots of extractant or reagent for three samples at a time. 25 ml of 1 M KCl solution or deionized water is added and the suspension is stirred at 400 r.p.m.

The suspension is allowed to stand for about 30 minutes, and the pH is measured with a gel-filled combination glass electrode while stirring.