Agricultural Research Centre, Wollongbar, NSW 2480
In a situation of increasing soil acidity with time, it is important for farmers to be conversant with the signs of acidity related infertility problems. Soil pH measurement is the most obvious means of monitoring the problem. However, different plants are affected by soil acidity at different pH values in the one soil. Furthermore, for a given plant, acidity problems occur at different pH values on different soils.
A more direct means of monitoring the onset of acidity problems is to observe the plant - the symptoms caused by acid soil problems, the chemical composition of the plant, and plant response to treatments increasing the soil pH.
In this paper I have emphasised the problems of nodulation failure in legumes, molybdenum deficiency, manganese toxicity and aluminium toxicity. Other potential problems in acid soils are deficiencies of calcium, magnesium, potassium, boron, zinc and copper. In respect of the last four, these problems are not typically acid soil problems -they are deficiencies that can occur at any soil pH level. Calcium and magnesium deficiencies are generally limited to acid soils. They are mostly of secondary importance to aluminium and manganese toxicities, however, except for very low cation exchange capacity sandy soils. It is noted here that zinc and boron deficiencies can be easily induced by liming acid soils containing just adequate supplies of these nutrients.
I also have not discussed phosphorus deficiency as an acid soil problem. Phosphorus is, however, generally deficient in naturally acid soils, but its nature and correction are well understood, so discussion is not warranted here. It is worth pointing out that phosphorus availability to plants is generally not increased when lime is applied.
In the following sections the acid soil problems of nodulation failure, molybdenum deficiency, aluminium toxicity and manganese toxicity are discussed under the four headings - effects on the plant, symptoms, plant analysis, and tolerance.
(i) Effects on the plant
Leguminous plants such as subterranean clover, lupins and lucerne, have the capacity to form a symbiotic relationship with rhizobium bacteria. The bacteria infect the root through root hairs or where young roots emerge from their parent root. The infecting bacteria multiply within root cortical cells to form a root nodule. The nodule bacteria receive their food requirements from the host plant. In return, the bacteria use some of the food energy to convert gaseous nitrogen from the soil air, to the ammonia form of nitrogen. The plant, in turn, uses the ammonia in the production of plant proteins, and thus can be independent of soil nitrogen.
Low soil pH, low soil calcium and high soil aluminium and manganese affect nodulation and nitrogen fixation in several ways. Low pH and calcium and high aluminium and manganese restrict the survival of rhizobium in the soil. Low soil p11 and calcium levels inhibit the infection process, and hence the establishment of nodulated plants. Lastly low pH and calcium, and high aluminium and manganese, can reduce the rate of nitrogen fixation by established nodules. These are illustrated in Figure 1.
Clearly nodulation and nitrogen fixation are difficult in acid soils. Figure 2 provides an example of the way subterranean clover nodulation is affected by soil pH, and of the effects of some treatments applied to improve nodulation.
The clearest symptom is the absence of root nodules, and typically nitrogen deficient plant. With some species (e.g. lucerne) nodules have a very weak connection to the root, so extreme care has to be taken when separating root and soil when looking for nodules.
When nodulation fails at establishment in low nitrogen soils, the seedling rapidly turns yellow. The older leaves and cotyledons are often more yellow.
In cases where soil acidity is not sufficiently severe to inhibit infection, effects of acidity may be less obvious. The only symptom may be a nodulated but marginally nitrogen deficient plant.
(iii) Plant Analysis
Non-nodulated or poorly nodulated plants growing on low nitrogen soils will have a leaf nitrogen level less than the normal level of 3-4%N. The low leaf nitrogen levels may also result from other factors such as molybdenum deficiency or the absence of a suitable rhizobium strain.
Both the rhizobium and the plant can be selected for tolerance to low soil pH and associated factors. There are broad differences between species however. Subterranean clovers tolerate soil pH levels down to about 5.0 (1:5, soil: water method), whilst lucerne and most medics require soil pH levels above 5.6 to 5.8 for successful nodulation.
(i) Effects on the Plant
Molybdenum is required in small quantities by plants for the process of nitrate conversion to ammonia, and for the process of nitrogen gas conversion to ammonia in legumes. It is thus essential before proteins can be formed and is required in greater quantities by legumes.
Nitrogen deficiency, molybdenum deficiency, and nodulation failure, all result in failure of the plant protein metabolism. Thus the symptoms of these disorders are similar - general plant yellowing occurs, with the youngest leaves being somewhat greener. The more severe the deficiency the yellower the plant.
FIGURE 1. The factors influencing nodulation and nitrogen fixation in legume roots,
FIGURE 2. The effect of soil pH on nodulation of subterranean clover. (Data from Roughley and Walker, 1973).
X Sites where slurry inoculation was as effective as lime pelleting or use of 50/50 lime/superphosphate.
• Sites where lime pelleting and 50/50 superphosphate treatments were more effective than slurry inoculation.
X ' An old subterranean clover pasture site where a symbiosis tolerant of acidity may have developed.
(iii) Plant Analysis
Critical leaf molybdenum levels vary from as low as 0.02 parts per million (ppm) molybdenum in grasses tolerant of low molybdenum levels, to values of 0.1 ppm for many non-legumes, and to levels as high as 0.3 ppm for nodulated legumes.
Molybdenum deficient plants may contain high nitrate nitrogen levels resulting from the inhibition of nitrate reduction to ammonia. The presence of high nitrate levels in a chlorotic, apparently nitrogen deficient plant, is thus evidence for molybdenum deficiency.
Legumes require more molybdenum than grasses because of the extra requirement for nitrogen fixation. This large difference in requirements is illustrated by a study which showed the grasses green panic buffel grass and setaria did not respond to molybdenum on a soil that required 100 g molybdenum per hectare for maximum growth of the legume -greenleaf desmodium. There are also differences in molybdenum requirements among grasses and legumes. For example phalaris has been shown to be more sensitive to molybdenum deficiency than perennial ryegrass.
(i) Effects on the Plant
Aluminium has not been shown to be essential for plant growth. However aluminium becomes increasingly soluble as the soil p11 decreases below 5.0. The soil solution aluminium reacts with root cell wall materials and cell membranes, restricting cell wall expansion and hence root growth.
Some aluminium enters the cells, probably after damaging the root cell membranes. Once within the cell it reacts with phosphorus compounds, and upsets the plant phosphorus metabolism. Aluminium also interferes in the process of cell division, and inhibits the nucleic acid metabolism (i.e. inhibits reproduction of the plants genetic material) of the plant. This is illustrated in Figure 3.
The plant tops of aluminium toxic plants appear typically phosphorus deficient. This reflects aluminium dislocation of the plant phosphorus metabolism. The occasional observation of yellow spots or pale flecking of the leaves of grasses or cereals, may reflect effects of aluminium on other metabolic processes.
The most characteristic symptom of aluminium toxicity in solution cultures is the development of thickened, stubby and distorted root systems. These symptoms result from the effect of aluminium restricting cell division and cell expansion in the roots. Under field conditions it is often difficult to
observe root systems because affected plants are very susceptible to moisture stress and die easily.
FIGURE 3. Diagrammatic representation of aluminium toxicity effects on plants
(iii) Plant Analysis
Plant analysis is of limited use in detecting aluminium toxicity in the field. Small amounts of dust contamination on the plant material can easily dominate the measured aluminium levels, even where aluminium is at toxic concentrations in the plant. Soil pH levels and soil aluminium analyses are more reliable than plant analysis in detecting aluminium toxicity.
Plants have two main mechanisms to tolerate high soil aluminium -including the soil solution aluminium - and inactivating absorbed aluminium. These are diagrammatically represented in Figure 4.
Most evidence indicates exclusion of aluminium at the root surface is achieved by maintaining the root surface pH above 5.0, by secreting alkaline compounds. To do this however the root must absorb more negatively charged anions (i.e. nitrate, chloride, phosphate and sulphate) than positively charged cations (i.e. calcium, magnesium, potassium and sodium). This is possible for non-legumes, especially those with a high supply of soil nitrate. In contrast it is very difficult for a legume fixing gaseous nitrogen and absorbing little nitrate.
Recent observation of a plant disorder in wheat on acid soils, was associated with low leaf magnesium levels. It is possible that magnesium deficiency was induced as the plant reduced cation uptake in an attempt to keep anion uptake greater than cation uptake. Was magnesium deficiency induced as a result of the plants attempt to overcome aluminium toxicity?
In the second tolerance mechanism the plant inactivates the absorbed aluminium, by forming organic complexes with the damaging aluminium ions. This is an important tolerance mechanism in woody species where the organic aluminium compounds are ‘dumped’ in unused xylem vessels (wood tissues) and in cell walls. This mechanism is also involved in tolerant herbaceous species such as subterranean clover, but the actual ‘dumping sites’ (cell walls of cell vacuoles) have not been identified in these species.
It is worth noting that both the tolerance mechanisms seem to involve compromises. The first requires the plant to either have a very high nitrate supply, or to exist on a very low level of absorbed cations. The second, which allows aluminium absorption by the root, means it is likely to he excreting acid at the root surface, making the soil at the root surface more acid and higher in aluminium. This is particularly true of nodulated legumes growing at low soil nitrate levels.
It is probably not coincidence then, that plant communities on very acid soils tend to be slow growing and relatively unproductive, even if they do tolerate the conditions.
FIGURE 4. Diagrammatic representation of aluminium toxicity tolerance mechanisms in plants.
These observations suggest we should not go too far down the road selecting plants for higher degrees of aluminium tolerance. Current evidence indicates the tolerance mechanisms have a cost to the plant. This cost can eventually be expected to show up as reduced yield potentials. Thus there is a point where liming the soil to counter acidification rates, will be a more profitable pathway than selecting tolerant species and varieties. The yield response by subterranean clover to lime under aluminium toxic conditions is shown in Figure 2.
(i) Effects on the Plant
Manganese is required for healthy plant growth. Plant roots have a manganese absorption mechanism that provides sufficient manganese for healthy growth in most soils. In some acid soils however, solution manganese levels may reach very high levels. Many plants will then absorb more manganese than they require internally. If they do not have some internal mechanism to control cellular manganese concentrations, toxicity effects occur.
The main role of manganese in the plant is as an activator of enzymes associated with phosphorus reactions, and with the plant energy system. Plants normally control the rates of these reactions within cells, by varying the manganese concentration at the reaction sites.
Under manganese toxicity conditions, the evidence indicates cell manganese concentrations are so high, that control of the manganese activated enzymes is lost. The normal regulation of plant biochemistry is sufficiently upset to cause cell chlorosis, and in the extreme, death. This is illustrated in Figure 6.
The symptoms of manganese toxicity vary widely between plant species. In general they reflect the way the plant responds to high internal manganese concentrations. Usually symptoms are more severe in the older leaves that have had the longest time to accumulate manganese. With the sudden onset of high levels of manganese however, the symptoms can be most prominent in the younger leaves.
The most common symptom is the formation of chlorotic grading to dead spots on the leaf. These spots are frequently near the ends of xylem vessels, so tend to be near the leaf margin and in interveinal positions. Lucerne, cowpea, lupins, barley and perennial ryegrass all tend to develop leaf spots.
Leaf crinkling and cupping is a symptom of manganese toxicity in rape, beans and soybeans. The cupping is thought to be caused by manganese accumulation in the leaf margin area, slowing the growth of that area relative to the rest of the leaf.
With oats and fescue, manganese toxicity causes interveinal yellowing giving stripy leaves. This is thought to be due to manganese induced iron deficiency.
Solution aluminium concentration (μg atoms/l)
FIGURE 5. Effects of aluminium on the yield response of subterranean clover and lucerne to lime on 30 soils in the glasshouse.
no - subterranean clover (Mt Barker)
lm - lucerne (Hunter River)
Solid symbols - minus lime plants showing manganese toxicity symptoms
Open symbols - no manganese toxicity symptoms
* Calcium deficiency symptoms without lime
FIGURE 6. Diagrammatic representation of the effects of manganese toxicity on plants.
(iii) Plant Analysis
Leaf analysis is a valuable means of detecting manganese toxicity. Various species and varieties of plants can tolerate leaf manganese levels from 300-500 parts per million (ppm) (e.g. lucerne, some soybeans, narrow leaf lupin, various medics, barley, some wheat varieties), from 500 to 1500 ppm (e.g. some wheat, oats, white and sub clovers, white lupin - L. Albus), and above 1500 ppm (e.g. cotton, some soybeans, lettuce, bananas, sunflowers).
There is also wide variation within species in tolerance levels, and bean species in particular can tolerate more manganese at higher temperatures. More detail is given by Cregan (1980).
Leaf manganese levels near or above these levels indicate a manganese toxicity problem.
Plant tolerance of high soil manganese involves mechanisms of exclusion, and of binding excess absorbed manganese in non-active forms. Some species exclude manganese at the root surface, others restrict manganese transport to the tops, probably by isolating the absorbed manganese in root cell vacuoles. Other species tolerate high manganese levels in the tops probably by isolating excess manganese in cell vacuoles or by binding manganese to the cell walls, possibly in combination with silica. (See Figure 7).
These tolerance mechanisms are clearly distinct from mechanisms of plant tolerance of aluminium. Thus it is not surprising, that tolerance to aluminium toxicity is not necessarily associated with tolerance to manganese toxicity. Some species are susceptible to both problems (e.g. lucerne), others tolerate both (e.g. subterranean clover), while others may tolerate one and be susceptible to the other (e.g. rape is reasonably tolerant of aluminium toxicity but susceptible to manganese toxicity).
Lastly, unlike aluminium tolerance mechanisms, the manganese tolerance mechanisms do not appear to be associated with reduced yield potential. Thus selection for increased levels of tolerance is a very practical means of reducing manganese toxicity effects on crop and pasture yields.
FIGURE 7. Diagrammatic representation of manganese toxicity tolerance mechanisms of plants.