Aust. J. Agric. Res., 1974, 25, 309-16

The Importance of Environmental Factors in Soil Fertility Assessments. 11* Nutrient Concentration and Uptake

R. C. StefansonA and N. Collis-GeorgeB

A Waite Agricultural Research Institute, University of Adelaide, Glen Osmond, S.A. 5064. Department of Soil Science, University of Sydney, N.S.W. 2006.

Abstract

Lettuce plants were grown in two soils under a wide range of controlled conditions in the glasshouse. Assessments were made of the effect of soil temperature, incident light and season, in terms of the nutrient concentration and nutrient uptake in the plant tissue, which was analysed for nitrogen, phosphorus, potassium, magnesium, calcium and sodium.

Both the concentration and the uptake of each nutrient were affected significantly by soil tempera- ture and incident light, both between and within seasons. The type of soil had an effect on these estimations, but its significance could not be tested statistically. A considerable number of first and second order interactions between components of the physical environment affected the quantities being examined. These interactions were statistically significant. The value of each nutrient analysed, which is an aspect of plant performance, reflected changes in the physical environment independently of the other nutrients.

Often a particular nutrient showed no coincident pattern of responses to the environment when these were measured in terms of dry weight, nutrient concentration in the tissue, or total nutrient uptake. Each nutrient concentration and each nutrient uptake varied as micrometeorological factors in the glasshouse changed. Hence, ambiguous assessments of soil fertility were obtained with all measured plant responses.

Introduction Nutrient concentration and nutrient uptake are often used to describe the inter-

action of soil and plant for a specific plant nutrient. The problem of plant material sampling and correlation of field behaviour with tissue analysis arises partly because of the interaction of soil and environmental factors and partly because of variation within the tissue (Bates 1971). Uptake of nutrients by plants allows a nutrient balance sheet to be struck; the amount of nutrient has been related to the application rate required to maintain soil fertility.

Temperature and light regimes are known to affect the concentration and accumu- lation of nutrients in plants grown under various conditions, e.g. in light chambers (Luxmoore and Millington 1971), in the glasshouse (Ketcheson 1957) and in the field (Bates 1971). Because each measurable biological quantity may have a unique response pattern to changes in the physical environment, the plant property used to assess the fertility or nutrient supply of a soil will largely determine the value of that assessment.

In part I (Stefanson and Collis-George 1974) it was shown that yields of lettuce reflected changes in soil type, chemical applications to the soil, soil temperature, light energy and seasonal conditions, when other physical conditions of the soil, namely its bulk density and the moisture regime, were optimal for growth. This paper considers

*Part I, Aust. J. Agric. Res., 1974, 25, 299.

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R. C. Stefanson and N. Collis-George

the responses by lettuce in terms of nutrient concentration and nutrient uptake under the experimental conditions described in part I.

Materials and Methods Material was obtained as described in part I. The plant tops from the three

replicates were dried at 60°C, then bulked, ground and stored. All chemical deter- minations were carried out on aliquots of this material. Nitrogen was determined by microdigestion (McKenzie and Wallace 1954) and distillation.

The method of plant extraction was one developed by B. G. Davey (personal communication): plant material was ashed at 450°C overnight, and the ash taken up twice with 3~ hydrochloric acid over a steam bath. The extract was used for atomic absorption determinations of the cations calcium, magnesium, potassium and sodium, and also for the determination of phosphate by the colorimetric method of Hanson (1954).

The analytical results were converted to nutrient concentrations and to total nutrient uptake by the plant on the basis of the dry matter yields given in part I. After routine testing for homogeneity of variance, the values for each nutrient con- centration and uptake were examined by analysis of variance. Each statistical analysis included the data for all the variable physical and chemical treatments on each soil in each of the three seasons.

Hereinafter specific treatments are referred to by a code showing component variables in the following order:

(1) the root temperature (15, 20 or 25°C);

(2) the light received by the plants (50,75 or 100 % of the incident light received on a horizontal surface in the glasshouse);

(3) the midpoint of the 8-week growing period (June, September or December);

(4) the nutrient treatment (control (C), complete nutrient (+), minus nitrogen (-N), minus phosphorus (-P), minus potassium (- K), minus sulphur (- S), minus molybdenum (- Mo), lime requirement (LR), or minus calcium (- Ca)) as described in part I;

(5) the soil (10 or 30).

The symbol 2 denotes data for the complete range of conditions in one of the above component variables. For example, C : 75 : Dec : -I- : 10 denotes plants grown at all root temperatures, at 75 % light, during the December growing period, and receiving the complete nutrient treatment in soil 10.

Results and Discussion Nutrient concentrations

The prime experimental variables, i.e. chemical treatment, soil temperature, incident light and season, significantly affected the nutrient concentration in the tops of the plants grown in both soils (Table 1).

For example, as shown in Fig. 1 for C : 50 : Sept : + : 30, increasing the soil temperature reduced the phosphorus concentration. Also, it is apparent that seasonal conditions were more favourable for higher phosphorus concentrations in Z : C : June: + : 30 than inC:C:Sep t : + :30.

Soil Fertility and Environment. I1

Table 1 shows that the prime experimental variables are of great importance and that many significant interactions occurred between the chemical and the physical treatments; there were fewer significant first order interactions in terms of nutrient concentrations in plants grown in soil 10 than in soil 30. Only two generalizations can be made about the first and second order interactions of Table 1: (i) light was less frequently a significant variable of the first and second order interactions than the other experimental variables for either soil; (ii) sodium concentration in the plants grown in soil 30 was insensitive to the interaction of the experimental physical factors except in the one instance of chemical treatment v. season interaction. By contrast, many interactions were found for sodium concentration in plants grown in soil 10.

Table 1. Levels of significance for a change in nutrient concentration of lettuce grown in soil 30 (and in soil lo)* under a range of environmental factors

The analysis of variance was carried out for each nutrient concentration separately

Environmental factors and their Nitrogen Phosphorus Potassium Magnesium Calcium Sodium

interactions concn. concn. concn. concn. concn. concn.

Chemical (C) Season (S) Soil temp. (T) Light (L) c x s C x T C x L S x T S x L T x L C x S x T C x S x L C x T x L S x T x L

* P < 0.05. ** P < 0.01. *** P < 0.001. NS, not significant. A The level of significance is given in parenthesis for soil 10 when it differs from that for soil 30.

The following discussion concerns only those phosphorus and sodium concentra- tions which showed large responses to changes in the experimental environment.

Phosphorus Concentrations

Figs. 1 and 2 show that the level of phosphorus concentration in the plant tissue was determined by the soil, the physical conditions and the chemical treatment.

The soil in the complete nutrient and minus nitrogen treatments received identical phosphorus applications (Fig. 2), but the "yield" ratio of minus nitrogen to complete nutrient, evaluated in terms of phosphorus concentrations, suggests a phosphorus deficiency in the minus nitrogen treatment. The use of this ratio for phosphorus concentrations, which parallels the use of the fertility ratio of the dry weight yields in part I, assumes that the percentage of phosphorus in the complete nutrient treatment does not reflect a 'luxury' supply of phosphorus. To illustrate the problem of inter- pretation, the ratios of minus nitrogen to complete nutrient, calculated in terms of phosphorus concentration for 15 : 50 : Sept : 30 and 25 : 100 : Sept : 30, were 0.70

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R. C. Stefanson and N. Collis-George

and 0.78 respectively whereas the equivalent ratios in terms of dry weights were 0.96 and 0.68. Thus in terms of phosphorus concentration, the minus nitrogen to complete nutrient ratio was not as sensitive to soil temperature and solar radiation as it was in terms of dry weights. Clearly, the two methods of defining this ratio lead to conflicting interpretations of plant response to soil phosphorus.

Complete nutrient

Minus nitrogen

December Minus phosphorus

$$ U 0

%,;l

Fig. 1. Phosphorus concentration (C) of lettuce grown at different seasons of the year (S) in soil 30 which received the complete nutrient treatment. There were three light treatments (L) expressed as percentage transmission of the light received on a horizontal surface in the glasshouse and three soil temperatures (T). Inter- actions: S x L, T x L, S x T, LSD (1%) = 0.02%. Fig. 2. Phosphorus concentrations of lettuce grown during September in soil 30 which received the complete, minus nitrogen or minus phosphorus nutrient treat- ments. Interactions: S x C, T x C, T x S, LSD (1%) = 0.04%.

Similarly, the fertility analysis of the C :C : Sept : -P : 30 treatments (Fig. 2), expressed in terms of phosphorus concentrations, has an entirely different relative magnitude to that shown by dry weights. (The ratios of minus phosphorus to complete nutrient for 15 : 50 : Sept : - P : 30 and 25 : 100 : Sept : - P : 30, in terms of phos- phorus content, were 0.53 and 0.83 respectively, while the equivalent dry weights analyses gave ratios of 0.10 and 0.13.)

Sodium Concentrations

Concentrations of sodium as compared with other cations were markedly different in plants grown in soils 10 and 30. Of cations determined, only sodium and magnesium

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had higher concentrations in plants from soil 10 than in those from soil 30. This suggests that it would be unwise to compare the fertility of two soils on the basis of only one plant property. (Note that Table 1 in part I shows that soil 10 was richer in exchangeable sodium and magnesium than soil 30). The sodium concentration of plants grown in soil 30 was slightly lower in June than in September and December. For all three seasons, the responses of sodium concentration to increasing incident light and soil temperature in the + : 30 treatments were small and non-significant, whereas the sodium concentration in plants from soil 10 was markedly sensitive to season and showed a pronounced maximum in September.

Table 2. Levels of significance for a change in nutrient uptake by lettuce grown in soil 30 (and in soil lo)* under a range of environmental factors

Each nutrient uptake was analysed separately

Environmental factors and their Nitrogen Phosphorus Potassium Magnesium Calcium Sodium

interactions uptake uptake uptake uptake uptake uptake

Chemical (C) Season (S) Soil temp. (T) Light (L) c x s C x T C x L S x T S x L T x L C x S x T C x S x L C x T x L S x T x L

* P < 0.05. ** P < 0.01. *** P < 0.001. NS, not significant. A The level of significance for soil 10 is given in parenthesis when it differs from that for soil 30.

Chemical Uptake

The experimentally determined quantities of nutrient concentrations and dry weights (part I) often responded in opposite ways to changes imposed in soil tempera- ture, light energy and season. Soil was an important factor in determining how much of an element was accumulated by the plant: plants grown in soil 30 had higher total uptakes of nitrogen, phosphorus, potassium and calcium than those grown in soil 10. With one exception, the uptake from both soils was significantly influenced by each of the prime factors-soil temperature, solar radiation, chemical treatment and season (Table 2).

Only the interaction between chemical treatment and season was significant for uptakes of all elements from soils 10 and 30. As was noted in part I, it is impossible to discriminate between the effects of the photoperiod, air temperature and absolute solar radiation energy levels with this experimental design. However, all other factors were kept virtually constant both throughout and between the seasonal growing periods. Excluding phosphorus and potassium, significant interactions occurred

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R. C. Stefanson and N. Collis-George

between chemical and soil temperature treatments as well as between chemical and light treatments for soil 30. Few of the other possible interactions were significant for soil 30. In comparison, many first order interactions between soil temperature, light, season and chemical treatment were significant for plants grown in soil 10.

Several second order interactions were significant with respect to the uptakes of nitrogen, potassium and sodium by plants grown in soil 10. These interactions show that the uptake of these elements is much more complex than is allowed for in many experimental designs.

(4)

25

i~ 15 50 0.5 $ - ] 93 September

0 Z

15 50

December December

Fig. 3. Phosphorus uptake by lettuce grown during June, September and Decem- ber in soil 30 which received the complete nutrient treatment. Interactions: S x L, S x T, T x L, not significant.

Fig. 4. Nitrogen uptake by lettuce grown during June, September and December in soil 30 which received the minus nitrogen treatment. Interactions: S x T , S x L, LSD (1%) = 8 mg; L x T, not significant.

Although the absolute quantity of phosphorus taken up by plants in soil 30 in the complete nutrient treatment varied from season to season, the response surfaces were similar (Fig. 3) and only one first order interaction was significant (Table 2). This similarity of response to change in the physical environmental factors allows com- parisons between the chemical treatments within a season. However, since the chemical x season interaction was significant (Table 2) for phosphorus uptake for soil 30, phosphorus uptake cannot be used to make comparisons between seasons.

Soil Fertility and Environment. I1

By contrast, the phosphorus uptake cannot be used reliably for any comparisons for soil 10. The variety of uptake response surfaces that can result from seasonal changes in light and root temperature is illustrated by Fig. 4 for one nutrient for lettuce grown in one soil under supposedly constant chemical conditions. The response surfaces for uptakes of the majority of elements varied in a similarly ill-defined manner, and in each case the soil type strongly influenced the shape of the response surfaces.

Conclusion

Because nutrient concentration changes as the plant ages, harvesting age is impor- tant (Loneragan and Snowball 1969). However, plants harvested at a common date cannot be assumed to be of the same physiological age if grown at different levels of nutrition. Offsetting this according to Bates (1971) is the tendency for plants growing with adequate nutrition to have relatively large and physiologically older leaves which dominate analyses of nutrient concentration in harvested material while plants grow- ing under conditions of mineral deficiency have fewer physiologically younger leaves. Consequently, differences in concentrations caused by differences in the level of nutrition are minimized.

Absorption of nutrients by plants is largely an active process which depends on the oxidation of carbohydrate to supply the energy requirements. Hence the concen- tration of nutrients in a plant is determined by the production of carbohydrate in the leaves and its subsequent translocation to the roots. These processes are controlled by soil temperature (Nielsen and Humphries 1966)' light intensity, and seasonal conditions.

The experimental results show that large differences in both nutrient concentration and uptake occur as a result of micrometeorological changes. Significant interactions were found between soils, added nutrients, and different attributes of the physical environment (Tables 1 and 2).

This is consistent with Walker's (1969) observations on the concentration of 16 nutrients in maize seedlings. He found that optimal soil temperature differed for each element and that the response curve of concentration to changes in soil temperature was unique for each element.

Youngberg and Dryness (1965) have postulated that an adequate experimental design is essential if nutrient interactions occurring during the biological assay of soil fertility are to be described. We suggest that this design must include the physical as well as the chemical environment, particularly in view of the experience of Voss et al. (1970). They showed that it was possible to improve the interpretation of nitrogen, phosphorus and potassium concentrations in the leaves of maize by including environ- mental factors in the multiple regression equations.

It has been shown that the analytical quantity chosen for its association with plant performance has a major influence on the interpretation of the assessment of soil fertility in terms of plant response. Both the absolute and relative values of each quantity determined experimentally changed as the physical environment altered. Moreover, many of these changes followed independent patterns. Such independent variations confuse the interpretation of soil fertility in terms of plant response.

Finally, it has been shown that the two soils responded differently to changes in the physical environment in terms of each of the measurements of plant performance.

R. C. Stefanson and N. Collis-George

Accordingly, comparisons of fertility between soils can have no absolute value even when made under standard conditions.

Acknowledgments

The program was supported by the Rural Credits Development Fund of Australia. The help of Dr B. G. Davey, Dr D. G. Lewis and Mrs G. Bishop is gratefully acknowledged.

References Bates, T. E. (1971). Factors affecting critical concentrations in plants and their evaluation: a review.

Soil Sci. 112, 116. Hanson, W. C. (1954). The photometric determination of phosphorus in fertilizer using the phos-

phomolybdate complex. J. Sci. Food Agric. 1, 172. Ketcheson, J. W. (1957). Some effects of soil temperature on phosphorus requirement of young corn

plants in the greenhouse. Canad. J. Soil Sci. 37, 41. Luxmoore, R. J., and Millington, R. J. (1971). Growth of perennial ryegrass (Lolium perenne L.)

in relation to water, nitrogen and light intensity. 11. Effects of dry weight production, transpor- tation and nitrogen uptake. Plant Soil 34, 561.

Loneragan, J. F., and Snowball, K. (1969). Calcium requirements of plants. Aust. J. Agric. Res. 20, 465.

McKenzie, H. A,, and Wallace, Heather S. (1954). The Kjeldahl determination of nitrogen: a critical study of digestion conditions-temperature, catalyst and oxidizing. Aust. J. Chem. 7, 55.

Nielsen, K. F., and Humphries, E. C. (1966). Effects of root temperature on plant growth. Soils Fertil. Harpenden 29, 1.

Stefanson, R. C., and Collis-George, N. (1973). The importance of environmental factors in soil fertility assessments. I. Dry matter production. Aust. J. Agric. Res. 25, 299.

Voss, R. E., Hanaway, J. J., and Dumenil, L. C. (1970). Relationship between grain yield and leaf N, P, and K concentration for corn (Zea mays L.) and the factors that influence this relationship. Agron. J. 62, 726.

Walker, J. M. (1969). One-degree increments in soil temperature affect maize seedling behaviour. Soil Sci. Soc. Am. Proc. 33, 729.

Youngberg, C. T., and Dryness, C. T. (1965). Biological assay of pumice soil fertility. Soil Sci. Soc. Am. Proc. 29, 182.

Manuscript received 26 April 1973