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II. Factors Affecting the Silica Content of Plants

II. Factors Affecting the Silica Content of Plants

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of soil solutions is entirely monosilicic acid, commonly present to the

extent of 30 to 40 ppm. SO,. The concentrations in solutions obtained

from a wide range of soils of the same p H ranged from 7 to 80 ppm.

SO,. In one field soil treated with various amounts of acidifying agents

the concentrations decreased from 70 to 23 ppm. SiO, as p H increased

in the range 5.4 to 7.2. A similar relationship has been found between

pH and the concentration of monosilicic acid in aqueous extracts of soils

(Beckwith and Reeve, 1963, 1964; McKeague and Cline, 1963a,b). In

general the concentration of silica in such extracts increases on either

side of a minimum which has been variously placed between p H 8 and

9 (McKeague and Cline, 1 9 6 3 ~ ) .

It is apparent from the foregoing that the solubility of silica in soils

contrasts with its solubility in water. Thus, the concentration of monosilicic acid differs among soils of the same pH, but the highest reported

concentrations are below that of a saturated solution of monosilicic acid.

In addition there is a marked effect of pH on the solubility of silica in

soils. These effects have been investigated by Beckwith and Reeve

(1963), Jones and Handreck ( 1963, 1965b), and McKeague and Cline

(1963b). Each group used simplified systems and found that monosilicic

acid was adsorbed by iron and aluminum oxides. Adsorption depended

on p H in a manner resembling the adsorption of monosilicic acid by

soils, i.e., adsorption decreased on either side of a maximum at about

pH 9.5. Jones and Handreck (1963, 1965b) found that aluminum oxides

were more effective, weight for weight, in adsorbing monosilicic acid

than iron oxides. While the degree of crystallinity of the iron oxides did

not greatly affect their adsorptive capacities, that of the aluminum

oxides did, and the most crystalline aluminum oxide adsorbed only onethird as much as the least crystalline.

From their separate studies Beckwith and Reeve (1963, 1964), Jones

and Handreck ( 1963, 1965b), and McKeague and Cline ( 1963a,b) all

concluded that the concentration of monosilicic acid in soil solutions is

largely controlled by an adsorption reaction dependent on pH. Although

silica is adsorbed on the surfaces of various kinds or inorganic compounds in soils the role of sesquioxides, especially aluminum oxide, is

almost certainly a dominant one. The mechanism by which monosilicic

acid is adsorbed on to sesquioxides is obscure. Because we are dealing

with a reversible adsorption it is unlikely to be through the formation

of a salt as suggested by Beckwith and Reeve (1963), Iler (1955), and

Jones and Handreck (1963). A more plausible explanation is that

monosilicic acid is joined through a hydrogen bond to an oxygen atom

that bridges two iron (or aluminum) atoms as follows:



. . O(Fe20aH4)



Since monosilicic acid is here acting as an acid, this would explain

the fact that it is repelled by increasing acidity. The reaction would also

explain the increased acidity which is observed when monosilicic acid

is adsorbed, because the hydrogens of the ferric hydroxyl groups are

thereby made more acidic.

Jones and Handreck (1963, 196513) added iron and aluminum oxides

to a soil in a pot experiment and thereby obtained more direct evidence

of their effects on the concentration of monosilicic acid in the soil

solution. The soil was a sandy loam containing 12 percent clay and 2.9

percent “free” ferric oxide and the iron and aluminum oxides were added

at the rate of 5 percent by weight of the air-dry soil. The pH of the

untreated soil and of the soil-oxide mixtures was adjusted to 5.9 and


Effects of Sesquioxides on Uptake of Silica by Oats from a Sandy Loam of p H 5.gU

SiOn in soil

SiOl in plants

Oxide added



Dry matter





Ferric oxide

Aluminum oxide

Aluminum oxide






2 80












Jones and Handreck (1965b).

the soil water was maintained at pF 2. Soil solutions were obtained

with a pressure cell using a procedure which simulated the system of a

soil in which the soil water is alternately removed by plants and replenished. In this procedure a given soil sample was subjected to successive

cycles which consisted of alternately removing the soil solution to pF

2.6 and replenishing with water to bring the pF back to 2.0. The concentration of silica in solution decreased progressively in the first few

cycles and then leveled off to a steady value which the soil could

maintain despite repeated withdrawals; this steady level was characteristic of the particular soil. The iron and aluminum oxides lowered the

concentration of silica in the soil solution (Table I ) in proportion to

their adsorptive capacities in simple systems.

The effects of sesquioxides are also shown by the widely differing

concentrations of silica in solution in two soils of the same pH and

clay content (Jones and Handreck 1963, 1965b). One of these was a

krasnozem with large quantities of “free” sesquioxides and contained

only 7 ppm. SiO, in solution. The other was a black clay, very low in

“free” sesquioxides and contained 67 ppm. SiO,.



The relevance of monosilicic acid in the soil solution to the uptake

of silica by plants was established by Jones and Handreck (1965b),

who grew oat plants on their various soils and soil-oxide mixtures.

Analyses of the tops for total silica (Tables I and 11) showed that the

concentration in the dry matter was directly proportional to the concentration of monosilicic acid in the soil solutions. The concentration of

silica in the plants varied linearly with the concentration of monosilicic

acid in the soil solution in the range 7 to 67 ppm. SiO,.

Some work with culture solutions also provides evidence for a linear

relationship between the concentration of monosilicic acid in the growth

medium and uptake. Thus, rice grown in culture solutions containing


Uptake of Silica by Oats in Relation t o the Level of Silica in the Soil Solution

and the Amount of Water Transpireda


(pH 5.6)




sandy loam

Penola black


S O 2 in


solution matter

(ppm.) (g./plant)



transpired expected

Mg./plant (kg./plant) (mg./plant)

Si02in plants



















26 1

Data from Jones and Handreck (1965b).

0, 5, 20, 60 and 100 ppm. SiO, had, respectively, 0.07, 0.54, 1.63, 3.99,

and 5.76 percent SiO, in the tops (Okuda and Takahashi, 1 9 6 1 ~ )Less


complete series of cultures used by Mitsui and Takatoh (1963) and

Volk et al. (1958) gave similar results.

At this point the widely quoted work of Whittenberger (1945) calls

for comment although it is of little relevance to soil-plant systems. He

studied the uptake of silica by rye and sunflower from culture solutions

containing 0, 110, 320, and 960 ppm. SiO,. The upper levels so greatly

exceed the limits of stability of monosilicic acid (120 to 140 ppm. SiO,

at ordinary temperatures) that it must have polymerized to form colloidal silica. Not surprisingly, less silica was taken up by the plants at

these upper levels than might have been expected from a linear relationship between the concentration of monosilicic acid and uptake.

Some reports ( Aberg et al., 1945; Kawaguchi and Hattori, 1959; Pope,

1945; Themlitz, 1941) which indicate an effect of soil on the uptake of

silica are difficult to interpret. Others however may be explained in the



light of the recent knowledge of the factors which influence the levels

of monosilicic acid in soil solutions. The effect of pH is shown by Ayres

( 1966), Deguchi et al. ( 1955), Grosse-Brauckmann ( 1953, 1956), and

Syasina (1960), who found that liming a soil decreased the uptake of

silica by various plants including oats, ryegrass, red clover, barley,

sugar cane, and rice. Conversely, the concentration of silica in oats was

increased from 1.68 to 2.77 percent SiO, by lowering the pH of the soil

from 6.8 to 5.6 (Jones and Handreck, 1965b).

There are also reports which indicate that the uptake of silica from

soils is related to their content of iron and aluminum oxides. Thus, the

leaf sheaths of sugar cane grown on ferruginous latosols contained less

than 1 percent SiO, in the dry matter, whereas those of cane grown on

siliceous “gravelly” soils contained about 5 percent SiO, (Halais and

Parish, 1963). The silica content of rice has been related to the silicasesquioxide ratio of soils (Ueda et at., 1957), but a better assessment of

the available silica in a soil may be obtained from the ratio of easily

extractable silica to the “free” or easily extractable sesquioxides. Kawaguchi and Matsuo (195&) and Kawaguchi et al. (1958) used an ammonium acetate buffer and 0 . 2 N HC1 as extractants and found that the

higher the ratios of Si/Al or Si/Fe in the extracts the greater the uptake

of silica by rice. There is evidence that higher concentrations of silica

in rice are associated with increased resistance to diseases, including

fungal attack, and siliceous blast-furnace slags are sometimes added

to soils so as to increase the concentration of silica in the rice plant to

“protective” levels. Soil extractions such as the foregoing are aimed at

predicting the amount of slag which is needed. The most comprehensive

study of this problem is that of Imaizumi and Yoshida (195&),who

found that the silica extracted from a soil by a sodium acetate buffer at

pH 4 gave the best indication of how much slag is needed. They proposed a regression equation to predict the amount of slag which should

be added to give an economic increase in the yield of rice and their

procedures have been widely used in Japan (Yoshida, 1965) and elsewhere (Lian, 1963; Shiue, 1964).

There are studies dating back to the last century which show that

additions of silica to soils increase the silica content of plants. The

silica has been added as soluble silicates of sodium and potassium (e.g.,

Hall and Morison, 1906; Schollenberger, 1922; Sreenivasan, 1936b) , as

silicate minerals ( e.g., Schollenberger, 1922; Themlitz, 196O), as amorphous silica (e.g., Ganssmann, 1962; Lemmermann et al., 1925; Schollenberger, 1922), and as quartz (e.g., Densch and Steinfatt, 1931; Lemmermann et at., 1 9 5 ) . It is clear that the more soluble the siliceous material,

the greater is the effect on plant uptake; quartz is practically ineffective.



Several reports (Baba, 1956; Baba et al., 1956; Hemmi, 1933; Sreenivasan, 1936a; Tanaka et al., 1965; Williams and Shapter, 1955) indicate

that the uptake of silica increases with increasing water content of the

soil. This effect is especially marked with rice in which, for example, the

concentration of silica in the leaf blades increased from 7.68 to 9.97 percent when the soil water content was increased from 50 percent of the

“moisture holding capacity” to a flooded state (Baba et al., 1956). Although this effect may partly reflect the need of rice for waterlogged

conditions, it may also be partly due to an increase in the concentration

of silica in the soil solution following flooding. Thus, Ponnamperuma

(1964) found that the concentration of silica increased with time of

submergence, in one case increasing from 24 to 41 ppm. SiO, in 50 days.

Factors other than a change in pH must be involved in this increase

in silica concentration because the pH of a submerged acidic soil rises

with the onset of reducing conditions, and this by itself would tend to

decrease the concentration of silica in the soil solution. Sreenivasan

(1936a) thought that the responsible agent was organic acids released

from organic matter under reducing conditions. This suggestion seems

plausible because acetate and citrate solutions dissolve silica from soils

(Imaizumi and Yoshida, 1958) and because soils with high contents of

organic matter show the greatest increase in soluble silica (Ponnamperuma, 1964). However Ponnamperuma (1964) discounts the role of

organic acids and suggests that the increase in soluble silica is due to a

release of silica from “ferri-silica” complexes under reducing conditions.

It seems that the only way this could occur is through conversion of

ferric to ferrous complexes of higher solubility or through the removal of

ferric iron from the silica complexes by organic anions. These may therefore play an important role in the release of silica from “ferri-silica”

complexes. It is clear from the uncertainties disclosed by this discussion

that there is little definitive information about iron-silica systems in

submerged soils, particularly when variable amounts of CO, and organic

and sulfur compounds are present. Because silica is important to the

normal growth of rice (e.g., Okuda and Takahashi, 1W4) these systems

would seem to merit further study.


Plants take up different amounts of silica according to their species.

Indeed, the contrast between gramineous and leguminous species has

been known since the first report of the occurrence of silica in plants

(de Saussure, 1804). It is generally accepted that Gramineae contain 10

to 20 times the concentrations of silica found in legumes and other dicotyledons (Russell, 1961, p. 536). Such differences have been recently



reported by Baker et al. (1961b), Dougall (1963), Forman and Sauer

( 1962), Keeler ( 1963), and Parker (1957), but it should be noted that

these authors did not take account of possible differences due to soil.

More rigorous proof of the differences between species comes from

work in which several species have been grown in the one soil. Thus,

Grosse-Brauckmann ( 1953) compared gramineous and leguminous species in a humic sand of p H 5.2 and found that barley and ryegrass contained 1.95 and 1.58 percent SiO,, respectively, whereas red clover and

blue lupin contained only 0.12 and 0.24 percent SiO,, respectively. Jones

and Handreck (unpublished) have also grown different species in the

one soil, a sandy loam of p H 6.0 containing 45 ppm. SiO, in solution.

They found that oats, rye, and ryegrass (Ldium rigidurn Gaud.) contained 2.04, 2.41, and 2.34 percent SO,, respectively, whereas crimson

clover, peas and mustard contained 0.12, 0.25, and 0.15 percent SiO,,

respectively. It seems certain that lowland rice contains much higher

concentrations of silica than other ( “dryland ) Gramineae because several authors (e.g., Imaizumi and Yoshida, 1958; Matsubayashi, 1963)

report that it commonly contains 10 to 15 percent SiO, in the straw.

The foregoing suggests that plants may be divided into three groups

which differ in their ability to absorb silica from a given level in solution.

Because the silica in soil solutions is undissociated monosilicic acid

it might be expected that it is absorbed through simple solution in the

transpiration stream. Indeed, Pfeff er ( 1900) wrote that transpiration

“aids in the absorption of silica,” and Frey-Wyssling (1930) also suggested that silica is absorbed in the transpiration stream by a nonselective

process. Recent studies with both soil and solution cultures have examined this possibility and at the same time have thrown more light on

the differences between species.

In their investigations Jones and Handreck (1965b) used the oat as

representative of “dryland Gramineae. It was grown in soils in which the

concentration of monosilicic acid in solution ranged from 7 to 67 ppm.

SiO, and the amounts of silica absorbed and water transpired were

measured. The ratios of water used to dry matter produced (i.e., transpiration ratios) were similar in the different soils, and at the various

stages of growth there was close agreement between the amounts of

silica in the tops and the amounts that could be expected from a passive,

nonselective uptake in the transpiration stream ( Table 11). Further evidence of a passive uptake of silica comes from work with oats in culture

solutions. In these the concentration of monosilicic acid remains practically constant as water is lost by transpiration. Also, the concentration

of monosilicic acid in the xylem sap has been found to be close to that

in the external solution (Handreck and Jones, 1967a). It seems likely



that the uptake of silica by other dryland Gramineae is also passive because their contents of silica are of the same order as that of oats.

Crimson clover (Trifolium incurnuturn L.) was chosen by Handreck

and Jones (1967a) as the experimental plant representative of the

legumes and the other dicotyledons which contain low concentrations of

silica. When grown in the soils containing 7 to 67 ppm. SiO, in solution,

clover had similar transpiration ratios to the oats but the concentration

of silica in the tops was only 5 to 10 percent of that in oats. Crimson

clover growing in culture solutions absorbs monosilicic acid more slowly

than water, and in the xylem sap (collected at u)minutes after decapiTABLE I11

Silica in Crimson Clover (Trifolium incarnatum L.) in Relation to the Concentration

of Monosilicic Acid in the External Solution”

Monosilicic acid

Total Si02 in dry matter



(ppm. SiOg)



(ppm. SiOd


















Data from Handreck and Jones (1967a).

tation) the concentration of monosilicic acid was only 5 to 7 percent of

that in the external solution. Analyses of the clover plants show that the

concentration of total silica in the roots was about 8 times that in the

corresponding tops (Table 111). In studies with tomato Kono and Takahashi (1958) and Okuda and Takahashi (1964) found that it also took

up monosilicic acid more slowly than water, and Okuda and Takahashi

(1964) found that the concentration of monosilicic acid in the xylem sap

was less than that in the external solution. The characteristically low

concentrations of total silica in the tops of legumes and other dicotyledons may therefore be attributed to an exclusion of monosilicic acid from

the transpiration stream, either within the root or at its external surface.

There is evidence to suggest that metabolic processes may be involved

in this exclusion of monosilicic acid. Thus, Handreck and Jones (1937a)

found that the concentration of monosilicic acid in the xylem sap from

crimson clover rose from about 6 percent of the external solution at 20

minutes after decapitation to a maximum of 60 percent at 21 hours after

decapitation. It subsequently showed a diurnal fluctuation with minimum

concentrations at times of maximum metabolic activity in the roots. The



mechanism by which the undissociated molecule of monosilicic acid is

metabolically excluded is, however, obscure and calls for further


When the high concentrations of total silica in rice are considered

along with the reports that its transpiration ratio is similar to that of other

plants (Hong, 1957; Takahashi, 1964), it appears that rice has a special

ability to accumulate silica. The nature of this special ability has been

investigated by several Japanese agronomists and plant physiologists. In

marked contrast to other plants in culture solutions, rice absorbs monosilicic acid at a much greater rate than it absorbs water (Kono and Takahashi, 1958; Okuda and Takahashi, 1964). The magnitude of this effect

is seen in data of Okuda and Takahashi (1964), which show that the

concentration of monosilicic acid in a culture solution decreased from

100 to less than 10 ppm. SiO, in the 37 hours following the introduction

of intact plants. The same authors showed that the concentration of

monosilicic acid in the xylem sap increased as the initial concentration

in the culture solution was varied in the range 10 to 1 0 ppm. SO,. In

the xylem sap the concentration of monosilicic acid was always many

times that in the external solution. For example, the plants introduced to

a solution containing 100 ppm. SiO, had 650 ppm. SiO, in the xylem

sap after 37 hours in the solution. It may be noted here that xylem sap

from rice in the field often contains 400 to 800 ppm. SiO, (Baba, 1957).

By comparing the behavior of excised roots and excised tops Okuda

and Takahashi (1964) have further examined the manner of silica uptake. With wheat and tomato there was little difference in uptake between tops and roots. The uptake by tops of rice was as low as that by

wheat and tomato tops, but uptake by the roots was very high. Thus, in

a 20-hour period excised tops and roots absorbed 0.5 and 6.8 pg. atoms

Si, respectively. Various metabolic inhibitors (sodium cyanide, 2,4dinitrophenol, iodoacetate ) blocked the uptake of silica by excised roots

of rice but they were without effect on uptake by the tops. The uptake

of water by both roots and tops was unaffected by the inhibitors. The

evidence from the various experiments of Okuda and Takahashi (1964)

led them to conclude that the uptake of silica by rice is closely linked

with metabolic processes in the roots and that these have a specific

ability to concentrate silica from the external solution.


If silica is absorbed passively in the transpiration stream one could

expect a change in the transpiration rate to be reflected in the amount

of silica in the plant. Germar (1934) was the first to study this relationship experimentally by growing sunflower plants in sand cultures at two



levels of humidity which produced transpiration rates differing by 62

percent. At the lower rate the tops contained lower concentrations and

total amounts of silica. Although Germar (1934) presented evidence that

lower light intensities also reduced the silica content of both whole plants

and individual leaves of various cereals, this factor cannot be considered

independently of the transpiration rate ( Monteith, 1966).

Recently Okuda and Takahashi (1964) obtained more definitive evidence of the effect of transpiration on uptake of silica using tomato as

the experimental plant. In a 48-hour period in which transpiration was

reduced from 150.0 to 63.5 ml. per plant, the uptake of silica was reduced

from 1.1to 0.4 mg. per plant.

The effect of transpiration on silica uptake by rice has been investigated by various workers. Okuda and Takahashi (1964) found only a

5 percent decrease in the total amount of silica in the tops when the rate

of transpiration was decreased by as much as 70 percent. A similar slight

effect of transpiration on uptake was reported by Baba et al. (1956), but

no effect was found by Kono and Takahashi (1958). In contrast to these

small effects of transpiration on total uptake, Baba et al. (1956) and

Baba (1956) found that transpiration had a big effect on the distribution

of silica in the rice plant. This was especially marked in the leaf blades

in which a 4.5 or 22.5 percent reduction in the amount of water transpired per unit area reduced the silica concentration by 9.6 or 22.9 percent, respectively. As the concentration of silica in the stems sometimes

actually increased when the transpiration rate was reduced, Baba et al.

(1956) concluded that the main effect of reduced transpiration was to

reduce the upward translocation of silica within the plant. This conclusion is compatible with the observations that shading, partial removal of

the root system, and lowering of the soil water content all reduced transpiration and in turn reduced the accumulation of silica in the leaf blades

(Baba, 1956). Conversely, when the transpiration rate was increased by

increasing the velocity of air flow around the plant the translocation of

silica and its accumulation in the leaf blades was increased. These observations suggest that although the overall uptake of silica by rice is largely

independent of transpiration, the subsequent translocation of silica toward the leaves may be affected by the transpiration rate.



Early work of the Rothamsted Experimental Station cited by Lawes

and Gilbert (1884) and Hall and Morison (1906) shows that fertilizing

with either nitrogen or phosphorus causes a decrease in the concentration

of silica in barley and wheat crops. Similar effects are consistently indicated by more recent work with soils (Coppenet et al., 1947; Delmas,



1960; Fletcher and Kurtz, 1964; Ganssmann, 1962; Grosse-Brauckmann,

1956, 1957; Knickmann, 1949) and with both sand and water cultures

(Engel, 1958; Gile and Smith, 1925; Lemmermann et aE., 1925; Rothbur

and Scott, 1957), but much of this work only presents information on the

concentration of silica in the dry matter. Grosse-Brauckmann (1956,

1957) and Ganssmann (1962) present, in addition, data on dry matter

yields and so make it possible to consider in more detail the effects on

silica uptake of increasing levels of nitrogen and phosphorus. The work

relating to these elements will be considered under separate headings

and the effects of other elements will be considered under a third


1. Nitrogen

Grosse-Brauckmann ( 1956) systematically investigated, in greenhouse

experiments, the effects of fertilizing summer wheat with different

amounts of various nitrogen compounds ( CaCN2, NaNO,, (NH4),S0,,

Ca ( NO, ) 2, NH,NO, ) . Increasing the supply of nitrogen produced systematic increases in the yield of dry matter which were accompanied by

decreases in the concentration of silica in the plant. These effects were

of the same magnitude irrespective of whether the additions were as


or NH4-N. In further greenhouse studies with oats GrosseBrauckmann (1957) extended the upper range of nitrogen, added as

NH,NO,. The results (Table IV) again show systematic decreases in the

concentration of silica with increasing yields of dry matter. However,

the concentration of silica in the plants levels off when nitrogen is no

longer limiting and yields of dry matter have reached a constant level.

The most likely explanation of these relationships may be found by

considering the effect of nitrogen supply on the transpiration ratio. It

has long been known (Wilfarth, 1905) that nitrogen fertilization leads to

a more efficient use of water by plants. This phenomenon has been

closely studied by Ballard (1933) and Trumble (19332), who show that

increasing the nitrogen supply to nitrogen-deficient plants decreases the

transpiration ratio by as much as 30 percent. In a recent review Viets

(1962) emphasized this effect of nitrogen on the water economy of plants

and cites considerabk fieId data to show even greater reductions in

transpiration ratios.

Although data on transpiration were not given by Grosse-Brauckmann

(1957) it would be reasonable to assign transpiration ratios of 600 to his

plants which received no nitrogen and 400 to those which received the

largest amount. On the basis that silica uptake by the oat is a passive

process, it may be calculated that 37.2 and 33.2 ppm. SiO, would be

required in the respective soil solutions to account for the amounts of



silica found in the plants. These calculated values for silica in the soil

solution are very close and suggest that its concentration did not fall as a

result of increased uptake. It may be concluded that the effect of increasing the nitrogen supply on the concentration of silica in the plant is an

indirect one because the more efficient plant produces more dry matter

for each unit of water and silica absorbed.

There is substantial evidence that fertilizing with nitrogen causes a

decrease in the concentration of silica in lowland rice (Izawa and Kume,

1959; Kid0 et al., 1958; Kid0 and Yanatori, 1963; Ota et al., 1957; Volk


Effect of N Fertilization on Uptake-of Silica b y Oatsfrom a-Soil-Sand (5 :3) Mixturea

SiOz in plants


(g. N/8 kg. soil)


0 0

0 15

0 3

0 9

1 5


2 4


Dry matter











0 55


0 67

0 80

1 07

1 11

1 14

N in plants






































Data from Grosse-Brauckmann (1957).

et al., 1958; Yamane, 1953). The results of Ota et al. (1957) show the

magnitude of this effect in various parts of the plant. For example, additions of 1, 2, and 3 g. N (as (NH,),SO,) to 12 kg. soil in pots decreased

the silica content of the leaves from 10.9 to 7.35, 4.92, and 4.60 percent

SiOz, respectively. This dilution of silica in the leaves might also be explained by a decrease in the transpiration ratio (Hong, 1957) which

would, in turn, decrease the rate of translocation of silica toward the


Many studies of the effects of nitrogen on rice show that heavy applications of nitrogen make the plant more susceptible to fungal attack.

Since this has been found to be directly related to a decrease in the silica

concentration in the straw (Volk et al., 1958), farmers in Japan and

elsewhere are advised to add siliceous slags to their fields when fertilizing heavily with nitrogen (e.g., Iwata and Baba, 1962).

2. Phosphorus

The most comprehensive study of the effects of phosphorus on the

uptake of silica by plants is that of Ganssmann (1862). He showed that

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