Surface Relationships of Roots and Colloidal Clay in Plant Nutrition

The recent increase in national concern in regard to the losses of soil by erosion may well lead us to appreciate the fact that the land has gone nude because cover growth is prohibited by man’s management and because of the declining store of fertility in the soil. This fertility consists of the chemical elements which the soil contributes to the plants. This contribution to any single crop is small. For many plants it represents but five per cent of the total dry matter in the plants, or even less. Air and water worked up into chemical combination by the energy of the sun constitute the bulk of most plants. This process of carbohydrate synthesis–the dominant one of all plant growth activities–can be carried out, or even initiated, only as the soil contributes from its store of essential plant nutrients. The declining supply of soil fertility must of necessity shift the plant population more and more to those kinds whose final composition represents less from the soil and more from the air and water. That is, considering rather broadly the functions of the nutrient elements within the plant, the crops must shift toward those with less of protein and mineral content and more of materials with mere fuel significance. Naturally, lessened possibilities for proper animal and human nutrition must accompany these changes. In the light of these considerations, the contributions by the soil to plant contents, the mechanisms through which such contributions are made, and the relative supplies within the soil become of much concern to all of us.

More Recent Concepts of Mechanisms of Nutrient Delivery by the Soil–With the increased knowledge of the colloidal behavior of the clay separate of the soil¹ and of the exchange of adsorbed ions between colloids through their contact,⁶ it is no longer necessary to consider the supply of nutrients in the soil as limited to those in the displaceable soil solution. The immediate stock is not only that which is in true solution or that which would leach out, but also that which is exchangeable by other ions, more particularly those of similar electric charge. This exchangeable stock is of far larger magnitude than that of the simple solution. For better understanding of plant nutrition we must understand this cationic exchange behavior in which the soil gives up its adsorbed nutrient elements of positive charge for hydrogen, which is a cation contributed in exchange by the plant. Whether anionic behavior is similar is a question that is awaiting specific information, though it is not unreasonable to anticipate some likeness.

Plant Nutrition May Be Mainly a Surface Phenomenon–If the cations of nutritional value are given up by the soil through this exchange in which the hydrogen, coming from the plant’s liberation of carbon dioxide, displaces them from the clay in direct contact with the root, then the extent of the performance resolves itself into one of surface nature and areas. The rate of reaction, as well as the total of cations taken from a given soil by the plant roots, is then a question of magnitudes of root and clay surfaces in contact, and of the kinds and concentrations of adsorbed ions on the clay.

Should all other factors be removed from consideration, it will be of theoretical interest, at least, to view plant nutrition as largely a surface phenomenon determined by the surfaces of the plant root, and the surfaces of the soil or colloidal clay on which the exchange activities are possible. The following discussion uses some recent root surface data and some clay surface values in an attempt to elucidate plant nutrition hypothetically as an exchange phenomenon.

Some Root Surface Values for Plants–The studies by Dittmer² give values for the root and root hair surfaces for soybeans, oats, rye and bluegrass in a specific soil. Since, in the last three of this group, more than 90 per cent of the surface of the root system is that of the root hairs, the values for the entire root system will be used, even though adsorptive activities are commonly attributed to the root hairs only. The values for the total surface of the roots are given in Table 1 with figures ranging from 1.0 to 25.6 square centimeters per cubic centimeter of soil.

Table 1. Total root surfaces of different crops in given soil volumes.

Crops	Square inches per
	42 cu. in of soil	Cu in. of soil*	Sq. cm. per cc. of soil*
Soybeans Oats Rye (winter) Bluegrass(Kentucky)	106.1  583.4 1,267.9 2,779.9	2.5 13.9  30.0 66.1	1.0 5.9 11.8 25.6

*Values are calculations from those by Dittmer.

 

Some Surface Values for Colloidal Clay–As for the surface offered by the soil for contact with the roots, this can be determined from the size of the particles of the colloidal clay. Numerous studies of this fraction of the Putnam silt loam subsoil have been made.⁷ If we disregard the sand and silt fractions of this well weathered soil for their exchange activities, and if we accept the general fact that this clay constitutes one-sixth of the surface soil of the Putnam profile, then the clay content will amount to .222 gm. per cubic centimeter of soil which weighs 1.33 gms. By placing the approximate general size of the clay particle at one tenth micron, or .00001 cm. (10-5 cm.), as the effective diameter, the value will be that into which about 35 per cent of the clay falls. The shape of the particles is of disc nature,⁸ but for the purpose of simpler concept, we may visualize the shape as cubical with faces of the above effective diametric dimensions, viz., 10-5 cm.

Small Portion of Soil’s Clay Content is in Root Contact–Should we visualize that these colloidal clay cubes are carrying their adsorbed nutrients on their surfaces and are in contact with the root with one face of the cube against it, then each particle would present a contact area of 10-10 sq. cm. According to the root areas given in Table 1, the numbers of clay particles which would be required to cover the entire root area in a cubic centimeter of soil are those given in Table 2 (column 1). With a specific gravity of 2.5 and a particle volume of (10-5 cm.)³, the weight of the clay in root context for the root surface per cubic centimeter of soil volume would be that given in the same table (column 2). Since there is but .222 gm. clay in this soil volume, then the clay in contact with the roots represents but a small percentage of the soil’s total clay, or those fractions of per cent given also in Table 2 (column 3).

 

Table 2. Clay in contact with plant roots expressed as numbers and weights of clay particles and per cent of clay in the soil.

Crops	Clay in root contact per cubic centimeter of soil expressed as
	Number of clay particlesa	Weight,b gms.	Per cent of total clayc
Soybeans Oats Rye (winter) Bluegrass(Kentucky)	1.0 x 1010 5.9 x 1010  11.8 x 1010  25.6 x 1010  (Column 1)	.25 x 10-4 1.47 x 10-4 2.95 x 10-4 6.40 x 10-4 (Column 2)	.011 .066 .133 .289 (Column 3)

a Root area in sq. cm. divided by area of face of particles (10-10 sq. cm.).

b Number of clay particles x (10-5)3 x 2.5.

c Weight of clay particles x 100/.222

 

Adsorbed Nutrients Represented by Contact Exchange–As a means of determining the amount of nutrients delivered by the clay surface in root contact, we may use the total exchange capacity of the clay, viz., .65 M.E. per gm., or .65 pound equivalents per thousand pounds of clay, as has been reported from many determinations.3 In a cubic foot of soil weighing 83.2 pounds (1.33, volume weight, x 62.4 pounds per cubic foot of water), of which one-sixth, 13.85 pounds, is clay, the exchange equivalents would be but .009 pounds (13.85 x .65/1,000). When considered per acre six inches deep, this would be but 196.02 pound equivalents (.009/2 x 43,560 sq. ft. acre area). Should this clay be saturated completely with calcium, then the clay per acre six inches deep would contain twenty times the equivalent, or 3,920 pounds of exchangeable calcium. If the percentage of surface in contact as given in Table 2 is applied to this calcium value and if the calcium on the clay surface in contact is completely taken by the plants, then the amount of calcium so obtained by the different crops would be those in Table 3 (column 1).

That removal to completion, or 100 per cent, by exchange is not easily conceivable as common occurrence was shown in the work by Ferguson and Albrecht,³ where only 85 per cent of the exchangeable calcium was taken by three successive crops, but not without significant irregularities in plant growth. A single crop took but 40 per cent. If we should assume as usable only 85 per cent of the adsorbed calcium on the clay faces in root contact, the amounts taken per acre would then be those in Table 3 (column 2). If only a single crop had been grown with 40 per cent of the exchangeable calcium taken, then only those amounts as given in the table (column 3) would be taken per acre six inches deep.

 

Table 3. Calcium available by root contact (pounds per acre six inches deep) and contained in normal crops.

Crops	With contact exhaustion of surface calcium at:			Content of normal crops
	100%	85%	50%	Pounds
Soybeans Oats Rye (winter) Bluegrass(Kentucky)	.43 2.58 5.21 11.33 (Column 1)	.36 2.19 4.43 9.62 (Column 2)	.17 1.03 2.08 4.53 (Column 3)	5.4 per ton forage 4.1 per 25 bu. grain and 1,200 lbs. straw 2.8 per 15 bu. grain and 1,000 lbs. straw 10.0 per ton forage (Column 4)

 

Normal Calcium Use by Crops is Greater than the Delivery by Direct Contact Only–When one considers the acre yields of these crops and the calcium contents of their above-ground parts as found by chemical analyses, then the fact is immediately evident that the soil delivers to this crop portion and particularly to the entire plants, roots and tops, more calcium than would be provided if only that adsorbed on the clay surface in immediate root contact were taken. In Table 3 (column 4) are given the amounts of calcium commonly found in these crops as harvested from conservative acre yields. 

Movement Between Clay Particles by Adsorbed Ions is Suggested–If these figures represent the facts, they suggest that there must be adjustments in concentration of the adsorbed ions, particularly nutrient ions, even on the colloidal clay surfaces of the individual particles and between the different clay particles. Calcium removal, according to these calculations for the commonly harvested parts of the crop, suggests adsorbed ion movement toward the roots through more than a few layers of clay particles, especially for the soybeans. When the entire plants are considered, the evidence is more convincing. Still further, the movement of ions cannot be wholly from clay in a silt loam of which only one-sixth is clay. Some of the root area is in contact with silt. From the mineral faces of the silt it would seem that the source would be the face in contact only. With the silt so little active, the clay must be all the more active in the movement of adsorbed ions over the particle, and through several layers of particles.

Movement of Cations Between Clay Particles Demonstrated–That such movements of ions from one clay area to another are possible has been demonstrated. By bringing a sand-clay mixture, the clay of which was saturated by calcium, into contact with a similar one saturated by hydrogen, the migration of these cations from one location to the other within a period of thirty days was demonstrated. Calcium had moved more than two inches into the hydrogen clay area, and the hydrogen had similarly moved into the calcium clay area. These migrations took place when all the calcium was adsorbed, and when no significant solution activity can be considered as playing a role in this exchange.

Cationic movement of adsorbed nutrients is a possibility, then, along the faces of the clay particles, if such activity may be visualized as occurring in the exchange atmosphere or in the adsorption layer on the face of the clay crystal, and if these plant and soil behaviors may be considered as evidence.

Movement of Ions From Mineral Crystal Into Colloidal Adsorption Atmosphere–Since three successive crops may reduce the supply of exchangeable nutrients of the clay to 85 per cent of exhaustion, we are immediately confronted with the fact that continuous cropping on many experimental fields has gone forward for more than half a century without even approaching such a high degree of depletion of the exchangeable nutrients in the soil. How then is the supply on the exchange atmosphere of the clay maintained? Graham,⁴ in his use of the colloidal hydrogen-clay in contact with pure minerals of silt size, has demonstrated that the adsorbed hydrogen on the clay is active in exchanging itself for the cations of the mineral. This exchange serves to nourish plants.⁵ The silt fraction with its mineral store is then the supply from which that of the clay is replenished after depletion by plant growth.

Ion Movement From Silt Particles Directly to Plant is Possible but Small–That ions can be taken in some measure by the plants directly from mineral particles of silt size without the intervention of the clay has also been demonstrated by Graham.⁵ Plants failed, however, to grow as well under such conditions as on the silt in the presence of other colloids less active than clay, and decidedly not as well as on the silt mixed with the colloidal clay. It was only when an acid clay was mixed with the minerals of silt size that the growth of the plants was most effective. Plants can then use the minerals directly and the silt size separates may serve, but their contribution is small. This may be a case of limitation strictly to surface contact area, since ionic movement from crystal to crystal does not seem so probable.

Exchange Concept Clarified Relation of Soil Development to Crop Production–Only in those soils, in which the mineral reserve is ample both as to the kinds and the amounts of necessary elements, will production be maintained for more than a three-year period of continuous cropping. Mineralogical studies of the silt fraction of the soil with its classification as dominantly quartz or “other-than-quartz” will contribute much to better understanding of the continued productivity of some of our lands. By knowing the extent to which the clay is exhausted of its nutrient cations, or the reciprocal, namely, the extent to which the clay has become saturated with hydrogen, and by knowing, in addition, the extent to which the reserve of mineral crystal nutrients in the silt fraction is exhausted, we can make some estimate of the degree of soil development and of the possibilities of crop production as to kind and quality. Such understanding of the soil in its practical significance is more easily obtained by aid of the concept of exchange between the colloidal clay and the root surface.

In the inorganic portion of the soil, then, the immediate supply of nutrients for plants would seem to be on the colloidal clay in the adsorbed form. The “other-than-quartz” minerals of silt size would then seem to be the reserve supply for either direct or indirect use in the future, if not for part of the immediate growing season. The colloidal clay, then, aids through two steps in nourishing plants. In the first, through its root contact, it serves to deliver nutrients by cation exchange. In the second, it serves in connection with the mineral breakdown of the silt fraction of the soil. By means of this view of the soil and root behaviors mainly as contact and surface phenomena, we may visualize more clearly and interpret more simply the processes of plant nutrition, of depletion of soil fertility and others connected with crop production and soil maintenance.

Summary

By means of some data giving the root surface per unit volume of soil and some giving the surface areas of colloidal clay, the calcium delivery to the crop through exchange phenomena was calculated. The calculations suggested that a crop gets more calcium than is present on only that clay surface in immediate root contact. The data suggested that exchangeable ions move from one clay particle to the next clay particle through several such layers.

Hydrogen movement from hydrogen clay into calcium clay and the reverse movement of calcium were demonstrated. Hydrogen clay contact with mineral crystals demonstrated similar exchange from the crystal to the clay colloid. Ionic movement from the mineral crystal to the root by direct contact failed to nourish the plants amply, yet the crystal in contact with the clay served effectively.

Thus in terms of these surface phenomena of the colloidal clay and the root, we may get a clearer concept of how the adsorbed nutrient supply of the clay is replenished from the mineral crystals of the soil. By means of this concept we can visualize more clearly the mechanism involved in plant nutrition, soil fertility depletion, and various aspects of crop production and soil maintenance.

 

References Cited:

1. Bradfield, R.: “Chemical nature of a colloidal clay.” Missouri Agric. Exper. Sta. Res. Bull., 60, 1923.
2. Dittmer, Howard J.: “A quantitative study of the subterranean members of the soybean.” Soil Conservation VI: 33-34, 1940.
3. Ferguson, Carl E., and Wm. A. Albrecht: “Nitrogen fixation and soil fertility exhaustion by soybeans under different levels of potassium.” Missouri Agric. Exper. Sta. Res. Bull., 330, 1941.
4. Graham, Ellis R.: “Primary minerals of the silt fraction as contributors to the exchangeable base level of acid soils.” Soil Science, 49: 277-281, 1940.
5. —”Calcium transfer from mineral to plant through colloidal clay.” Soil Science, 51: 65-71, 1941.
6. Jenny, H., and R. Overstreet: “Contact effects between plant roots and soil colloids.” Proc. Nat. Acad. of Sci., Washington, 24: 384-392, 1938.
7. Marshall, C. E.: “The chemical constitution as related to the physical properties of the clays.” Trans. Ceramic Soc., 35: 401-411, 1936.
8. —”Studies in the degree of dispersion of clay. IV. The shapes of clay particles.” Jour. of Phys. Chem., 45: 81-93, 1941.