Agriculturists have long appreciated the important influence that the physical condition of soil has on plant growth. Old English expressions still commonly used in England refer to land as being in “good heart,” when the soil is in prime condition, and as being “out of heart,” when the soil condition is poor.
“Land out of heart,” wrote Thomas Fusser in 1573, “makes thistles a number forthwith to upstart.” In this country, we have no counterpart of these terms except possibly the expressions, good and poor soil tilth. These latter terms, however, are more restricted in their meaning, and usually refer to soil conditions at planting time or shortly after during the growing season.
Texture and structure are the two most important physical properties of soil associated with tilth or soil conditions. The texture of a given soil and the relative proportions of sand, silt, and clay which it contains is always the same.
The structure of the soil, the arrangement of individual soil particles into aggregates or granules, on the other hand, is constantly changing. When we speak of soil condition and soil conditioners, we are concerned with soil structure.
Technical Side of Soil Structure
The mechanism whereby individual soil particles, chiefly clay particles, are held together in aggregates or granules, the environmental conditions which favor granulation and those which do not, have been studied by soil scientists for many years. Much has been learned, and several plausible theories have been proposed.
In 1934, E. W. Russell, in England, suggested that water molecules, together with certain cations, especially calcium, served to link clay particles together in a definite pattern. He proposed a scheme for the linkage of clay particles by a sequence of clay particles — water molecule — calcium ion — water molecule — clay particle.
Organic matter in the soil has been known to play an important role in maintaining soil structure for nine years. In general, soils adequately supplied with organic matter, for example, a satisfactory humus content, have excellent soil structure and good physical condition, while soils low in humus have poor soil structure and are difficult to manage.
“All grim gardeners,” wrote the authors of “Garden Rubbish” with realistic humor, “should possess a keen sense of Humus.”
Organic Matter And Soil Structure
In 1947, J. B.Peterson of the Iowa Agricultural Experiment Staikas attempted to explain the role that soil organic matter plays in the soil structure mechanism. Drawing on the work of previous investigators, he knew that some of the products of decomposing organic matter persist in the soil much longer than others.
Many of these residual substances are complex, long-chain organic compounds that belong to groups in which the organic chemist culls polyuronides. Ile drew further from the work of a German chemist, Henglein, and a Swedish chemist, Stiverborn, who had suggested a pattern for the linkage between component unit parts of some of those complex polyuronide substances.
Their proposal involved the linking, or “bridging” the action of certain polyvalent cations, particularly calcium. Finally, he took the work of several investigators in this country, which showed that clay with divalent calcium attached, was more active chemically than clay with monovalent sodium.
Integrating the results of these fundamental researches, Peterson postulated a scheme for a calcium linkage or “bridge” between uronide particles and between uronide particles and clay particles. According to his theory, the linkage from one clay particle to another might be clay — calcium uronide particle — calcium — uronide particle — clay.
Experimental Theory
To test his theory experimentally, Peterson used pectin, a polyuronide substance commonly found in plants and plant material. He was able to demonstrate the effectiveness of pectin in promoting the granulation of clay particles from a water suspension of clay; mud found that pectin was particularly effective in the presence of a weak concentration of calcium ions.
His work suggested the definite “possibility that the polyuronides found naturally in the organic matter of soils and occurring in the surface layer of root hairs may be active in soil granulation.”
Pectin Capacity
The capacity of pectin, the same substance the housewife uses in the jelly making, for promoting soil structure development can be easily demonstrated. One has only to take a small handful of soil, mix in a small quantity of pectin powder and moisten it with water until a firm mud ball can be formed.
If this mud ball, together with one prepared without pectin, is carefully lowered into a glass of water, it will be observed that the pectin-treated ball will retain its shape much longer and that water surrounding it will remain much clearer.
Now we come to the new chemical soil conditioners which have received all of the publicity and fanfare that modern publicity and promotional techniques have been able to give them. What are these materials and how do they fit into our scheme for explaining the mechanism of soil structure development?
Development Of Synthetic Soil Conditioner
What has the organic chemist done in the development of synthetic soil conditioners? It would seem that consciously, or otherwise, the organic chemist has been able to synthesize long-chain compounds similar in structure to substances found naturally in soil organic matter and possessing the same important functional chemical groups.
It seems entirely likely that these new “polyelectrolyte” soil conditioners function in the same way in soil structure development as Peterson proposed for pectin or as do various other natural structure-producing substances found in the soil.
Since it is probably only the general long chain configurations of the molecule and the existence of certain essential functional chemical groups that are important in the soil structure-producing properties of a substance, it is obvious that there are hundreds of such substances occurring naturally in the soil and also hundreds of compounds which can be synthesized which would have this capacity.
Of course, some substances of either natural or synthetic origin would be much more effective in their soil structure-producing capacities than others.
Soil Conditioners
When properly used, chemical soil conditioners are very effective in promoting good soil structure development in clay soils and soils with a substantial clay content. It is with soils of this type that such excellent results have been obtained through their use.
It is often difficult, if not impossible, to keep the organic matter content of such soils high enough to develop and maintain good soil structure. The most promising area for the wide use of chemical soil conditioners is “heavy” soils.
Chemical soil conditioners
Chemical soil conditioners have another advantage over soil organic matter in that they are more stable and their effects longer lasting in the soil. Organic matter in the soil is continually being destroyed through microbial action and the supply must be continually replenished, whereas these synthetic materials, thus far, at least, appear to be resistant to microbial attack.
Synthetic soil conditioners
Synthetic soil conditioners, however, have not been particularly effective on sandy soils or soils high in organic matter. One would not expect them to be very effective on greenhouse soils, for example, and thus few reports of their effectiveness on greenhouse soils have appeared.
If the conclusions reached above in the discussion of the mechanism of soil structure are essentially correct, the reasons for the ineffectiveness of chemical soil conditioners on sandy soils and soils high in the organic matter should be readily apparent.
In addition to playing an important role in soil structure relationships, soil organic matter also serves in many other useful capacities in the soil. Outside of their structure-promoting properties, chemical soil conditioners, on the other hand, are of little or no value.
Soil organic
Soil organic matter is one of the most active constituents in the soil. If present in adequate quantities in well-fertilized soils, it serves to hold certain important mineral nutrients, such as calcium, magnesium, and potassium, in an exchange relationship so that they are readily available to growing plants, yet are not easily leached from the soil.
The colloidal clay fraction of the soil also functions similarly, but in sandy soils the clay content is low and the action of organic colloids here is important.
Many of the favorable effects of organic matter result not from its mere presence in the soil, but from its being continually decomposed or broken down into its simpler constituents.
The ultimate complete breakdown of this material releases quantities of plant nutrients that can be readily taken up by growing plants. In this way, not only nitrogen is released, but also calcium, magnesium, potassium, and many other elements essential to plant growth.
Organic Matter
To say that organic matter breaks down or decomposes in the soil is another way of saying that the soil microflora, the fungi, the bacteria, and other life forms in the soil are simply utilizing organic matter as a source of energy and essential nutrients to multiply and carry on their life processes. The many beneficial effects of soil microorganisms in the soil in turn are too numerous to list.
Years ago, the important role of organic matter in soil was discovered. Dale II. Siding and his co-workers at the Massachusetts Agricultural Experiment Station have found that certain breakdown products of decomposing organic matter in the soil, principally organic acids, serve to prevent the formation of relatively insoluble or “fixed” forms of phosphorus.
Acid Soils
In acid soils, phosphates tend to combine with free iron and aluminum to form insoluble iron and aluminum phosphates. These “fixed” forms of phosphates are not readily available to plants. Certain organic acids can combine with or “complex ” free iron and aluminum in its soil and thereby prevent phosphate fixation.
Organic acids are even effective in releasing phosphates from fixed forms, making them readily available to plants as they may need them.
The discovery and development of chemical soil conditioners is an important one. The full potentialities of these materials have not yet been fully investigated, but for reasons enumerated and discussed above, chemical soil conditioners should be looked upon as supplements and not as substitutes for soil organic matter.
44659 by William G. Colby