Good soil means good roses. What makes soil good is not a simple thing… it is highly complex. There are three possible sources from which rose roots can extract their nutrients; the soil solution, the soil particles and the readily decomposable minerals.
All are equal in importance and relationship to the rose. Therefore a general review will explain this relationship more fully.
Water is universally a component of soils. The amount may vary from the merest trace to a quantity sufficient to completely fill all the spaces between soil particles.
The soil solution’s nutrient concentration in any given soil varies with the proportion of water present, the plant growth in the soil and the activity of micro-organisms.
The proportion of water present depends upon the pore spaces (the irregularity of soil particles in size, shape and arrangement) which varies directly with the physical and chemical conditions to which the soil is subjected.
Soil texture is defined according to size of particles; clay the smallest, sand the largest and loam in between. The volume of air space depends upon the character of the pore space.
As a rule sandy soils have larger air volumes than heavier textured clay soils and also have a greater number of large pore cavities from which gravitational forces can remove water.
The field capacity of a soil is a degree of soil moisture. It is a point of equilibrium between the downward flow of water (gravitational) and the upward flow of water (capillary). Some clay loams and clays have such uniformly fine pore spaces they are nearly saturated at field capacity.
The result is an insufficient supply of oxygen for rose roots. Roses growing in these tight soils suffer not from too much water but from too little oxygen.
Water absorption requires a good supply of oxygen. The absorption of nutrients and water is part of the respiratory process of roots and they can only live in a soil adequately aerated.
Climate — Organic Matter
Plants differ in their ability of transferring nutrients to their tissues. The variation in the plant’s mineral composition is due to differences in demands made by the plant·upon the mineral constituents of the soil.
Climatic conditions affect the respiration of the roots, movement of nutrients through the plant, photosynthesis and other physiological processes. For this reason the composition of both the mineral and organic matter of roses may be greatly modified even though grown upon identical soil types but in different geographical locations.
Under natural conditions there is an equilibrium between the formation of organic matter by vegetation and its decomposition by micro-organisms. The bulk of organic residues in every soil is furnished by plants.
Consequently the degree of vegetation will be a major factor in determining the quantity, distribution and a general quality of soil organic matter, including humus.
Climatic conditions, the amount of vegetation and the activity of microorganisms collectively determine the nature of the cycle through which elements are taken by the plant from the soil, incorporated into living matter and finally returned to the soil again.
Season, temperature, water supply and micro-organisms determine whether decomposition of residues will be rapid and complete, or slow and partial.
Plant Removable Nutrients
Inorganic soil clay colloids are the residues of rock particles that have been washed for ages by water. Through this washing they have lost much of their mineral content. The mineral matter retained by the soil clay colloid contains plant nutrients needed for growth which become available for the rose.
The typical clay colloidal properties are very pronounced: they swell when water is added and disperse when agitated with greater quantities of water. When they are dispersed and the water content reduced, they form a jelly mass which when further dried becomes almost impervious to water.
The colloidal process by which cations (i.e., Ca,Mg.K,etc.) from a solution go into insoluble form and another ion comes out into solution to take its place is known as base exchange. Soil tests for acid soils determine the base exchange capacity of a given soil.
The soil clay colloid contains varying quantities of cations that may be removed completely or partially without destroying the colloid.
The extent of separation is chiefly dependent upon the composition of the clay colloid and the quantity of water present. Cations moving from the colloid surface will be absorbed on the root surface, absorbed into the rose, or the clay colloid will remove cations from the root surface.
Thus the amounts and kinds of nutrients which reach the surface of the rose root becomes a major factor in the nutrient uptake.
The surface of the root is covered with hydrogen ions while that of the colloid contains varying percentages of all exchangeable cations. These colloids are like small magnets which can pull various cations in varying degrees.
Thus certain cations are found at different distances or rings around the clay colloid depending on how active they are. The most active are close to the colloid while the least active are found further out.
Due to the differences of activity hydrogen, calcium and magnesium are found closest to the clay colloid. Sodium and potassium are found in the outer level furthest away from the clay colloid.
The exchangeable cations are held tight by the magnetic clay colloid and can only be removed in exchange with a hydrogen ion on the root surface of an ion exchange in the solution in the root cells.
Rose roots coming in contact with clay colloids promote a potential for cation exchange between the two. The hydrogen ion, therefore represents an effective means by which the rose may remove nutrients from the colloid.
These hydrogen ions come in part from carbonic acid which is formed when carbon dioxide is liberated by the roots during respiration and mixes with soil water.
The exchange of ions from the soil to the rose root is not dependent solely on the quantity of a specific ion but upon the variety of ions available. It is this variety of ions that the rose root must obtain which are essential to growth.
Decomposition of Minerals
Soil material is the product of physical and chemical weathering of rock and minerals; sands and clays are more or less weathered chemically. They may serve as parent materials of soils without having to go through further physical and chemical changes.
The fate of the products of decomposition depends mainly on the rate of water movement through the soil. The greater amount of downward leaching (percolation) the higher proportion of the products of weathering removed from the place where they are formed.
They can either be redeposited in the lower layers of soil or else carried with the percolating water into ground water.
As water percolates through the soil it loses some of its dissolved oxygen and becomes enriched with carbon dioxide. This water causes reactions on the surface of mineral particles over which it moves.
The speed of decomposition depends on the constitution of the mineral particles concerned and increases with temperature and the quantity of percolating water.
Some minerals are very soluble in soil water, for example chlorides and sulfates. As fast as these are produced they appear in the soil solution and move with it.
Other products such as the principal cations, sodium, potassium, magnesium and calcium can be held by the clay colloid, but are easily removed if the soil is subjected to much leaching.
Through this continuous weathering of minerals the rose has a continuous supply of exchangeable ions.
Because the hybrid rose is a foreign introduction to most soils it may not have sufficient minerals for growth, thus added applications of minerals in the form of a high analysis fertilizer may be needed to offset the deficient soil minerals.