Roots may grow upward to a moist layer maintained at the soil surface. Frequent light sprinklings of water on lawns tend to keep the roots of grass near the surface of the soil and thereby impair the ability of the grass to withstand drought and to absorb nutrients present in the soil below the surface a few inches.

Roots of most plants will not enter wet, saturated soils.

High water tables limit root penetration and may even kill roots that had previously penetrated below the water table. High water table conditions that restrict roots to a shallow surface layer during the spring impair the capacity of the plant to stand a drought during the summer.

Wet soils impair root activity to the extent that they restrict aeration exposure to air. Soil pores filled with water leave no room for air. Roots are actively alive and need a continuous supply of oxygen to remain healthy.

There is also a need for dissipation of the carbon dioxide given off by root respiration. An accumulation of carbon dioxide at the root surface can be toxic. Facility for gaseous transfer in a soil is a good measure of its suitability as a physical medium for root growth.

Plants killed by flooding of soils usually die from physiological drought.

Flooding keeps air from the roots. Unaerated roots of most plants cannot absorb water.

THE TEMPERATURE of the soil also affects the development of roots, but it does so through its influence on cell activity, particularly respiration. It is pertinent therefore to consider in more detail how the cells in the surfaces of roots absorb water and nutrients.

The drawing illustrates a small section of the absorbing surface of a root. Note that root hairs are tubular extensions from the wall of the surface cell located one-fourth inch to 1 inch back from the growing point of the root. Note also that the root hairs make close contact with soil particles as the hairs protrude in an irregular course through interstices among the particles. Furthermore, the film of water that surrounds each soil particle touches the surface of the root hair. There is also illustrated the vital air pore spaces that must maintain oxygen supply to the root cells and transport carbon di-oxide away from the roots. Parts of the root not covered by root hairs also maintain intimate contact with the soil particles and their surrounding moisture films.

We have noted that a single plant may have an astronomical number of root hairs, which provide a seemingly enormous surface for contact with soil particles. The roots of a winter rye plant were found to have a total surface of 6,870 square feet. The plant was grown in a cubic foot of loam soil. Such a soil will have a specific surface of about 20 square feet to a cubic centimeter. The surface on all the millions of particles contained in a cubic centimeter of loam will add up to about 20 square feet. Calculations show that the particles in a cubic foot of such a soil will have a total surface of about 560,000 square feet. That is, what we regarded as an enormous root surface was really only a little over 1 percent of the soil surface available for contact. In other words, even this highly proliferous root system did not come in contact with 98 percent of the soil particle surface.

Water enters the roots from the soil particle films as a result of osmotic forces in cells or through the pull of transpiration transferred from the leaves. Cells must be alive and active to absorb water. If the oxygen supply is deficient, the root-absorbing cells may become asphyxiated and stop absorbing water, and the plant wilts.

Unduly low temperatures may also impede water absorption by root cells. Sudden hot spells in spring when the soil is still comparatively cool cause cotton plants to wilt, even though soil moisture is adequately available. Too low a temperature around the roots limits absorption of water. Toxic substances that impair the health of root cells also impede the absorption of water.

As the roots absorb water from the films surrounding the soil particles, the films become thinner, and the remaining moisture is held by the soil particle with greater tenacity.

At the moisture content of field capacity the soil moisture content a few days following a heavy rain when free internal drainage has stopped the soil particles hold the moisture with a force equivalent to 0.01 atmosphere.

When the plant has removed the soil moisture down to the permanent wilting percentage, the soil particles hold the moisture with a force equivalent to about 15 atmospheres.

The movement of moisture along the surfaces of particles towards the root surface is slower as the moisture films become thinner. Thus both energy of retention and movement may contribute to impeding water entry into roots at low levels of soil moisture.

The moisture held by the soil between field capacity and the permanent wilting percentage is termed the available soil moisture.

Sandy loams hold about 1 inch of available water per foot of depth. Clay loams hold about 2 inches of available water per foot of depth. The soil moisture reservoir available to a plant is equal to the depth of the active root zone of the plant times the amount of available water per foot of soil. Thus spinach growing in a sandy loam will have a soil-moisture reservoir equivalent to a 1-inch layer of water, but a deep-rooted alfalfa field in a clay loam will have a moisture reservoir of 12 inches. Obviously the status of the soil-moisture reservoir is of key importance in enabling growth of plants during extended periods of drought.

The mineral nutrients absorbed by the roots are adsorbed on the surfaces of the soil particles or dissolved in the moisture films surrounding the particles. Maybe you assume that the minerals move into the absorbing cells of roots along with the entry of water. That is not necessarily so. Although minerals move in along with water pulled by transpiration, they also will move in even if transpiration is nil.

The accumulation of mineral ions by root cells is an active life process effected by energy released by respiration of the cell. Any condition in the root environment such as low temperature, lack of aeration, and the presence of toxins that impair respiration will likewise adversely affect absorption of mineral ions.

As is evident in the drawing, in order for minerals to reach the vacuoles of a cell, they must move through the cytoplasm around its inner wall.

The cytoplasm cytoplasm plus nucleus equals protoplasm has remarkable powers of accumulating and retaining certain minerals. For example, a root cell may accumulate potassium in its vacuoles to 20 to 3o times its concentration in the soil solution. The cells retain the accumulated potassium against the high tendency for outward diffusion as long as it is actively alive. If the cell is full, the potassium will immediately diffuse out.

Although potassium and sodium are quite alike chemically, the cytoplasm in the roots of many plants differentiates them accumulating potassium while excluding sodium. Since nutrient absorption depends on energy released from respiration, it also follows that the supply of respiratory substrate (sugars) in the root cells regulates mineral intake. The sugars providing this energy are transported down from the leaves. In this manner, conditions of the aerial environment affecting photosynthesis and sugar accumulation may have a direct bearing on absorption of mineral nutrients by the roots.

The mineral reservoir available to the plant is conditional, as is the soil moisture reservoir, upon the extent and proliferation of the root system. Since the plowed layer is the main source of minerals in most soils, it is obviously important that the roots develop maximum permeation of this layer.

Thus we see that the vegetative growth of a plant is the resultant of a complex interrelationship of a large number of environmental factors upon the constituent cells in the plant. In a number of crops, the grower has no economic interest beyond the phase of growth termed vegetative development.

Potatoes, sugarcane, sugar beets, hay crops, and the leafy vegetables are examples, but the farmer growing cotton, soybeans, fruits, grains, and seeds must produce something besides stems and leaves.

Why does a plant go into the flowering stage? We do not know exactly. When a plant has reached a certain age and the proper environmental conditions prevail, certain cells in the growing points initiate the development of tissues that become flower parts.

Investigations by Department of Agriculture scientists some 20 years ago showed that the flowering of many plants is controlled by length of day. Some flower during the long days (short nights) of early summer and others flower during the shorter days (longer nights) of late summer and early fall. Other species of plants are induced into flowering by exposure to low temperature. For example, sugar beets, cabbage, and cauliflower must go through a cold period before they will flower.

Soil-management practices may affect flowering through their influences on vegetative vigor. Vigorous plants are usually delayed in flowering but this may be compensated for by increased intensity of flowering.

The maturation of seed is largely influenced by the previous vegetative status of the plant, weather conditions, and soil moisture. The problem of soft corn is a good illustration of the economic importance of the right growing conditions and good weather and seed maturation.

The production of our crop plants is determined by their inherent capabilities to grow within the limitations imposed by environmental conditions. The farmer can do relatively little to improve the weather conditions prevailing on his fields, but he can have a major influence through soil management upon the environment of the roots of his crops: A plant is a living entity derived from soil, sunshine, air.

The soil is not the least of these three.