Fertilization of Garden Soils

To the average gardener the fertilization of garden soils becomes more confusing every day. The organic gardening enthusiasts declare that those nasty chemical fertilizers rob both vegetables and fruits of essential enzymes rendering them of little value as human food. Then the fertilizer companies show how judicious use of their products can double the yield from your small garden space. How can an ordinary person make sense of such contradictory claims? Though this is a problem which should be referred to our most learned experts possibly a rank amateur can at least guess at some of the answers.

How do plants obtain nutrients from the soil? All soil chemicals, whether organic or inorganic, either must be dissolved in the soil water or remain in a gaseous state in order to be available as a plant food. Those that are insoluble in weakly acid, neutral or basic soil solutions are unavailable to the plants. All students of botany will remember the study of osmosis where chemicals in solution will pass through a semi-permeable membrane from a weak solution to a stronger. Now the outer "skin" of the fine root hairs is, in reality, a semi permeable membrane. As long as the cell solution inside the root hair is stronger than that outside the soil solution will move into the roots. If, however, the addition of too much fertilizer has made the soil solution stronger than that within the root hairs the cell sap will move outwards and the plant will die. We say that the plant was burnt by the fertilizer. Instead it was dehydrated.

Although we all know that leaves have breathing pores, or stomata, which allow air and other gases to enter and assist in photosynthesis less mention is made of the fact that roots also require oxygen and other gases as well as water. In fact the roots of most plants prefer to live in a "moisture chamber" that is well aerated yet moist enough to allow a thin layer of water to adhere to the soil particles.. The tiny roothairs cling to the moisture-coated particles and absorb this water as well as the gases of the "moisture chamber". Therefore it is important to have a continuous, but not excessive, supply of both moisture and gaseous chemicals in that chamber.

Now every gardener has been told about the necessity for a good rich loam as a garden soil. Technically a loamy soil is one that contains a mixture of all sizes of soil particles, sands being coarse and clays very fine. Practically we gardeners always consider humus, or organic material, as a necessary constituent of garden loam.

If the soil is a coarse sand the water simply drains through it. There is plenty of air but little moisture left clinging to the soil particles. If the soil is very fine the addition of water can turn it into a gluey mass. There may be plenty of water but no air around the root hairs. Then if this gluey mass dries it can become as hard as brick and impenetrable to root hairs. Though a mixture of soil particles of differing sizes is better than either a soil of coarse particles or one of fine even it may be unsuitable for plant growth.

The leavening factor is organic material. The organic material with its great water holding capacity retains moisture in coarse sandy soils and separates the grains of finer clays leaving air spaces instead of the gluey mass or the bricklike structure. In other words it increases the water holding capacity of the coarser soils and the aeration of the fine clays. H organic material did nothing else these two reasons would justify its addition to garden soils.

But the organic material is also food for soil organisms such as "fish" worms and millions of bacteria as well as being of chemical value itself. The worms and wormlike creatures help to aerate the soil and the bacteria not only break down the organic material into a form useful to plants but also tend to change many soil chemicals from a form unavailable to plant life to one that is available.

This sounds as though the organic gardeners were right and there is really no need for chemical fertilizers. But this is only part of the story. If the basic chemicals are not in the soil, even in a form unavailable to plants, the addition of organic material can only add the plant food chemicals within itself. Moreover if fresh organic material, in contrast with well-rotted, is added the bacterial action actually drains nitrogen from the soil. If fall leaves or other fresh organic material has been dug into your garden it will be necessary to add a high-nitrogen fertilizer. Fresh manure can also deplete the soil of nitrogen.

In summation whether your soil is sandy or clayey add organic material to improve the soil texture and its moisture-holding capacity for this assists in the creation of the necessary "moisture chambers" between the soil particles. Also you add organic material to provide a home for soil bacteria which, according to the well-known micro-biologist Dr. Selman Waksman, not only convert unavailable soil chemicals into a form available to plants but also give off CO2 and other gases necessary to plant growth.

Finally, if your soil lacks necessary chemicals do not hesitate to add them. Be an organic gardener-plus.

Please contact the OHS or the author if you wish to republish these articles. © Ottawa Horticultural Society

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