Chemically synthesized, inorganic fertilizers became widely available after World War II , and were welcomed as a major advancement resulting in higher yields. Over time and with their overuse, detrimental effects became evident. These included the negative effect on soil structure, increased salinity, and a decrease in soil microorganisms. This is not to say that they don't have their place in modern gardening. When used in conjunction with organic fertilizers in a prudent manner they can be beneficial, and an important tool in the gardener's arsenal.
Soil is an aggregate, made up of mineral particles such as sand and clay, and decaying organic matter. Microorganisms break down these particles, producing nutrients, as well as other chemicals that help maintain soil structure. The overuse of inorganic fertilizers leads to a drop in the population of microorganisms and a change in soil acidity. The structure of the soil begins to break down, affecting its ability to retain air and moisture and leaving it more susceptible to erosion.
Chemical fertilizers are either manufactured, or are the purified salts from natural underground deposits. Their use year after year leads to an accumulation of sodium in the soil. Carbonic acid, which is present in soil, combines with the sodium to form sodium carbonate. It acts as a bonding agent causing soil particles to bind together and compact. The soil becomes hard, difficult to work, and plant roots can't penetrate it. The soil sheds water rather than absorbing it, robbing plants of moisture.
Effects On Plant Growth
In the natural environment, plants live in a symbiotic relationship with the soil microorganisms. The microorganisms produce nutrients, hormones and antibiotics that plants can absorb in small quantities. Inorganic fertilizers allow plants to bypass this process, and absorb nutrients directly in larger quantities that can burn plant roots, especially if the soil is dry. The rapid growth that results from the use of inorganic fertilizers can also be more susceptible to insect and disease attack.
There are times when the use of inorganic fertilizer is warranted. During periods of prolonged wet weather, or in mid season, when plants have depleted the soil, inorganic fertilizer applied at half strength can be helpful. Slow-release fertilizers are also a better choice. Water the soil thoroughly before application to protect plant roots.
Soil nutrition should be considered a year-round endeavor. In the fall, after the garden has finished growing, slow-acting organic fertilizers should be worked into the soil along with organic matter such as leaves, compost and peat moss. In the spring, faster-acting organic fertilizers and more organic material should be worked in prior to planting. Finally, just before flowering and fruiting, another application should be made and a top dressing of compost or manure should be added around the plants.