Maintaining soil fertility
The other night, I became frustrated with my football coverage so switched between documentary channels on the cable network. I managed to catch the second half of a program about earthworms. Prior to final credits, the narrator emphasized the significance of the earthworm’s contribution towards maintaining soil fertility. He then suggested that soil fertility was crucial for the long term sustainability of life on earth. Unfortunately, he repeated the words monotonically and they became divorced from any reasonable sense of urgency.
As a keen student of organic principles, I’ve been concerned about the long term consequences of poor quality soils for several decades. Irrespective of the demand for organically grown produce, there are large numbers of consumers who remain misinformed about the importance of soil quality in determining the nutrient values of their basic foods. While vegetables raised in deficient soils may appear and taste adequate, they will not necessarily provide the complex balance of nutrients required for optimum health. For those interested in raising organic vegetables, a basic understanding of factors influencing soil fertility will provide the most reliable foundation for positive results.
What are the underlying causes of soil degradation and infertility? In terms of natural environments, erosive actions caused by wind, sun, water, and seismic disruptions are all potential contributors to soil degradation.
How do erosive forces influence soil fertility? Physical and chemical changes can disrupt the soil’s living organisms and their capacity to perform certain beneficial functions. This may occur as a consequence of habitat destruction or the depletion of important resources. The evidence of habitat destruction may be concealed. Consider the example of a subtle surface compression restricting a soil’s ability to deliver water and oxygen.
On other occasions, habitat destruction is externally obvious. For example, large quantities of topsoil might be stripped from one location. While external signs of physical erosion can be observed in many regions, they tend to develop over extended periods and appear more obvious in extreme climate situations like those experienced in desert, tundra, and alpine locations.
Soil degradation is also caused by a broad range of human activities including intensive agriculture and the appropriation of marginal and fragile environments for cropping and food production. During the latter half of the previous century, the steady accumulation of political and technological influences coerced significant numbers of food producers to adopt an intensive approach to agriculture. Instead of focusing on the complex determinants of natural soil fertility, intensive agricultural production depends on chemical assistance to regulate plant growth.
Water soluble fertilizers feed plants directly, bypassing the complex nutrient cycles which plants have traditionally depended on to remain healthy. The widespread use of synthetic chemicals is associated with a marked decline in the quantity and diversity of soil organisms. Consequently, many of these chemicals decrease natural soil fertility and plant health. The symptoms are frequently observed in private gardens, including those devoted to vegetable production.
Ecosystems and soil fertility
Optimum soil fertility depends on establishing ecosystems which facilitate the efficient recycling of nutrients. Ideally, soil nutrients are absorbed by living plants and animals which must eventually restore the balance of organic materials by means of excretion and physical decomposition. Under such conditions, healthy plants will develop beneficial relationships with soil bacteria and other organisms. An example of this are rhizobial organisms which attach themselves to the roots of legumes. While feeding on root secretions, they release compounds which enable the plant to obtain and absorb nitrogen from the atmosphere. Similarly, there are fungal organisms which penetrate plant roots and enable important soil nutrients to be transferred directly into the root system. Growers resorting to chemical fungicides and other treatments inevitably shift the balance between these organisms and risk losing their positive functions.
A powerful magnification of soil samples from an organic garden should typically reveal a complex diversity and abundance of microbial organisms. The majority of these organisms contribute to their ecosystem by breaking complex organic materials into simpler components which release energy and facilitate the storage of soil nutrients. Several classes of organism influence soil fertility by improving the structural composition of soil particles. They excrete various substances which function as an important cement between soil particles. Ideally, this enables soil particles to be combined in matrix arrangements which provide adequate surrounding spaces for the distribution of water and air.
When attempting to raise vegetables organically, it’s important to appreciate soil as a dynamic ecosystem with broader capabilities than a simplified growth medium for plants. In order to sustain fertility, the soil ecosystem requires continuous energy which is supplied by the combination of sunlight and the decomposition and eventual recycling of complex organic materials. There are three broad phases in the decomposition of organic materials which, in most gardening situations, will directly influence a soil’s fertility. The first of these relates to the availability of partially decomposed organic substances. The second relates to substances which have been recombined into new compounds following an advanced stage of decomposition. The final category is used to describe high molecular weight compounds which have been fully decomposed and stabilized against further decomposition. To remain viable over the longer term, these phases of decomposition must occur naturally within the soil.
A majority of cultivated vegetables have high nutrient demands which must be provided within relatively short growth periods. To what extent can these short term demands be sustained by pursuing the organic principle of routine soil replenishment? The best answer begins with a further question. How efficiently can organic materials be converted into the basic nutrients which are then absorbed by plant roots? This primarily depends upon the availability of energy. Throughout the active phases of organic decomposition, the speed of nutrient release is partly determined by localized temperatures within surface layers of soil. Warm soils are generally required to promote rapid decomposition and nutrient recycling. A soil’s warmth is generated from sunlight and the biological processes of numerous living organisms.
Soil chemistry and structure
The efficiency of nutrient release is also influenced by aspects of soil chemistry. Living vegetables have the capacity to absorb nutrients in the form of ionic compounds. While organic material represents the ideal collection point for many of these electrically charged compounds, a nutrient deficiency or imbalance can be worsened by the indiscriminate application of unsuitable organic materials. If a soil becomes severely deficient in calcium and magnesium, for example, the addition of potassium rich compost or fertilizers will generally slow the release of deficient minerals and further distort the soil’s profile. When a sustained application of organic principles fails to provide positive results, a professional soil analysis is a worthwhile investment.
After several seasons of organic vegetable production, most gardens will benefit from a fallow period and the addition of soil conditioners to replenish those nutrients depleted through repeated cycles of plant growth. Naturally sandy soils will benefit from the addition of moisture holding materials. Depending on what is readily available and cost efficient, choose organic cow or horse manures, peat moss, spent mushroom or garden compost. Heavy clay soils are best improved with mixtures of dry organic material such as seaweed, leaf mould, shredded straw, sawdust, wood shavings, and worm casts. Such dry materials build soil structure by loosening and separating some of the tightly bound clay particles. Subsequent additions of coarse grade river sand will reinforce this process.
When the vegetable garden is maintained with generous amounts of organic material, most plants will grow superbly without requiring additional fertilizer or nutritional supplementation. There are however, some occasions when high quality organic plant supplements can be useful. Vegetables sown a week or two late in the season can be encouraged to catch up with an extra dose of nitrogen rich fertilizer. This promotes rapid leaf growth, and is particularly useful for salad greens, lettuce, and spinach. Potassium based organic supplements are designed to encourage flowering and fruiting so may apply to certain varieties of zucchini, eggplant, and cucumber for example. I used to make my own plant foods in the form of liquid extracts but am currently sourcing a granulated mix from some local plant breeders who combine large quantities of natural organic ingredients to create well balanced garden fertilizers. Their standard mix contains equal amounts of linseed meal, kelpmeal, and guano. Their key ingredient is the kelpmeal. It costs several times more than the others but contains the natural hormones and growth regulators which assist vulnerable plants and seedlings to resist stresses like sudden cold spells and mild dehydration
Commercial products which claim to increase the size and yield of garden vegetables should be evaluated cautiously. The treated vegetables may be large in size but bland of flavor due to their higher than usual cellulose and water contents.