Soil Fertility Factors

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Soil Fertility Factors

By George Giltner, Advanced Master Gardener

You don’t have to battle through botany, chemistry, cellular biology and physics to understand that when essential nutrients are not available to plants, they will not grow.  Common garden logic is in the “Law of Return”.  When plants are removed from a garden, along with them goes a portion of the nutrients in the soil.  Therefore the gardener must return that complement of nutrients in the form of fertilizer back into the soil to maintain its fertility.  Other nutrient-loss factors like water leaching, volatile gases lost in decay, pest grazing, mineral insolubility, even removing weeds, and others contribute to fertility losses.

Soil testing can identify these losses.  Also soil test results supply information on soil mineral corrections.  Soil test sample boxes are available in a convenient mail-in form that can be obtained from the AgCenter.  Results are usually emailed or mailed within a week.

Organic matter additions are an excellent means returning nutrients and minerals back to the soil.  Consider making compost year round for garden amendments.  The compost is important to the soil structure, the microbes to insects in the soil-food web, moisture and mineral retention, and to the environment.  Mineral fertilizers do return mineral nutrients quickly to the soil, but it is in a leachable form that is destructive to soil life and the long term detriment of soil fertility.  Organic fertilizers provide a wide range of macro and micro nutrients that chemical fertilizers do not have.

Plant nutrient intake is influenced by temperature, mainly from 42 to 95 deg F for most plants, due to limits on photosynthesis and microbial produced nitrogen.   Also mycorrhizal fungi are very important for most of plant’s phosphorus uptake.  During early spring, one may notice purple leaves on tomatoes exposed to cool temperature soils.  This is probably due to lack of phosphorus transport activity of the fungi due to cool temperatures.  In summer, exposed soil around plants can reach temperatures around 120 deg F, thus limiting photosynthesis.  Mulching can reduce these temperatures by a significant 30 degrees, thus allowing for moderate temperatures for photosynthesis.

Problems with pH are typical with chemical gardeners.  Additions of ammonium are converted to nitrates by nitrogen-fixing soil bacteria, resulting in a lowering of the soil pH.  As the soil pH goes more acid, less and less of soil minerals are available to plants.  If lime is added during the plant growth cycle, “lime shock” occurs which leads to further problems with nitrogen loss, and a lock up of phosphorus in insoluble (unavailable) calcium phosphate.  Plant microbes are affected and other minerals become unavailable for plant absorption.  With a healthy organic soil, plants synthesize and release exudates that adjust the pH through action of the microbiological community.  Therefore organic soils are much less pH complicated to the gardener.

Poor soil aeration can devastate beneficial microbes in soils.  A compacted soil results in trapped carbon dioxide reacting with water to form carbonic acid. Excess carbonic acid then reacts with organic matter to form deadly alcohols and other noxious chemicals that kill root cells.  Aerobes in the soil are replaced with anaerobic life which ties up nutrients that would be going to plants.  Organic soil will hold its loose structure even after rains, whereas mineral soils will collapse and become compacted. A fluffy soil that can allow oxygen and water to flow easily is ideal. Water and oxygen movement is necessary to maintain the microbes and to transport soluble nutrients to plant root systems.

Chemical balances influence availability of individual mineral nutrients.  There is completion among ions of minerals for absorption on root sites.  Example: If you have too much potassium, magnesium, or sodium in your soil, plants will take up less calcium.  As a result, blossom-end rot would be much more common in your tomatoes, squash, and other plants.  This is why frequent soil tests are very important to chemical gardeners, but less so to organic gardeners.

Many people want to become gardeners, especially with rising food prices, problems with food safety, and reports of lack of nutrients in food items.  Too many are taking the “modern path” by pouring on N-P-K fertilizer without knowing how the fertilizers work.  These people use three times the nitrogen that farmers use.  This results in excess nutrients (esp. nitrogen and phosphorus) that are washed into waterways and harm aquatic environments, plus their garden is a flop.  To become a responsible and knowledgeable gardener, take the Master Gardener Classes at the LSU AgCenter. For more information about Master Gardeners, call the AgCenter at 337-463-7706 or email khawkins@agcenter.lsu.edu.

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