CINDY MURRAY
As most of us know, the three primary macronutrients essential for plant nutrition are nitrogen, phosphorus and potassium, or NPK. These are the nutrients needed in the greatest quantities and the ones most often limiting plant growth. For decades, farmers and gardeners have been growing food crops by supplying these nutrients annually, often based on soil tests.
Agriculturalists have long recognized there are other nutrients required for optimal plant growth. Farmers and gardeners add them when they are aware a nutrient is lacking, but a whole system approach to soil building rather than simply supplying what is limited can provide better yields and healthier soils.
To understand how a system approach benefits the biology and health of the soil, we need to know that plants undergo a natural biochemical sequence of nutrient assimilation and metabolism. Only supplying NPK often sidesteps some vital links in the chain.
This biochemical sequence kick starts with the absorption of boron, in the form of boric acid or borate, through the roots of every plant. Boron plays a major role in sap pressure, which is vital in the transportation of nutrients and food throughout the plant.
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The second link commences with boron stimulating the root system to secrete sugars into the soil. The sugars stimulate growth of beneficial microorganisms, which in turn transform silicates (a form of silicon) into silicic acid.
The third link comes about by silicic acid inducing the uptake of calcium, organic nitrogen, magnesium, phosphorus and potassium through capillary action. Silicon may also provide some resistance to plant diseases, promote strong stems and enhance yields in vegetable crops.
Calcium’s role in plant nutrition is to strengthen cell walls and membranes and promote good soil structure. But calcium has low mobility in plants, causing some plants like tomatoes and squash to show symptoms of deficiency (blossom end rot) even when the soil contains adequate amounts. The key to increasing the mobility of calcium to all parts of a plant is to keep the soil evenly moist throughout its root system.
Calcium then brings nitrogen into the sequence by binding to it. Now the plant is able to form DNA, undergo cell division and manufacture proteins like chlorophyll.
Magnesium is a crucial component of chlorophyll, which absorbs energy from sunlight through photosynthesis. Equally important, magnesium is also a phosphate carrier.
Phosphorus helps convert sunlight into energy. It is also a vital component in the transfer of energy, including the energy required to take the carbon atom from the carbon dioxide molecule to manufacture sugars.
Lastly, potassium carries the sugars to feed the entire plant.
All the while the plant nutrition sequence is taking place, Actinomycetes bacteria and mycorrhizal fungi are breaking down clay and humus, providing the perfect medium for nitrogen-fixing and other beneficial microbes. In reaction, the roots exude more sugars beneficial to the microbes. If you were to view a small mass of these plant roots, you’d view them as dense, fine and intricately branched. Soils heavily treated with nitrogen, phosphorus, and potassium, however, don’t display these attributes, as inorganic NPKs induce salty soil conditions inhibiting Actinomycetes and mycorrhizae.
Additionally, soils heavy in inorganic NPKs inhibit microbial growth required for boron to commence the process, so the entire nutritional sequence may never catch up. High levels of nitrogen and phosphorus may also block the absorption of calcium, rendering the plant calcium-deficient.
I believe the lesson we learn from studying this biochemical sequence is how to feed our soils rather than simply feeding our plants. The best way we may accomplish this is to incorporate plentiful organic matter like compost into our gardens every year. Other practices may include employing cover crops and/or applying mulch to the surface. All of these promote growth of beneficial fungi, bacteria, protozoa, nematodes and earthworms.
We’re fortunate to reside in Northern Arizona where all required minerals for plant growth are naturally in our soils, but we may need to replenish nitrogen or other elements annually. Soil tests are the best way to tell us what nutrients and micronutrients are insufficient in our fields and garden beds.
Once we take these actions, the nutritional biological sequence can roar to life, ultimately rewarding gardeners and farmers with bountiful harvests.
Cindy Murray is a biologist, co-editor of Gardening Etc. and a Coconino Master Gardener with Arizona Cooperative Extension.
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