As most of us know, the three main macronutrients essential for plant nutrition are nitrogen, phosphorus and potassium, or NPK. These are the nutrients needed in the greatest quantities and the ones that most often limit plant growth. For decades, farmers and gardeners have grown food crops by supplying these nutrients every year, often based on soil testing.
Agronomists have long recognized that there are other nutrients necessary for optimal plant growth. Farmers and gardeners add them when they are aware that a nutrient is lacking, but a whole systems approach to building the soil rather than just supplying what is limited can provide better yields and healthier soils.
To understand how a systems approach benefits soil biology and health, we need to know that plants undergo a natural biochemical sequence of nutrient uptake and metabolism. Only the supply of NPK often bypasses certain vital links in the chain.
This biochemical sequence begins with the absorption of boron, in the form of boric acid or borate, by the roots of each plant. Boron plays a major role in sap pressure, which is vital in transporting nutrients and nutrients throughout the plant.
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The second link begins with boron stimulating the root system to secrete sugars into the soil. Sugars stimulate the growth of beneficial microorganisms, which in turn convert silicates (a form of silicon) into silicic acid.
The third link is ensured by silicic acid inducing the absorption of calcium, organic nitrogen, magnesium, phosphorus and potassium by capillarity. Silicon can also provide some resistance to plant diseases, promote strong stems and improve vegetable crop yields.
The role of calcium in plant nutrition is to strengthen cell walls and membranes and to support good soil structure. But calcium has low mobility in plants, causing some plants like tomatoes and squash to show deficiency symptoms (blood end rot) even when the soil contains adequate amounts. The key to increasing calcium mobility 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 make 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 element in the transfer of energy, including the energy needed to extract the carbon atom from the carbon dioxide molecule to make sugars.
Finally, potassium transports sugars to nourish the whole plant.
As the plant nutrition sequence unfolds, actinomycete bacteria and mycorrhizal fungi break down clay and humus, providing the perfect medium for nitrogen fixation and other beneficial microbes. In response, the roots exude more microbe-beneficial sugars. If you were to see a small mass of these plant roots, you would see them as dense, fine, and intricately branching. However, soils heavily treated with nitrogen, phosphorus, and potassium do not exhibit these attributes, as inorganic NPKs induce saline soil conditions inhibiting actinomycetes and mycorrhizae.
Additionally, soils rich in inorganic NPKs inhibit the microbial growth necessary for boron to begin the process, so the entire nutrient sequence may never catch up. High levels of nitrogen and phosphorus can also block calcium uptake, making the plant calcium deficient.
I believe the lesson we learn from studying this biochemical sequence is how to feed our soils rather than just feed our plants. The best way to do this is to incorporate abundant organic matter like compost into our gardens every year. Other practices may include using cover crops and/or applying mulch to the surface. All of these promote the growth of beneficial fungi, bacteria, protozoa, nematodes and earthworms.
We are fortunate to reside in northern Arizona, where all the minerals necessary for plant growth are found naturally in our soils, but we may need to replenish nitrogen or other elements each year. Soil testing is the best way to tell us which nutrients and micronutrients are lacking in our fields and flowerbeds.
Once we take these steps, the nutritional biological sequence can come alive, ultimately rewarding gardeners and farmers with bountiful harvests.
Cindy Murray is a biologist, co-editor of Gardening Etc. and Coconino Master Gardener with Arizona Cooperative Extension.