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Soil Fertility Basics: Essential Plant Nutrients

by Steve Peters, Seeds of Change Agricultural Planner

The major nutrients required by plants can be divided into two basic groups, cations and anions.

Cations
Cations are ions (atoms) that carry a positive charge. The major cation nutrients, which are used in relatively large quantities by plants, are:

• Potassium (K)
• Calcium (Ca)
• Magnesium (Mg)

Soil clay particles and organic matter (OM) contain negatively charged "exchange sites," which attract and hold these cations. The cation exchange capacity (CEC) is a measure of a soil's ability to attract cations and keep them in an exchangeable form. These cations drift back and forth between the soil and organic matter particles and the soil water solution. A soil with a high CEC (predominantly clay or high OM soils) are able to hold more nutrients in reserve than are low CEC soils (sandy and low OM soils).

Potassium (K), a major cation nutrient, is needed in greater quantities than any other nutrient, except perhaps nitrogen. It is vital in numerous plant functions including protein and starch formation, cell division, photosynthesis, cellular structure of stalks, and disease-resistance. Most soils contain high levels of K but most of it is unavailable to plants. Biologically active, non-compacted, high humus soils are best for assuring the availability of adequate K.

Compost and cover crops are the best K sources. Another excellent organic source of K are the kelp extracts such as Maxicrop which are applied directly to the leaves as a foliar spray. Inorganic K sources include wood ashes (too much can burn plants and excessively raise the pH), granite dust, greensand, sul-po-mag (sulfate of potash magnesia), and potassium sulfate. Liberal doses of almost any organic residue usually preclude the need for inorganic K fertilizer.

Calcium (Ca), another major cation nutrient, is the most important element in the soil for assuring good soil structure. It is present in the soil in much greater quantities than any other nutrient. C is also critical for maintaining a balance between the cation nutrients and the acid-forming, non-nutrients (hydrogen and aluminum). The relative amount of hydrogen ions in the soil water solution is known as the pH. A pH of 7 (logarithmic units) indicates a neutral solution, greater than 7 is alkaline, and less than 7 is acidic. If the pH becomes too acidic, the increased aluminum and manganese levels become toxic to plants. If the pH becomes too alkaline, phosphorus and many trace elements become deficient. Therefore, Ca becomes critical in maintaining a moderate pH (6.0 to 7.5), which enables most plants to thrive. Calcium is needed only in small quantities in the plant, but is essential for cell wall building, seed formation, nitrogen uptake, and for eliminating waste products. The best source for Ca is high-calcium limestone.

Magnesium (Mg), the final major cation, is a primary constituent of chlorophyll, the green pigment in plants which is the receptor of light energy in photosynthesis. Mg is also closely linked to all metabolic processes involving phosphorus. In wet climates, Mg can sometimes be deficient. The best remedy for MG deficiency is to add dolomite (high magnesium limestone). In arid regions, Mg is rarely deficient, and in fact is often present in excess. This can create gumbo or adobe-like, compacted soils which are highly deleterious to plants. The best ways to reduce Mg are to add compost, grow cover crops, or add gypsum (a mined material containing calcium and sulfur).

Anions
Anions, the other group of major nutrients, are negatively charged ions. The major anion nutrients are:

• Nitrogen (N)
• Phosphorus (P)
• Sulfur (S)

Anions, in contrast to cations, are not held on exchange sites, but are constituents of microorganisms. Anions are released to plants through the decay of organic matter and from rainwater and soil water. As the major constituents of proteins and carbohydrates, anions are required in much greater quantities than are cations. Also, unlike cations, anions are constantly changing in form and quantity, and are subject to leaching or gaseous loss (nitrogen, sulfur), or become unavailable by reverting to an insoluble form (phosphorus).

Nitrogen (N) is an essential component of plant proteins which control the movement of energy within the plant and overall plant health. Chlorophyll, enzymes, and hormones are all proteins. N is probably the nutrient most likely to be deficient in a plant, causing stunted and poorly developed plants. N can also occur in excess, causing a delay in flowering and fruiting, susceptibility to disease and insects, and reduced storage life of harvested crops. All soil life requires N in abundant amounts. Unlike other plant nutrients, N does not exist in the soil in mineral form, but rather is derived from the atmosphere, which contains 78% N.

Plants, however, cannot utilize this gaseous form of N. The conversion of atmospheric N to plant-available N forms is initiated by free-living bacteria species, such as azotobacter and clostridia, or by rhizobia bacteria. These bacteria live on the roots of leguminous plants such as peas, clover, beans, vetch, and alfalfa. This initial conversion process is known as nitrogen fixation. In a subsequent process known as nitrogen mineralization, other species of bacteria convert the nitrogen captured by the N-fixing bacteria into ammonium- and nitrate-nitrogen. These forms of N are taken up directly by plants, but they also are very readily lost via leaching and volatilization. Therefore, the biggest challenge is providing an adequate quantity of N while simultaneously preventing N losses.

It is best to grow a nitrogen-demanding crop (e.g., corn, cabbage, potato) immediately following the incorporation of a legume. Applying mature compost prior to growing the crop is the best means of supplying N gradually throughout the growing season, to minimize losses. However this initial application may not provide enough N during peak crop demands. Therefore, an additional application during the season of any organic material rich in N may be necessary. More compost would be best, although fish meal, alfalfa meal, or blood meal could all be used. Fish emulsion can also be sprayed directly on plant leaves in a liquid form.

Phosphorus (P) is essential in virtually every metabolic process including fruit, flower and seed formation, protein synthesis, and cell division. It is a constituent of DNA. Phosphorus is often in large supply in the soil, but most of it is unavailable to the plant. At a low pH, P is bound by aluminum or iron. At a high pH it is held by calcium. As mentioned above, a mid-range pH will increase the availability of P to plants. Also, soils with a high biological activity will lead to greater P availability, because P is a major component of microorganisms. In addition, cover crops such as buckwheat and sweet clover can extract P from lower soil layers and bring them near the soil surface for plant uptake.

Obviously, compost is a great source of P. An excellent inorganic P source is colloidal rock phosphate, which is a fine powder. Phosphorus is very immobile so the rock phosphate should be broadcast and incorporated thoroughly throughout the upper 6-8 inches of the soil to ensure that the plant roots will come in contact with it.

Sulfur (S) is an essential ingredient in many of the amino acids. Legumes require large amounts of S as do many beneficial soil microorganisms. It acts in similar ways to N in the plant, such as yellowing leaves when it is deficient. Compost is an excellent source of S. Gypsum is a good immediate source of S and is particularly valuable when soil organic matter levels are low.

Micronutrients
In addition to the major nutrients discussed above, there are at least six micronutrients (nutrients required in small amounts) that are critical for plant health. Four of these are cations. These include:

• Iron (Fe)
• Copper (Cu)
• Zinc (Zn)
• Manganese (Mn)

The other two micronutrients are anions:

• Boron (B)
• Molybdenum (Mb)

High quality compost contains a balanced mixture of micronutrients and is the recommended means of supplying these elements without causing toxic excesses. Foliar feeding with a proprietary micronutrient formula like Earth Juice Microblast can quickly supply plants with these trace elements but will do little to correct long term imbalances in the soil.