Healthy soil and sustainable growing

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Soil Fertility Basics #2: Essential Plant Nutrients by Steve Peters
Steve Peters
http://www.seedsofchange.com/enewsletter/issue_21/issue_21.asp#Succession

The soil fertility discussion in last month's newsletter focused on 
management of organic matter. This month we will take a closer look at the 
major nutrient elements in the soil that are critical for maintaining healthy 
plants.

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

Another major cation nutrient, Calcium (Ca) 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.

The final major cation, magnesium (Mg) 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, the other group of major nutrients, are negatively charged ions. The 
major anion nutrients are nitrogen (N), phosphorus (P), and 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 rain water 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.

Another major anion nutrient, 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 insure that the plant roots will come in 
contact with it.

Lastly, 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.

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), and manganese (Mn). The other two micronutrients are the 
anions boron (B) and 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.