Soil fertility is a complex topic. It's one that initially seems simple; what could possibly be complicated about dirt and what plants eat? Especially when one has the background, as I did, coming from a University system that, when I was there in the late 80's, catered to a growing agribusiness mentality and was still entrenched in the mindset of soil testing, charts showing a crop's desired diet, and then adding any elements in short supply in the most economical and directly available way. Don't get me wrong: I strongly advise testing your soil. The problem is that the result of this mindset has been the continued degradation of large tracts of some of our most precious soils using what I would consider to be brutal fertilizers. While these are not generally used in the home garden, they help make a good point: We should be feeding our soils and promoting a healthy soil ecology which in turn takes advantage of the soil's natural ability to hold water, nutrients, and increase good tilth through increased humus content and the resulting aggregation of soil particles. Feeding the plants with fast acting and strong fertilizers can do harm to the soil ecology and over time, reduce the amount of humus, lowering soil fertility.
Anhydrous ammonia, a vapor that has a whopping 82% nitrogen content, can be injected into the soil to boost the amount of yield you get out of each acre. Unfortunately, pumping that much nitrogen directly into the soil, although it can be argued that it DOES increase microbial activity after several weeks, has some side effects that are not so great: The excess nitrogen promotes rapid decomposition of whatever organic matter is in the soil. The acidity of the soil also plummets and is usually neutralized by adding lime -1.8 times as much as the nitrogen added! These reduce soil aggregation and results in compacted soils.
Muriate of Potash, containing up to 60% Potassium also seems like a magic bullet until you realize that it (Potassium chloride) also contains over 47% chloride -salt! Sodium nitrate, sometimes referred to as Chilean nitrate, is also highly saline. Salt degrades aggregation. It also can literally suck water out of plant tissue through osmosis.
So if the "big guns" of conventional fertilizer backfire on us, and the answer instead really lies in building and maintaining humus, the desire to understand what is really going on leads us to the complex web of life in the rhizophere- the magical area surrounding plant roots where interactions of root secretions, microorganisms and mycorhizal fungi- and even interactions with earthworms! The more I learn about this complex system that is always in flux, the more I realize how little we really understand. Instead of the conventional mindset of replacing mineral elements, the new mindset is more like micro wildlife management where we have to supply food for all the organisms, keep them protected from excess solar exposure, erosion, drastic swings in moisture content, pH, or temperature extremes.
Earthworms enjoy a steady food supply. In agriculture this means reducing or eliminating discing, plowing, and tilling, adding manure, growing cover crops, and keeping as much crop residue on the surface as possible. In the garden it means not tilling and adding lots of compost and regular layers of mulch. A happy population of earthworms are said to process 20,000 lbs of top soil per acre per year, increase the water infiltration rate by up to 10 times, and stimulate plant growth (from: ATTRA Sustainable Soil Management by Preston Sullivan)
"... it may be doubted if there are any other animals which have played such an important part in the history of the world as these lowly organized creatures." -Charles Darwin's take on earthworms after careful study.
While we are now shifting our thoughts from supplying nutrients to plants to feeding the soil organisms, we still need to understand and be aware of the nutrients in play. There are a few ways to remember the "essential plant nutrients", but the way I learned it was with the slogan:
"C Hopkins cafe mighty good, but Clara's zany cup has many more!"
This is fairly easy to remember and actually maintains the elements in order of decreasing amounts actually used by plants. Let's break it down:
- C -Carbon, used in photosynthesis to create hydrocarbons, used as part of sugar, starches, and cellulose.
- H -Hydrogen, supplied from water, also used in photosynthesis to form sugars and many other elements
- O -Oxygen, used to generate ATP (energy) through the breakdown of sugars (respiration)
- P -Phosphorus, a component of ATP, used in flower and seed formation, and the control of enzymes
- K -Potassium, used in stomata to regulate water loss
- N -Nitrogen, used in all proteins
- S -Sulphur, used in amino acids and building chloroplasts
- Ca -Calcium, used in translocating nutrients and activating enzymes
- Fe -Iron, used in photosynthesis
- Mg -Magnesium, a part of chlorophyll for photosynthesis and used in ATP production
- B -Boron, used in sugar transport, cell division and creating enzymes
- Cl -Chlorine, used in osmosis and photosynthesis
- Zn -Zinc, used in DNA transcription
- Cu -Copper, used in photosynthesis, creating lignis for cell walls, and enzymess
- Mn -Manganese, used to create chloroplasts
- Mo -Molybdenum, used in amino acids and enzymes.
(-all paraphrased from wikipedia)
Other elements have gotten on the list since I learned this, such as Silicon (cell walls), Nickel (Nitrogen metabolism), Cobalt(Nitrogen fixation in legumes), Selenium, and Sodium.
The Big Three: N P K; Nitrogen, Phosphorus, and Potassium are considered Macronutrients. They are reflected as the three numbers on all fertilizer containers, always in that order. In very broad strokes, fertilizers high in N will cause the plant to grow quickly, putting on vegetative growth. Fertilizers high in Phosphorus will help promote flowers and roots. Fertilizers high in Potassium help with reducing diseases and improving fruit production.
A rule of thumb is that fertilizers higher than a value of 10 in any of the N P K designations is most likely NOT an organic fertilizer. There are a few exceptions where you see a 12 or a 15, but nothing much higher. Inorganic fertilizers are usually in a concentrated form and are more readily available to plants, but they are also leeched out of the root zone quickly. According to the Cooperative State Research, Education and Extension Service, USDA, organic fertilizers "have many advantages". Soil crusting is reduced, water movement and structure is improved, beneficial microbes are fed, and nutrients are more slowly released over time. Some possible problems with Inorganic fertilizers are that it's possible to apply too much at once. Problems with Organic fertilizers noted are related to the possibility of putting on too much non-composted manure and the possible introduction of weed seeds. The real problem with organic fertilizer is that they are generally, per pound of nutrient, more expensive.
"OOOOOOMMMMMM" Organic Matter: Compost Compost Compost! Mulch Mulch Mulch!
Organic matter can come in bulk from regular mulching, or it can be added with compost, composted manures, or specialty organic fertilizers. A discussion of specialty organic fertilizers and some other soil amendments will be posted on the next blog. I'll also be talking about fertilizers on the 20th and 27th at local Sloat Garden Centers, Sunday, 10am.