Tuesday, March 24, 2009

Plant Nutrition.

Plants require essential raw materials to keep them going, to provide both energy and building blocks for growth. This is true of all living organisms, including humans. Carbon dioxide is absorbed from the air along with water from various sources, mainly the soil, and together the elements carbon (C), hydrogen (H) and oxygen (O) are provided. In addition to these basic units, some thirteen essential nutrients are also required for a crop to thrive: three major nutrients, three secondary nutrients and seven micronutrients.

During the past half century, there has been a depletion of the amount of micronutrients present in plants and thus available to those creatures including humans, who eat them. There is a sanguine quote from Prince Charles, who is a keen organic gardener, and runs an organic farm on his Highgrove Estate in Gloucestershire:

"The New Scientist recently reported alarming research results from a study of the long term effects of the so-called 'Green Revolution' in South Asia. New plant varieties fed with high levels of artificial fertiliser have dramatically increased food production, to no-one's surprise. But it now becomes clear that those intensively grown crops are nutritionally deficient. They lack vital trace elements and minerals, particularly iron and zinc. This deficiency has been passed on through the food to such an extent that an IQ loss of 10 points has been observed in a whole generation of children who have a diet based largely on crops grown in this way."

Actually, years ago as a child, I lived on the Elmstree Estate which is next door to Highgrove, and whose elm tree population was devastated by Dutch Elm Disease, a scourge of the British countryside in the late 1960s/early 1970s. We lived there in a rented part of the main farmhouse (which was the original manor house), since my family are hardly gentry, having fled South Wales where I was born, in the aftermath of my father's bankruptcy. Not such a big deal now but it certainly was then.

As plants grow they remove these essential elements to a varying degree and rainwater leaches out more, so from time to time they need to be replenished and so in conventional farming/gardening this is usually done by adding artificial fertilizers. In permaculture systems, plants die and rot-down and the nutrients are returned to the soil as part of the natural recycling process. The availability of nutrients and their uptake by plants is assisted by mycorrizal fungi which are found in the rootballs of most plants.

The three Major Nutrients are Nitrogen (N), Phosphorus (P) and Potassium (K). Nitrogen (N) is required for healthy stems and leaves. It is an essential component of the amino acids which form the proteins and of the chlorophyll molecules that harvest light to drive photosynthesis. It is normally taken up into plants in the form of Nitrate (NO3-) and to a lesser degree as Ammonium ions (NH4+). Nitrates are easily leached from soil by rainfall during the winter, but when spring comes and the soil warms, nitrogen is extracted from the air and converted to nitrate by nitrogen-fixing bacteria.

When the soil is waterlogged, denitrification occurs by anaerobic bacteria. This is why plants grow better in well drained soil where air can percolate through it. Earthworms play a vital role too, in burrowing through and processing soil, thus increasing the availability of its nutrients and creating drainage channels and spaces for root-systems to grow into.

Phosphorus (P) is taken up as phosphate ions (PO4(3-)), and is a critical component of the nucleic acids, DNA and RNA. The ATP-ADP energy transfer process within plant cells requires phosphorus. It is moved around within the plant, being recycled from older parts to points of new growth. The Carbon Dioxide released during respiration reacts with water to produce carbonic acid and this assists the uptake of PO4(3-) by plant roots. The secondary root-system provided by micorrizal fungi greatly extends the reach of the primary roots and more effectively remove the phosphate ions from the insoluble soil salts.

Potassium (K) is not an essential building block of plants but plays a central role in protein synthesis and in maintaining the balance of water. It also makes plants winter hardy and improves their resistance to disease. Taken up as K+ ions, the ratio of N to K has an important effect on plant growth, the ideal being N:K = 1 for most crops and 2:3 for root crops and legumes. Magnesium (Mg2+) ions compete with K+ for uptake, but so long as the K:Mg ratio is about 3:1 or 4:1 there is no problem.

The three Secondary Nutrients are:- Magnesium, as Mg2+ ions, is the key metal element in chlorophyll, where it forms the centre of the molecule and its light-absorbing process. It is involved in the production of the cellular energy-transfer molecule ATP.
Calcium in the form of Ca2+ ions is required for the healthy growth of new stems as it is used to give cell walls their strength. Sulphur (S) is taken up as sulphate ions (SO4(2-)), and is an essential constituent of all proteins, including enzymes. Legumes have higher requirements for S than most other plants do.

As the name implies, smaller amounts of the seven micronutrients are required but they nonetheless cannot be ignored for healthy plant growth, and are usually present sufficiently in most soils. These are boron (B) as H2BO3- ions, chlorine (Cl) as Cl- ions, copper (Cu) as Cu2+ ions, iron (Fe) as Fe2+ ions, manganese (Mn) in the form of Mn2+ ions, molybdenum (Mo) as molybdate (MoO4(2-)) ions and zinc (Zn) as Zn2+ ions.

Artificial fertilizers are manufactured using fossil fuels and have been responsible for massive increases in the yield of crops achieved in the last century - "The Green Revolution". There are estimates that the yield could fall by about 75% if we stopped using them. Accordingly, it is argued in some quarters that feeding the world's population without modern farming methods and its inputs of energy and fertilizers would require much more land than is available. Others, however, including many aficionados of permaculture dispute this, and argue that if the soil is brought back to its natural state there will be plenty of food for all, albeit not the cereal-based diet we are now used to.

Interestingly, there was a news report (B.B.C. March 5th) to the effect that most of us in the U.K. are deficient in selenium because for the past 30 years we have eaten bread made from European wheat rather than from wheat imported from Canada and the U.S. The problem is the different soil, which this side of the pond is low in selenium but rich in the element in North America and Canada. Apparently selenium levels can be restored to soil by adding selenium-enriched fertilizer, but this is part of the energy intensive process that we are seeking to avoid in preparation for declining oil and gas supplies. On a personal basis, eating a daily handful of Brazil nuts maintains healthy selenium levels but these are grown and imported of course by means of gas and oil, so this is not a long term solution.

If we convert to permaculture and regenerative agriculture in general, we will need to get by without much cereal and provide more of our diet from nuts, fruits and vegetables, and from animals whose grazing helps to till and nourish the land naturally on open-plains. Another good source of selenium is garlic, however, so long as it is not cooked for too long which denatures the compounds that contain it.

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