by Hugh Lovel
In the uptake of nutrients from the soil foodweb, sulphur is the catalyst for nutrient release. Ever at work at the surfaces of things, sulphur, as sulphate, infiltrates the interstices between the soil’s colloidal particles and exposes their surfaces. In short, sulphur is the ‘open sesame’ to the soil’s mineral storehouse.
Silicon follows, and forms a partnership with the sulphur containing amino acids, methionine, cysteine and cystine in the structure of cell walls and connective tissues. In fibrous tissues, particularly in plant stems, this includes the linings of capillary vessels, as these do double duty as connective tissues, as in the stems of fruits.
Boron, as borate, embeds itself in these silica linings and establishes an unresolved electron bond in the surrounding silica network that draws water and nutrients into the plant. As a result calcium, magnesium, potassium and amino acids are taken up from the soil and delivered to cell division and chlorophyll rich sites in growing tips and leaves. As chlorophyll captures light waves, phosphorous transfers this energy into sugar production—after which a mix of sugars and carbohydrates follow potassium through the silica pathways to store or provide energy wherever required in the plant.
This also means the role of silica, allied with boron, has enormous importance for photosynthesis.. The rapid transportation system of C4 grasses makes them the most photo-efficient plants—as they can really move things. Through their silica transport system they transfer the energy from chlorophyll to sugar faster than other plants. And abundant photosynthesis depends on how fast the reactions occur. The previous energy catching event must move out of the way for the next one to occur.
It might seem this would happen at the speed of light. In the leaf, however, the magnesium/chlorophyll complex that catches light is stationary. It has to send the energy it captures via phosphorous to where sugar is made, and the speed it transports this energy boosted phosphorous chemistry determines the rate of photosynthesis. C4 grasses are also most efficient at moving carbon dioxide and water into the process while speedily getting sugar out of the way so there are no bottlenecks. This is why with grasses like sugar cane, maize or sorghum, brix readings may need to be taken from the bases of leaves or stems rather than from leaf panels, as these plants rapidly move sugars away from where they are made.
In looking at this picture, we want to be aware that the sulphur containing amino acids associated with silicon work very differently from nitrates, which cannot be excluded from plant water uptake. High levels of nitrates upset this process. While there will always be some nitrate uptake from the oxidation of amino acids within the soil, excess nitrate is closely associated with low brix. Brix is a measure of dissolved solids; and carbohydrates normally account for roughly nine tenths of the dissolved solids in plant sap.
If the plant can’t exclude nitrates, it must convert them to amino acids or they are useless—even toxic. If anything, nitrate’s affinity for water dilutes plant sap, protoplasm and chlorophyll, impairing photosynthesis and compromising plant vitality.
The conversion of nitrate to amino acid takes time and nearly the same energy as it took to fix nitrogen biologically. This means if plants use up their sugars plant converting nitrate to amino acids, this limits what is left over for root exudation and microbial nitrogen fixation around plant roots.
If nitrate uptake is too abundant or the plant’s conversion is too slow, its protoplasm stays watered down and nitrate reduction lags. Excess nitrate may even scald the plant’s silica transport system resulting in a low brix plant that is difficult to boost.
Winter Builds Complexity Into Soils.
As winter begins, photosynthesis slows down and nitrogen fixation becomes more problematical, while what grew in summer digests back into the soil. This process builds complexity. We can contribute or impede this, and sometimes we do—perhaps unwittingly, or perhaps deliberately. We have to learn what’s best. What emerges is it is particularly important that decomposing proteins are incorporated into humic acids and built into soil organic matter for future reference—along with sulphates, phosphates, borates and various major and minor nutrients.
Come spring, this complexity will start to break down and release a complex nutrient stream to feed plant growth. If one uses nitrate rich fertilisers—including organic ones—the importance of low nitrates and high amino acids in the root zone will show up in reduced vigour and crop quality.
Brix testing in the middle of crop cycles and trying to rescue processes that were less than optimum during winter, is no substitute for preparing over the previous winter to achieve high brix throughout the crop cycle. For many growers taking the appropriate steps in winter might see a welcome change from crops losing their oomph after the summer solstice when the days start getting shorter and sap doesn’t flow quite as strongly as it should.
This returns to the question of the importance of strong sap flow.
Plants readily take up amino acid nitrogen, unless nitrates get in the way. Plants are healthiest getting most of their nitrogen as amino acids from the interface between their roots and the soil food web, rather than taking up nitrates in their water. But either way they must get nitrogen. The limiting factor in amino acid uptake usually is root exudation, which supplies energy to nitrogen fixing microbes. These microbes require abundant energy to manufacture amino acids. And lest we forget, soil animal life, particularly protozoa, must digest the nitrogen fixing microbes and release amino acids if this process is to support robust growth.
If boron is sufficient and the uptake of water and nutrients from the soil is strong, photosynthesis will be productive, and in turn root exudates will be abundant. Otherwise nitrogen fixation may slow down and stall. One of the worries is that nitrate uptake reacts with boron. Thus it can flush boron out of the capillary linings and reduce sap pressure, nutrient uptake and root exudation.
Advocates of chemical nitrogen say fertilising with artificial nitrogen is efficient because plants don’t have to supply the energy. They reason that if artificial fixation uses ten units of methane to make one unit of ammonia, and still more to convert this into other forms, this is carbon energy the plant does not have to supply. However, when artificial nitrogen is applied as urea, half volatilizes as N2O gas while the remainder oxidizes to nitrate. Moreover, plants use up nearly as much energy converting nitrate to amino acid as was required to fix nitrogen as amino acid in the first place, so where is the savings?
The clincher is that nitrate suppresses nitrogen fixation—nitrogen fixing microbes drown in their own waste, as nitrate is the final waste product of their activity. This means that artificial nitrogen fertilisation—even if from organic sources—shuts down biological fixation. Then plants must depend on applied nitrates rather than on feeding nitrogen fixation and receiving amino acid uptake.
Ironically, the methane required for artificial nitrogen fixation is a non-renewable resource. We don’t want to become dependent on its use to artificially produce nitrogen because whenever it becomes scarce we’ll be in a fix.
In a low nitrate soil, microbes living around plant roots depend on strong sap flow, rich in amino acids and low in nitrates, into the plant by day. In return the plant gives off energy rich root exudates by night. With plenty of energy to fix nitrogen, the nitrogen fixing microbes and the protozoa which digest them in the soil foodweb will provide ample amino acids in each new day’s sap uptake. Strong sap uptake assures rich photosynthesis which assures more energy given off as root exudates. Then there is increased nitrogen fixation and protozoal digestion the following evening. This feeds richer amino acid uptake, stronger photosynthesis, more root exudation and so forth.
The less plants take up nitrate and the more they take up amino acids, the more efficiently they photosynthesise and share their life energy with their microbial symbiotes in the soil; then the more complex and vigorous they tend to be. In the final analysis, it not only matters how we build life processes into our soil, but also whether we impart these in an appropriate, balanced way. There are up and down processes. What goes on above adds energy and complexity to the growth and foliar processes which supply root exudates. Then if what goes on in the soil goes up, then what goes on in the leaf goes back down as root exudates.
Nitrogen comes into these processes in the soil, while carbon enters via the leaf. So we must see to the activities of sulphur, boron and silicon that open up the soil and provide transport for calcium, amino acids, magnesium, phosphorus, etc. so they arrive in the leaf and the processes of turning water and carbon dioxide into sugar take place.
The dynamic is that a certain amount of sugar is required to provide the energy for initiating microbial release of sulphur, boron and silicon for plants to deliver nutrients to the leaves. This is why crop seeds have large, carbohydrate rich cotyledons while weeds have tiny seeds with next to no carbohydrates. Conversely the amino acids and minerals delivered from the soils are required for the leaves to capture energy and make carbohydrates in the leaves. This delivers carbohydrates to the soil’s microbes as root exudates and feeds more and more nitrogen fixation. The dynamic interplay between what goes on below ground and what goes on above depends on boosting each activity at the right times, morning and evening—as if we were pumping our farms or garden up on a swing set. Timing and balance are key, and that means there’s no substitute for doing the right thing at the right time. We need rhythm and feeling as well as a modicum of substance.
It becomes clearer and clearer that we cannot meet all our challenges in agriculture without understanding both processes and substances. Substances play their parts, but we need an understanding of life process as well. These differ with the seasons, the phases of the moon and various other factors. Without doing the right things at the right times we will never turn our farms around to improving instead of running down. Ultimately what this means is operating our farms or gardens as unique organisms within their own boundaries and contexts. These can be the property boundaries and natural cycles, within which energy and complexity builds up out of the surroundings; but, without boundaries and closure of cycles, life forces leak away.