Boron, Overgrazing and Soil Building




Boron, Overgrazing and Soil Building

Hugh Lovel

All too often I find paddocks which once yielded excellent growth now have declined to the point the grasses, clovers and other plants look like bonsai miniatures. I just did a soil test for someone who had enough of every other nutrient except boron, which was severely deficient. Growth was poor, despite an abundance of nutrition in a soil that should have been a model of fertility. Why should boron be missing so often? How does something like this happen?


Time and time again I find the most limiting nutrient in pastures is boron. Boron provides sap pressure, and when it comes to trucking calcium, amino acids and all the other nutrients down the good old silicon highway one doesn’t get far without sap pressure. If calcium and amino acids don’t reach key cell division sites, growth is stunted and dwarfed.

All too often I find paddocks which once yielded excellent growth now have declined to the point the grasses, clovers and other plants look like bonsai miniatures. I just did a soil test for someone who had enough of every other nutrient except boron, which was severely deficient. Growth was poor, despite an abundance of nutrition in a soil that should have been a model of fertility. Why should boron be missing so often? How does something like this happen?

First, nitrate is the enemy of boron, as there is no stronger affinity in the periodic table than nitrogen and boron. Nitrate is famous for leaching under wet conditions, and it only takes a little nitrate to strip the boron out of the topsoil. Artificial nitrogen fertiliser is the usual suspect if boron is missing, and usually such fertilisers as ammonia or urea oxidize toward the nitrate unless they are converted by soil microbes to amino acids and proteins. On their way approximately half of such nitrogen fertiliser evaporates into the atmosphere as nitrous oxide, which is no laughing matter since nitrous oxide is an extremely potent greenhouse gas.

20040323_fieldOvergrazing This makes a good argument for applying low rates of nitrogen along with high carbon microbial foods, such as liquid humic or fulvic acids, so the nitrogen will be held by soil microbes and released in a gentle, steady way instead of evaporating as nitrous oxide or leaching as nitrate. Since one of the more troublesome side effects of artificial nitrogen ferts is they burn up the soil carbon which otherwise might hold onto boron, it makes sense for all nitrogen applications to include carbon food sources for soil microbes which can utilize the nitrogen and provide steady slow release. However, if nitrogen fertilisers are applied without microbial foods such as humic or fulvic acids, they tend to simply oxidize, and as far as boron is concerned leaching is assured in the next wet cycle. Thus the practice of putting urea and super on pastures is famous for shutting down microbial activity and allowing boron to leach. The reason this is not the ‘common’ knowledge taught in university classrooms is university science has become so expensive that funding from governments and non-aligned philanthropic foundations no longer is sufficient, and science must sell itself to commercial interests that can grant a few million dollars here and there for favourable research into products such as chemical fertilisers. Of course, unless the results of such funding favor sales of the products in question the money dries up. If it were ‘common’ knowledge that artificial nitrogen inputs could be cut by more than half if these ferts were applied along with brown coal extracts such as humic and fulvic acids, a huge shift would occur in the fertiliser industry, but so far no one in that industry has stepped up to bat on this one. 

Why Australia
When it comes to nitrification Australia has another feature that allows boron to leach even without buying in nitrogen. Most of Australia experiences periodic droughts and wet cycles. During a drought soil microbes dry up like tea leaves, and with fencing and set stocking, to say nothing of bank overdrafts, there is a tendency to overgraze during droughts, which not only stunts top growth but causes root die-back. Then when a soaking rain follows, the microbial protoplasm is all released at once, and the amino acids in the resulting broth usually oxidize to nitrates within the next 48 hours. If the pasture was overgrazed, its roots may be too shallow and sparse to soak up these nitrates before leaching occurs, and it only takes a couple of such seasons in succession for boron to become so deficient that growth becomes stunted. When that happens only deep rooted plants are able to bring the boron back up to the topsoil, and most of these, such as capeweed, Patterson’s curse, heliotrope, blackberry and thistles are considered weeds and tend to be herbicided when found in pastures.

Observation Is the Basis of Intelligence
Sometimes I think what is happening should be obvious, but people need to think back and review the sequence of events as well as observing the present to sort out how boron deficiencies and stunted pastures occur. It is not rare to see a pasture that is stunted right up to the fence, and yet outside the fence the roadside—though it may be poorly planted, maintained and fertilised—is not stunted but rather is home for many deep rooted weeds.

‘Lucky’ Clover
Clover, which might otherwise be highly desirable because of its ability to unlock calcium and feed nitrogen fixing microbes, is far more seriously affected by boron loss than grasses, even though both may be stunted in a boron deficient paddock. Because boron is necessary for sap pressure and sap pressure is required to carry calcium, amino acids and other nutrients to where cell division occurs in plants, then clover may either be severely stunted by overgrazing during drought, or it may die out altogether. On the other hand I happened to encounter a cattle and horse operation in North West Victoria where capeweed was doing a brilliant job of bring boron (along with sulphur, calcium and nitrogen) back into the topsoil after a period of drought and severe overgrazing. The cell division was proceeding so well that not only were the clover leaves quite large, there were numerous four and five leaf specimens. Presumably four leaf clovers are considered ‘lucky’ because they are a sign of robust soil fertility and good cell division, which can only occur where boron is sufficient.

Building or Rebuilding Soil
It might be nice if all we needed to do is put a bit of boron back into the topsoils of degraded paddocks. We could boom spray with a kilo of soluble boron per hectare mixed with 10 litres of liquid humate and, perhaps, 5 kg of urea, in a few hundred litres of water and spray down every paddock where boron deficiency is an issue. This wouldn’t overdo anything and could be repeated wherever necessary.

The truth is an ounce of prevention is worth a pound of cure. What should be obvious is that overstocking, set stocking where a pasture is kept eaten down, and especially overgrazing during drought are all situations that can lead to boron deficiency and stunting of regrowth. Can we just destock and lock up these pastures until they recover?  

Actually it is erroneous to assume that livestock are harmful to the soil. Balance is the real key. To restore pasture ecologies plant growth should be maximized, as this catches carbon and stores energy. It also is easy to see that whenever plants grow to their maximum and set seed, they go dormant. This is especially true of grasses, and unless something either eats them or knocks them flat, regrowth is inhibited by the simple fact that sunlight is blocked by dead vegetation. This is especially true for grasses. For regrowth to happen and for plants to spread out and completely cover the soil, sunlight has to be available for photosynthesis at the soil’s surface. Back when the environment was more robust it was common practice to ignite dead vegetation and burn it off, whereupon regrowth would occur. The lush, tender vegetation was a magnet for herbivores such as the kangaroo, and fire was a useful tool for hunter gatherers to create a happy hunting ground. The question is how much soil did this build?

Nature shows that the digestion of vegetative growth is an important part of building soil. Digestion returns to the soil the better part of the organic matter in a nutritive form, which provides habitat and nutrition to a wide range of soil building organisms. Far from wrecking grassland ecologies, livestock are an essential part of building grassland soils, and people should gain a better appreciation of what animals do.

A common false assumption is that trees build the soil. Trees are important because they encourage rain, which is often much needed. Nevertheless the carbon and energy trees catch mostly builds up above the soil rather than in the soil. Thus a fire during a drought can send nearly all of this stored carbon and energy straight back into the atmosphere, and little if any net gain is achieved.

Glen Bunter's pastureOn the other hand, pasture ecologies build root mass that is busy cycling the products of photosynthesis into the soil to feed a teeming, diverse ecology of bacteria, fungi, protozoa and other micro-organisms living around these roots and releasing minerals, fixing nitrogen, solubilizing phosphorous and potassium and digesting a steady stream of nutrients which in turn feeds pasture plants. This means a robust interaction between what goes on above ground and what goes on below is achieved. Because such a quantity of sugary carbon compounds from photosynthesis are fed back to the soil around pasture plant roots, where herbivores graze the soil building processes historically are the most robust on the planet. Understandably this process falters when grazing populations are high and growth is halted by drought. Destocking and protection of ground cover and soil ecologies becomes an imperative, and farmers who do not destock at such times commonly pay the price in boron deficiency.

Alternatively, as is the case for dairies whose stocking rates require stability, forages must be harvested and stockpiled when growth is abundant and in dry times these stockpiles must be fed where the soil fertility is too weak to create a nutrient blanket for good growth when moisture returns.

Somewhere in our national debate about greenhouse gasses, carbon sequestration and global warming—which drives the world’s weather to greater and greater extremes—we must find the wit to realize that farmers have the greatest potential to reverse these trends if they only adopt the right practices.

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