The entire film may have been a prescient metaphor for the looming soil health crisis. If we build soil conditions that perfectly suit soil microbes, they will indeed come.
The question is: what comprises the field of dreams for soil microbes? It is here that we can easily be confused or misled by what we hear and read.
Daily, it seems, my inboxes are inundated with litter-ature about tools and strategies to improve soil structure — composts, cover crops, microbial amendments, lime or gypsum, and even a bunch of ‘new’ mechanical interventions.
Each may genuinely offer some benefit, or even a nice benefit for a while, however, inevitably leads to disappointment. This is simply because the underlying cause of the soil structure problem has not first been addressed.
Soil bugs and plant roots are much like us in the sense that they too wish for a cosy home and some staples in the cupboard. Our job as farmers is to provide them.
This is the nitty gritty of soil restoration — the not-so-sexy stuff — as an ideal environment must be prepared for its inhabitants to thrive.
The ideal environment requires roughly equal volumes of soil solids (mineral and organic particles) and pore space. About half the pore space should contain water and the other half air as in the diagram. About 10 per cent of the solid fraction (five per cent of total) should be organic matter, with 10 per cent of the organic bit humus.
While much is made of humus (and rightly so), the most important bit is the balance between air and water in soil pore-spaces.
Pore size is critical to soil functionality as it controls the surface area at which the air and water interface which determines the rate of oxygen dissolution into soil water — essential for delivering oxygen while protecting sensitive organisms from toxic O2.
Soil pore size is, in turn, controlled by the ratios of the two dominant exchangeable minerals in the soil — calcium (Ca) and magnesium (Mg).
Thus, it is with the balance of these to minerals that the secret to soil structure solutions is held, and maintaining this balance is particularly challenging on dairy farms.
Uniquely, dairy farms are constantly pumping a whole heap of calcium out the gate.
Have you worked out how much? Milk contains 400mg of calcium per litre, and cows 25-30kg. Does this routinely get replaced — year after year?
By contrast, milk contains only 50mg/litre, and the cow herself 2-3kg, of magnesium.
Since the ratio of these two minerals control the way our soils behave we are, through milk and growing and culling cattle, mining Ca relative to Mg and in so doing, constantly compromising the structural chemistry of the farm’s soil.
Pasture pugging, low milk fat, low conception rates, high bloat risk are a few more clues that the soil’s structural chemical balance is not right.
The perfect ratio for exchangeable Ca and Mg is 68:12, with a little latitude given for soils with more clay (less Mg:more Ca) or more sand (more Mg:less Ca).
This makes sense — exchangeable Mg pulls soil colloids together reducing pore size while Ca draws these same colloids apart expanding pore space.
Thus, the importance of regularly correcting this ratio cannot be underestimated as it ultimately determines how water, oxygen, humus, and ultimately, the entire soil ecosystem behaves.
Dr Les Sandles is a renowned thought leader and provocateur in the dairy industry. Best known for his role in revolutionising nutritional and pasture management practices, Les has turned his attention to the ‘last frontier’ — transmogrifying the forage production system into a C-munching machine. Contact him at: info@4sight.bioif
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