The science behind white clover decline

17:43, Dec 03 2013
A wild bee pollinates white clover.
FARM FOOD: A wild bee pollinates white clover.

I'm hearing a cacophony of denial out there in farm-land. I am not talking about the local sports teams or politicians. I am referring to my pet hobby-horse - white clover.

We give ourselves so many reasons to justify why white clover no longer thrives on our farms like it did back in Dad's day - it must be the dreary droughts, or the fickle flea, the evil weevil, miss's management or mister drug, fertiliser N. The list goes on.

I have no doubt that these events, practices and insects have some effect - sometimes all of them - but I'm not willing to concede that we should take an early shower, pack the kit and retire to our clover-less farms.

These are just excuses, and I say excuses because the science behind growing clover is mature, thanks to our scientific forebears like the late great Professor TW Walker.

Clover like most plants needs 16 nutrients. Thankfully, for us, most of these are abundant in our soils.

Some are ubiquitous like hydrogen, oxygen and carbon. Others are abundant in our soils - for example, calcium, iron, manganese zinc.


The ones typically lacking in our soils are: nitrogen, phosphorus, potassium, sulphur and on some soils magnesium and the trace element molybdenum.

There are some qualifications required. Selenium and cobalt are added to fertilisers in specific cases, not because the plant needs them (they are not one of the 16 essential nutrients) but because animals require them.

A further important clarification is to do with nitrogen (N). All soils and this is universal are N deficient. Thus fertiliser N is essential for all non-leguminous crops (eg cereals, maize, brassicas).

That is why it is the dominant nutrient traded and sold around the world. It is estimated that 48 per cent of the world's population can and are being fed as a consequence of fertiliser N.

This simple fact, of course, is hardly likely to dampen the enthusiasm of the fertophobic green fringe.

Our pastoral system in New Zealand is different because we have that mighty forage legume white clover.

It is a great food for ruminants and adds "free" nitrogen to the system. An ideal clover-based pasture containing 30-40 per cent clover should add about 150 to 200kg N/ha/yr.

I say "free" in inverted commas because there is a cost. White clover, this otherwise marvellous plant, has a serious flaw.

It was born with a weak root system - its roots are shallow and are non-fibrous, quite unlike grasses. Compared with grasses, it struggles to compete for water and nutrients and hence it needs higher soil nutrient levels than grasses, except of course for N. The clover plant is biophysically retarded. Like a shy but otherwise prodigious child, it needs tender loving care.

That is why it is the first component of the pasture to struggle and ultimately disappear if one of those 16 essential soil nutrients is not present. The exception is N - because it fixes its own N it will easily out-compete grasses when the soil N levels are low, like after cropping, providing it is fed plenty of phosphorous (P), potassium (K), sulphur (S) etc.

And clover, like all plants, is subject to Leibig's famous law: A plant can only grow as fast as the most limiting nutrient. Putting more and more super (P and S) on a soil that is K deficient for example is a waste of money. You would be surprised how frequently this occurs.

We ignore these facts at our peril and that is of course exactly what we are doing these days in pasture-land. Clover-based pastoral farming in New Zealand is, in my view, in a perilous state.

A sad anecdote will illustrate the point. I was asked to audit the fertiliser programme on a large dairy operation. I inspected the pasture, collected clover samples for analysis and summarised their very good soil test records.

It was clear that despite above-optimal levels of all other nutrients, the farm was severely deficient in K. I reported this fact to the owners and indicated what was required to rectify the problem together with information on the likely production and economic benefits.

In turn they sought an assessment of my recommendation from their fertiliser co-operative. Amazingly, they were told that it was not economic to correct K deficiency. Imagine going to the doctor and being told after suitable testing that you had Type 1 diabetes but it was not economic to give you insulin. If you are a pastoral farm, then correcting any nutrient limitation is always economic.

The outcome of the inevitable squabble was a field trial to determine whether the pasture on this soil (Soil Quick Test Level 4) would respond to increasing rate of fertiliser K.

Surprise, surprise. Eliminating the K deficiency increased pasture production over three years by about 35 per cent. This result was entirely predictable from the known science.

That is the true value of mature science - we can use it to accurately predict future outcomes.

So, thanks to Prof Walker and all the other scientists who contributed to accumulating all of this science know-how. As Isaac Newton, that irascibly brilliant mind, once quipped referring to his scientific predecessors; "I can see further because I am standing on the shoulders of giants."

Is it more giants we need or just more people prepared to climb on science's shoulders?

* Dr Doug Edmeades is managing director of agKnowledge and was Federated Farmers' Agriculture Personality of the Year in 2012.

The Timaru Herald