BY: Dr Doug Edmeades

I see a number of advertisements for fertilisers, soil conditioners, biostimulants and the like that make a virtue of claiming they contain many, many elements – up to 50 in one case!  This fact alone should alert the unwary. However, I do get asked a more serious but related question: how many nutrients are required for pasture plants to achieve optimal production. The answer is 16. Here is why.

The 16 essential nutrients (see table) are normally divided into major nutrients (i.e. present in “major” amounts) and the micronutrients (required in lesser amounts and hence sometimes referred to as minor elements). In this sense, major and minor do not mean more or less importance – all are required at the same time.

These are the only elements required for plant growth, and if you try to grow a plant without one of these nutrients it will not grow to its potential if at all. So what about all the other elements in nature (120 of them at last count on the Periodic Table)? It turns out that if a plant has all of the 16 elements listed above, then adding one of these other 104 elements makes no difference – they do not further enhance plant growth and some are toxic.

Now for some qualifications: There are some agriculturally important plants (e.g. sugar beet, fodder beet and mangolds) that do require sodium (Na, as in common table salt). And there are others (e.g. sugar cane) that appear (there is still scientific doubt about this) to benefit from silicon (Si) applications (as in, say, serpentine, a magnesium silicate).

Also, the bacteria on the roots of clover – the ones that convert atmospheric N into clover protein – need tiny amounts of cobalt (Co), but Co is not included above because it is required by the bug and not the host plant. Apart from these exceptions plants, and particularly pasture plants, need only 16 nutrients.

So why do we add Co and Se in our fertilisers if they are not required for plant growth? The reason is simple. These two trace elements are required by animals and the simplest most cost-effective way of doing this is to add it to the soil and let the plant and animal do these rest.

Okay then, why do we not add Mn, Fe, Zn, B and Cl to our soils? The reason is that at the normal soil pH levels we deal with in New Zealand (i.e. 5.0 to 6.5), there is enough of these nutrients already in the soil for most situations. Indeed, the only known case of Mn deficiency in NZ occurred in wheat on an over-limed soil (pH < 6.5). Boron is required, however, on brassica crops and on white clover crops grown for seed production, but no cases of B deficiency in pastures per se have been reported.

So the list is down from 16 to 11. Of these, carbon, oxygen and hydrogen are obtained by the plant from the soil as water (H2O) and from the atmosphere as carbon dioxide (CO2). Via the process of photosynthesis they are made into carbohydrates (the stuff that gives the plant its structure). So, provided that there is sunshine (energy), water (normally in the soil) and carbon dioxide (in the atmosphere), the problems of C, O and H supply are solved.

We are now down to eight: N, P, K, S, Ca, Mg, Mo and Cu.

Calcium we can eliminate immediately: thanks to our young soils (i.e. not strongly weathered), and the fact that we use lots of super (20% Ca) and lime (35% Ca), Ca deficiency is unheard of in NZ. Note:

  1. a) the active ingredient in lime is the carbonate, not the Ca
  2. b) hypocalcaemia in animals is not due to low soil and plant Ca levels – it is a hormonal disruption within the animal that prevents it from releasing bone Ca in early lactation.

Down to seven.

Most NZ soils currently have good reserves of Mg – at least those sedimentary soils derived from the sea. Mg deficiency for pasture growth first appeared on the coarser pumice soils that had little or no reserves of Mg and hence Mg had to be applied via the fertiliser. However, Mg deficiency is becoming more widespread for the simple reason that after farming for 50-100 years we are mining down the originally adequate soil reserves. Within another generation it is likely that most farmers on volcanic soils will also need to add fertiliser Mg.

A similar logic applies to Mo and Cu. When pastoral farming started in NZ you could tell by the soil group whether one or other of these two trace elements was required. Peat soils were deficient in Cu and most sedimentary soils needed Mo. The same pattern does not necessarily apply today. Certainly, these two soil groups require ongoing inputs of Mo and Cu but we have exhausted the original supply of these trace elements in other soils – for example, I have found three cases of Mo deficiency in the Waikato! The only way to be sure about whether these trace elements are required on your farm is with regular pasture testing (clover-only samples for Mo and Cu and mixed-pasture samples for Cu intake into the animal).

Down to the big four: N, P K and S. Most NZ pastoral soils in their virgin state were deficient in N, P and S and most soils used for dairying needed K. Maintenance inputs will continue to be required on these soils. This is the reason why so much discussion about fertiliser revolves around these big four nutrients, and incidentally this quirk in nature is the reason that scientists like me get labeled – wrongly – as NPKS addicts!

There are two sources of N in our clover-based system. Clovers should be nodulated. These nodules contain bacteria that convert atmospheric N into clover plant protein, which is subsequently returned to the soil as dung, urine and plant residues. This process adds about $1.3 billion worth of N to our pastoral soils but we top this up – especially dairy farmers – by adding extra fertiliser N.

We typically think of the clover N – the $1.3b of N from the atmosphere – as free but it does in fact come at a cost. Clovers have higher requirements for P, K and S (and indeed all the other 13 nutrients) than grasses. So while we do not need to add fertiliser N to the clover-based pastures we must add sufficient P, S and K to maximise clover growth.

Several last points.

All 16 nutrients are required for plant growth but the rate of growth of the plant – assuming adequate sunshine and water – will depend on the rate of supply of the most limiting nutrient. This is Leibig’s (he was father of plant nutrition) famous “Law of the Minimum”. No point in adding, say, more P and S to a soil that is K deficient or Mo deficient or whatever. This is the meaning of “balanced nutrition”.

Table: The 16 essential plant nutrients and their chemical symbols 

Major nutrients                                     Minor nutrients

Carbon (C)                                                    Copper (Cu)

Hydrogen (H)                                               Zinc (Zn)

Oxygen (O)                                                    Boron (B)

Nitrogen (N)                                                 Manganese (Mn)

Phosphorus (P)                                             Iron (Fe)

Sulphur (S)                                                    Molybdenum (Mo)

Potassium (K)                                                Chlorine (Cl)

Calcium (Ca)

Magnesium (Mg)

  • Dr Edmeades is an independent soil scientist and consultant.