Soil pH and Plant Nutrients

Doug Edmeades gives out sound advice on pH.

It used to be believed (going back to the early days of soil science) that the ‘ideal’ soil was neutral: neither acid nor alkaline it had a pH of 7.0. This early belief still prevails especially in Charlatanville. However, with the benefit of much subsequent research our view of the ideal soil pH has changed.

First, it is now known that different plants have different tolerance to acidity. Restricting the discussion to pasture species, browntop is very tolerant to acidity which is one reason why it thrives in undeveloped soils. Ryegrasses are more sensitive and like a higher pH. Clovers are more sensitive again and, of the legumes, lucerne is very fussy. Our pastoral agriculture is focussed on growing clover-ryegrass and the optimal pH is 5.8-6.0 – this is the pH at which pasture production and especially clover production is optimised. In contrast, a straight lucerne stand requires a pH of about 6.5.

Liming pastoral soils above pH 6.0 is not recommended for several reasons. First, there is no benefit in terms of production and it can have detrimental effects on both pasture production and animal health. As the soil pH increases the availability of soil molybdenum (Mo) increases and thus the pasture Mo content increases. This can, in some cases, induce copper deficiency in animals. Also, increasing the soil pH above 6.0 reduces the concentration of soil zinc (Zn) and manganese (Mn) concentrations. This can result in induced Zn and Mn deficiency. Liming soils, contrary to popular belief, is not always beneficial!

He also works up a sensible New Zealander's criticism of liming to "fix" the Ca:Mg:K ratio.

Lime is typically calcium carbonate. For us in New Zealand the active ingredient in lime is the carbonate not the calcium (Ca). Our soils fortunately are rich in Ca – indeed often awash with Ca – a result of their origin (from the sea) and youthfulness (not very weathered).

Given that the benefits of liming are related to the change in soil pH then it should be obvious that the only useful guide and hence measurement for the requirement of lime is the soil pH. This is the adopted science-based approach used in New Zealand.

So what about all this base saturation ratio argument? In the 1930s there were two competing theories about plant nutrition. One said that the ratio of the nutrients Ca, Mg, K and Na was important. These were measured as the proportion of the soil cation exchange capacity (CEC) – the ability of a soil to store these nutrients called cations. Thus your hear some say that the Ca saturation of a soil is 50% meaning that 50% of the CEC was occupied by Ca. The other theory was that plants did not care what the ratio of nutrients were – the plant was fine providing the minimum amount of each nutrient was present. This is called the Sufficiency Theory distinguishing it from the Ratio Theory.

After almost 80 years of research the jury is definitely in. The Ratio Theory is not consistent with observations and hence is now set aside. Indeed we now know that using the Ratio Theory as a basis for fertiliser recommendations can be and often is misleading. For example the Base Saturation Ratios of Ca in most New Zealand soils would suggest they are Ca deficient. The fact is they are not and Ca deficiency has never been recorded in New Zealand.

There are other problems with the Ratio Theory. It applies to only 3 nutrients (Ca, Mg and K – appreciating that Na is not required for plant growth (except on some crops such as sugar beet).

What about all the other 13 plant nutrients? Also we now know that soils have variable charge – this realization has occurred within my 30-year career. The consequence is that the CEC depends on the pH at which it is measured. The old method still used by the quack brigade measures the CEC at surprise, surprise the “ideal” soil pH of 7.0. This inflates the CEC thus reducing the base saturation ratios, especially for Ca. By sticking to this now disproved methodology the quacks can be certain that the soil test results will say the Ca base saturation ratio is low therefore apply my product because it contains Ca.

My region's soils are similarly well supplied with calcium. My agricultural consultancy mentors taught me to be skeptical of the Ca:Mg:K approach to evaluating soil nutrient status. In my region it was used to justify expensive formulations of foliar applied applied calcium, or to justify adding expensive soluble calcium to the irrigation water on soils with a good supply of calcium. Normally on high value crops in good years when adding extra nutrients for insurance has legs. Charlatans is not too strong a word. Back in the 1980's these folks would use A&L Laboratories, well established, amny offices, with an excellent professional reputation, and which reported Ca:Mg:K in a ratios friendly format. I'll bet this is still the case. You can't beat something like that for conferring legitimacy, can you?

The originator of the ratios approach, soil scientist William Albrecht was a brilliant observer of nature with a considerable body of work which still gets a lot of play. The basic premise of Albrecht's 1938 Loss of Soil Organic Matter and Its Restoration is solid: it takes a ready supply of soil calcium and nitrogen to build soil organic matter. His concepts continue to be stretched beyond to the breaking point both by well meaning folks exchanging advice on organic farming methods, as well as in efforts to sell product to the unsuspecting. Yet we don't read much in the way of criticism of the ratios approach. It is excellent of Doug Edmeades to voice his concern.