The Mikhail System of Sustainable Soil Management

Transcription

1 The Mikhail System of Sustainable Soil Management E.H. Mikhail 1 1 SWEP Analytical Laboratories, 45-47/174 Bridge Rd, Keysborough 3173 Introduction During the 1960s research began (at first in Egypt and then in Australia) into ways of improving certain problem soils. This research found that soils with a high proportion of Magnesium and relatively low Calcium had a tendency to set hard when dry. It resulted in the first definition of the Calcium-Magnesium ratio in terms of soil physics rather than plant nutrition and led on to the development of methods for effectively correcting problems. This balance approach was also extended to work on Sodic soils and this led to a more complete understanding of the relationship between soil chemistry and soil physics. The Mikhail System Perhaps the most important thing to come from the Australian research is a recognition that the concept of Balance goes beyond cation proportions alone. Getting the cations right is an important first step, but we need to look at all three aspects of soil balance chemical, physical and biological. This is what underlies the view of Soil as a Living System, which can also be explained by using the Human Body as an analogy. To have a healthy human, you must first begin with a strong skeleton. To have strong bones you need Calcium, Magnesium and Phosphorus in the right proportions. So too in the soil the proportions of Calcium, Magnesium, Sodium, Potassium and Hydrogen are important for the basic structure or skeleton of the soil. But, a human being is more than just a collection of bones. The skeleton supports the body and the development of strong muscles. This process requires Carbohydrate, Protein and Fat in the right proportions, throughout life. Likewise, in the soil, productivity is built on having Nitrogen, Phosphorus, Potassium and Sulphur available to plants in the right proportion throughout their growing season. Of course, human health relies on more than just carbohydrate, protein and fat; we also have requirements for various vitamins and minerals in the proper amounts. In similar fashion, productive soils must provide the complete range of Trace elements in suitable amounts. Still, we all realise that healthy people need even more. We need the right balance of good bacteria on our skins and in our digestive systems to help us take up the nutrients we require and help protect against infection. Soil is no different and the biological component of the soil also needs to be balanced. And just as the requirements for humans will vary somewhat for each individual and at every stage throughout life, so too, each soil will have its own individual needs and characteristics. However, in researching the Ca:Mg ratio it became clear that this alone was not enough. The impact of this ratio depended on both the relative amounts of Calcium and Magnesium and their percentages of the Cation Exchange Capacity (CEC). For example, a clay loam with a Ca:Mg ratio of 1.5 could well be hard-setting when dry, while a Medium Clay with the same ratio could have poor structure but not be hard-setting at all. Results like were puzzling, but not uncommon.

2 The problem lay in defining what made up the CEC. If there were only the four base cations (Ca, Mg, Na, & K), you would expect these two soils to show little or no difference in their levels of friability, and yet clay loams and medium clays are clearly very different soils. Obviously, there must be something else contributing to the CEC. It was clear that there should also be other cations present and basic chemistry suggested that exchangeable Hydrogen would be the most important. Unfortunately, only estimates of exchangeable Hydrogen could be made until 1968, when a reliable laboratory test was developed and this was used to show that a very accurate figure for the CEC in any soil could be provided by adding together the levels of five exchangeable cations (in milliequivalents/100g of soil) Calcium, Magnesium, Sodium, Potassium and of course Hydrogen. It was also shown that, even in highly acidic soils, all other cations were present in amounts too small to significantly contribute to the value of the CEC or substantially alter the physical character of the soil. Here then was the answer to the riddle. A Clay loam could have a CEC of about 15me/100g, while Medium clay could be perhaps 30me/100g. If we assume the two soils have the same amounts of Calcium and Magnesium (say 6me/100g Ca & 4me/100g Mg, giving a Ca:Mg ratio of 1.5), then the difference between the two soils lies in the cation percentages the clay loam would be 40% Ca and 26% Mg, while these percentages for the medium clay would be 20% and 13% respectively. In short, the medium clay would have levels of Ca and Mg that are too low to significantly impact the physical character of the soil other than failing to support development of good structure. In general, the significance of the Ca:Mg ratio as an indicator of the physical condition of soil will decline as the Calcium and Magnesium percentages are reduced unless Magnesium is 20% or more of the Adjusted CEC (see below). 7,000,000 Desirable Total Active Population 6,000,000 Total Active Population (cells/g) 5,000,000 4,000,000 3,000,000 2,000,000 1,000, Adjusted CEC Above: A schematic representation of the relationship between the Total Active Population of soil micro-organisms and the Adjusted CEC when sorted according to the Complete Soil Balance Percentage. This shows how well balanced soils have active microbial populations that conform closely to the predicted desirable level. < 40% balanced 40-50% balanced 50-60% balanced 60-70% balanced > 70% balanced

3 Having reached this point, yet another discovery was made. It was found that for soils with a high organic matter percentage, the balance proportions were still not right. Of course, organic matter is high in exchangeable Hydrogen, and it was found that as the organic matter levels increased, the calculated Lime and Dolomite requirements to properly balance all the Hydrogen became enormous (sometimes more than 100 tonnes per hectare). Clearly, this could not be correct. After more research it was found that organic matter requires a certain amount of exchangeable hydrogen as an intrinsic part of its composition. This needs be left out of the balance relationship for recommendations to be equally reliable on all soils. Consequently, an adjustment factor for this proportion was determined (relative to the Organic Matter percentage), which resulted in the Adjusted Hydrogen that is now used to calculate the Adjusted CEC on SWEP soil tests. It is from this figure (rather than the total CEC) that the cation percentages in the Mikhail System are calculated. Of course, soil balance requires more than just cations. It became clear that it was important to have BOTH good cation balance and levels of available nutrients sufficient to provide for Balanced Plant Nutrition. This is in contrast to the usual system of using the soil as a nutrient sponge filling it up to Luxury levels with a few major elements, squeezing it dry and filling it up again. Instead, a system was developed that adjusts the levels of all essential nutrients to a point where they are sufficient to provide the needs of a specific Land Use through the period of its growing season. This approach (when combined with proper cation balancing) has proved successful in maintaining high levels of productivity without the need to maintain excessive levels of soil fertility. Recent research has further strengthened this system by revealing that properly balanced soils show a consistent and predictable balance in the proportions of certain key indicator groups of micro-organisms, with the total active population of these indicator organisms (in well-balanced soils) being proportional to the Adjusted CEC. The graph opposite shows the distribution of microbial populations from the research database (in relation to the Adjusted CEC), when sorted according to soil balance percentage. It shows clearly that the microbial population will be closest to the desirable level only in the most well balanced soils. Again, we can compare this to the situation with other living organisms including people. Healthy people do not live in sterile bubbles. We all need the right balance of good bacteria in our digestive systems and on our skin to make the most of available nourishment and help ward off infection. I have always believed that the same would be true in soil and now our research has proved it. Soil has Needs of its own In terms how we manage sustainable production, the Mikhail System focuses on cation balance as the first important step. This is an important foundation for all that follows and many problems encountered in using the Mikhail System can be seen to stem from shortcuts taken here. Cation Balance optimises soil structure, ensures the greatest possible availability of all essential plant nutrients and provides an environment suitable for the maintenance of a stable biological community. This is why it is important not to talk about cation balancing in nutrient terms the proper balance of cations in dependent upon characteristics of the particular soil in question not what is being grown in it. In other words, optimum productivity requires first meeting the needs of the soil itself.

4 Structure and other physical characteristics of soil depend upon the relative proportions of five Exchangeable Cations Calcium, Magnesium, Sodium, Potassium and Hydrogen as shown in the Table below. However, many people are aware that these are not the only positively charged elements in the soils and so wonder why they are the only ones the Mikhail System considers. An example is Aluminium. The soil chemistry of this element is different from other Cations, in that it is mostly locked up within the chemical lattice of soil minerals and is only converted to a plant available form by acid conditions resulting in turn from an inappropriate cation balance. Also, even in very acid soils, Aluminium makes only a very small contribution to the soil Exchange System being mostly present in the soil solution (where it can be toxic to plants) rather than on colloid 1 surfaces. This means that including it will not significantly improve the calculation of CEC in any soil. However, even if all this were not true, Aluminium (like all so-called Other Cations ) usually has no great impact on the Physical Character of the soil. Given that Soil Physics is what this aspect of the Mikhail Balance System is all about, there is simply no point including it. Exchangeable cation Calcium Magnesium Sodium Potassium Hydrogen Ideal proportion of Adjusted CEC 65-70% 12-15% < 5% 3-5% 10% or less The same is not true for Exchangeable Hydrogen. Unlike Aluminium, which becomes available to plants under acidic conditions, Hydrogen is actually the cause of those acidic conditions. And its contribution as a component of the CEC is also very different. This is because the CEC is measured in milliequivalents not parts per million. This measurement is used because it reflects the amounts of each cation needed in a reaction (in this case adsorption onto an exchange surface). The Table below illustrates the difference between ppm and milliequivalents by comparing Aluminium and Hydrogen at the same concentration. Note that a soil with this amount of extractable Aluminium may well produce toxic effects on plants in any soil, but the same amount of exchangeable Hydrogen, while high, would need to be judged relative to the CEC of a particular soil. Cation: Hydrogen Aluminium 130 ppm = 13.0 me/100g 1.0 me/100g Not all that is Exchangeable requires Balance As discussed previously, one of the most important Jellybeans is Hydrogen. Ted has been measuring exchangeable Hydrogen since the test method first became available in 1968, but along the way he also made an important discovery: Soil Organic Matter is naturally high in exchangeable Hydrogen, but this cannot be completely balanced out. To cope with this, the Mikhail System adjusts the amount of exchangeable Hydrogen included in the soil balance relationship by 0.5 milliequivalents per 1% Organic Matter. Thus, both the Adjusted Hydrogen and the Adjusted CEC depend on the Organic Matter percentage. This has proved to be one of the most significant factors responsible for the overwhelming reliability of the Mikhail System when applied to soils around the World. 1 A Colloid is defined as any particle small enough (< 2 microns) to remain suspended in water. In soil the main colloids are clay and humus particles.

5 Earlier soil balancing systems are often based on research done before a reliable test for exchangeable Hydrogen was available and so used estimates of Hydrogen & other cations that often produces a misleading result. Also, they do not take soil organic matter into account and so tend to over-balance the soil.

6 The consequences of Poor Cation Balance Many so-called Problem soils can be shown to be suffering from a Cation Balance problem and (more importantly) will be responsive to appropriate corrective measures. Problems of this kind include: Hard setting soils Soils forming a surface crust after wetting or with poor moisture infiltration Soils that become either soft and sloppy, or sticky and porridge-like when wet Soils that are especially prone to compaction Soils that are easily damaged by cultivation Often people try in vein either to bash these soils into shape with heavy equipment, or apply treatments that seem to work, but overuse them to the point of creating a whole set of new problems. The bottom line is that if it cannot be measured, then it cannot be managed. But with the right information, fixing the problem is as simple as getting the right coloured Jellybeans back in the jar! Common Misunderstandings Sometimes problems can arise with apparently simple things like the Ca:Mg ratio and Exchangeable Sodium Percentage (ESP). The desirable levels for the Ca:Mg ratio (that are now widely accepted throughout Agriculture in Australia) were one of the first outcomes of Ted s research in the early 1960s, yet many people still do not appreciate the significance of this simple measure for soil physics (instead, they tend to think of it in terms of plant nutrition). Remember that it is important to think of the Ca:Mg ratio in combination with the Cation percentages. As mentioned earlier, the significance of the Ca:Mg ratio as an indicator of the physical condition of soil will decline as the Calcium percentage is reduced unless Magnesium is 20% or more of the Adjusted CEC. The Exchangeable Sodium Percentage (ESP) also causes trouble. This is because its impact on soil (called Sodicity ), or the tendency for soils to be sloppy and dispersive when wet (resulting in reduced hydraulic conductivity and root penetration), relies on the percentages of both Sodium and Magnesium. Thus, in the USA, soils are not considered to be Sodic until the ESP reaches 15%, while in Australia the appropriate level is 5%. This reflects a difference in thinking about soil function. That is, the American standard is based on very old research that considered Sodium only, while the Australian standard is based on more recent work with both Sodium and Magnesium. However, since it is most often the case that when Sodium is high, Magnesium is also, people have tended to speak about Sodic soils in terms of Sodium only. Things become even more confusing, because the Sodium percentage is often calculated from the sum of only four cations Ca + Mg + Na + K, whereas the Sodium percentage used in the Mikhail System is the percentage of the Adjusted CEC that is: Ca + Mg + Na + K + Adjusted H! Again, because people have misunderstood the importance of exchangeable Hydrogen, they have tended to simply leave it out.