The role of clay minerals in determining the behaviour of soil-applied fertilizers is well recognized. Often farmers complain that they don’t get response to adequate fertilizer doses especially potassium applied on their fields even under best management conditions. Several factors are responsible but the mystery relates to the nature of clay minerals in our soils, which varies from region to region.

All soils contain minerals in sand, silt and clay. Under our soil conditions, sand and silt fractions consist of primary clay minerals like quarts, feldspar and mica while clay fractions dominate the secondary clay minerals in decreasing order of illite, kaolinite, montmorillonite, chlorite and vermiculite. Clay fraction consists of illite, kaolinite and montmorillonite while chlorite and vermiculite are less abundant. Sand and silt fraction largely consists of mica and feldspar, which are actually ores of potassium.

The chemistry of soil clay minerals are of active compounds like heavy metals, pesticides and herbicides residues which may be adsorbed or retained on surface and thus render inactive or if conditions become favourable, these might be degraded or released to soil solution for active uptake by crop roots, therefore making their entry into food chain. Moreover, clay humus complexes also play a greater role in better soil structure development, which indirectly holds fertilizer nutrients efficiently.

Due to its important role, the clay mineralogy must be taken into consideration for fertilizer management. Another important aspect is the adsorptive ability of some clay minerals which decreases loss of crop nutrients especially of fertilizers like urea, DAP and SOP/MOP. At the same time, plant nutrients like potassium, ammonium, calcium, and phosphates are held by the clays in an exchangeable form that can easily become available to crops. Another important implication of soil clay minerals especially illite and vermiculite is their ability to fix certain nutrients over others, thus rendering them unavailable to crop roots. Thus during fertilizer management programmes, one of the most challenging problems is the fixation of ammonium, phosphates and potassium nutrients.

The fertilizer efficiency for these nutrients is very poor in our farming systems i.e., 30-60 per cent, 10-20 per cent and 40-50 per cent for nitrogenous, phosphatic and potassic fertilizers, respectively. Kaolinite and chlorite clay minerals: These minerals don’t contain potassium and have low nutrient retention capacity; their importance as sources of potassium nutrient for plants, and their interaction in soil solution with other nutrients like ammonium, calcium etc., are negligible.

However, chlorite clays can release low amounts of magnesium, and copper nutrients as well as some heavy metals like cobalt and nickel. In Pakistan’s semi-arid environment and poor drainage conditions, these minerals can be weathered to saponitic smectites; in this case, the clay-fertilizer nutrients relationships will be completely changed.

In case of phosphatic fertilizers like DAP, MAP and SSP etc., the interactions of these clay minerals with phosphate nutrient are more important: Aluminium and iron present as structural components of these minerals are easily accessible to the phosphatic ions in soil solution especially in paddy fields. After application of phosphatic fertilizers to soils inherently rich in these two clay minerals, all the absorbed phosphate is exchangeable (easily available to crop roots).

During subsequent addition of muriate of potash (MOP fertilizer), a portion of the adsorbed phosphate nutrients becomes non-exchangeable, or ‘fixed’ i.e., not available to crop, this portion being dependent upon the amount of phosphatic fertilizer adsorbed, amount of MOP fertilizer used, and the nature of the previously applied phosphatic fertilizer adsorption cycle; if in such soils phosphate nutrients are adsorbed in very small increments then some portion of the applied phosphatic fertilizer will be converted directly into fixed form.

To such soils, high doses of potassium fertilizers should be applied in splits. These soils also demand higher doses of phosphatic fertilizers, a portion of which will be fixed thus becoming unavailable to crop roots especially if farmers apply muriate of potash fertilizer as potassium source, subsequently. For such soils, nutrient fixation capacity should be assessed beforehand and considered while making fertilizer recommendations.

Mica and illite clay minerals: These minerals are the main component of clay fraction (mica is present in silt and sand fractions also) in our soils, and so they influence in determining soil solution chemistry which affects the fertilizer’s behaviour. Both minerals are most important source of potassium nutrient in our alluvial soils. The mica in our soils, although least weathered but major potassium bearing mineral can be traced back to the Himalayan rocks. Our soils have large potassium reserves as mica (and potassium feldspar as well) of which potassium contents vary from 1-4 per cent. The potassium contents of mica in sand, silt and clay fractions are 70 per cent, 93 per cent, and 97 per cent, respectively.

As mica releases potassium ions to the soil solution during the processes of weathering, and if weathering has proceeded enough it is transformed to expandable illite (hydrous mica) after interlayer entry of water molecules. Mica thus serves as precursor for hydrous mica clay mineral which is mainly of two types i.e., muscovite and biotite mica. From soil fertility point of view biotite mica is more important as it releases almost 100 times more potassium to soil solution than muscovite.

Thus a soil rich in biotite clay mineral will usually not respond to applied potassium fertilizer. Illite clay mineral can fix potassic fertilizer both under wet and dry conditions e.g., in rice-wheat farming system. However, potassium ion fixation, in the interlayer spacing of biotite mineral is less strongly bonded.

In biotite rich soils if crops are grown for few years without application of potassic fertilizers and then if farmer applies potassium fertilizer, it will be strongly fixed which might be due to the oxidation of structural iron. This kind of process can occur frequently in our semi-arid climate where higher potassium concentrations are often more probable, may reduce the natural potassium supplying power of soil as a consequence of the formation of same but modified mineral, bonding the potassium nutrients more strongly than the original ones. Almost similar behaviour can be faced by ammonium nutrients supplied from urea fertilizer application in such soils.

In general illite clay minerals (vermiculite and montmorillonite also as well) show no particular tendency to interact strongly with phosphatic and nitrogenous fertilizers. It is obvious that after applying phosphatic fertilizers to soils rich in the above mentioned clay minerals, some interactions of minerals can result. For example after partial breakdown of an illite with loss of silicon and potassium, there will be formation of insoluble phosphates.

Thus in our soils with low use rate of phosphatic fertilizers, there may be valuable phosphate sorption by iron and aluminum hydroxide coatings on the mineral surface especially under good quality water irrigation. However, nitrate ion adsorption is generally negligible after urea application to well drained soils due to formation of soluble compounds with metal ions i.e., iron and aluminum hydroxides.

Vermiculite and smectite clay minerals: Both the minerals although found in minor proportions in our soils but due to their swelling and shrinking as well as higher nutrient exchange capacity, play a crucial role in the fixation and release of ammonical and potassic fertilizers. After intensive weathering of illite clay mineral, along with release of potassium, the clay mineral is transformed into vermiculite. It should be taken into account that, as a consequence of high nutrient exchange capacity (more availability of nutrients to crops) of vermiculites, and strong adsorption selectivity for potassium and ammonium over other nutrients present in soil solution (e.g., calcium, magnesium, sodium etc.,) fixation phenomenon of really some importance can occur when vermiculites are the predominant clay mineral, giving rise to problems pf nutrient deficiency to various crops or lack of response to applied fertilizers i.e., low fertilizer use efficiency. As a matter of satisfaction, soil clay rich in vermiculite are proportionally rare except in Gujranwala, Wazirabad, Satgarah and Murree soil series.

Smectite group of clay minerals can be broadly divided into montmorillonite and beidellite, both of expanding nature under wet conditions along with shrinking characteristic on drying. Beidellite clay minerals almost show behaviour in ammonium and potassium nutrient fixation, similar to that of vermiculite, whereas montmorillonite has almost 50 times less fixation capacity than beidellite.

Thus in our soils, occurrence of vermiculite and beidellite is highly related to potassium deficiency while that of montmorillonite and illite to adequate potassium supplying status. Best soils are those that contain both mica/illite and montmorillonite because the released potassium from mica/illite is held as an exchangeable nutrient by montmorillonite clay mineral with less fixation but more easy availability to crop roots.

During 1st or 2nd crop cultivation, proportion of fixed potassium by vermiculite and beidellite is hardly available, while that of fixed by montmorillonite becomes available soon. It is interesting to note that with heavy doses of potassium fertilizers vermiculite clay minerals can be converted to illite clay minerals.