Hi,
After some research, I have found recommended soil potassium to magnesium ratios ranging from 0.25 to 0.7 in agricultural soils (ratios calculated from CEC concentrations). Which one do you use or recommend and why ?
The K/Mg ratio can be indicative of how available the potassium and magnesium are in the soil. If for example your ratio is less than 0.25, it typically indicates that the magnesium in the soil is over powering the potassium in the soil making it harder for the plants root system to uptake potassium. If the ratio is >0.35 it can indicate that the plant may have trouble accessing magnesium.
High potassium demand crops also require attention to K:Mg balance and as we increase potassium application we must pay attention to Mg balance. The ideal K:Mg ratio in meq is 0.2 to 0.35 for most crops and crops such as potato 0.3 to 0.4. Often times in trying to satisfy a crops K requirements yield loss is experienced with over application of K by inducing Mg deficiency. Percent saturation levels of K and Mg in the soil are key to understanding when Mg needs to be applied with K to avoid this K:Mg imbalance in a fertility program.
Mg- and K contents of soils depend on the soil type, the pH and the horizon which the soil solution has been taken from. Both elements are strongly correlated with the parent material of the soil type. Mg is present in sandy soils at an amount of as much as 0.05 wt. % and in clayish ones in as much as 0.5 wt. %. It is bound to Mg silicate such as pyroxene, amphibole, olivine and biotite as well as soil chlorites. On calcareous bedrocks Mg is also contained in dolomite. It responds to the amount of Ca cations which can replace each other and is of effect on the phosphate fertilizers sprayed across the field. The same results may be observed for K which is high in argillaceous and low in arenaceous soil type with maximum values of as much as 3.3 wt. % K. It is bound to micaceous clay minerals such as illite and K feldspar. The K and Mg mobility, their bonding, their way of extraction and determination are rather complex particularly in view of the cation exchange capacity which is strongly controlled by the organic matter and the swelling clay minerals such as smectite and swelling chlorite so that far-reaching explanations are hard to be given in such a post in a general way without a knowledge of the morpho-climatic zone and the hydrological regime.
The ratio might also be in influenced by the crop that you are growing. Even within a crop there can be differences between varieties.
It depends on many other parameters such as pH, CEC, your study area ecological conditions. Generally, the K to Mg ratio is taking the base saturation of potassium and dividing it by the base saturation of magnesium.
Eg: A soil test with 4.5% K and 18%Mg means 4.5/18 = 0.25. This means the ratio is 0.25.
Thank very much for your answers which precise the concept of K to Mg ratio. My question however is about the recommended (optimal) ratio that you use or trust to assess and/or fertilize agricultural soils.
Thanks immensely to Zia Ullah Khan who has given me some values for specific crops. I can't wait to read other answers from this fantastic community :-)
i think , Prof Dill has given an excellent account of the issue , we are deliberating through this thread . Potassium to magnesium ratio in a given soil is predominanatly a function of mineralogy .
- Badges
- Science topic
- Similar topics
- Agriculture
- Soil Science
- Soil