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Biological farming - mineral management

As I said last week, I recently spent a week studying biological agriculture.  On the first day we learnt about mineral management.  I have prepared a table with a summary of the signs of mineral deficiencies, which you can download here, as it doesn't fit on a blog page!

First a paradigm shift….
For decades NPK (nitrogen, phosphorous and potassium) have been regarded as the essential ingredients for plant growth and are the main components of most synthetic fertilisers.  This was due to work conducted in the 1800s by Justus Von Liebig, in a very crude experiment by modern standards, which showed that the main constituents of plants are N, P and K.  While this was technically correct, there are actually at least 9 other minerals also required by plants in varying quantities.  Plants that have access to a soil with balanced minerals will be more resistant to pest and diseases and will be more nutritious for stock and humans to eat.  Balancing the soil minerals is therefore the first step to farming without chemical inputs.

General points about mineral requirements of plants
Plant minerals must be absorbed from relatively dilute solutions or the plants will be stressed by the salty water.  This can be a problem when chemical (NPK) fertilisers are applied in large quantities as the solution is usually too concentrated.  In addition, it must be recognised that minerals are held in the soil in the clay and in the organic matter.  The clay can only hold minerals that form a positive charge (calcium, magnesium, potassium and sodium), and negatively charged minerals (for example nitrate, phosphorous, boron and sulphur) are susceptible to leaching unless the soil has sufficient organic matter (hummus), which can retain both positively and negatively charged minerals.

Some elements can slow down the absorption of others into the plant, for example, calcium slows down potassium and vice versa. The phenomenon is known as "antagonism" (see Mulder’s chart).  Healthy plants result when the nutrients are absorbed in certain relative, "balanced", proportions (see William Albrecht’s work on base saturation).  When ratios between nutrients are extreme, deficiency conditions are created.  For example, if a high proportion of nitrogen to potassium is absorbed, the plant will suffer from potassium deficiency.

Minerals in the soil solution may not be absorbed by the plant if the pH of the soil is too far away from 6.4 (the pH at which all minerals are soluble, see diagram).  Nutrients are absorbed by plants more easily in a chelated form, that is, using a large organic molecule to surround the mineral atom to facilitate transportation into the plant.  Chelating agents include organic sources such as compost, humic acid and fluvic acid (the latter two being derived from brown coal) or synthetic chemicals such as EDTA – often included in synthetic fertilisers.

The nutrient medium must contain an adequate supply of oxygen, which is achieved through sufficient aeration of the soil.  This will depend on soil structure and organic matter.  High magnesium soils will tend to be “tighter” than soils with the ideal ratio of magnesium to calcium, as calcium is a larger atom that creates more space between clay particles.

Balancing the soil
Now that you understand the importance of soil minerals for your growing plants, you are probably wondering where to start in correcting the balance.  This will depend on the scale of your operation.  We used a soil test to understand our soil deficiencies and bought some products to help us to add some minerals to the soil, but will never afford to add the amount of lime that would be required to perfectly balance our soil. Where the success of a crop is essential, commercial orchardists and market gardeners may also go to the extent of using plant tissues tests to target foliar sprays at all stages of plant development.  In a home garden, it is probably more cost-effective to look for signs of deficiency in the plants and add minerals as required.  In all cases, adequate soil organic matter will buffer any deficiencies by making minerals more available to the plants and feeding beneficial microbes.  Using cover crops with deep roots can also help to “mine” minerals from the sub-soil.  The most important thing to remember is that not all minerals are required in large quantities and not at all stages of plant growth.  Sometimes a seed coating, foliar spray or liquid injection (or a sprinkle of minerals when planting, in the home garden scale) will be sufficient, rather than trying to correct ALL the soil in an area.

What have you done to manage the mineral balance of your soil?  


  1. Love your factual information , just soaked it all up. We have been using crusher dust on our home vege garden and some dolomite.Our soils are very magnesium deficient and selenium deficient here.
    The use of rock dust to balance soils is an interesting new thing to me.It is also interesting how a variety of common weeds tell us about soil imbalance.Fire weed is one such weed that tells us our soil is crying out for help.

  2. In our area we tend to get frequent rain (if not large amounts) and this leeches the soil of its nutrients as well.

    Thanks for an informative post.

  3. gosh thanks for that information, I know that when I added trace elements and sulphate of potash to my veggies I noticed a jump in production. Not sure what that added though..... we also get a lot of rain in the wet season so find foliar feeding small quantities the best thing.

  4. Interesting! I am so NOT scientific about my fertiliser application and should probably spend more time thinking about it and figuring out what I need to do. I just chuck stuff on when I have it. Stuff like seaweed, vermicast and worm tea, compost, coffee grounds - whatever I can get my hands on free!

  5. hi all, I don't worry about any of this in my vege garden, except for the occasional pH test. If you are adding plenty of organic matter and trace minerals (and seaweed, wish I could get a good supply!) you can't go too wrong. The problem is with larger scale farming, imbalances can cause major problems with crops and livestock may not be getting what they need. If you are keeping animals, you should get a soil test done even if its just so you can supplement them rather than fix the soil.


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