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?