A Short Introduction to Essential Plant Nutrients: Part 3

Factors effecting the availability of nutrients


In general, most plants grow by absorbing nutrients from the soil. Their ability to do this depends on the nature of the soil. Depending on its location, a soil contains some combination of sand, silt, clay, and organic matter. The makeup of a soil (soil texture) (Figure 1) and its acidity (pH) determine the extent to which nutrients are available to plants. Soils should be tested at least every 3rd year to ensure nutrients are in balance and the soil pH is still in the correct range for the crop cultivated. 

Soil Texture (the amount of sand, silt, clay, and organic matter in the soil)

Soil texture affects how well nutrients and water are retained in the soil. Clays and organic soils hold nutrients and water much better than sandy soils. As water drains from sandy soils, it often carries nutrients along with it. This condition is called leaching. When nutrients leach into the soil, they are not available for plants to use. That is why the use of fertilizers with enhanced efficiency is so important. COMPO EXPERT’s range of nitrification inhibiting and controlled release fertilizers is a very good example.

Soils vary in their texture and nutrient content, which makes some soils more productive than others. Sometimes, the nutrients that plants need occur naturally in the soil. Other times, they must be added to the soil as lime or fertilizer.

Soil pH (a measure of the acidity or alkalinity of the soil)

Soil pH is one of the most important soil properties that affect the availability of nutrients (Figure 2).

  • Macronutrients tend to be less available in soils with low pH.
  • Micronutrients tend to be less available in soils with high pH.

Lime is added to the soil to make it less acidic (it also supplies calcium and magnesium for plants to use) and raise the pH to the desired range of 6.0 to 6.5. In this pH range, nutrients are more readily available to plants, and microbial populations in the soil increase. Microbes convert numerous elements to forms that plants can use.

Nutrient Imbalances

A wide variety of problems can effect plant growth and health. Excess levels of nutrients in the soil leads to toxicity, or a lack of nutrient within the soil leads to deficiencies within plants. The relationship between soil nutrient levels and plant nutrient levels is complex. The ability of a plant to absorb any given nutrient is affected by pH, temperature and moisture as well as the amounts of other nutrients within the soil. For this reason, it is important to correctly diagnose symptoms of deficiency and toxicity before taking corrective action otherwise you risk solving one problem only to create another one (see https://plantprobs.net/plant/nutrientImbalances/1%20NutrientDeficiencyAndToxicity.html).

Figure 3 below (know as a Mulder’s Chart) display the various interactions that can occur between the plant-soil nutrient system. 

Antagonism: If one nutrient is at a too high level in the soil it can inhibit the availability and the uptake of other nutrients as shown in the chart. For example, high nitrogen levels can reduce the availability of boron, potassium and copper; high phosphate levels can influence the uptake of iron, calcium, potassium, copper and zinc; high potassium levels can reduce the availability of magnesium. It is therefore important to remember that too high levels of the big three elements (nitrogen, phosphorous and potassium can induce plant deficiencies of other essential elements. 

Stimulation: This occurs when the high level of a particular nutrient increases the demand by the plant for another nutrient. For example, increased nitrogen levels create a demand for more magnesium. 

Our enhanced efficiency fertilizers, from nitrification inhibitors to controlled release fertilizers have been designed to maintain the nutrient balance and ensure the nutrients applied are not lost, but utilized by the crop.

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