Soil can supply 13 essential plant nutrients. Based on the amounts plants need, these fall into three categories:
Primary nutrients
N P K
nitrogen phosphorus potassium
Secondary nutrients
Ca Mg S
calcium magnesium sulfur
Micronutrients
B Cu Fe Mn Mo Zn
boron copper iron manganese molybdenum zinc
The need for primary and secondary nutrients is still small relative to the need for the carbohydrate components carbon (C), hydrogen (H), and oxygen (O), which are obtained from water and air. The areas of the circles in the image below are proportional to nutrient need.
Carbohydrate Components
Primary Nutrients
Secondary Nutrients
Nutrient levels
Nutrient availability and utilization is context-dependent, and nutrient ratios can be as important as absolute levels. In particular, the N:K ratio determines whether a plant produces vegetative growth or flowers and fruit (high N/low K favors vegetative growth).
Deficiency
Insufficient soil levels of an essential nutrient compromise plant growth and health and in extreme cases can lead to plant death. The most commonly seen deficiencies are for primary nutrients. These are in the greatest demand by plants and can be depleted quickly from soil. Our acidic soils can also create deficiencies in secondary nutrients (see Soil Acidity). Finally, in addition to plant utilization, some nutrients are easily leached by excess rain or irrigation.
Liebig’s Law of the Minimum states that growth is controlled not by the total amount of resources available, but by the scarcest resource: the limiting factor.
P deficiency leaves marked reddish purple, particularly on young plants
K deficiency firing or drying along the tips & edges of lower leaves
N deficiency yellowing that starts at the tip & moves along the middle of the leaf
Mg deficiency whitish stripes along veins, underside of lower leaves often purplish
Drought grayish-green color, rolled leaves
Disease
Chemical burn tips & edges of leaves die
Excess
Direct problems
Direct problems include frank toxicity or undesirable metabolic and environmental consequences. Examples:
Boron toxicity
high N — excessive vegetative growth and delayed flowering or fruiting as well as ground water pollution
Indirect problems
Because nutrients interact both chemically and functionally, excess levels of some nutrients indirectly cause deficiencies of others.
Functional interaction: competition for plant uptake
Chemical interaction: nutrients are charged and can bind one another, so an excess of one nutrient can in some cases sequester its binding partners and prevent their uptake
The image below illustrates the major, generally observed nutrient interactions that can occur when one nutrient is present at high levels. Often, consequences appear only when the target nutrient is effectively present in marginal amounts (either the soil concentration is low or the soil pH has made the nutrient mostly inaccessible).
Nutrient availability
Chemical Form
Nutrients must be in the correct chemical form to be utilized by plants. For example, nitrogen, which makes up approximately 70% of the atmosphere, must be converted by soil organisms into ammonium (NH4+) or nitrate (NO3-) before it can be utilized.
Solubility
Nutrients are only available to plants when dissolved in soil water, because water transports the nutrients to the plant roots. Most of the essential nutrients found in the soil are not present in soluble form, but locked up within soil minerals and/or organic matter. Weathering of the soil minerals and the decomposition of organic matter releases these nutrients, but this process is quite slow.
Binding
A nutrient can be in water-soluble form but still unavailable because it is too tightly bound to the mineral surface or to another fertilizer ingredient. This can occur for some nutrients if the soil pH is too high or low or if another nutrient is present in excess.
Nutrient mobility
Soil mobility determines how long an applied nutrient will persist in the plant root zone and whether the nutrient can be applied at the soil surface or should be incorporated into the root zone.
Plant mobility determines where nutrient deficiency symptoms appear.
Primary nutrients
Primary nutrients tend to be the focus for most gardeners. However, primary nutrients work only if secondary and micronutrients are present in sufficient quantities.
Melspring Green Care Prevention through nutrition
Healthy corn leaf
P deficiency leaves marked reddish purple, particularly on young plants
K deficiency firing or drying along the tips & edges of lower leaves
N deficiency yellowing that starts at the tip & moves along the middle of the leaf
Mg deficiency whitish stripes along veins, underside of lower leaves often purplish
Drought grayish-green color, rolled leaves
Disease
Chemical burn tips & edges of leaves die
