Limiting factors

There are many factors that can limit crop yield. For example, soil structure, fertility status, and texture will all impact crop yield. In Figure 2, the mean zone yield is 75 tonnes/ha in the red zones and 112 t/ha in the green zones. Overall, the yield ranged from 0 – 145 t/ha.

Figure 1 A Yield Map from a Paddock of Onions in Hawke’s Bay

Understanding what is limiting crop growth, and therefore yield, is important. “Liebig’s Law of the Minimum” states that yield is affected by the most limiting essential nutrient. This is often shown as a barrel with shorter staves as in Figure 2. The maximum allowed by the shortest stave (in this case water) sets the maximum that can be achieved. Adding more nitrogen will not increase yield.

Figure 2 Liebig’s Law of the Minimum states yield is determined by the most limiting factor (Source: http://www.greencare-concept.nl/eng/pagina/141/prevention-through-nutrition.html)

Nutrient deficiencies in the crop can occur for a number of reasons and adding more of nutrient(s) via fertiliser application may not fix the underlying problem. It may not always be an essential nutrient/s that are limiting crop growth and yield. There are other factors such as pH, soil compaction and drainage that must be kept in mind. For example, the graph in Figure 3 stresses that applying more and more Nitrogen will not compensate for water-stress. Or if it is too cold, the plant won’t grow regardless of nutrient status.

Figure 3: Potato Yield Response to N Fertiliser at Varying Levels of Water-Stress (Source: Nutrient Management for Vegetable Crops in New Zealand)

KEY POINT:         The yield of a plant is limited by a deficiency of any one essential element, even though all others are present in adequate amounts.