Overview

Extended periods of very dry soil conditions can reduce Nitrogen (N), Phosphorus (P) and Potassium (K) availability to plants.

Nitrogen

For N, low soil moisture decreases soil microbe activity. Microbes play an important role in breaking down organic matter and converting organic N to inorganic ammonium and nitrate nitrogen, a process called mineralization. 

In dry soils with low N mineralization, there could be less plant available N in the form of either ammonium (NH4+-N) or nitrate (NO3--N) nitrogen. 

However, the risk of NO3--N loss through leaching or denitrification is reduced in dry soils. Although losses are reduced, plant uptake if often reduced as well. 

When the rains return there can be a sudden increase in soil nitrogen, which can lead to increased NO3--N losses. If this occurs late in the season it may be detrimental for perennial crops going into dormancy.

Phosphorus

Reduced soil microbial activity in soils with low moisture can reduce organic matter decomposition and the mineralization of organic P to inorganic P. 

Phosphorus moves from higher concentrations in the soil to lower concentrations in plant roots by diffusion. 

As soils become drier, less diffusion occurs. This is because the water film around the soil particles becomes thinner, making diffusion to the plant root more difficult.

Potassium

Decreased movement of K to the plant roots occurs in dry soil. 

As soil dry, clay minerals become dry and shrink, trapping K tightly between mineral layers. Once trapped, K is unavailable to plant roots for uptake. This K is released and plant available again when the soil moisture increases. 

Reduced K uptake during extended dry soil conditions can show up as low K levels in tissue samples or high K levels in a post-harvest soil sample. 

Taking soil samples in a dry year and comparing it to normal years can provide valuable information for what to expect in future dry years.

Crop nutrient removal in dry conditions

Plant N, P and K uptake under extended periods of dry soil conditions may be less. As a consequence of this, it is likely that some nutrients applied from fertilizers and organic sources may still be available the following year.

Soil testing is the best method to estimate the remaining nutrients available for the next year's crop. Especially in dry years, a recent soil sample is a better estimation for next year's crop nutrition program than a sample that is older (for example, more than 3 or 4 years old).

A soil test will help in accounting for nutrients carried over from one year to the next.

Nutrients that could be carried over and should be credited from one year to the next include:

  • immobile nutrients previously applied like phosphorus, potassium and zinc
  • nutrients from organic sources like manure
  • nutrients in crop residues

An extremely dry winter could see carry over of mobile nutrients like nitrate nitrogen, sulfate, and boron.

Conduct a spring soil test to identify levels of mobile nutrients that may have carried over from previous years.

References

  1. Soil and Fertility Handbook. OMAFRA Publication 61, 2006.
  2. IPNI Plant Nutrition Today, W. M Stewart, No. 7, Fall 2012.