Dry soil conditions impact on nutrient availability

Deanna Németh, Horticulture Sustainability Specialist

Although the past week brought some much needed rain, many locations in Ontario were experiencing prolonged dry soil conditions.  These 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 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.  In dry soils, the risk of NO₃^–-N loss through leaching or denitrification is reduced.  While this means there is more soil N available to crops, plant uptake can be reduced.  When the rains return there a can be a sudden increase in soil nitrogen.  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 reduced 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.  Reduce 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 to for what to expect if future dry years occur again. 

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 nutrients applied from fertilizers and organic sources may still be available the following year.

Soil testing in the fall of 2016 is the best to estimate the remaining nutrients available for the next 2017 crop. In dry years, a recent soil sample is a better estimation for next year’s crop nutrition program than a sample that is older (e.g. more than 3 or 4 years old).

A fall soil test will help in accounting for nutrients carried over from 2016 to the 2017 crop.

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

• Mobile nutrients like nitrate nitrogen, sulfate, and boron in the soil
• Immobile nutrients previously applied like phosphorus, potassium and zinc
• Nutrients in crop residues

For more information, contact:

Deanna Németh, Horticulture Sustainability Specialist, OMAFRA

Christoph Kessel, Soil Fertility Specialist, Horticulture, OMAFRA 

 *Reference:

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