Nutrient Management Research Database

http://www.ipni.net/publication/bettercrops.nsf/0/6241AF80DC0DB1F2852579800081E15B/$FILE/Better%20Crops%202005-2%20p11.pdf

Calcareous soils (containing free lime) are common in many arid and semi-arid regions of North America and occur as inclusions in more humid regions. Phosphorus (P) is very reactive with lime. Following fertilizer application, P undergoes a series of reactions that gradually reduce its solubility. In most calcareous soils, there does not appear to be a strong agronomic advantage of any particular P source when managed properly. Organic matter can inhibit P fixation reactions to some extent. Some fertilizer recommendations call for additional P to be added when the soil contains high amounts of free lime.

Calcareous soils are those identified by the natural presence of calcium carbonate (also known as lime). The pH of these soils exceeds 7 and can be reach as high as 8.5 or above.

Calcareous soils can be extremely fertile when correctly managed.

Phosphorus availability is often an important factor that needs consideration for agricultural use of calcareous soils.

Through the process known as P “fixation,” P becomes unavailable to plants due to being stuck to soil particles or contained within compounds with calcium that limit P mobility.

This paper covers the effect of the following factors that control P availability in calcareous soils:

1)      Time

• P becomes less available to plants as time passes from the moment of application.

2)      Phosphorus Fertilizer Source

• No differences have been identified between the range of commercially available fertilizers.
• Work in underway to identify application methods and other technologies that increase the percent of P fertilizer that becomes plant available.

3)      Organic Matter

• The presence of organic matter can increase P availability by interfering with the “fixation” process.

4)      Temperature

• On the one hand, higher temperatures mean that fertilizer P is transformed to unavailable forms more rapidly.
• On the other hand, increased soil temperatures raise the solubility of P, therefore increasing plant available P.
• The importance of these two conflicting occurrences largely depends on the time of planting and fertilizer application.

• Fertilizer application rates need to be adjusted when working with a calcareous soil. A general recommendation of 10 extra lbs P2O5 per/acre for each 1% increase in soil lime should be accounted for when developing soil P management plans.