Biological and Chemical Properties of soil

Chemical Properties

• Cation Exchange Capacity (CEC)
  The soil colloids are the soil particles with the greatest level of cation exchange.
  Cation exchange is where cations leave colloidal surfaces to replace those removed from the soil solution or they are replaced by other ions more abundant in the soil.
  Negative charges on colloids comes from chemical weathering.
  Important feature of CEC is that it is greatest on smallest particles and it holds onto soil nutrients.


• Soil pH
  Soil pH is the negative log of the hydrogen ion concentration in the soil solution. In other words the more H+ the lower the pH.
  The pH of Irish soils generally range from 5.0 to 7.5 The optimum soil pH is between 6 and 7.
  Al+++ and H+ cause acid soils and these occur where the soil is weathered from granite or sandstone or where there is acid in the soil.
  Ca++ and Mg++ cause neutral or basic soils and these occur where the soil is weathered from limestone soils or where the farmer spreads lime.


• Biological Properties


• Soil Organisms
  Grouped into plants and animals, see box pg 46. The majority are plant and most are microscopic.
  Soil biomass is the total amount of living matter in an area of the soil.
  High levels of biological activity = good soil and good crop growth.
  All soil organisms help in some way to:-
  1.Convert plant and animal debris to humus.
  2.Physically churn the soil to create and maintain soil structure


• Soil Organisms cont.
  Bacteria, fungi and earthworms are the most important organisms and most of what they do is beneficial to the soil.


• Earthworms
  Earthworms like a moist, warm(>10OC) soil with a pH of 6 – 8 and plenty of palatable organic matter.
  Benefits of earthworms to soil:-
  1.Breakdown and incorporate organic matter into soil.
  2.Make nutrients in organic matter more available to plants.
  3.Improve soil drainage and aeration by burrowing.
  4.Help to improve soil structure
  5.Bring organic matter down to lower levels in the soil and generally mix soil up.


• Organic Matter Degradation
  Organic matter is mainly broken down in the soil by bacteria and fungi.
  The more organic matter there is in the soil the more of the microbes there will be.
  Humification is the breakdown of organic matter into humus with the release of other substances such as water, carbon dioxide, amino acids and nutrients along the way.
  Humus is what is left when organic matter is fully decomposed. It is made up of various things such as fats, waxes and other compounds.
  Mineralisation is the breakdown of organic matter into minerals such as nitrate and phosphate.
  Organic matter breakdown is favoured by moist, warm, well aerated, high pH (>6) soils

 

• Nitrogen
  Needed to make chlorophyll and so increased nitrogen gives increased photosynthesis which leads to increased growth.
  Most important and widely used fertilizer.
  Deficiency causes reduction in yield and a yellowing of the crop.
  Plants can absorb nitrogen in the form of nitrate ions (NO3-) but they cannot absorb nitrogen in the form of ammonium ions (NH4+) or urea.

 

• Adding Nitrogen to Soil
  We can apply nitrogen fertilizer as nitrate, urea or ammonium.
  Nitrogen can also be added to the soil by Rhizobium bacteria that live on the roots of legumes e.g. clover. The bacteria form nodules on the roots and they live off the plant while the plants benefit from the added nitrogen. This converting of atmospheric nitrogen to a form that plants can use in the soil is known as nitrogen fixation.
  When organic matter (plant and animal debris) is degraded ammonium ions are also produced.


• Removing Nitrogen from Soil
  Nitrification is the converting of the ammonium ions that cannot be used by plants into nitrate ions that can be taken up by plants. It is done by 2 bacteria, Nitrosomonas and Nitrobacter, that like moist, well aerated, warm, high pH soils.
  Denitrification is the converting of nitrate ions into nitrous oxide and nitrogen gases which are lost from the soil. Denitrifying bacteria are anaerobic and they like wet or water-logged soils.
  Nitrogen is also lost from the soil by the leaching of nitrate ions. This is a waste of money and also causes pollution.