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Thursday 8 December 2016

Soil texture crucial along with fertility and acidity analysis

Gerry Bird

Published 09/11/2010 | 05:00

Farmers attend the Bord na Mona biomass open day held at the Edenderry Power Station, Co Offaly
Farmers attend the Bord na Mona biomass open day held at the Edenderry Power Station, Co Offaly

The recent heavy showers have made field visits challenging -- and what a contrast to a couple of weeks ago. The current wet conditions have increased slug activity in the rougher seedbeds, particularly after rape crops. Weed control will have to wait until ground conditions improve and will require a good combination of contact and residual activity.

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Walking some fields -- drilled with winter wheat -- in north Kildare and south Meath provided a stark contrast in soil conditions. The main reason for the difference was the soil texture and how both soils dealt with the rainfall. Farmers have always soil sampled for lime requirements and fertility levels, but few have a soil texture analysis result for their farm.

Percentage

Soil texture, which is the percentage of sand, silt, clay (mineral elements) and organic matter in a soil, is fundamental to fertility, structure, and drainage characteristics. Soil fertility can be influenced by fertiliser and lime application, rotation and good tillage practice, but soil texture -- in reality -- cannot be changed, with the exception of organic matter. The classification of a soil as a clay loam, a medium loam, a sandy loam, etc, is based on the various proportions of sand, silt and clay.

Soils with a high percentage of clay are potentially more fertile relative to sandy/silt soils, hold more water, drain slowly and compact easily.

Clay soils have the ability to hold more fertiliser due to a complex reaction where inorganic fertiliser -- phosphorous and potash -- lock or bind onto the clay particles, similar to the reaction of iron filings to a magnet.

The N, P and K in fertiliser has a negative or positive charge. The clay particle also has a charge (negative), facilitating the nutrient lock-on. P and K lock on, whereas N does not very well and remains in the soil water and can be washed out.

A soil with a lot of sand will drain well, but will have low fertility as fertiliser will not lock-on due to reduced soil- to-nutrient attraction. Soils with high silt levels have low fertility potential, a tendency to compact and have poor drainage characteristics.

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Finer soil particles of sand and particularly clay and silt can be washed down through the soil as heavy rain will cause problems with soil structure and drainage. A healthy soil should have a distinct network of large and small channels to regulate water content (drainage and plant requirements) and provide air for root growth.

The small pores can hold water, with the larger macro pores draining away excessive water. Applied chemical fertilisers are water soluble and need soil moisture to dissolve and lock-on to the clay or be taken up by the roots. If the soil channels become blocked, with silt or fine clay, water builds up, all channels become saturated, water replaces air, the soil become waterlogged and crops therefore suffer.

Saturated soils, with excessive water, flush fertiliser out, especially nitrogen, into watercourses. A wet soil is a cold soil, so plant growth is reduced. Soil pH decreases where soils remain wet and this has a major influence on nutrient uptake.

The performance of organic manures (slurries and sludges) is greatly influenced by the levels of soil water levels. The bulk of the nutrient content in organic manures is not water soluble and needs to be broken down by soil microbes. Waterlogged soils, with poor aeration, will result in reduced breakdown of organic manures and crops will suffer -- in fact, the quality of soil structure will deteriorate.

Warm and dry summer days are now a nice memory as we become accustomed to the winter rains, but the foundations of crop yields are based on how your soils manage water. Knowing your soil texture can be the key to improved crop management.

Gerry Bird is an agricultural consultant and an Irish Tillage Consultants' Association member. Email : gjbird@eircom.net

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