Senwes Scenario June / July 2017 - Page 38

•••• T RADE N E W S Management of sub-soil acidity SUB-SOIL ACIDITY CAN OCCUR NATURALLY IN HIGH RAINFALL AREAS, OR CAN BE THE RESULT OF ILL-CONSIDERED OR EXCESSIVE APPLICATION OF NITROGEN, AND LACK OF REGULAR LIMING OF THE TOP SOIL. A dvisors find that sub-soil acidity increasingly occurs in the lower rainfall areas, which usually goes hand-in-hand with the practice of applying nitrogen before planting. The placing of nitrogen is usu­ ally deeper than in the case of lim­ ing. Lime has to be worked deeper into soil where underground acid­ ity is observed in order to neu­ tralise the soil acidity, the reason being that neutralisation is carried from the top soil to the sub-soil at a very slow pace. The deeper placement of lime is an expensive process which has to be done with specialised or adjusted equipment. The high cost of this specific action and the irregular liming of the top soil result in a worsening of the problem of underground acidity, which has a negative impact on crop production. The depth of the soil profile becomes more shallow due to the chemical limitation in respect of the devel­ opment of roots, which means that the moisture in the sub-soil cannot be utilised. If this problem is not addressed, it can result in serious consequences, such as lower yields. Table 1 reflects the quantity of pure CaCO3 which is required for the neutralisation of the resultant acid per kilogram pure nitrogen (N) applied as fer­ tiliser (Moore, 1998). Nitrogen is applied annually, but not everything is taken up by the crops, which means that the remainder seeps through the soil profile. Table 1 reflects the Table 1: Neutralisation requirements relating to the acetification of nitrogen- containing fertiliser with CaCO 3 . Nitrogen fertiliser CaCO 3 needed to neutralise the addition of nitrogen (kg CaCo 3 /kg N) No leaching CaCO 3 needed to neutralise the addition of nitrogen (kg CaCO 3 /kg N) 100% leaching Ammonium sulphate 3.6 7.1 Ammonium nitrate 0 3.6 Urea 0 3.6 DAP 1.8 5.4 Potassium nitrate -3.6** 0 Sodium nitrate -3.6** 0 ** Neutralisation by the fertiliser itself 36 JUN/JUL 2017 • SENWES Scenario neutralisation requirements and the quantity of lime required. The lime which is applied, moves very slowly through the soil profile if it is not worked in. In the case of normal application, lime is applied to the top part of the soil profile. The slow movement of the lime in the profile is approximately 8 to 12 mm per year, depending on the texture of the soil and on whether the limed part of the profile main­ tains a favourable pH. The down­ ward movement or neutralisation takes place by means of root action and insect action, e.g. earthworms. Should liming programmes be maintained over years, it is proba­ ble that sub-soil acetification will not take place as fast as is the case at present. An acceptable practice is to apply half of the sub-soil lime requirements with the top soil lime requirements. This management practice should be maintained due to the slow movement of the lime in the soil profile. A gypsum and lime combina­ tion can be applied to accelerate the neutralisation of sub-soil acidi­ ty, should sesquioxides be present. The application thereof should be done with care since seeping of magnesium and potassium can take place from the top soil to the sub-soil in low percentage clay soil. A safe ratio of gypsum and lime is 20% gypsum and 80% dolomitic lime. It is advisable to mix dolo­ mitic lime, which contains potas­ sium, with the gypsum to counter the seeping of magnesium.