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Subsoil acidity harms yields

June 2017

FERTASA was approached by Grain SA and a private consultant, Mr Pietman Botha, about a suspicion that subsoil acidity may restrict the development of plant roots in water-table soils in the Free State. This suspicion arose when a producer’s experience in this regard was discussed at an editorial meeting of SA Graan/Grain. It is common knowledge that acidic soils considerably reduce the production of maize in particular.

Confirmation of possible problem
Enquiries to FERTASA members’ agricultural experts confirmed that a significant percentage of subsoils – not only the water-table soils, but also other high-potential soils – have pH (KCl) values of less than 4,5. The general impression is that it occurs quite widely. Agricultural experts among FERTASA members are ready to assist their clients to identify and quantify such cases. There were speculations about the reason why soils have acidified considerably over the past two years.

Further investigation necessary
The above is very general and farmers will have to establish and assess their individual situations in collaboration with experts in the fertiliser industry and elsewhere. If it is seen as a focus area, plans can be made.

Experience has taught that there will definitely be spots where the pH is even lower than 4,5 pH (KCl). At these levels of acidity aluminium becomes soluble and can stop root growth through this layer completely.

If soil samples are taken in the normal manner, the extremely acidified layer may be overlooked, as it is sometimes only a few centimetres thick and is ‘diluted’ if a thicker layer of soil is included in the sample.

It is wise to dig profile holes and study the root zone thoroughly. Mechanically compacted layers are sometimes confused with chemically restrictive layers. Aluminium poisoning of maize roots is very characteristic. The roots stop very suddenly at a relatively shallow depth and do not turn sideways. The total absence of hair roots is a further sign. With mechanical compaction the main root turns sideways and still has hair roots.

In many cases subterranean acidification can occur at a deeper level than that normally ploughed. In most cases it takes place at the same depth as that at which N fertiliser was placed beforehand, for example.

Possible causes of acidification
The reason why subsoils acidify is usually that topsoils were not limed properly. This occurrence is described in the chapter regarding liming in FERTASA’s Fertiliser Manual.

The practice of managing unbuffered grounds at certain acidity levels, together with injudicious nitrogen fertilising, can undoubtedly lead to this situation.

The compromising of nitrogen absorption during the previous dry years definitely contributed to the acidification of the soils. It is common knowledge that when ammonium nitrogen oxidises to nitrate nitrogen, hydrogen ions (H+) are released. The soil will acidify if the plant does not absorb the nitrate ion (NO3-) and release a hydroxyl ion (OH-) for every nitrate ion that neutralises H+. This can occur when NO3- leaches with heavy rain or during drought, when plants cannot absorb plant nutrients. Heavy rains after drought, as was the case this season, will definitely lead to soil acidification.

Subsoil acidification can certainly occur if very fine lime was applied only in the fertiliser band at reduced levels without managing the overall acidity levels of the soil.

Producers should evaluate practices like prior fertilisation and determine the impact of this on soil acidification.

Possible resolution of the problem
Guidelines for dealing with this problem are given in FERTASA’s Fertiliser Manual.

FERTASA members’ agricultural experts, with their wide knowledge and experience, are also geared to give advice on dealing with the problem.

A few aspects to take into account include determining the exact extent and position of the acidification.

Some people feel that gypsum applications can neutralise subsoil acidity in the sandy soils of the Free State. This can be a risky practice, as gypsum can only have a self-liming effect in subsoils with sufficient sesquioxides. In sandy soil the magnesium can leach even if the gypsum is applied with dolomitic lime. Potassium is also easily stripped from the topsoil with gypsum applications. The amount of gypsum added to the lime reduces the amount of lime being applied and this needs to be taken into account.

Unfortunately it may be necessary in many instances to lime deeper than usual if the soil is acidified below the cultivation depth. One would have to look at suitable implements and operations to apply the lime effectively. No till could also hold its own challenges if the subsoils are acidified.

The effect of topsoil liming can also move downward in the profile, but this takes several years and crop losses could be suffered in the meantime. However, re-acidification can be avoided with slight over-liming of the topsoil.

It may be necessary to discuss all the variables at a workshop or conference where farmers and advisers are present. FERTASA members have declared themselves willing to add their inputs in this regard.

Soil acidification is a natural process that needs to be managed correctly. If liming is approached scientifically, it can be profitable in the long term.

It is very important that ordinary fertilisation of crops is not neglected. Lime can never be applied instead of fertiliser.

Liming and fertilisation supplement each other.

Article submitted by Pietman Botha, SA Grain contributor and Pieter Hauman, Fertasa. For more information, send an email to pietmanbotha@gmail.com or pieter@fertasa.co.za.

Publication: June 2017

Section: Pula/Imvula