Managing no-till soil acidity and fertiliser requirements

No-till may be a sustainable and cost-effective crop production system, but it comes with challenges of its own. Guy Thibaud, veteran soil scientist with the KwaZulu-Natal Department of Agriculture and Rural Development, explains how to deal with these.

An important principle of no-till is having a crop residue covering of at leat 30% over soil after planting. Photo: FW Archive

As its name describes, no-till requires minimum soil disturbance. In no-till commercial cropping enterprises, a specialised planter is used to cut and open shallow and narrow slots in the soil surface. The planter then drops seed and fertiliser into these slots before covering them with soil and plant matter.

At the opposite end of the spectrum is conventional tillage, which typically uses a mouldboard plough to invert cropping soil. A disc is then used to break up the resulting clods into a fine tilth into which seeds are planted.

Another important principle of no-till is ensuring that abundant – at least 30% after planting – and permanent plant residues cover the soil surface. A commercial maize crop produces large quantities of organic residue to cover the soil surface and to build soil organic matter. Soya bean, by contrast, while a useful crop due to its protein values, oil content and nitrogen-fixing abilities, produces very little residue that can benefit soil health.

“Crop rotation is therefore an integral part of no-till,” explains Guy Thibaud, soil scientist with the KZN Department of Agriculture and Rural Development. “Using an economically viable and diversified crop rotation in a no-till cropping system mitigates possible weed, disease and pest problems. It also enhances microbial biodiversity in the soil. And utilising legumes in a crop rotation results in beneficial biological nitrogen fixation from the atmosphere into the soil. Crop rotation also spreads risk for the farming business.”

Thibaud points out that while some farmers claim that no-till does not work for them, this is not due to any inherent failure of the no-till system. It is as a result of the farmers making mistakes or taking shortcuts in the implementation of no-till in their cropping enterprises.

“There are many South African farmers, including Ant Muirhead and Tony Matchett, who have profitably and sustainably used no-till continuously on 100% of their soil for as long as 25 years. There’s no reason why all South African crop farmers should not be implementing no-till. If farming is not sustainable, it’s not profitable,” he says.

The traditional tillage myth

According to Thibaud, one of the most difficult aspects of converting to no-till from conventional tillage is having to “unlearn what you thought was true”.

He is convinced that blindly following outdated family farming practices – “Grandpa ploughed, Dad ploughed, and therefore I must plough” – is what has led to the relatively slow adoption of no-till in South African agriculture. In the words of US conservation agronomist, Ray Archuletta, “excessive tillage leaves soil naked, hungry, thirsty, and running a fever”.

Thibaud explains that disturbed and uncovered soil results in more than just erosion. During rain or irrigation, it loses valuable organic matter – approximately 99% of its nitrogen content, 99% of its sulphur content, and about half its total phosphorous content, amongst other soil nutrients.Other negative effects are environmental damage from excess chemical and fertiliser use, the silting up of freshwater resources, and the loss of crop productivity.

By contrast, according to Thibaud, no-till has numerous direct and indirect benefits, particularly in a water-scarce country such as South Africa. These include enhanced moisture conservation through reduced run-off and reduced evaporation loss.

“The improved soil moisture levels and retention times resulting from no-till mean that crops are less likely to experience water-stress between rainfall events or irrigation applications. This means that the crop plants have improved resistance to pest and disease threats. More moisture in the soil also allows these plants to grow bigger leaves that can then convert sunlight into larger crop yield.”

With correct implementation, he explains, no-till can achieve the same or better crop yield than conventional tillage – and on a sustainable basis. In isolated cases where no-till crop yield is marginally smaller than those of conventional tillage crops, the net profit and return on investment from the no-till crops are almost always greater.

This is due to lower production costs. Fewer machinery hours are required on the land, resulting in savings on fuel, labour, machinery wear and tear, and time.

No-till challenges

Thibaud cautions that no-till is not without its own challenges, but these can be dealt with effectively using correct management and informed strategies.

“Acidification of the soil surface in no-till grain production is probably the biggest problem found so far. It has a negative effect on the efficacy of a number of herbicides, particularly those that are atrazine-based. It has an adverse effect on crops’ surface root development and a negative effect on earthworm populations. But the soil surface acidity can be managed without the use of a plough,” he says.

The major cause of soil acidification in any cropping system, no-till included, is commonly applied nitrogen fertilisers. When coming into contact with warm and moist soil, the ammonium nitrate in these fertilisers is converted into nitric acid over two to three weeks.

This lowers soil pH, which in turn results in increased soluble aluminium in the soil. It is this soluble aluminium, not the nitric acid, that is toxic to plants.

Thibaud says that it should be kept in mind that some soils are naturally acidic, but this has come about over thousands of years, unlike the rapid acidification of soils through nitrogen fertiliser applications.

Lime and acidification

In an 11-year no-till cropping system trial in KZN’s Howick area, Thibaud and others annually applied nitrogen fertiliser to a land, but did not apply any lime. It was found that soil acid saturation in the top 5cm increased from less than 5% in the first year to just below 60% in the 11th year.

“This nitrogen fertiliser-induced soil acidity found its way down to a depth of between 50cm and 60cm. The mobility of this acidity in the soil, and the problems that it can cause, should not be taken lightly,” he stresses.

The trial also demonstrated that regular applications of surface-applied lime to no-till soils can prevent this soil acidification problem. On an adjacent plot, Thibaud and his colleagues surface-applied 750kg/ha lime to the soil each year before nitrogen fertiliser was applied. Even when as much as 180kg/ ha of nitrogen was applied to the lime-treated no-till trial plot, soil acidity did not worsen.

In lime-treated areas where 60kg/ ha nitrogen was applied, soil acidity was reduced. Surface-applying 1,5t/ha lime each year to no-till lands before nitrogen fertilisation applications produced even better results. “But this data does not mean that surface-applied lime is able to move down into the soil profile,” Thibaud explains.

“Instead, the neutralising effect that lime has on soil acidity is actually as a result of a physiological process called alkalinity transfer. The lime on the soil surface neutralises the nitric acid on the soil surface. The resulting calcium nitrate is able to percolate down through the soil. Since maize plant roots take up an excess of nitrate over calcium, they release alkalinity into the soil which increases pH and lowers soil acid saturation.”

This alkalinity transfer cannot take place in the absence of plant roots. Ironically, highly acidic soils are not conducive to the development of roots in most commercial crops. It is therefore essential that these highly acidic soils be adequately treated with lime and planted to acidity-tolerant plants before being utilised for no-till crop production.

Improved yield

This no-till maize trial found that liming the soil resulted in no significant grain yield improvements in the first four summer production seasons.

Thereafter, however, yield improved dramatically. In the 2012/2013 summer production season, for example, the trial achieved a dryland no-till maize harvest of about 13t/ha using a combination of 180kg/ha nitrogen fertiliser and 1,5t/ ha surface-applied lime. Interestingly, when 180kg/ha nitrogen fertiliser was applied in combination with 750kg/ha of surface-applied lime, the maize yield was almost the same.

“Remember, it’s not surface liming that’s increasing the no-till maize yield. Surface liming prevents an increase in soil acid saturation and it is this that results in improved no-till maize yield,” Thibaud explains.

“It’s pointless to keep increasing nitrogen fertiliser application rates to boost no-till grain yield if soil acidity is not properly managed. In fact, our trials found that 180kg/ ha of nitrogen fertiliser applied to a land of no-till maize where no surface lime was applied yielded less than a no-till field where 120kg/ha fertiliser was applied in combination with surface-applied lime.”

Applying additional nitrogen fertiliser to crops growing in acidic no-till soils is a waste of money. Correctly balancing lime applications with nitrogen fertiliser applications to achieve optimal no-till grain yield is the most cost-effective and sustainable system for any cropping enterprise. This is an adaptation of a presentation given at Pannar Seed’s Karkloof Farmers’ Day on 12 April 2016.