Messages from the soil fertility conference

A huge crowd attended Teagasc’s soil fertility conference in Kilkenny last week. Most of the topics were tillage related, with a mix of research and advice. The event also saw the launch of a new version of the Green Book, Teagasc’s nutrient advice publication.

Potash – the forgotten nutrient

Potash might be described as the forgotten nutrient and deficiency has been traditionally described as the hidden hunger. A crop might still look OK when there is insufficient supply, but it will come up short on yield, said Teagasc’s Mark Plunkett.

It can be described as forgotten, because it has not received nearly as much attention in recent years, as it is not subject to legislative controls like N and P, and also because farmers have generally not applied sufficient amounts.

Mark reported on some recent Teagasc research on potash in spring and winter barley, which posed some interesting questions. On a particular trial site in Oak Park, Mark conducted a simple trial on potash application in index 1 and 2 areas in the same field. The low-index areas were created by their previous history in grass variety testing and alternating areas of index 1 and 2 were very visible before fertilisation.

This K application trial confirmed some long-standing findings. The lower the soil index, the lower the yield, regardless of the amount of fertiliser applied, as shown in Figure 1. This has long been the basis of soil fertility recommendations to get soils into index 3, which is where maximum yield potential is produced.

Potash deficiency hits the crop early in the season, as seen by leaf and tiller dieback. For this and other reasons, higher indices facilitate higher yield potential in the absence of applied K.

Yield potential is established very early in the life of a crop and for this reason there was no benefit in applying 100kg versus 50kg K/ha on the index 1 soil, but there was a yield benefit from the extra K on the index 2 plots. Saying this another way, the higher the soil K index, the higher the yield potential and the greater the chance of a yield benefit from higher K application.

While potash is important for growth and yield, it also has other functions. One that was particularly noticeable was its role in plant defence mechanisms. In these trials, high K availability improved the plant’s natural defence mechanisms, which was clearly seen in the high level of mildew in the low-K plots.

In a winter barley trial in 2016, potash rates were tested from zero to 200kg K/ha in 40kg intervals on an index 2 soil. All the K was applied in spring to both a two-row and a six-row variety.

On average, the two-row variety produced a yield benefit of 0.97t/ha while the six-row variety produced an average yield benefit of 0.7t/ha across all K rates. The six-row produced and additional 2.1 t/ha, on average, over the two-row and as each tonne of grain removes 10kg K/t the six-row removed an additional 21kg K/ha which must be reflected in fertilisation rate, Mark concluded.

One other factor examined in 2016 was the type of potash used. Muriate of potash was directly compared with the more expensive sulphate of potash, but the trial found no crop yield benefit from sulphate over muriate.

Carbon is organic matter

Carbon in its various organic forms fulfils many functions in a soil. Carbon is organic matter which is present at varying stages of degradation and when that process is completed, it releases various nutrients via mineralisation to supply plant growth.

Having more carbon in any soil, but especially in a tillage soil, helps sequester carbon and slow or prevent its release to the environment with the potential for climate change effects, Gary Lanigan of Teagasc told the meeting.

Such decisions should not be about the environment. Gary said that organic matter helps fulfil very many productivity requirements and affects the chemical, physical and biological properties of the soil.

For example, having higher soil organic matter (SOM) levels helps to increase P availability at high and low pH levels. It basically functions as a slow-release fertiliser which is very important during the yield formation stages.

Higher SOM helps soil aggregation, which is the natural process for restructuring damaged soils. It also helps increase a soil’s cation exchange capacity (CEC), which is its ability to retain nutrients. And, most importantly, it helps to decrease the yield gap with chemical fertiliser to reduce the rates required to produce maximum yields.

SOM is influenced by soil management. Tillage or cultivation acts to lower SOM levels because there is no cover present and organic matter decays when air is added during cultivation. Meanwhile, reduced tillage, straw incorporation or use as grassland will help hold or increase SOM levels depending on the starting point.

Gary said that SOM levels will tend to hit an equilibrium at both the high and low end for any land use system. But these can be modified to help raise SOM level or prevent its fall.

Good cropland gains and loses SOM on an annual basis and management governs which one has the biggest effect. Gary said that only about 20% of the organic matter content of straw remains in the soil, but that the remainder is used to fuel microbial biomass, which is also very important.

Reduced tillage systems tend to reduce SOM loss, but while levels can increase near the soil surface, Gary said that levels lower down in the soil may not increase.

For those who try to improve SOM, Gary said that cover crops will be a definite benefit, straw will help, as will rotations, but grass remains the best single practice.

Urea for spring barley

Leanne Roche has looked at the potential for losses from varying sources of nitrogen for spring barley. Three main sources were tested – CAN, urea and protected urea (using NBPT at 660 ppm).

Leanne said that the individual fertilisers had strengths and weaknesses on different measurements, but that the biggest estimated loss was through ammonia emissions and urea fared worst in this regard, with protected urea having less than 20% of the loss level.

Loss measurement is for environmental purposes. Growers use yield as their measure and, in this regard, there was no significant difference between the three fertilisers over the three years. Both urea sources were at least as good as CAN, with protected urea appearing to have a slight edge on output.

Nitrogen timing for winter barley

It’s important that crops look well in spring, but looks do not always result in tonnes. Having more nitrogen (N) at any stage will result in increased leaf area production, but the greatest uptake takes place between GS31 and heading. So does very early spring nitrogen benefit winter barley yield – Richie Hackett reported.

This question was asked in recent trials, which applied spring N either in late February, mid-March or late-March/early-April. The total used was 180kg N/ha in two splits, with either 30%, 50% or 70% applied as the first split to test rate by timing interaction. The crop used was Cassia sown in late October.

While early N and higher amounts of N in the first split helped the appearance of the crop and the canopy size, this did not translate into a yield benefit. If anything, the mid-March application appeared marginally best and the late first N timing often had maturity challenges due to unevenness of ripening.

Richie reported that the smaller the proportion of the total applied in the first split, the higher yield tended to be, with one exception in one year.

A separate trial looked at two versus three N splits, using a 30:50:20 split, with the third application at GS37. Richie said that the third split showed a slight but very small advantage on a two-row variety, but this was not tested on a six-row.

In another trial, Richie looked at the impact of nitrogen timing on variety and seeding rate in an effort to mimic poor spring plant populations.

These trials showed very little benefit in straying from the standard mid-March timing and quantity and there was very little difference between variety types. But he commented that perhaps a low seed rate is not a true test of a poor spring plant population.

Assessing soil structure

Structure is a hugely important aspect of any healthy soil. Eileen Jeuken explained that structure is the formation of aggregates that help in many different soil functions. It helps make a soil friable when cultivated and also helps water and air movement.

Eileen is working on a project called SQUARE, which aims to produce a simple evaluation of soil structure in grassland, called GrassVESS. This is a visual soil evaluation (VSE) which is to be used by farmers as a monitoring tool for management consequences. It requires very basic equipment – a 25cm spade, a tray, a trowel, a measuring tape and an assessment chart.

The system enables each sample to be scored and a high score represents poor structural quality. An equivalent test is also being assessed for tillage soils.

The soil test and testing

Soil testing attempts to estimate the level of available nutrient in a soil to guide fertiliser use and lime requirement. Dr Karen Daly from Teagasc explained the various challenges in trying to do this and the fact that certain soil characteristics can interfere with local tests, or how they should be interpreted.

Karen explained that the nutrients in a soil can be categorised as:

The fixed element is currently unavailable because it is locked up in the soil component and might not become available for decades or centuries. The available reserves are potentially available to fuel plant growth at some point in the future, while the reserves are what is available now and this is largely what is seen by the soil test. In relative terms, Karen said that a soil might have 500 to 1,500mg/kg of fixed phosphorous (P) with perhaps 100mg/kg as available reserves, but only 10mg/kg as reserves.

Basically, our P test using Morgan’s reagent only sees 1% to 2% of the total P in the soil and the test measures the ability to supply the nutrient rather than the amount. Some soils are slower to release P, while pH can affect P availability as well as a soil’s ability to release P.

So an individual soil can influence a test result. Soil testing is attempting to look at more characteristics to provide soil-specific results and advice.

Karen said that new testing tools are now available to replace wet chemistry to evaluate soils. A portable soil spectrometer can measure multiple soil properties in one scan and this might be used to also provide a measure of the factors which influence test accuracy so as to help provide better results and advice. This can also measure soil texture.

However, while these tools are now available and relatively affordable at around €2,000, Karen said that they are not yet fully calibrated for Irish soils and so of limited use until this is completed.

Teagasc launched a fourth edition of its major and minor nutrient advice booklet, now better known as the Green Book. David Wall of Teagasc is a co-author of the new edition, which is the major source of advice on nutrient requirements for different crops and land uses. David said that the provision of such advice extends back as far as the 1940s and has since evolved to provide relevant and up-to-date advice on nutrient requirements.

David reported that, in 1958, the standard advice was for 25kg to 50kg N/ha for cereals, but by 1986 this had increased to 185kg N/ha for winter wheat. Advice evolved with practice and knowledge and this must remain the case.

Nowadays, nutrient recommendations have been refined to reflect off-take and so recommended rates can reflect historic yields. Basically, rates evolved with productivity over time.

The Green Book must be calibrated against field performance and soil testing. David stated that nutrient use must now be advised in the context of legal requirements under the nitrates directive.

This forced a more holistic approach to nutrient use on farms, but lime is still a major issue. Annual use fell off considerably since the mid-1980s when it dropped from around 1,700,000t to around 800,000t.

David told us that the new edition must build on the experiences of the past, while taking into account the latest soil and fertiliser research. It must also take account of increased nutrient demand due to increasing output and off-take and it must balance agronomic and environmental optima with production economics.

In doing this, David said that the new book will have four new sections which look at:

There are also a number of sections that have been revised due to new information. These include:

The actual book will be available online from early November and hard copies will be available after that.