Soils may supply less N than we think

With the nitrates directive up for review this year, this Teagasc study provides information on the amount of soil nitrogen that is available from our soils for our crops. Soils can supply large amounts of nitrogen in some instances but these trial results show how little soil nitrogen is supplied in some soils.

The amount of fertiliser nitrogen required by any crop will be influenced by the amount of nitrogen that is likely to originate from the soil. The more nitrogen that the soil supplies, the less fertiliser nitrogen will be required.

However, the amount of nitrogen that soils can supply varies considerably and is influenced by many factors including previous crop, soil type and weather. Developing a better understanding of these variables would help improve fertiliser nitrogen recommendations.

Soil survey

Research was carried out in conjunction with Teagasc, Oak Park, and UCD over a three-year period. This was effectively a soil survey to investigate soil nitrogen supply to winter wheat crops in Ireland and the factors affecting it.

The survey involved setting up 110 plots 25m2 in size in commercial winter wheat fields. These crops followed different preceding crops which included grass, oilseed rape, potatoes, maize, continuous cereals, peas and beans.

The plots were also established on different soil types covering the main tillage region of the country from Ardee in Louth to Ballycotton in Cork. Soils were divided into three categories of heavy, medium and light. The three-year duration also allowed different weather factors to be considered in the observations.

In the commercial crops the plot areas were covered with plastic when the nitrogen fertiliser was being spread. This was to ensure that the nitrogen accumulated by the plants in that plot came from the soil and not the fertiliser. Then by measuring the total amount of nitrogen in the crop (straw and grain) at harvest, which was deemed to be the soil nitrogen supply, the effect of various factors (previous crop, soil type and weather) could be studied.

Previous crop

Knowing how much soil nitrogen is in a soil following different previous crops can allow more or less fertiliser nitrogen to be applied to make up the balance of its requirement.

As expected, soils following grass had the highest soil nitrogen supply at an average of 119kg N/ha. This was true for soils freshly out of grass and two to four years out of grass. It should be noted that these soils after grass were after permanent pasture.

Soils following maize were the second-highest contributors of soil nitrogen supply. This may be due to the large amounts of slurry and/or farmyard manure applied to maize crops in this country. This is in contrast to the current soil nitrogen index used in Ireland which indicates that maize leaves little nitrogen behind.

This study showed low soil nitrogen supply from soils following peas and beans and actually placed them in the same range as sites where the previous crop was a cereal (52kg N/ha). In our current soil N index system, peas and beans are regarded as providing index two, although there were fewer sites with peas and beans.

Soil Type

Soil type clearly affected the supply of nitrogen from the soil. Heavy soils (eg in Meath and Dublin) supplied the most soil nitrogen, while medium (eg Laois and mid-Kildare) and light (eg south Kildare and Carlow) soils followed.

This is to be expected as light soils, as well as tending to have lower soil organic matter levels, tend to be free-draining and more prone to leaching while heavy soils tend to have higher soil organic matter levels and are less prone to leaching loss.

Figure 2 indicates the differences in nitrogen supply due to soil type, but it should be noted that the crop soil nitrogen supply is on soils following different crops. Soil type is not included as a factor under the current nitrogen index system in Ireland.

Weather

Winter rainfall had a clear effect on soil nitrogen supply. In the three years of this trial, winter rainfall increased each year. As winter rainfall increased, soil nitrogen supply decreased as reserves were depleted during the winter months. From the graphs it can be seen that where soil nitrogen supply was high, winter rainfall was low. Increasing winter rainfall in 2013 and 2014 led to lower soil nitrogen supply. Soil temperature did not have a significant impact on the soil nitrogen supply.

Can soil nitrogen supply be predicted?

As stated previously, a lot of factors can influence soil nitrogen supply. The next step in this work was to see if the amount of nitrogen that a crop would get from the soil could be predicted early in the growing season, at a time when it could help fine-tune fertiliser nitrogen decisions.

To do this, the factors outlined previously (previous crop, weather and soil type) were tested in a number of different prediction models. These were tested, either alone or in various combinations, to see how well they predicted the final availability/supply.

A prediction model that used only previous crop groupings, similar to the current nitrogen index, explained just 20% of the variation in soil nitrogen supply and so was found not to be a very good model for predicting soil nitrogen supply. A model based on previous crops alone, which used different groupings, did perform somewhat better but was still relatively poor at predicting soil N supply (see Figure 4).

When more of the factors were taken into account, the prediction ability increased. For example, a model where previous crop group and soil type were included improved the prediction ability considerably to 50% (Figure 4).

The prediction model which fared best at 72% in explaining the majority of the variation in soil nitrogen supply included previous crop group, soil mineral nitrogen content in the springtime and spring crop nitrogen uptake. This model gave relatively good predictions. However, it required the amount of mineral nitrogen in the soil in spring to be measured.

This measurement requires soil samples taken to 90cm, which then need to be analysed within a day or two of being taken. This makes it laborious and expensive and impractical on a wide scale. It is hard to justify this test. Other soil nitrogen tests were also examined, but from the samples gathered in this study, these tests were not suited to Irish conditions.

This trial work has once again confirmed that to get a good prediction of the amount of nitrogen that a crop will get from the soil requires a lot of information, some of which is difficult to obtain. There is, therefore, a trade-off between precision and cost.

In Figure 4, the model which uses previous crop group and soil type only requires a knowledge of the field’s history. But still this can explain 30% more of the variability in soil nitrogen supply than our current nitrogen index system. This also means that it has the potential to more than double the accuracy of soil N supply and prediction to the crop.

This study has identified valuable information and it shows that there is potential to update and improve our current nitrogen index system, with benefit for the farmer and the environment. These results suggest that we could go back to criteria that were used in the past and include soil type in our decision-making process for the amount of fertiliser nitrogen to be applied to winter wheat and cereal crops in general. To do this would require very little additional information, or cost, relative to the system currently being used.

Acknowledgements

My thanks to all of the farmers who provided access to their fields and prevented application of N to the 25m2 plots used in this study. Thanks also to the Teagasc Walsh Fellowship Scheme for funding the research.