Measure to manage nitrogen inputs successfully in 2022

Author: Alli Grundy

17th February 2022

NRM

Agriculture

Autumn and winter rainfall has a seasonal impact on the amount of available soil nitrogen in early spring. However, it is risky to assume that supplies are always high or low because the preceding months have been drier or wetter than average. Individual farm factors also influence the availability of soil nitrogen and should be accounted for when estimating spring levels. Field history and management, soil type, and the established winter crop can also have a large bearing on spring available soil nitrogen supply (SNS).

Influence of field history and management

Farms with a long history of manure use or that grow forage within the rotation tend to maintain higher levels of soil nitrogen. This is because the supply of organic nitrogen often exceeds crop demand and builds over time. More and more arable farms are also being encouraged to establish ground cover during the winter months ahead of spring sown crops. This is to mop up any excess nitrogen before heavy rainfall can leach it out of the system. Businesses that switch or expand arable enterprises away from dairy/livestock systems are likely to have larger nitrogen reserves for an extended period of time, particularly if crops are grown on medium to heavier textured soils.

Underestimating the impact of previous management on future SNS will only increase the inefficiencies of crop production and conflate the impact of challenging autumn and winter weather patterns.

Influence of soil type

The nitrogen content of lighter textured soils is influenced by higher rates of rainfall over winter. It is also often reduced in spring compared with a more clay rich soil, particularly if fields are left bare over winter. However, farms between and within regions don’t all receive the same amounts of rainfall and nitrogen retention on the lighter soil isn’t always as low as we might expect.

Variation in nitrogen content naturally occurs within soil textures of all types. Lighter textured soils warm up faster in early spring and begin to mineralise nitrogen earlier than those with a higher clay content. So, management plans for autumn or spring sown crops on these types should be adapted to account for this natural variation.

Influence of crop on available SNS

The SNS calculation includes not only the proportion of available soil mineral nitrogen (SMN) and organic nitrogen a soil contains in spring, but also how much of it the crop has recovered over the preceding autumn and winter months. Cereal crops sown in the autumn have often not taken up much nitrogen by spring—maybe 30-40kg N/ha for the very forward crops, but typically less. However, it is still important that this contribution is included in the SNS estimation.

Spring crop nitrogen assessment is critically important for winter sown oilseed rape. Larger canopied oilseed rape crops can easily have taken up 100kg N/ha by February and understanding how much remains in the soil is important because it influences the management and timing of applied nitrogen. It may be that the crop canopy is large, and so the soil is depleted, or the supply remains high. On the other hand, a crop canopy can be small with a high or low SNS. These combinations can occur within the same field and/or between fields. It is important to note that the SNS can still be large when the soil nitrogen content is low because both components contribute to the SNS assessment. Nitrogen is found either in the (autumn sown) crop or in the soil, and each component must be included for an accurate estimation of SNS.

Impact of crop size and soil supply on the overall SNS

This table illustrates the impact of crop size on the overall SNS. Spring crop nitrogen content can be small, but the soil reserve remains medium or high, resulting in a large SNS overall and reduced spring crop requirement.

SNS = SMN + est. mineralised N + spring crop N

Why do we only want to know if soil supply is low?

As soon as January arrives, the industry has been keen to understand and predict what the average supply of spring soil nitrogen is likely to be against the backdrop of either a wet or dry winter. A wet autumn and winter give rise to the easy message that “nitrogen inputs should increase” to account for a diminished SNS. A drier autumn and winter means that soil nitrogen retention should be increased, and is particularly important in the current climate of increased fertiliser nitrogen costs.

To use applied nitrogen efficiently, every business should understand the variation in soil nitrogen supplies across their farm’s rotation regardless of rainfall. By sampling fields that represent the number of soil types and crops grown, real efficiency gains can be made. This is particularly true if livestock manures are used, and soil fertility is suspected to be high. Determining the variation in SNS should be planned at a resolution that represents the scale of the enterprise and is essential if operational costs are to be effectively managed. The cost of production has been brought into focus because of the sharp rise in input costs, and the move towards more nature-based approaches to producing our food. Measuring available SMN is a part of the toolkit to help us meet our environmental objectives and work with the soil rather than trying to master it. This is the best way to increase soil health and therefore productivity and profitability.

Measuring to calibrate the RB209 SNS index

SMN measurement is incredibly cost effective compared to the cost of over or under fertilising crops and can easily be used to calibrate the SNS index system in the nutrient management guide RB209. NRM’s N-Check Plus service completes this step by not only reporting the amount of soil nitrogen measured but correlating the result to the corresponding RB209 SNS index number. As a farmer or adviser, it is very easy to find the appropriate crop recommendation table and determine the optimum nitrogen rate.

Don’t speculate on what the SNS might be on your or your customer’s farms. Soil measurement means being in control of costs, increasing farm efficiencies, profitability, and protection of the wider environment.

NRM’s new N-Check service analyses soil for Nitrate-Nitrogen, Ammonium-Nitrogen, soil mineral nitrogen, and gives a total value for the combined soil depth. N-Check Plus adds soil organic matter (SOM) to estimate mineralisable nitrogen, spring crop N to determine the soil nitrogen supply (SNS), and the SNS index based on RB209 guidelines.

It is important to note that you must take your samples from fields ahead of applying any nitrogen fertiliser or organic manures/materials.

For more information, visit our website or contact me directly allison.grundy@nrm.uk.com.