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Post-Amelioration Fertiliser Strategy: Managing Nutrient Needs in a Mixed-Up Soil Profile
Research on this is still in the early stages, but there are a few trends emerging.
Amelioration tends to increase yield potential, which suggests a need for more nutrients. However, if crop roots can grow better and access previously unavailable nutrients, it might balance out somewhat in the short term. Amelioration probably won’t reduce fertiliser needs immediately. Monitoring nutrient removals and soil tests will be vital to assess fertiliser requirements.
Amelioration can redistribute nutrients in the soil, and the more intense the tillage, the more they are moved. A detailed study of soil relocation by different implements showed that mouldboard ploughs create nutrient-rich seams in the subsoil, whereas rotary spading creates a screw-like pattern, but both machines create patchy subsoils. This animation shows how soil moves during ripping.
Soil movement makes post-amelioration soil testing to work out fertiliser rates a challenge. For example, ploughing creates a very heterogeneous soil, mixing layers and potentially soils of different textures and densities. It is very hard to get a representative soil sample that reflects the treated area. You might pick up a patch of clay in one sample but no clay in another. In order to achieve a more representative soil sample, more cores are needed, along with a more strategic approach. It’s likely that you know where the soil types change, so sample strategically within zones.
Nitrogen
Amelioration tends to increase your chances of achieving the economic yield potential, which may require more nitrogen (N) than you’ve been using in the past. Soil organic carbon will also likely be buried or diluted (see how soil carbon changes after amelioration in the tables above), so you might not mineralise as much N in the first year, depending on the tillage equipment used. Before sowing, mineral N soil tests are crucial in the second year after amelioration to gauge your N stocks and where the N is sitting in the profile. Consider sampling at 0-10 cm, 10-30 cm and 30-60 cm depths to understand the impact of amelioration, even if only across a few small zones.
Phosphorus
If the crop was responsive before ameliorating, the response is likely to be similar after amelioration i.e., phosphorus (P) rates will not change.
Given that P is relatively immobile, applying it before soil amelioration is an opportunity to boost soil stocks and plant availability. Soil mixing can also help redistribute bands of P that have formed near the soil surface over the years. In sandier soils, where P levels are similar throughout the soil profile, mixing will have little impact on P distribution.
If the soil is inverted, for example, by mouldboard or Plozza ploughing, and the subsoil brought to the surface is low in P, the crop may need more starter P.
Soils that require amelioration are usually sandy soils which have very low P fixation potential (PBI) at the surface. But, if you flip the soil, you might bring calcareous sand to the top, which has a higher PBI, so replacement P rates may no longer be suitable for recently ameliorated sand. That’s why it’s important to re-test your sandy soils after amelioration. Give them a year to settle and then sample the top 10 cm to see if your nutrients have been diluted and what P replacement strategy you’ll need to use going forward.
Trace elements
Most trace elements are immobile. Like P, amelioration could redistribute trace elements deeper in the soil. In this case, be mindful then to apply trace elements in the first year to top up the surface layers, particularly if you have inverted or mixed the surface.
