Mechanical soil amelioration can change crop fertiliser needs. Increased yield potential from amelioration suggests an increased need for nutrients. However, as amelioration creates a more favourable environment for crop roots, better root systems can access previously unavailable nutrients.

This balance between a greater need for nutrients and improved access to soil nutrient stocks might explain why in the short term at least, pre and post-amelioration responses to fertiliser do not differ much on soils with reasonable nutrient status.

Mechanical soil amelioration also theoretically moves nutrients within the soil profile, particularly relatively immobile nutrients such as phosphorus and trace elements. When the soil is mixed, these nutrients have more chance of being distributed throughout the root zone and might also be moved into more wettable soil.

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 (Scanlan and Davies, 2019).

In water-repellent soils, amelioration can move nutrients from dry soil into more wettable soil, which usually improves nutrient uptake. Soil mixing or inversion also allows the soil to wet up more evenly, which improves nutrient uptake.

Research to date suggests that ameliorated soils are at least as responsive to fertiliser as non-ameliorated soils. Using the same fertiliser rates should not reduce the return on investment, but you might not get as much yield uplift from amelioration if soil and fertiliser stocks cannot meet increased crop needs.

In the long term, the higher yield potential should increase fertiliser needs, especially where soil nutrient stocks are low. Amelioration is unlikely to reduce fertiliser needs.

Deciding whether or not to change fertiliser rates depends on the expected increase in yield, amelioration quality and post-amelioration management. If the amelioration is done correctly and other potential limitations such as crop establishment and herbicide rates are managed properly, it is a good idea to reconsider fertiliser rates.

Nitrogen

Data on changes in crop nitrogen use is still limited; however, mechanical soil amelioration appears to increase crop response to nitrogen. One trial on a compacted deep sandy duplex found that nitrogen response increased in the second year after deep ripping. Non-ripped soil with added nitrogen did not have the same increase in gross margins. There could be an increase in mineralisation from mechanical amelioration; however, this research is still in progress.

Phosphorus

Responses to phosphorus do not change much. If the crop was not responsive to phosphorus before ameliorating, it is not likely to respond afterwards. If the crop was responsive before ameliorating, the response is likely to be similar after amelioration; in other words, phosphorus rates will not change.

Because phosphorus is a relatively immobile nutrient, applying it prior to soil amelioration is an opportunity to boost soil stocks and plant availability. Mixing can also redistribute bands of phosphorus that have formed near the soil surface over the years. In lighter soils where subsurface phosphorus levels are similar to those nearer the surface, redistribution of phosphorus by mixing is minimal.

If the soil is inverted – for example, by mouldboard ploughing – and the subsoil brought to the surface is low in phosphorus, the crop might need more starter phosphorus. Knowing subsoil phosphorus stocks before inversion will guide phosphorus fertiliser decisions.

Potassium

Soil amelioration appears to improve access to soil potassium, particularly in sandy water-repellent soils. However, be prepared to apply more potassium. Although tillage can improve nutrient availability, there might not be enough in the soil to support the increased crop growth. A 2020 analysis of ameliorated paddocks found that potassium was deficient in 20 to 40 per cent of flag leaf samples collected.

If a crop is not responsive to potassium before amelioration, it might respond post-amelioration. If it was already responsive to potassium, its potassium needs will be at least the same or increase further.

Trace elements

Responses to trace elements are unlikely to change after amelioration, mainly because supply of trace elements is good in most situations before and after amelioration.

Even if soil stocks of trace elements are adequate, tillage amelioration is a good time to top up less-mobile nutrients such as copper and zinc. Tillage will likely distribute them better through the root zone, ensuring better availability to plants for years to come. Also consider extra manganese if applying lime as part of the amelioration, as manganese deficiency can occur in high-pH or recently limed soils.

Acknowledgements: Craig Scanlan (Department of Primary Industries and Regional Development)

This article was produced as part of the GRDC ‘Maintain the longevity of soils constraints investments and increase grower adoption through extension – western region’ investment (PLT1909-001SAX). This project is extending practical findings to grain growers from the five-year Soil Constraints – West suite of projects, conducted by the Department of Primary Industries and Regional Development (DPIRD), with GRDC investment. The research summarised here is part of current work in ‘Nutrient re-distribution and availability in ameliorated and cultivated soils in the Western Region’ (DAW1801-001RTX).

References

Potassium intrigue after sandy soils amelioration

Soils research offers glimpse into plant reaction to nutrients

Scanlan C A and Davies S L 2019. “Soil mixing and redistribution by strategic deep tillage in a sandy soil”. Soil & Tillage Research 185, 139-145.