After the harvest

Shorter days, cooler nights – it’s harvest time for summer cereal crops. Where this may once have meant land lying fallow over winter, nowadays crop paddocks are more likely to go straight back in to production so that farms can maintain the high dry matter levels that are essential in modern agriculture.

Keeping productivity high almost inevitably means using some form of fertiliser to replace those nutrients removed in the crop. In the past, when fertiliser costs were relatively low, it was common to apply enough of each nutrient to support maximum crop growth. Nowadays, though, a more cautious approach is required, both from an economic and environmental perspective.

So, after you have harvested your summer cereal crop, what is the impact on the fertiliser strategy you need to follow to get the best out of the land over the next six months?

What have you lost?

Growing any crop removes nutrients from the soil. The exact quantities removed depend on the crop, yield and, in the case of potassium, the amount of nutrient present in the soil (plants will take up more potassium than they need, if it is available, a phenomenon known as luxury uptake). Figures 1 and 2 show nutrient removal for maize silage and maize grain crops, and illustrate how the quantities of nutrients removed increase with yield.

Crops that are harvested remove more nutrients than crops that are grazed, since nutrients are returned in dung and urine. Management of crop residue also affects nutrient removal rates – ploughing residues back in to the soil returns some of the nutrients taken up by the crop; burning residue returns far less.

It’s been common wisdom that burning results in the loss of nitrogen and sulphur to the atmosphere, but that most of the other nutrients are returned to the soil in the ash. However, some interesting work done in Canada casts doubt on this assumption.

Researchers there studied what happened when straw from wheat, oats and flax crops was burnt. The result for wheat and oats is shown in Figure 3. The graph shows that more than 90% of the carbon in the straw was lost on burning, along with nearly all of the nitrogen, and around three-quarters of the sulphur. What they didn’t expect to find was that up to a quarter of the phosphorus and some 35% of the potassium was also lost.

Where did these nutrients go? The most likely explanation is that the nutrients drifted away with the smoke. This would contain particulate matter, and that in turn would contain the nutrients. It’s possible – indeed, likely – that the particulate matter would eventually settle down again, but where and when that happened would depend on the wind and other factors that influence smoke dispersion.

Soil sampling after burning

If land is cleared by burning, rather than being subjected to ploughing, anomalies can arise when the land is soil tested. This happens because the material to be burned is usually not evenly distributed over the paddock, so when it is burned, the ash is concentrated is discrete locations, e.g. along windrows. This means that the nutrients in the ash are also concentrated in these locations.

Samples taken from these ash-dense parts of the paddock will likely return high results for nutrients such as potassium. In one example (from Canada), the soil where windrows had been burnt returned potassium readings of 325 ppm, while the rest of the field gave results of 114 ppm. The composite sample (223 ppm) would have given a misleading picture of the nutrient status of this soil. This illustrates the importance of understanding the history of the paddock and taking this into account when soil sampling.

The next step

While it is important to have an appreciation of the relative amounts of nutrients removed by a crop and the factors that affect this, when it comes to determining what post-harvest fertiliser strategy to adopt, it is more critical to focus on the next crop you’re going to plant.

Soil testing is essential and should reflect both the root depth of the crop to be planted and the cultivation plans for the land. If the land is going back into pasture and is not going to be cultivated, then samples should be taken at 75 mm depth. However, if cultivation will be carried out after soil tests are taken, then sample at 150 mm. This compensate for the nutrient dilution that occurs when soil is cultivated, and so will give a more accurate assessment of fertiliser requirements.

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