Lime – The Misunderstood Crop Input

In recent weeks you may have read that lime is underutilized, that low pH leads to wasted fertiliser, and not all lime is the same. You may have read that Granulated lime is easy to store, easy to use, easy to apply, lands on target and is quick to react. However, because we do not necessarily understand how lime works to raise soil pH and how soil pH relates to soil nutrients and their availability to the plant, it is easy to not pay attention to how vital lime and pH are to crop production.

The further we get into the subject of soil fertility, the more complicated things seem to become. We know that nitrogen, phosphorus and potassium are the nutrients required in the largest amount to achieve desired yield. We understand that there are other nutrients such as magnesium, calcium and sulphur that are also required to produce a viable crop. Beyond that, we know there are many other nutrients that the crop requires, but it begins to get far less clear. In fact, most businesses offering soil testing services do not even offer comprehensive soil tests to measure soil fertility beyond P, K, Mg & pH. Then when it is time to order fertiliser, it is easy to get confused as to what type and how much fertiliser to apply. Especially when it comes to fertilisers containing S, Zn, Na, Se, Ca, Mg etc.

Our soil tests may not be telling us much about these other nutrients that we are potentially adding to the soil with fertiliser, but they are telling us something that we have a tendency to ignore: our pH in Northern Ireland is generally well below the recommended 6.4 for cereals and 6.2 for grassland. We don’t necessarily understand what this means however and as a result, we are generally happy if our pH is near 6.0. This can no longer be acceptable. The recommended levels exist for a reason. Close is not good enough and yield/quality can be severely impacted if pH levels fall outside of the recommended ranges. The availability of each nutrient varies depending on the pH. Some are more available at low pH, some are more available at higher pH. The majority of nutrients are most available to the crop for uptake at a pH between 6.2 and 6.8. As soil pH moves away from this range, the soil locks the nutrients up and the plant is unable to extract them from the soil.

This results in money being wasted on fertiliser nutrients that get locked into the soil and cannot be extracted. In 1828, Carl Sprengel published “The Theory of Minimum” which states that ‘yield is proportional to the amount of the most limiting nutrient, whichever nutrient it may be’. Therefore, if we supply whichever nutrient that is needed, we can increase yield up to the point of the next deficient nutrient. What if however, we could supply a sufficient amount of all nutrients?

By applying lime to the soil, we can raise our pH to the optimal level between 6.2 and 6.8, making the most use of the nutrients that we apply to the soil, and maximizing the plants ability to extract the nutrients it requires to achieve the best potential yield it can. If soil pH is correct, you will not only attain higher yield, but often you will have a higher quality crop.

Therefore, if your soil is out of the optimal range, yield and quality will suffer as a result and money will be wasted on fertiliser that is not used.