Soil fertility refers to the ability of soil to supply nutrients and sustain plant growth. It is the combined effect of three major interacting components; the chemical, physical and biological characteristics of the soil.
Fertile soil will contain all the major nutrients for basic plant nutrition:
- Nitrogen (N)
- Phosphorus (P)
- Potassium (K)
- as well as other secondary nutrients needed in smaller quantities:
- Calcium (Ca)
- Magnesium (Mg)
- Sulphur (S)
- and trace elements: Iron (Fe), Manganese (Mn), Zinc (Zn), Copper (Cu), Boron (B) and Molybdenum (Mo)
- as well as other secondary nutrients needed in smaller quantities:
There is no soil test for N. Phosphorus and Potassium are the most common nutrients tested along with soil pH. The most limiting nutrients for plant production are N, P and K.
Secondary nutrients can be tested but you must specify this when submitting your soil samples. Generally, there is no need to test secondary nutrients unless your advisor identifies a specific problem.
Table 1: The soil index system
Soil index | Index description | Response to fertiliser |
1 | Very Low | Definite |
2 | Low | Likely |
3 | Optimum | Unlikely – but it is important to apply maintenance levels of fertiliser |
4 | Sufficient / Excess | None |
It has been shown that over 80% of Irish grassland soils are suboptimal in either pH, P or K. This is severely limiting potential grass production from these soils.
Soil testing is the first step in meeting nutrient requirements to maximise grass or crop production. It is probably the most cost-effective investment you can make in order to increase the output from your farm.
How to soil sample
Sample the field in a W pattern. Take one sample for each 2-4 ha if the area is uniform. Avoid gateways, ditches, troughs etc. The sample should be representative of the field or area being sampled.
Take a minimum of 20 soil cores to a depth of 10 cm, mix them together, and take a representative sub-sample for analysis, making sure the soil sample box is full.
Label the sample box correctly. Avoid soil sampling if the ground is saturated or unusually dry.
When sampling, divide the farm into fields or areas that can be easily managed individually to apply fertiliser.
Figure 1: When soil sampling, the field should be sampled in a W pattern.
When to sample your soil
It is best to soil sample at a similar time each year for comparison and avoid sampling when soils are extremely wet or dry.
The ideal time to sample soils is from September to March and there is no time like the present. This will allow you to put a fertiliser plan in place for 2019 to target specific fields for specific nutrients.
Ideally, the entire farm should be sampled every 2-4 years (or a proportion of the farm e.g. 30% per year to spread fertiliser costs).
Do not sample until 3-6 months after the last P, K or slurry application; or 2 years after lime for pH.
Results
There is no point in sampling your soil unless you utilise the results. This will save you money, by applying the correct rate of fertiliser in the right field and improving yield performance from the whole farm. When you get your soil sample results it is important to then develop a fertiliser program specific to each field. Many farms create a farm map showing which fields are low in P and K and this is very useful when deciding where to spread slurry.
Soil pH – The most important deficiency to correct and the cheapest one! For grassland, target a soil pH of 6.2 to 6.5 (pH 5.5 on peaty soils). Apply lime according to soil test recommendations. A maximum of 7.5 t/ha (3 t/acre) should be applied in any one year. For recommendations greater than that, split the application and apply the balance 2 years later.
Benefits of correcting pH
- Release up to 80 kg of nitrogen from the soil compared to low pH soils
- Make P and K more available in the soil
- Increase efficiency of applied N, P and K fertiliser
- Increase grass DM yield by approx. 1.5 t DM/ha (compared to low pH soil)
Phosphorus (P)
Phosphorus is necessary for the physiology of the plant, including photosynthesis, root and tiller development. Like other nutrients P can get “locked–up” in the soil if the pH is low; another reason why correcting soil pH should be the first step in improving soil fertility. Phosphorus use is restricted within the Nitrates directive.
Potassium (K)
Potassium can be spread all year round (there is no restriction on its application), nonetheless there are a few points to consider. In spring K applications should not exceed 90 kg k/ha in a single application. If you have a requirement above that it would be best to apply the balance in autumn. On rapidly growing swards that receive high levels of K grass tetany can occur as it prevents Mg uptake by the grass crop. Regardless, it would be best practice not to apply any fertiliser in high amounts in one application and split the application to get the best results. On silage ground, there is a large demand for Potassium. For every 1 t DM/ha grass removed, 25 kg K/ha will be removed. A 10 t/ha yield will remove 250 kg K/ha.
Use your slurry wisely
You should target slurry to first cut silage ground or fields low in P or K (Index 1 or 2). Slurry nutrients can vary widely so you need to estimate N, P and K content. Use a slurry hydrometer to determine slurry DM content, or speak to your agricultural advisor who may be able to test it for you.
Knowing the DM % of your slurry will allow you better estimate the N, P and K levels. This will help avoid leaving crops short of nutrients and losing yield or over-application of costly purchased fertiliser.