Agricultural soils are tested to measure the soil quality and nutrient levels for monitoring fertiliser programmes and to assist with identifying nutrient deficiency or toxicity.
Why test soil?
Most New Zealand pastoral soils are deficient in phosphorus, sulphur and to a lesser extent potassium and some trace elements can also be deficient. Soils under intensive cultivation may have abnormal levels of some nutrients due to previous management practices and fertiliser programmes. Soil testing takes the guess work out of nutrient management and allows cost effective fertiliser programmes to be produced. .
The value soil testing delivers
Measure whether soil nutrient levels are high enough to sustain the desired level of plant growth.
Indicate the existence of any deficiency, excess or imbalance of major nutrients.
Provide a scientific basis on which to assess fertiliser and lime requirements of crops, pastures and turf.
The Organic Soil Profile measures the soil quality, testing the soil organic matter, available nitrogen, total nitrogen and carbon:nitrogen ratio.
Soil tests measure only a fraction of the total pool of nutrients available to plants (immediate and long term) so it is important that standard NZ methods of sampling and soil analysis are used so the results are meaningful and can be related to plant growth under NZ conditions. A single soil test is of limited value as it is essentially a snapshot in time of a dynamic environment. Trends in soil test values over time will give a better indication of how well your fertiliser program has worked. To obtain a more complete picture of farm nutrient status, soil testing should be complemented by plant testing which measures the mineral nutrients that the livestock are eating.
Even when soil nutrient status is at optimum levels other factors such as soil temperature, rainfall and compaction can influence the rate of uptake of nutrients by plants.
The soil tests to order
For Pastoral Soils: a Basic Soil Profile and sulphate-S test are recommended. Where reactive phosphate rock (RPR) fertiliser has been used, the resin phosphorus (RP) test should be added.
For Arable Crops like Maize: a Basic Soil Profile, sulphate-S and available-N are recommended.
Other Soil Tests include: organic sulphur, total sulphur, the organic soil profile for soil quality, aluminium, boron, EDTA trace metals, Mehlich 3 trace metals and more.
For agriculture samples, collect twenty 7.5 cm cores from across the area. For a representative sample, aim for 500g of soil. Avoid dung and urine spots and recently grazed pastures and do not sample within three months of applying fertiliser or lime. For arable samples, collect twenty 15 cm soil cores. Sampling at the same time of year in the same way will give better nutrient monitoring.