Checking the health of your soils – what to measure first
NRM
Agriculture
There are a wide range of parameters that can be measured to evaluate soil health, but there are some key analyses that will help check the capacity of the soil for sustainable crop production in the future. These fall into three categories: the chemical, physical and biological status of the soil. They are inter-related, and all have an impact on soil health.
Chemical analyses
In the past growers have regularly measured soil pH as well as phosphate, potash and magnesium availability. Today, with more focus on the total nutrition of the crop, it is also useful to check the levels of micro-nutrients in the soil. It is very easy for sub-clinical deficiencies to go unnoticed and reduce crop yields. At the same time, the cation exchange capacity (CEC) should be assessed. This is a guide to the quantity of the main cation nutrients, potash (K), magnesium (Mg), calcium (Ca) and sodium (Na) that the soil can hold. If we measure the availability of the broad spectrum of nutrients in the soil, our crop nutrition plans can reflect their availability and can include micro-nutrients in both foliar and solid fertilizers. Remember, soil pH has a major effect on nutrient availability for both macro and micro-nutrients. For arable cropping most nutrients will be readily available to the crop at a pH level of around 6.5.
Physical assessments
A key physical measure for every field is analysis of the soil texture, which can now be efficiently measured in the laboratory. The proportions of sand, silt and clay are recorded, and the soil classification reported. This valuable information, which doesn’t change over time, will provide the foundation for all cultivation plans. Ideally the composition of the soil volume should be around 45% mineral material, 5% organic matter, 25% air and 25% water
The starting point for assessing the physical status of any soil is to take a spade and dig inspection holes, initially in the topsoil layer down to 30 cm where possible. Look for signs of water logging, which would turn a soil from aerobic to anaerobic, as well as signs of compaction which will inhibit drainage and root development. It is vital that our soils remain aerobic to enable the roots to access oxygen and for the soil microbes that require oxygen to thrive, as they are so important in making nutrients available to crops.
The extent of compaction should be identified to enable remediation to be planned. However with wet soils being vulnerable to compaction, it is essential to pay attention to the effectiveness of drainage systems, both land drains and ditches. The degree of compaction can be benchmarked using the VESS scoring system (Visual Evaluation of Soil Structure). A spit of soil 20 cm X 20 cm is removed and examined to identify compacted layers and the soil given a score, from 1 (very good) to 5 (very poor).
Biological Measurements
Understanding the organic matter content of each field is critical. Many soil functions rely on adequate organic matter, which provides a food source for microbes, helps resist compaction and improves the water and nutrient holding capacity of soils. Building organic matter is a slow process, but many arable soils have become depleted, affecting soil health and crop performance. The warmer and lighter soils are at the highest the risk of organic matter loss. The organic matter levels in fields should be monitored every four to five years by laboratory analysis.
Increasing organic matter will also increase the amount of carbon stored in the soil and it is worth measuring Active Carbon if you are trying to improve your soil health. This test measures the readily available portion of soil carbon that provides a food source for soil microbes and which is therefore considered a key soil health indicator. Regular measurement of active carbon will highlight the impact of different soil management techniques quickly and easily.
Another useful test to assess soil health is to measure carbon dioxide released from the soil (CO2 burst test), which indicates the potential level of microbial activity. If this is found to be low, it can indicate low organic matter content or water-logging due to compaction causing soils to be anaerobic. Microbial populations are also very sensitive to acid conditions and their development is severely restricted in acid soils, reducing nutrient availability. This reduces nutrient use efficiency and reduces crop yields.
The chemical, physical and biological factors mentioned above are all interconnected in their effect on yield. Acid soils restrict microbial activity, nutrient uptake is then inhibited, and crop yields reduced, highlighting the need to keep soil pH at optimal levels. To make sure crop yields will not be jeopardised by an unhealthy growing environment, check the health of your soils now.
Talk to your agronomist or contact NRM to arrange for samples to be tested for the full range of factors above. The NRM Soil Health report will provide the data to identify the health of your soils and highlight problems where they exist.
