A Complete Guide to Soil Fertility and Testing

Soil fertility is a crucial factor in determining the success of any agricultural operation. It refers to the ability of the soil to provide essential nutrients to plants for optimal growth and production. However, soil fertility can vary greatly depending on factors such as climate, soil type, and management practices. Therefore, it is essential for farmers to regularly assess the fertility of their soil to ensure healthy and productive crops.

Why assess soil fertility:

• Optimal crop growth: Plants require a variety of nutrients to grow and produce healthy yields. By checking soil fertility, you can determine which nutrients are lacking in your soil and make necessary amendments to ensure optimal crop growth.

• Cost-effective: Fertility testing can help you avoid over-fertilization, which not only wastes resources but can also harm the environment. By knowing the exact nutrient needs of your soil, you can save money by only applying the necessary fertilizers.

• Prevent nutrient deficiencies: Nutrient deficiencies can lead to stunted growth, reduced yields, and even crop failure. By regularly assessing soil fertility, you can identify and address any nutrient deficiencies before they become a problem.

Plants require 17 essential nutrients for their growth and development, which they obtain from soil, water, and air. In Punjab, where cropping intensity is high at nearly 200 percent, there is a deficiency of certain essential nutrients in both soil and plants. This is mainly due to the imbalanced use of one nutrient, which can lead to the unavailability or toxicity of other nutrients. Further, it is important to note that not only does soil serve as a storehouse of nutrients, but continuous removal of these nutrients without regular replenishment will result in a shortage of essential nutrients.

Notably, the Government of India launched the Soil Health Card (SHC) Scheme in February 2015. This scheme aims to provide complete information about the right timing, amount, type, and method of fertilizer application through soil testing of farmers’ fields. To assist farmers in increasing production through the prudent use of inputs, the government intends to provide soil health cards to farmers. By promoting judicious use of fertilizers, the SHC contributes to sustainable agriculture practices.

Moreover, Punjab Agricultural University (PAU), Ludhiana is also playing an important role in providing SHCs to farmers through testing of their field soil samples. Department of Soil Science, PAU, Ludhiana, and Krishi Vigyan Kendra’s in almost every district of Punjab offer soil testing services at very nominal charges. SHC provides valuable information about soil quality and fertility, enabling farmers to make informed decisions about fertilizer application.

These cards will include crop-specific recommendations for fertilizers needed for each unique field. The comprehensive data on soil pH, soil EC, soil texture, soil organic carbon, phosphorus, potash, zinc (Zn), iron (Fe), manganese (Mn), and copper are all provided by the soil health card. In addition, it offers details on the issues with sodic soils (kala kallar), saline soil (chitta kallar), and appropriateness for orchard plantations. The technique used for soil sampling has a major impact on the accuracy of soil testing. The only way to get a clear picture of the soil fertility of a field is to take a representative soil sample from it. To prevent using too much fertilizer, farmers should have their soil evaluated before adding any organic or chemical fertilizers.

Soil Tests and More:

Detailed information on soil pH, electrical conductivity, soil organic carbon, available phosphorus, available potash, and soil texture are provided by the SHCs. Thus, farmers are advised to employ a balanced combination of chemical and organic fertilizers based on the information gathered.

1. Soil pH: Soil acidity or alkalinity is determined by the pH of the soil. A soil's pH can be classified as acidic (below 6.5), normal (6.5 to 8.8), alkaline (from 8.8 to 9.3), or kala kallar (pH above 9.3). In Punjab, acidic soils are quite uncommon and can be recovered by applying lime. Green manure crops and well-rotted farm yard manure can be used to restore alkaline soils. Depending on the degree of alkalinity, gypsum may be administered to kala kallar soils. It is advised to apply a 25% higher dose of nitrogen than usual to these soils.

2. Soil EC: An essential indication of soil health, soil electrical conductivity (EC) is a measure of the number of salts in the soil. It has an impact on plant nutrient availability, crop suitability, crop yields, soil microorganism activity etc, Excess of salts hinders plant growth by affecting the soil-water equilibrium. EC below 0.8 milli mhos/cm is normal for crops and where EC is more than 0.8 milli mhos/cm soils are saline. Saline soils require a 25 percent additional dose of nitrogen over the recommended quantity. It is advantageous to add agricultural crop residue, green manures, and organic manures. In such soils, farmers are advised to avoid gypsum.

3. Nitrogen: The organic carbon (OC) content of soils is taken as an index of nitrogen availability and thus used to make fertilizer recommendations for nitrogen (N). Soils are categorized as low (less than 0.4% OC), medium (0.4 to 0.75%), and high (more than 0.75%) based on their OC content. Because they have a lower potential to supply nitrogen than normal soils, low-OC soils need 25% more nitrogen from fertilizers. However, compared to medium soils, use 25% less N in high OC soils.

4. Phosphorus: Based on the amount of available phosphorus (P) in the soil, soils are categorized as low (less than 5 kg P/acre), medium (5-9 kg P/acre), high (9-20 kg P/acre), and very high (more than 20 kg P/acre). If the soil has low P content, apply 25% more P fertilizer than recommended, and if the soil has high P content, apply 25% less. If the soil has a very high P status, omit applying P for two to three years and then retest the soil. OC affects the amount of P needed in fertilizer in addition to the soil's P status (Table 1). As a result, fertilizer P dosage needs to be modified based on both available P status and OC content, as indicated below:

Tale 1: Recommendations for fertilizer P based on OC content and available P in soils:

5. Potash: Soils are categorized as sufficient (more than 55 kg K₂O/acre) or deficient (less than 55 kg K₂O/acre) based on the amount of soil available potash (K). It is imperative to conduct soil testing for potassium deficiency to ensure that crop productivity is not impeded by the application of potassium fertilizer, which is only advised in soils lacking in potash.

6. Micronutrients: As agriculture expands, more high-yielding crop varieties are cultivated, and higher analytical fertilizers are used, micronutrient deficiencies are starting to limit crop yields in many regions. Depending on the crop in question, a soil application of 10–25 kg zinc sulfate is advised in zinc-deficient soils (less than 0.6 kg Zn content/acre). In sandy soils, manganese deficiency usually manifests itself in wheat and berseem crops that come after rice. Spray the wheat crop with a 0.5% MnSO₄ solution 2-4 days before the first irrigation, and then do three more sprays every week if the soil has less than 3.5 kg of manganese per acre. Similarly, under paucity of water, chlorosis among paddy/basmati seedlings starts appearing due to iron deficiency which can be rectified by giving 2 or 3 sprays of 1% FeSO₄ solution at weekly intervals.

The data indicates that SHC would provide information about a particular nutrient content whether deficient or sufficient. Finally, detailed information about the method of soil sampling, for knowing fertilizer requirements, for problematic soils as well as for orchard plantations is given below for a better understanding:

Know the Method of Soil Sampling:

Soil samples may be drawn from the field after the harvest of the crop so that the need of the fertilizer requirement for the next crop can be assessed. Make a V-shaped cut with a spade or khurpa up to a depth of 6 inches after clearing away any surface debris, if any (as indicated in Figure 1). Remove about 1-inch-thick uniform slice of soil from any side of the cut.

Similarly, collect soil samples from 7-8 randomly selected spots in a crisscross pattern in the field having a similar appearance. But if the fertility or texture of the field looks different then take samples from each field separately. Mix thoroughly the soil collected from different spots of the field. If soil contains some concretions, stones, or gravel, do not throw them out but include them in the sample. Take approximately 500 grams of the mixed soil. If the soil is wet, dry it under shade. Put this soil in a clean cloth bag along with a slip of information like the name of the farmer, address, date of sampling, cropping sequence, source of irrigation, kind and quantity of fertilizer used, etc. In case of wet soil use a small separate polythene bag for the slip so that it will not get damaged with moisture.

In case of problematic (Kallar) soil, dig a foot-deep pit in such a way that one side of the pit should be vertically straight and the other slanting as shown in Figure 2. On the vertically straight side put marks at 6 inches, 1 foot, and 2 feet depth. Remove about 1-inch-thick soil layer from 0-6 inches, 6 inches to 1 foot, 1 to 2 feet, and 2 to 3 feet soil layers with the help of khurpa or kahi to collect about 500-gram soil from each layer. A total of four soil samples will be collected. Put the samples in a separate clean cloth bag depth-wise. Mention the field number, address, depth of samples, etc. in a slip and put that slip in the respective sample.

Furthermore, before planting orchards soil samples should also be tested for pH value and calcium carbonate content along with general fertility status. For this, dig a 6-foot-deep pit in the center of the field as shown in Figure 3 in such a way that one side of the pit should be vertically straight and the other side should slant. Collect 500 g of soil by taking a 1-inch-thick layer from the surface to 6 inches, 6 inches to 1 foot, 1 to 2 feet, 2 to 3 feet, 3 to 4 feet, 4 to 5 feet, and 5 to 6 feet of soil layers separately. A total of seven soil samples will be collected. If there is any concretion layer at some depth, sample it separately note down the depth and width, and mention it on the slip accompanying the sample. Put a slip with all the information regarding the soil sample as mentioned earlier.

After collecting the soil samples, they should be sent directly to the Soil and Water Testing Laboratory, Department of Soil Science, PAU, Ludhiana, or nearby Krishi Vigyan Kendras or Department of Agriculture and Farmers’ Welfare, Punjab laboratories for testing, It is advisable to get the soil tested after every 2-3 years interval and also for each crop rotation so that current data on changes in soil nutrient status may be used to maintain soil health while also obtaining good economic returns from crops.