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Soil Analysis: How to interpret your soil test

If you use your soil to earn your living, soil test should be a routine part of your management. A soil test gives you a snapshot of some of the nutrients in your soil and helps you decide which ones to add to make your soil more productive. However, it is not a magic formula, and test results need to be considered together with plant tissue tests, and your farm’s cropping, pasture and fertilizer history.

If you sell produce off the farm (including milk), you need to test soil annually because crop removal rapidly depletes the soil of nutrients. If you graze animals you need to test every 2–3 years to ensure nutrients are in balance.

 Soil test results

This article contains information to help you understand the most significant results, but it is important that you back up this broad interpretation with advice from your horticulturist or agronomist.

pH

Preferred level pH (CaCl2): 5.0–5.5

During soil test, soil acidity is measured on a pH scale from 0 (most acid) to 14 (most alkaline), with 7 as neutral, that is, neither acid nor alkaline. The scale is logarithmic, that is, going down the scale from pH 7 (neutral), each number is 10 times more acid than the one before it. For example:

  • soil with a pH of 6 is ten times more acid than soil with a pH of 7 (neutral);
  • soil with a pH of 5 is one hundred times more acid than soil with a pH of  7.

The term CaCl2 after the pH figure signifies that the pH was measured in a solution of calcium chloride, a test preferred by most soil scientists. When soil pH is measured in a solution of CaCl2, the pH is 0.5–0.8 lower than if measured in water.

Kenyan soils are less acidic, so pH 5.0–5.5 (CaCl2) is suitable for most agricultural and horticultural purposes. Soil pH levels above 5.5 are costly to maintain in this environment. Soil pH below 5.0 can be raised by applying lime to the soil.

Cation exchange capacity (CEC)

Preferred level: above 10

This is a measure of the ability of the soil to hold the nutrients calcium, magnesium and potassium. Good fertile soils with high clay content and moderate to high organic matter levels usually have a cation exchange capacity of 10 or higher. (Note: a ‘cation’ is a positively charged ion.)

Exchangeable cations

The major cations are calcium, magnesium, potassium, sodium and aluminium. These are held in the soil by organic matter and clay. The preferred percentages and the suggested quantities of exchangeable cations in the soil are given in Table 1.

Table 1. Preferred percentages of exchangeable cations as a proportion of CEC, and suggested quantity values
Cation Preferred percentage (%) Suggested quantity (meq/100 g)
Calcium >5
Magnesium >1.6
Potassium 2–6 >0.5
Sodium 0–1 <1.0
Aluminium 0 0

Note: meq/100 g or meq% is the same as cmol/kg.

If your soil test report does not provide percentages, you can calculate them yourself by dividing the quantity of each cation (the meq figure) by the CEC figure, and multiplying the result by 100.

  • Sometimes the level of hydrogen cations is reported, but this should not be added to your total CEC.
  • If any other cations are reported, such as manganese, this may indicate a toxicity problem.
  • High levels of aluminium are toxic to some plants, and this situation is usually associated with more acidic soils.
  • High sodium levels can indicate sodicity problems (i.e. soil structure problems), or salinity problems.

When your soil test report gives quantities in parts per million (ppm), you can use the following conversions to obtain meq figures:

Cation Number to divide by
Calcium Divide by 200
Magnesium Divide by 120
Potassium Divide by 390
Sodium Divide by 230
Aluminium Divide by 90

Calcium/magnesium ratio

Preferred level: above 3

The calcium/magnesium ratio is found by dividing the quantity of calcium (meq/100 g) by the quantity of magnesium (meq/100 g). If the figure is below 2, it is more difficult for plants to take up potassium, and there can be problems with soil structure breaking down due to dispersion.

If you use dolomite (2 parts calcium to 1 part magnesium) regularly, your soil’s calcium:magnesium ratio will fall because too much magnesium is applied compared with calcium. Calcium can be added in the form of gypsum or lime. High calcium:magnesium ratios up to 20:1 have not been shown to adversely affect plant yields.

Phosphorus

There are two different tests for phosphorus in NSW: Bray and Colwell. Since they give very different results, it is important to know which one is used in your report.

Bray phosphorus levels vary with land use:

  • 15–20 mg/kg for dryland pastures
  • 25–30 mg/kg for irrigated and improved pastures
  • 30–50 mg/kg for tree crops
  • 50+ mg/kg for vegetables.

Note: ‘mg/kg’ is the same as ‘parts per million (ppm)’.

Colwell levels vary from 20 to 100 mg/kg depending on soil texture.

The Bray test tends to be more suitable for the North Coast’s acid soils. Because phosphorus tends to tie up with aluminium and iron and become unavailable to plants in acid soils, it is important to keep your pH at around 5 if your soil is to benefit from phosphorus.

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Nitrate nitrogen

Preferred level: none calibrated

Nitrate levels fluctuate widely, depending on the season or rainfall. No levels have been calibrated for Kenya, but agronomists generally like to see a level of 10 mg/kg or more in pasture soils, and a level greater than 20 mg/kg in horticultural crops.

Conductivity (salt)

Preferred level: below 0.15 dS/m (EC1:5)

Electrical conductivity is a measure of salts in the soil. A productive soil’s conductivity should be below 0.15 dS/m (decisiemens per metre).

Plants vary in their reaction to salt stress, from ‘sensitive’ to ‘tolerant’, and the degree of reaction is less in clay soils than in sandy soils. For this reason, soils affected by salt should also have a saturation conductivity test (ECse). However, these results should not be compared with EC1:5 figures.

Salinity problems can be caused by too much fertiliser, salty irrigation water or saline ground water. Salts can be leached out with rainfall or low salinity irrigation water without affecting soil pH. Because of its high rainfall, the North Coast generally does not have a great problem with soil salinity except in some low, poorly draining soils close to tidal rivers.

Trace elements

Preferred level: varies according to crop

The extraction procedures for trace elements can vary between laboratories, resulting in different figures. A general guideline for the preferred level of trace elements in the soil is given in Table 2:

Table 2. Preferred level of trace elements in soil
Trace element Preferred level in soil (mg/kg)
Arsenic <20
Boron 0.5–4
Cadmium <1
Copper 2–50
Lead <35
Molybdenum 2
Mercury <1
Nickel 1–20
Silicon >10
Sulfur 10–20
Zinc 1–200

If you suspect your soil has a trace element deficiency, have a plant tissue test done.

Organic carbon

Preferred level: above 2%

Organic carbon is a measure of the organic matter in the soil. It includes undecomposed plant litter, soil organisms and humus. Soil organic carbon stores important nutrients, stabilises soil structure and feeds soil microbes. If soil organic carbon is declining over time, then consider practices such as green manure crops, minimum tillage, mulching or strategic grazing.

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Other soil properties

Laboratory tests are important but they will not alert you to soil compaction, structure decline, erosion or subsoil problems. These types of degradation are much harder to fix than a nutrient deficiency, and recognising the problem early can make a big difference. When collecting your soil samples, note the following:

  • condition of the soil surface;
  • depth of the topsoil;
  • structure of the soil;
  • penetration of plant roots.

A soil health card is a simple guide to this type of soil examination.

Contact us if you want your soil tested all over Kenya!

 

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Don’t Just Add Fertilizer Know Its Components

According to agricultural experts, farmers ought to learn the components used in fertilisers and if they are beneficial to their soils. Its noted that many farmers just buy fertilisers without knowing what their soils need, defeating the purpose of using them.

The experts have noted that there was improved logistical infrastructure for inputs and outputs in Africa, but more needed to be done.

Kenya is one of the countries that import fertiliser from different companies, with National Cereals and Produce Board (NCPB) being one of the main buyers.

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Fertilizer analysis

Understanding the fertilizer analysis is essential when choosing the right fertilizer to purchase and apply.

Fertilizers, such as 10-20-10, are identified on their package by their chemical analysis.
The three numbers on the bag or container refer to the percentages of nitrogen, phosphorus, and potassium components in the fertilizer.
Chemical fertilizers for the home garden are available in two forms. The granular form is sprinkled on the soil and worked in with a tiller or hand tool. Water soluble types are mixed with water and the feeding is accomplished by sprinkling onto the leaves of the plant (foliar feeding) or used as part of the watering process, in which the plant takes in the nutrients systemically (through the roots).

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Fertilizer components and what each does for your plants

  • The first number in a fertilizer formula is the nitrogen content. Nitrogen is used by plants for producing leaf growth and greener, lusher leaves.
  • The second number in a fertilizer formula is the phosphorus content. Phosphorus is used by plant to increase fruit development and to produce a strong root system.
  • The third number in a fertilizer formula is the potassium (potash) content. Potassium is used by plants for flower color and size. It is also important to the strength of the plant.

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Using Fertilizers Properly

The easiest way to explain this would be that a 100# bag of 10-20-10 converted to component weight would equal

  • 25# of nitrogen,
  • 50# of phosphate, and
  • 25# of potash.

Before applying fertilizer to the garden, it is best to test your soil (or have it tested by sending a sample to http://oxfarm.co.ke/ .
These soil tests will tell you the level of nutrients that are already in the soil, as well as the acidity (pH) of the soil. Adjusting the pH of your soil is essential, because some nutrients may become unavailable to your plants if the soil pH is above or below a certain range.
If your test is done by us, the test results will be much more detailed, and will also provide our recommendations for any needed additions of lime and fertilizer to your soil.
The timing of the application of fertilizers is very important. Too much fertilizer, or applying it at the wrong time can lead to an overabundance of foliage, delayed flowering, leaf and root burn, or even plant death due to excess fertilizer!

Always read the manual of any garden product.

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Take guess-work out of agribusiness by conducting soil test

Why is it important to conduct soil test ?

Soil test is a valuable tool for your farm as it determines the inputs required for efficient and economic production of fruits and vegetables. A proper soil test will help ensure the application of enough fertilizer to meet the requirements of the crop while taking advantage of the nutrients already present in the soil. It will also allow you to determine lime requirements and can be used to diagnose problem areas. It is very important that your sampling technique is correct as the results are only as good as the sample you take. Soil testing is also a requirement for farms that must complete a nutrient management plan.

Getting a soil test is a great way to measure its health and fertility. These tests are generally inexpensive, though well worth any cost when it comes to growing and maintaining healthy plants in the garden. So how often should you do a soil test and what does a soil test show? To answer these questions, it may help to learn more about the soil testing process in general.

Why Test Soil in the Garden?

Soil test Most soil nutrients are readily found in the soil provided that its pH level is within the 6 to 6.5 range. However, when the pH level rises, many nutrients (like phosphorus, iron, etc.) may become less available. When it drops, they may even reach toxic levels, which can adversely affect the plants. Getting a soil test can help take the guesswork out of fixing any of these nutrient issues. There’s no need to spend money on fertilizers that aren’t necessary. There’s no worry of over fertilizing plants either. With a soil test, you’ll have the means for creating a healthy soil environment that will lead to maximum plant growth.

What Does a Soil Test Show?

A soil test can determine the current fertility and health of your soil. By measuring both the pH level and pinpointing nutrient deficiencies, a soil test can provide the information necessary for maintaining the most optimal fertility each year. Most plants, including grasses, flowers, and vegetables, perform best in slightly acidic soil (6.0 to 6.5). Others, like azaleas, gardenias and blueberries, require a somewhat higher acidity in order to thrive. Therefore, having a soil test can make it easier to determine the current acidity so you can make the appropriate adjustments. It will also allow you to fix any deficiencies that may be present.

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How Often Do You Do a Soil Test?

Soil samples can be taken at any time of the year, with fall being preferable. They are normally taken annually or simply as needed. While many companies or gardening centers offer soil testing kits, you can usually obtain a soil test at low cost through Oxfarm Organic Ltd. Avoid having the soil tested whenever the soil is wet or when it’s been recently fertilized. To take a sample for testing garden soil, use a small trowel to take thin slices of soil from various areas of the garden (about a cup’s worth each). Allow it to air dry at room temperature and then place it into a clean plastic container. Label the soil area and date for testing. Now that you know the importance of getting a soil test, you can better manage your garden plants by making the appropriate adjustments from your soil test results.

Take the guesswork out of fertilizing by testing garden soil today. Contact our offices for more details.