By Erica Faber – Technical Manager
Optimum plant growth, performance and production, depends on the balance, combination and concentration of mineral nutrients available in the soil. Plants often face significant challenges in obtaining an adequate supply of these nutrients to meet the demands of general metabolism, growth and production due to the many soil nutrient constraints I will explain in the following. If your soil and leaf analysis is not reflecting adequate, balanced levels of all the nutrients, it means that plant health, production and profits will be affected.
For a fertiliser programme to be successful it needs to be cost effective, able to keep up with the crop demand and be flexible in order to overcome soil and root constraints or plant nutrient deficiencies. Regular leaf sampling is an important management tool enabling us as growers to be more aware of the nutrient status of our crop and giving us an opportunity to address any problems before they have an effect on tree performance, production and our pockets!
IMPORTANCE AND BENEFITS OF SOIL AND LEAF ANALYSIS
- Allows us to monitor the pH of the soil and the nutritional status of the soil and crop.
- Enables us to adjust our fertiliser programs and address any nutritional deficiencies, excesses and imbalances before they adversely affect production.
- Allows us to meet the crops nutritional requirements at critical phenological times in order to support optimal tree performance and maximise yield.
- Gives us an opportunity to determine and address soil nutrient constraints to ensure a balanced supply of essential nutrients for optimum yields.
- Enables us to evaluate the effectiveness of our fertilizer programs. Has our fertilizer program yielded results physically? Can we see a difference in tree health, performance and yield? Is the leaf analysis showing improvements? If not, we need to evaluate why, reassess our program and make the necessary adjustments for an improved outcome for next season. If you are not seeing results or positive changes, your program is not effective and both time and money have been wasted not to mention the lost production potential and income.
- Always remember … input dollars should be spent where there is the greatest chance for the largest return on investments.
- If done properly, tissue and soil analysis will lead to more efficient and economical use of fertilisers. Excessive or inefficient applications, that drives costs up and profits down, can be avoided.
- Plants may not show any visible symptoms, but the nutrient content maybe insufficient enough to reduce the yields. Leaf analysis alerts us to this. If the deficiency can already be observed on the tree, the crop has already lost some potential yield.
- Leaf nutrient concentrations are the most accurate indicator of crop status AND there is often not a strong relationship between the nutrient levels in the soil and in the plant tissue. It highlights all the factors that might influence, limit or restrict nutrient availability and uptake, as well as nutrient antagonisms that may be occurring. This is the reason why our soil and leaf results don’t correlate.
- Allows us to make quick, in-season corrections to nutrient deficiencies through foliar applications of specific deficient nutrients.
- It highlights the nutrient retention capacity of the soil and whether we need to improve this to improve the effectiveness of our fertiliser applications?
- Enables the correction of problems BEFORE establishing an orchard. Trying to do corrections after planting is far costlier and time consuming and also affects overall tree establishment, performance and initial yield.
In order for analytical results to be meaningful, sampling guidelines should be followed. The information is meaningless if the sample has not been taken correctly. One of the basic principle of sampling is to return to the same sampling trees or sites from year-to-year. These trees or sites must be representative of the entire orchard or major portion of the block based on tree observation, past experience, crop yield, soil type etc.
Using sampling trees or sites eliminates year on year variability and results in more accurate analysis. This then gives us more clarity on the effectiveness of our fertiliser programs and the nutritional status of the soil and crop and what adjustments need to be made.
IDENTIFYING YOUR SOIL NUTRIENT CONSTRAINTS
- Soil pH: The availability of nutrients to the plant is affected by the soil pH. Most minerals are most available to plants at a soil pH of 6.4, so this is considered the ideal soil pH. Soil where pH is either too low or too high will results in certain nutrients not being available for uptake to plants even though you are applying them.
- Root health: Any factor that restricts root growth and activity will lead to a reduced capacity for the roots to take up nutrients from the soil. Trees with poor root health or an inadequate root to canopy ratio will not be able to take up sufficient nutrients to support the tree for its optimum performance regardless of how much you apply to the soil.
- Root flush: As root tips mature, a waxy layer forms in the roots that limits water and nutrient uptake. Therefore, to have good nutrient uptake, there needs to be constant regeneration of white root tips.
- Soil fertility: This is a measure of the soils ability to supply essential plant nutrients and soil water in adequate amounts and proportions for optimum plant growth and production. Soil fertility is affected by mineral composition, organic matter, soil water, soil air and living organisms.
Poor soil fertility will therefore affect the availability and uptake of applied fertilisers.
- Soil organic matter (SOM): SOM is a very important factor in soil fertility. It is a reservoir of plant nutrients, has a high CEC, buffers soil pH, and chelates micronutrients. Organic matter exists in different forms in the soil, ranging from living soil organisms to fresh, readily decomposed plant residues to humus. Low SOM translates to less effective use and uptake of applied nutrients.
- Soil microbiology: The uptake of most nutrients is controlled by the relationships between soil microorganisms and plants. A significant percentage of applied fertiliser, gets “processed” through microbes before being available to the plant for uptake. A healthy, active population enhances nutrient use efficiency and also acts as bioagents for the control of pathogens such as Phytophthora. Looking after this aspect of soil fertility is just as important if not more so, than budgeting for and applying fertilizer, yet is often overlooked.
- Cation Exchange Capacity (CEC): Simply explained this is your soils ability to hold onto certain nutrients to prevent them from immediately leaching beyond the root zone. As the roots use up the nutrient supply in the surrounding soil solution, the clay particles and organic matter release more nutrients back into the soil solution, available for uptake again. Cations refer to positively charged ions which is the source of potassium, calcium, magnesium, zinc, manganese and even nitrogen in the form of ammonium. A low CEC would mean that you have soils that are not able to hold onto these nutrients, resulting in a low nutrient reserve. This will dictate how much and how often you fertilize in order for your applications to be effective and available for uptake. Improving soil organic matter levels is important for improving the effectiveness of your fertilizer applications.
- Soil Water: Soil water is necessary to dissolve nutrients and make them available to the plant. Excess water however causes runoff and leaching of nutrients as well as depletes root oxygen supplies and insufficient water will not enable the nutrients to be available for uptake due to limited soil solution.
- Soil temperature: This determines the speed of soil chemical processes that are responsible for the nutrients being available for uptake. Under cool soil temperatures, chemical reactions and root activity decrease resulting in a decrease in nutrient availability and uptake.
- Rate of transpiration: Transpiration is the movement of water through the plant and evaporation from leaves, flowers, fruit etc. It enables the flow of mineral nutrients from the soil via the roots to the plant. Soil water supply, temperature (soil and air), number of leaves (defoliation through mite damage) etc all affect the rate of transpiration. The cooler air temperatures during mid-autumn to early spring will result in a reduced rate of transpiration and with it a reduced flow of nutrients from the soil to the root. The cooler air and soil temperatures, coincides with the period from flower initiation to fruit set as well as fruit sizing and maturation – a period heavily reliant on adequate and efficient nutrients.
- Nutrient mobility within the soil and plant: A lot of nutrients are wrongly applied or are wasted because they never reach their target tissue. The speed at which nutrients can move throughout the soil profile impacts nutrient uptake. Mobility varies from nutrient to nutrient. Very mobile nutrients like nitrates (N) and sulphur (S) can move quickly through the soil profile and reach plant roots more easily than immobile nutrients like phosphorus (P) and potassium (K). It is also important to understand how nutrients move within plants in order to make sure that soil- or foliar-applied nutrients are taken up and moved within the plant to the part of the plants where they will be used. Knowing how nutrients move also can help us apply nutrients at the right time during the season. All nutrients can move in the xylem from the roots to the leaves but then need to be redistributed to all parts of the plant where it is needed via the phloem. Phloem mobility is therefore important for ensuring a nutrient is redistributed. Mobile nutrients, include nitrogen, phosphorus, potassium, magnesium, and molybdenum. In contrast, immobile nutrients do not have the ability to translocate from old to new growth. These include calcium, boron, iron, manganese, zinc, copper and sulphur. If, for even a short time, the soil or roots are not able to supply what the plant requires, rapidly growing plant parts quickly become deficient in the non-mobile nutrients because they cannot be moved from older leaves, roots, stems or wood to the nutrient sink where the plant needs it. A common example is low calcium plant levels growing in soils that have adequate or high calcium levels. This is because calcium is an immobile nutrient. Foliar sprayed calcium is hugely beneficial in fruit crops, to counter the poor mobility of this important mineral. One of the functions of calcium is for the cell division that drives fruit size and quality. A foliar spray of chelated calcium can make a significant difference.
- Nutrient imbalances: It is important to understand the interactions and antagonisms between nutrients to ensure that your fertiliser program is not contributing to poor or limited nutrient uptake. Efficient nutrient uptake requires nutrients to be BALANCED. Imbalances which affect nutrient uptake, will affect tree performance, yield and fruit quality. There are many interactions that can influence availability. These need to be considered when high levels of particular nutrients in the soil interfere with, and have a negative effect on, the uptake of other nutrients. Those nutrients which behave in this way are said to be antagonistic. These nutrient interactions and antagonisms can be the result of imbalances caused by the over application of a nutrient. For example, if you have soils with high levels of zinc, you may find low leaf levels of phosphorus even though the phosphorus level in the soil is satisfactory or high levels of calcium in the soil could be the cause of low leaf levels of magnesium when the soil levels of magnesium are adequate. The diagram below gives a little insight into how complicated and significant the interactions and balance between nutrients are.