How should I fertilize my walnut orchard?

Start with the evidence that this nutrient is short in this block. Choosing a product comes later.

Pale leaves, weak shoots, poor nut fill (kernels that do not fill the shell) and a disappointing yield all look like a lack of nutrients. They are just as often caused by something else:

• too little water
• too much water
• salt
• sodium
• compacted soil
• root disease
• pests
• uneven irrigation
• a heavy crop
• a soil pH that makes the nutrients unavailable to the tree.

Fertilizer will not fix any of these problems. It will cost you money, and it will hide the real problem for another year.

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Check the root zone before you add fertilizer

A tree can only take up what its roots can reach. The root zone is the soil where the roots grow. Before you add any input, check that the roots are able to work.

• Check whether the soil drains, or whether water sits in the root zone.
• Check whether the soil is compacted, or whether there is a layer that the roots cannot get through.
• Check whether your irrigation reaches the whole root zone, and reaches it evenly.
• Check whether salt or sodium is building up.
• Check your soil pH, and which nutrients become unavailable at that level.

If any of these checks shows a problem, fix it first. If the root zone is not working, the fertilizer you apply is wasted. The money you spent on it is the smaller part of the loss. The bigger part is the season you lose, because you believe the problem is solved when it is not.

Split the orchard into blocks before you sample

Sample and feed by block, not by farm. A block is a part of the orchard that you manage as one unit. Make separate blocks wherever soil type or depth, variety, tree age, irrigation design, yield history or management differ.

A composite sample (one sample mixed from several places) is an average. If you mix a strong area with a weak area, the result describes neither of them. You get a middle number that looks acceptable, and you have no idea what to do with it.

Leaf analysis is your main tool

To find out what the tree has, leaf analysis is more reliable than soil analysis. The soil test shows what is present in the soil. The leaf test shows what the tree took up.

Use leaf analysis for routine monitoring every year, not only when something looks wrong. Yield falls before symptoms appear. A block that gives the same result three years in a row tells you something that a single test never can.

How to sample

This is the Californian procedure. It shows the principle:

1. Sample in July. From late June to the end of July, leaf nitrogen, phosphorus and potassium levels are stable enough to compare one year with the next.
2. Take terminal leaflets from fully expanded spur leaves. The terminal leaflet is the one at the tip of each leaf. Take them at about 1.8 to 2.4 m above the ground, from all around the canopy (the branches and leaves of the tree).
3. Collect about 50 leaflets from 5 to 10 trees in each uniform block. Choose the trees at random.

Your own laboratory may use a different walnut procedure. Follow theirs. Which procedure you use matters much less than using the same timing, the same part of the leaf and the same laboratory every year. Then you can compare this year's result with last year's.

Some thresholds to use as a guide

A threshold is the leaf reading at which a nutrient counts as short, enough or too much. These are Californian figures. Check them against the guidelines that your own laboratory uses before you act on them.

Californian guidance. The ranges that count as enough, and the sampling procedures, differ between laboratories. A report from a Turkish or Balkan laboratory may therefore not be directly comparable with these numbers. Ask your laboratory for its own walnut guidelines.

Nitrogen is worth a second look. The old limit for a nitrogen shortage was 2.1%, but more recent work suggests that yields may already be limited below 2.3%. Both figures are in use. If your block is between them, treat the result as a warning, not as a sign that all is well.

Potassium shows why a threshold is not an instruction. The limit for a potassium shortage is 0.9%. But research puts the bottom of the optimum range (the best range for the tree) nearer to 1.4 to 1.5%. A tree at 1.1% does not have a shortage, but it is not in the optimum range either.

Soil and water tests

The soil test

A soil test describes the root zone. It measures pH, salt, texture and nutrient supply. Sample at the depth where the roots are. Walnut roots can reach 1.8 to 2.1 m, but most of them are in the top 60 to 90 cm. Adjust the depth for your own soil layers and for your wetting pattern (the soil that your irrigation water reaches).

The right phosphorus test depends on your pH. In acid soil, iron and aluminium hold the phosphorus so that the tree cannot take it up. In alkaline soil, calcium does the same. The same extraction method (the way the laboratory removes the phosphorus from the soil sample) therefore does not work for both.

Bray P1 is the usual method for acid to neutral soils, and Olsen is the usual method for neutral to alkaline soils. Ask your laboratory for the method that is calibrated for your soil (tested on soils like yours). Never compare a result from one method with a threshold that was written for the other method.

Potassium is usually measured by ammonium acetate extraction. Under Californian conditions, the tree is likely to respond to added potassium when the soil reading is below 150 ppm (parts per million). That number does not apply everywhere. Use the calibration for your own soils.

The water test

Iron and manganese in the water clog micro-irrigation systems. This is a nutrition problem as much as a maintenance problem, because a blocked emitter starves the tree that it serves. The tree lacks water first, and then it lacks everything that you inject after that.

Count what your water already gives you

Irrigation water often carries nitrate (a form of nitrogen that the tree can take up), particularly groundwater in a farming district. This nitrate is fertilizer, and it arrives whether you counted it or not.

For example, an orchard receives 900 mm of water at 4 mg per litre. It gets about 36 kg of nitrogen per hectare during the season, before you spread any fertilizer at all.

Treat the result as an estimate for your nutrient budget, not as a guarantee that the trees took up this nitrogen. Uneven water distribution and leaching (nitrate washing down below the roots) still affect nitrogen that arrives through the irrigation system. They affect it in exactly the same way as nitrogen that you spread.

Count everything else that you put in the water. If you use phosphoric acid to treat your irrigation water, you are applying phosphorus. Include it in the nutrient budget. Do not file it under water treatment.

Nitrogen: how much

Start from the amount that the crop removes. Then adjust for how little of your fertilizer the tree will take up.

Most of the nitrogen goes into the crop. About 80 percent of the nitrogen that a mature walnut tree uses each year goes into the fruit. The fruit is the hull (the outer covering around the shell), the shell and the kernel. When you harvest, that nitrogen leaves the orchard. Leaf fall, flower drop and root turnover (old roots dying and new roots growing) account for much less, and most of that returns to the soil.

Removal figures are a check on whether your plan is realistic. They are not an application rate. Recovery is explained below.

Then allow for poor recovery

In mature walnut orchards, measurements found that the trees took up only about 30 percent of the fertilizer nitrogen that was applied. This share is called recovery. The rest was lost by leaching, escaped into the air as gas, or was taken by the weeds and the ground cover.

This is why sensible applications are larger than the removal figures. It is also why improving your recovery is usually worth more than buying more fertilizer. To improve recovery, split the applications, apply the fertilizer through drip irrigation, and water carefully.

More nitrogen is not better beyond a certain rate

Yield trials with a wide range of nitrogen rates show that yields rise at first and then stop rising. Beyond that point, the extra nitrogen stays in the soil as nitrate, where it is at risk of leaching away.

In several long trials, no nitrogen was applied to established orchards for four to six years. The result was no significant yield loss at all. These orchards were getting nitrogen from the soil, from the water and from their own recycling. This is not an argument for feeding nothing. It is an argument for knowing your sources of nitrogen before you buy fertilizer.

Correcting your rate from the leaf test

• Leaf nitrogen is rising over several years and is above 2.3%. You are supplying more than the trees need. Reduce the rate.
• Leaf nitrogen is falling from year to year. Either apply more nitrogen or improve your recovery. Look at recovery first, because it is usually cheaper.
• Leaf nitrogen is above 3%. Halve the rate, then test again next summer. If the reading has not come down, consider applying no nitrogen at all for one season.

Nitrogen: when

Apply nitrogen when the tree is taking it up. A walnut tree can take nitrogen from the soil from the start of the spring flush until the leaves stop working in late October. The spring flush (the first big burst of new growth) starts around April.

• Apply half to two-thirds of the nitrogen in spring, for the main flush of shoot growth.
• Split the rest into several doses during the growing season.
• Do not apply any nitrogen from mid-autumn through winter. The tree has almost no ability to absorb it. These are exactly the months when rain leaches it away and wet soil turns it into gas.

Late summer is better than after harvest. Nitrogen that you apply by late summer is absorbed before the leaves fall. The tree stores it over winter in the roots, the crown (the base of the trunk, where the roots begin) and the limbs (the large branches).

Next year, this stored nitrogen builds the catkins (the male flowers), the spring flush of growth, the female flowers and the new leaves. A tree that goes into winter with these reserves starts faster in spring.

Decide the dates from bud break (the buds opening in spring), shoot growth and nut development. Also consider whether the soil is moist enough to move the nitrogen down to the roots. Do not decide them from a calendar.

Where nitrogen goes when you lose it

There are three different losses, and each one has a different fix. Find out which loss you have before you change anything. The fix for one loss is the cause of another.

Which nitrogen product?

For growth, every form of nitrogen works equally well. Trials compared ammonium sulfate, calcium nitrate, potassium nitrate, a blended granular fertilizer and a slow-release product. They found no difference in how the trees grew. Do not pay a higher price for a form that promises better growth.

The real differences are in leaching, soil pH and micronutrients (nutrients that the tree needs in very small amounts, such as zinc and boron).

How the forms change in the soil

Walnut trees take up nitrogen mostly as nitrate, and every other form of nitrogen changes into nitrate in the end:

• Organic nitrogen (manure, compost) turns into ammonium over weeks to months.
• Urea turns into ammonium over days.
• Ammonium turns into nitrate over days to weeks.

Because every form ends as nitrate, leaching is a risk with every product. The form only decides how much time you have before the nitrogen becomes nitrate.

• Nitrate is available to the tree immediately and moves easily with the water. It is useful when the tree needs the nitrogen now, but it is risky if you over-irrigate afterwards.
• Ammonium is held by the soil near the place where you put it, and it cannot leach until it changes into nitrate. It is useful in late spring where rain is unpredictable, and on coarse soils.
• Urea, in its urea form, moves almost as easily in the soil as nitrate does. In warm soil, it converts to ammonium within a few days. In cold or coarse soil in winter, a lot of urea can leach before it converts. In that case an ammonium product is the better choice.

The effect on soil pH, and why it matters

When ammonium converts to nitrate, it releases acid. For this reason, ammonium and urea products lower the soil pH over time. Calcium nitrate raises it.

Above pH 7.5, zinc, iron, copper and manganese all become much harder for the tree to take up. If your soil is already above 7.5, then choosing an ammonium or urea-based fertilizer works slowly in your favour. A pH this high is very common on the chalky and limestone soils around the Mediterranean. Trials confirmed that leaf zinc, boron and manganese change with the nitrogen source that is used.

Controlled-release fertilizers

These are coated granules. The nitrogen is released slowly as it moves (diffuses) through the coating, and it is released faster in warm soil. They keep the nitrogen in the root zone, limit how much is available at any one time, and make accidental over-application harder.

In trials, one application of controlled-release fertilizer gave the same growth as six split applications of ordinary fertilizer. In a small orchard, or wherever fertilizer is spread by hand, the saving in labour is often worth more than the higher price of the product.

Manure and compost

Manure and compost are useful, but they are far less predictable than a bag of fertilizer. Their nitrogen content varies from about 1 to 3 percent. How much of that nitrogen becomes available in the first year also varies enormously.

Roughly 90 percent of the nitrogen in chicken manure is available in the first year. Other manures may release less than half of their nitrogen over the same period. The rest is released over several seasons. Allow for what you applied in previous years before you decide this year's rate.

How to apply it

Apply nitrogen in a band, not by broadcasting. Broadcasting means spreading it over the whole orchard floor. This feeds your grass and your weeds as well as your trees. A band down the tree row, roughly 30 to 45 cm wide, puts far more of the nitrogen where the roots are. Broad banding down the weed-free strip is the common commercial practice. It is one more reason to keep that strip clean. Chapter 9 covers the orchard floor itself.

Applying nitrogen through drip irrigation gives the best recovery of all. The measured recovery of fertilizer nitrogen is higher with drip and micro-sprinkler systems than with conventional application. The likely reason is that low-volume irrigation concentrates a dense mat of fine roots in a small volume of soil. That mat takes up the nitrogen far more effectively. This is one more argument for the surface drip system that we recommend in chapter 6.

Soil moisture is part of nitrogen management. The right amount of water moves nitrogen into the root zone. It keeps air in the soil, so that bacteria do not turn the nitrogen into gas. It also helps to convert forms of nitrogen that the tree cannot use into forms that it can use.

Too much water carries your nitrogen down below the roots. Two things matter: even distribution of the water, and irrigation scheduling that matches what the trees really need.

Young trees

A young walnut orchard is a different situation from a bearing one. There is no crop that takes nitrogen away, and the root system is tiny. The damage from over-feeding is structural and permanent, not only a waste of fertilizer. Structural damage means damage to the framework of branches (the tree’s permanent structure).

From year four, the nuts begin to take nitrogen out of the orchard. Calculate that part from the removal figure and your recovery fraction, in the same way as for a bearing orchard. The crop is small at this stage, so the addition is small too.

Walnuts do not start to crop as early as almonds. For this reason, the nuts themselves make no real demand for nitrogen until about the fourth year. Until then, everything that you apply goes into building the tree.

Your first-year trees may need nothing at all. Depending on the site, much or all of the first year's requirement can already be met. The nitrogen in the soil and the nitrogen in the irrigation water can supply it. Check both before you buy anything. Nitrogen is normally the only nutrient that a first-year tree needs, and sometimes it does not need that either.

Split the nitrogen into small doses. Smaller, more frequent doses are far more efficient with young trees, because the root zone that they can draw from is so small. The trials behind the year-one figure divided it into six applications during the season. Controlled-release fertilizer suits a young orchard particularly well for the same reason. It gives a season's nitrogen in one pass, with much less risk of over-applying at any one time.

Over-feeding without burning is the more common problem and the more expensive one, because it ruins the branching structure that you are trying to build. A young tree that is pushed hard with nitrogen grows tall and weak, and you cannot correct this afterwards by pruning. The framework that you build in the first few years is the one that the orchard carries its crop on for the next thirty years. Chapter 7 is about building it.

Beyond nitrogen

Potassium

Potassium is a soil programme, not a spray. Use leaf and soil results together. Then allow enough time. A block that is short of potassium can take more than one season to respond fully to correction of the soil. Start early, and continue the programme.

Potassium sprayed on the leaves (foliar potassium) can help for a short time, but it does not fix the problem. The evidence for it in walnuts is thin, and one trial found no benefit. Use it, if at all, to keep a block stable while you rebuild the soil supply.

Phosphorus

The place where you put phosphorus decides whether it works. Spreading it over the surface is often ineffective, particularly in soils that fix phosphorus (soils that bind it, so that the tree cannot use it). The phosphorus has to go where wetted roots can reach it, inside the irrigation wetting zone (the soil that your irrigation water reaches).

On soil that is high in phosphorus, adding more is rarely the answer. Poor colour and weak growth on a soil that is already high in phosphorus have another cause. Look at water, roots, pH, salt, disease, crop load (how much fruit the tree carries) and micronutrients.

Zinc and boron

Zinc and boron matter most on chalky soils and soils with a high pH, on soils low in organic matter, and in blocks that repeatedly test low.

The signs in both lists can also be caused by water stress, salinity, pests and root problems. Treat these signs as a signal to test, not as a diagnosis.

If a shortage is confirmed on a chalky soil that is low in zinc and boron, foliar zinc and boron applied together improve both growth and cropping. This is one of the clearest responses to foliar feeding in walnut, and one of the few that are worth planning for.

The rest, briefly

• Calcium travels with the water that moves through the tree. Salinity or poor water uptake can leave a tree short of calcium even where the soil has plenty. Fix the water, not the calcium.
• Magnesium is most likely to be short on acid soils and on soils with a low CEC (cation exchange capacity, the soil’s ability to hold and supply nutrients). It is also likely to be short where calcium, potassium or sodium compete with it.
• Sulfur is most likely to be short on coarse soils, on soils low in organic matter, and on soils that have been heavily leached. Test the soil. Do not guess from symptoms.
• Iron is usually abundant in the soil, but the tree cannot use it at high pH. A foliar spray makes the tree green again for a while. It does not deal with the reason why the iron is unavailable.

Foliar feeding

Foliar feeding is worth using in four situations:

• a shortage is confirmed
• you need a response this season
• soil chemistry blocks uptake from the soil
• you need a temporary measure while a correction of the soil takes effect.

The strongest walnut evidence is for zinc and boron on a site where a shortage has been diagnosed. The evidence for everything else is weaker.

A foliar spray cannot permanently replace fixing the soil. A leaf can absorb only small amounts. Use the spray to gain time, and then deal with the cause.

Fertilizing through the irrigation system

Fertigation (applying fertilizer through the irrigation system) is only a method of delivery, and it is efficient only if the system works. A partly blocked system gives you too much fertilizer in one zone and too little in another. The leaf test then averages these zones into a number that describes no tree in the orchard.

The objective: spread the nutrient evenly through the active root zone, without pushing water below it. Water that goes past the roots takes the nitrogen with it.

Placement still matters, even when the fertilizer is fully dissolved. Nitrate and sulfate move easily with the water. Phosphorus and potassium do not. Phosphorus reacts with the soil, and potassium is held on exchange sites (places on the soil particles that hold nutrients). Both therefore stay close to the place where the water enters the soil.

Do not irrigate for longer, or apply more water, just to deliver fertilizer. Do not inject at all when the system cannot keep the nutrient in the root zone.

Mixing stock solutions

• A stock solution has to stay dissolved at the concentration that you intend to use.
• Do not put fertilizers that contain calcium in the same tank as phosphates or sulfates, unless the manufacturer explicitly says that they are compatible. These fertilizers form solids when they are mixed.
• Use a separate tank for acid, with its own controlled injection, so that you can manage pH separately from your fertilizer mix. Follow the safety instructions for the product.

Mistakes worth avoiding

In diagnosis

• Fertilizing “to be safe”. Surplus nitrogen costs money, leaches away, forces weak growth of shoots and leaves, and hides the real problem.
• One programme for the whole farm. Averages hide the differences that you need to see.
• Diagnosing from symptoms alone. Nearly every nutrient symptom looks like a symptom of water, salt, root and pest problems.
• Treating a leaf threshold as an order. Look at the trend, the crop and how far the reading is from the threshold.
• Applying the removal figure as a rate. It is only a check on whether your plan is realistic.

In the nitrogen plan

• One large dose, or one badly timed one. Split the nitrogen over the period when the tree is taking it up. On sandy soil, use six to eight small doses.
• Nitrogen from mid-autumn through winter. The tree cannot absorb it, and in those months it leaches away or is lost as gas.
• Treating “late” as “too late”. Nitrogen that you apply in late summer is absorbed and stored for next spring. Nitrogen that you apply after harvest is not.
• Buying a fertilizer form for better growth. All forms grow the tree equally well. Choose by leaching risk, effect on pH and price per kilogram of actual nitrogen.
• Comparing prices per bag. Compare prices per kilogram of actual nitrogen.
• Buying more fertilizer when the real problem is recovery. Trees take up about 30% of the nitrogen that you apply. Splitting the application and applying through drip is usually worth more than buying more fertilizer.
• Ignoring what your water carries. Nitrate, boron and phosphorus all arrive through the irrigation system.

In the field

• Broadcasting nitrogen over the whole orchard floor. You are feeding your weeds. Apply it in a band down the tree row.
• Broadcasting phosphorus on the surface. Place it in the wetted root zone, or do not apply it at all.
• Assuming that manure is the safe option. Manure leaches like any other fertilizer, and it carries sodium and chloride that build up and damage walnuts.
• Adding boron from several sources. The sources are water, a soil product, a foliar product and last year's application. Add them up before you add more.
• Forgetting hull potassium. If the hulls leave the orchard, a great deal of potassium leaves with them.
• Using foliar sprays as a permanent correction for the soil. A spray is a temporary measure only.
• Injecting into a system that you have not checked. Blocked emitters make the result of your fertilizer plan a matter of chance.

With young trees

• Feeding a newly planted tree before it has 15–30 cm of growth. The tree has almost no roots to take it up.
• Over-feeding young trees with nitrogen. You get long, thin growth, wide gaps between the buds, and a poor framework that you cannot correct by pruning.
• Applying too much at once. You get burned leaves and shoot tips bent into a shepherd's crook. Stay under 28 g for each year of the tree's age, in each application.

Your season, in order

Work through this list block by block, not for the whole farm at once. Send us your leaf analysis, your soil test and your water analysis. We will read them with you and calculate what your block needs.